Potential of suspended growth biological processes for mixed wastewater reclamation and reuse in agriculture: challenges and opportunities

ABSTRACT

Pollution and increasing water demand, especially for agriculture, put severe stress on freshwater sources, and as a result, there is progressive deficit in the global water supply and severe water scarcity is projected in the coming decades. Discharges from domestic, industrial and agricultural activities are potential sources of water pollution, impacting human and environmental health. In the face of growing water scarcity and droughts, coupled with the increasing water demand for irrigation, integration of high water-volume and nutrient-rich industrial effluents, into the existing water management plans for agriculture, could play an important role in tackling the problem of water scarcity. However, there is a gap in knowledge about integration of industrial effluents to sewage treatments and the reuse potential of biologically treated mixed industrial and domestic wastewater in agriculture. This study, therefore, provides a critical review on biological treatment of industrial effluents, including petroleum, textile and pharmaceutical wastewater to better understand the capability of bioprocesses and conditions for efficient degradation of pollutants. The effectiveness of activated sludge-based processes, for the treatment of mixed industrial and domestic wastewater, was critically examined, and biomass acclimation plays a vital role in enhanced biodegradation performance. Finally, the reuse potential of mixed industrial and domestic wastewaters for crop irrigation was assessed by studying the reuse outcomes in different cases where industrial effluents were utilized for crop production. Management practices, such as cultivation of salt- and metal-tolerant crops, blending and dilution of industrial wastewater with freshwater and sewage, could make industrial effluents valuable for irrigation.

GRAPHICAL ABSTRACT

KEYWORDS:

• Industrial wastewater
• domestic sewage
• mixed wastewater
• biological wastewater treatment processes
• water reuse in agriculture

1. Introduction

Global water resources are under increasing pressure from pollution, rapidly growing demands and climate change. With the overwhelming population growth, rapid industrial development, haphazard urbanization and agricultural expansion, freshwater resources are severely impacted, leading to serious deterioration in quality and global water scarcity [1,2]. The release of untreated industrial and municipal wastewaters, from expanding anthropogenic activities into the environment, not only generates physical, chemical and biological pollution that poses ecological and health risks, but also creates serious global freshwater scarcity. Water pollution and scarcity are among the leading challenges facing mankind that require urgent solutions. Wastewater treatment and reuse, particularly for irrigated agriculture, which is rated the largest consumer of available freshwater provides a long-term sustainable solution to water scarcity and water pollution issues across the globe.

The reuse of wastewater in irrigated agriculture is gaining huge popularity as an alternative source of water and nutrients for food production, and it is expected to increase significantly especially in water-drought regions and countries where access to freshwater is declining due to pollution. In the records of wastewater application in agriculture, the majority of the reuse water originates from domestic and municipal sources, while industrial effluents are classified unsuitable for irrigation due to their known chemical origin which unfairly undermines their water-volume and nutrient-rich potential for agriculture. Cost of treatment and public health concerns are constraints for the adaptation of industrial wastewater to irrigated agriculture. To implement industrial wastewater reuse in agriculture, adequate and effective treatment is required to adjust the composition of industrial effluents to obtain approved water quality for irrigation. Mixing of industrial wastewater with domestic sewage dilutes the concentration of recalcitrant and toxic compounds in industrial effluents and enhances their biodegradability amenable to resource-efficient and cost-effective microbial degradation. However, in the developing countries where only 8% of wastewater generated receives some form of treatment before discharge [3] due to serious lack of sewerage facilities and treatment infrastructures, unethical disposal of untreated domestic and industrial wastewaters into the environment has remained the norm, thus creating a complex wastewater matrix. Industrial wastewater mixtures of common occurrence in the developing countries could consist of, but not limited to, effluents from oil refinery and petrochemical industry, textile mill and pharmaceutical operation.

In the face of growing water scarcity, treatment and reuse of mixed industrial effluents and domestic sewage, which represents the scenario in the developing countries, is an emerging area with high research interest. The physico-chemical treatment methods, such as the advanced oxidation processes (AOPs) [4], carbon adsorption and membrane filtration [5], are efficient for industrial wastewater reclamation for reuse purposes; however, they are generally cost prohibitive owing to the relatively high energy, chemical and material requirements. Biological processes, utilizing the biodegradation capability of different microbial strains, are robust, highly efficient, relatively economical and eco-friendly [6]. Activated sludge-based processes, otherwise referred to as suspended growth biological processes (SGBPs), have been extensively studied for the treatment of industrial effluents and degradation of recalcitrant industrial compounds, and by adequate biomass acclimation, promising results have been reported [7,8]. However, there is a lack of knowledge about the effectiveness of activated sludge-based techniques for the treatment of mixture of industrial effluents with domestic sewage. This is because most studies are focused on biodegradation of individual industrial effluents rather than in complex mixtures, and as a result, there is limited literature on this topic. For this study, three industrial effluents, including petroleum refinery, textile and pharmaceutical, were considered to depict a complex wastewater mixture as would be obtainable in the developing countries. Considering the pressure on water resources from wastewater discharge issues in developing countries and increasing water demand which are likely to intensify, reusing mixed industrial and domestic sewage for agricultural irrigation could potentially contribute considerably to improve the sustainability of irrigated agriculture especially in developing countries and water-drought regions, and enhanced access to freshwater supplies.

Therefore, this structured review paper aims to provide a critical review of the potential of mixture of industrial effluents with domestic sewage for agricultural irrigation. Due to the limited literature on the topic, individual industrial effluents, including petroleum refinery, textile mill and pharmaceutical effluents, were first reviewed with respect to water consumption, effluent generation and characteristics to provide a glimpse on the complexity of industrial effluents mixtures, followed by a review on the characteristics and quality of mixed industrial wastewaters from the available literature. The treatment of the individual industrial effluents and domestic/municipal sewage, by activated sludge-based techniques, was critically examined to identify and better understand process conditions and factors that are critical for efficient biodegradation of pollutants in the respective wastewaters. Finally, the potential for reuse of mixed industrial effluents and domestic sewage in irrigated agriculture after biological treatments was assessed by critical review of the literature where activated sludge-based processes were used to treat individual/mixed industrial and complex wastewaters and where such treated wastewaters were reused for irrigation.

2. Mixed wastewater: wastewater generation and characteristic

Mixed wastewater is a heterogeneous mixture of waste streams emanating from various human activities, including industrial operations, domestic activities, and agricultural operations. Most of these liquid wastes, especially the industrial discharges, contain substances that are characterized as toxic and bio-recalcitrant, leading to their classification as persistent organic compounds (POCs) of environmental concern. As industries continue to proliferate rapidly along with population growth, the release of hazardous chemicals and refractory organic compounds into the environment is increasing substantially with alarming threat to public health and aquatic ecosystems. Some of the compounds and elements with priority concern for their potential public health threat include hydrocarbons, phenolic compounds and heavy metals [9], pesticides [10], chlorinated organic compounds and dyes with aromatic ring structures [11,12]. Industries, identified as potential sources of POCs and emerging pollutants, include but not limited to, oil refinery and petrochemicals, textile mill, metallurgical, rubber and plastics, pharmaceutical, tanning industry, pulp and paper board, chemical and paint facilities [13–15].

While there are many industries, whose effluents present serious threats to the public and environment, this review focuses on petroleum refinery effluent, textile mill wastewater and pharmaceutical stream discharge. Knowing that there are very little publications on mixed wastewater in general, and perhaps with none addressing the specific mixed wastewater scenario considered for this study, the individual industries are briefly discussed in relation to water consumption, effluent generation and wastewater characteristics to provide a glimpse about the complexity of their mixture. Giving the limited publications on this topic, industrial wastewater mixtures, containing at least one of petroleum, textile and pharmaceutical effluents and outside the three industries, are considered for the mixed wastewater scenario.

2.1. Petroleum refinery effluent

The petroleum refinery industry consumes a huge amount of water in crude oil refining and other oil-related operations, and consequently, a large volume of liquid wastes is generated. Specifically, crude oil refining consumes nearly 100 gallons of water in processing a barrel of crude oil, and generates about 1.6 times the volume of crude refined as liquid wastes [16,17]. Effluent from oil refinery has several origins, including streams from desalting operation, distillation, thermal cracking, catalytic and hydro-treatment processes, and cooling units [18]. The chemical composition of petroleum refinery effluent may vary considerably depending on crude oil characteristics, plant configuration and process designs, chemical treatments applied, and which units are in operation at any given time [18]. However, in general petroleum refinery effluent is characterized by high organic loads ranging between 3600 and 5300 mgCOD/L [16], and this is as a result of its enrichment in hydrocarbons, phenolic compounds, emulsified oil and grease, sulphides, ammonia, cyanides and other dissolved substances [16,19]. Effluents from oil refinery operations also contain a substantial amount of heavy metals (HMs), adding to the complexity of the waste streams. The characteristics and composition of a typical petroleum refinery effluent are presented in Table 1.

Table 1. Characteristics and composition of petroleum refinery, textile, pharmaceutical and domestic wastewaters.

Parameters	pH	Temp (^oC)	BOD5	COD	TOC	DO	NO3-N	NH4-N	TKN	TN	P	TP	References
Petroleum refinery wastewater	6.0–10.0		205–3378	744–7896	184–217		0.15–4.9	13.5–45	40.6–95		0.67–1.73		[9,17,20–22]
Textile mill wastewater	3.0–13.0	26.7-58.0	157–645	232–2430		1.2 −7.0	0–8.0	0.05-50.8		20	0.002-1.31	0–3	[23–26]
Pharmaceutical wastewater	1.5–9.5		22–6000	80–32500	860–4940	3.0–3.8		148 – 363	156 – 1000	582	0.08 – 5		[27–30]
Domestic wastewater	6.2–7.4	22.5–33	101–300	256–592				40–52	7.1–70		1.56–8.60	4.8	[31–34]
Parameters	TSS	TDS	TS	EC (µS/cm)	Turbidity (NTU)	Colour (Pt-Co)	Cl-	Free Cl	S^2-	SO4^2-	Salinity	Alkalinity	Reference
Petroleum refinery wastewater	280- 340	2200–6267		9430	402–419	3400–3650	2575		14–23	40–137		133–990	[9,17,20–22]
Textile wastewater	23–1200	14–11564	39.33–11689	690–13810		170–4637		0.01 −1.14	0.1–1.94			761.2	[23–26]
Pharmaceutical wastewater	30–2000	134–9320			40–120		80–4200			7.0–1500	0.02–0.03	30–40	[27–30]
Domestic wastewater	220–421	625–794			61–94		388–689		0.76-1.15	195–230		58.8-73.5	[31–34]
Parameters	Total hardness	Oil & grease	Ca^2-	Mg^2-	K^-	Na^-	Al	As	Cr	Cd	Ba		References
Petroleum refinery wastewater	985	97–1057	84	61	80	3369							[9,17,20–22]
Textile wastewater	189.6	0–8.3	0.26–9.2	0.17–3.58		4617	0–0.61	<0.001	0–0.5	0.01–0.05			[23–26]
Pharmaceutical wastewater		35–2000								0.003–0.05	0.04–0.06		[27–30]
Domestic wastewater	275–333	1.7–12			15–27	75–82			0.015-0.043				[31–34]
Parameters	VFA	Phenol	Benzene	Toluene	Ethylbenzene	Xylene	Pb	Cu	Zn	Fe	Ni	Mn	References
Petroleum refinery wastewater	198	98–128	33.31–34.36	38.58–41.08	1.80–1.90	30.03–33.04			0.027–4.3	6.48			[9,17,20–22]
Textile wastewater							0.08–0.09	0–5.14	0–2.93	0–2.14	0.0074–0.04	0–1.65	[23–26]
Pharmaceutical wastewater							0.003–0.04	0.009–0.28	0.01–2.62	0.2–20	0.005–0.02	0.05–5	[27–30]
2Domestic wastewater	35–53							0.015–0.038	0.04–0.08	1.5-2.5			[31–34]

All parameters are in mg/L, unless otherwise stated. pH is dimensionless.

2.2. Textile mill effluent

Textile manufacturing industry uses large amounts of dyes, water and a range of chemicals in the production of different fabrics; and about 20% of the dyestuffs that remained unfixed are washed into the effluent [35]. On global scale, nearly 3.0 × 10⁵ tons of textile dyes are discharged in textile industrial wastewater yearly [36]. Different processes, such as sizing, desizing, bleaching, mercerizing, dyeing and printing, are involved in the production of textile products, and each stage consumes a large volume of water, and different chemicals, such as acids, alkalis, surfactants, hydrogen peroxides, dispersing agents, are utilized [37]. An estimated 0.2 m³ of water is consumed in processing one kg of fabric in the textile manufacturing industry [37]. Consequently, textile manufacturing generates a large volume of liquid wastes characterized as complex mixtures of dyes and a wide range of chemicals, organic compounds and HMs, which are potentially toxic to the environment and humans [37]. The characteristics of textile wastewater, reported in the scientific literature, are presented in Table 1. Most of the dyestuffs used in textile mills today are synthetic as they offer brighter shades and a wider range of colours. These synthetic dyes have aromatic structures, and as a result, they are biologically stable, and tend to bioaccumulate in the environment [12]. Some of the chemical compounds in textile effluents with potential health hazards include amines (from azo dyes), sulphur, chromium, naphthol, acids, hypochlorite, formaldehyde, organic solvents, heavy metals and auxiliary chemicals [38].

2.3. Pharmaceutical effluent

Pharmaceutical industries are engaged in the production of a wide range of pharmaceuticals and personal care products (PPCPs) formulated for the treatment or prevention of diseases and illness for general well-being of humans and animals. Similarly, the sector consumes a huge amount of water in their various operations, including production, formulation, processing of PPCPs and washing of manufacturing equipment and general laboratory cleaning. Consequently, a huge volume of wastewater, containing a wide spectrum of pharmaceutical products, is generated [28], and the products end up in the environment due to their bio-resistant properties. The frequent detection and ubiquity of PPCPs in natural and built water environments, including surface water, groundwater, and wastewater treatment plants [39,40] adds up to their characteristic as biologically active organic micropollutants. Pharmaceutical wastewaters are characterized by high organic loads, nutrients, solids, oil and grease, and residual pharmaceutical products and micropollutants [27]. Table 1 presents the characteristics of a typical pharmaceutical wastewater, as reported in different scientific literature.

2.4. Domestic sewage

Domestic wastewater (sewage) is household-generated liquid wastes, which include blackwater (mainly urine and faeces) and greywater (kitchen, bath and laundry) [41]. It is characterized by high levels of organic compounds, nutrients, solid materials, and fat and oil, as presented in Table 1. Household wastewater may contain some trace metals, such as cadmium, lead, chromium, copper, zinc, and iron [32], and PPCPs. Domestic wastewater, discharged into the environment via drainage systems or septic tanks, reaches the water environment directly or diffusely, and by seepage, thus resulting in eutrophication of receiving water bodies, oxygen depletion, water quality impairment and groundwater pollution.

2.5. Mixed industrial effluents and domestic sewage

Considering the characteristics of the individual industrial effluents presented in Table 1, a mixture of industrial waste streams with domestic sewage, although it will allow dilution of active and toxic substances of industrial wastewaters, will generate a complex matrix with treatment difficulties for biodegradation [42,43]. Mixed wastewater generation is more of a characteristic of the developing countries where sewerage facilities and wastewater treatment infrastructures are in serious lack due to financial constraints. However, the detection of toxic organic compounds of industrial origin, such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and pesticides in sewage and sewage sludges generated in the United Kingdom [44] and USA [45], is an indication of industrial wastewater infiltration into municipal sewers, thus highlighting the extend mixed wastewater generation is practised across the globe. According to [13], about half of 10,200 m³/d industrial wastewater generated in Jorden is released to public sewers and the industries, including food industries, textile, metal and chemical enterprises, oil refinery and power plant produce toxic substances that have serious hazardous implications on the environment [13]. Similarly, the expansion of industries in Eastern China has put enormous pressure on municipal wastewater treatment plants initially designed to treat organic and nutrients-laden domestic wastewater, to begin to receive effluents from industries, such as paper making, tanning, printing and dying that have the potential to upset the biological process [42]. Although this shift in practice allows only pre-treated wastewater, the influents are generally poorly biodegradable with less than 0.1 biodegradability.

Although mixing industrial wastewaters with domestic sewage may provide some degree of dilution to the toxic compounds in industrial streams, treatments by biodegradation will be difficult because of the complexity of potentially toxic compounds that can interfere with the effectiveness of the treatment system. A mixed industrial wastewater with domestic sewage may be characterized as totally recalcitrant and non-biodegradable or poorly biodegradable, depending on the composition and proportion of domestic and industrial components in the matrix. The composition and characteristics of mixed industrial effluents and domestic sewage are presented in Table 2. The table reveals that mixed wastewaters are characterized by nutrients and a remarkable number of hazardous materials, including complex organic compounds, heavy metals, salts, and oil and grease, which vary depending on industrial wastewater sources. In general, a greater percentage of industrial chemical compounds is biologically stable and they tend to bioaccumulate in the environment and exert hazardous impacts. Liquid discharges from heavy industrial facilities are characterized by HMs, hazardous chemicals and organic compounds, such as PAHs, phenols, PCBs, polybrominated diphenyl ethers (PBDEs), phthalate esters (PAEs), pesticides, etc. [46,47]. These compounds are toxic, carcinogenic and mutagenic, and have been classified as priority pollutants by the United States Environmental Protection Agency and European Union, and their discharge into publicly operated treatment works is restricted [46,47]. A mixture of industrial waste streams with domestic sewage presents complex composition that poses grave threats to public health and aquatic ecosystems if it is not adequately treated. The release of nutrient-rich wastewater into the environment is the principal cause of algal bloom and eutrophication in rivers, and which adversely impact aquatic habitants [48]. Similarly, discharge of organic-rich wastewater into the environment results in drastic depletion of dissolved oxygen concentration in receiving water bodies with aquatic kill and ecosystem imbalance as the consequences.

Table 2. Characteristics and composition of mixed industrial effluents and domestic sewage.

Wastewater mixture	pH	Temp (^oC)	TOC	COD	BOD5	BOD/COD	NO3-N	NH4-N	TKN	TN	PO4-P	TP	References
Paper making, tanning, printing and dyeing industries, and domestic	6.75–7.08	11–22		182–636	150–210			9.1–51.6		36.2–75.7	0.32–0.90		[42]
Textile, chemical, food, metal finishing industries and domestic	7.3–9.8			1596–2598	297–780			8.2–14.1	28.7–46.5		4.10–7.40		[49]
Industries not specified and domestic				463									[50]
Dairy and domestic			1379	3383	1941				51			22	[51]
Phosphate treating factory and domestic	3			1470^a	1903^a								[52]
Pharmaceutical industry and domestic	6.4–6.8	20		200–300	90–130			21.38			2.77		[53]
Soup and food industry	6.8–7.5			638–3788	360–1710				7.3–17.9			0.33–1.6	[54]
Oil refinery, petrochemical and domestic				182–497	175–295			63.4–71.2					[55]
Industrial sources and domestic wastewater 60:40% volume ratio	6.7–7.5		30–300	87–800	28–234		<0.01	26.2–38.1					[56]
Pharmaceutical, dye and textile	7.32		274	1152	400	0.34	83						[57]
Wastewater mixture	TSS	TDS	EC	Chloride	Colour	Oil & grease	Cr	Cu	Ni	Zn	As	Others	Reference
Paper making, tanning, printing and dyeing industries, and domestic	302–410	2500											[42]
Textile, chemical, food, metal finishing industries and domestic	320–476	2200–3320					0.2–3.5	1.5–5.0	1.5–3.4	3.5–6.4	<0.05	CN: 0.2–3.4	[49]
Industries not specified and domestic	1375	1545			0.071^b								[50]
Dairy and domestic	831					263							[51]
Phosphate treating factory and domestic							21.1	0.52		12.46		Fe: 6.57 Cd: 1.97 Pb: 0.17	[52]
Pharmaceutical industry and domestic	900												[53]
Soup and food industry	1363 – 3628					40–288							[54]
Oil refinery, petrochemical and domestic		1956–2562				7.1–17.5	0.198					Urea: (24-32);CN: (0.1–0.3);H2S: (1-5.7); phenol: (1.4)	[55]
Pharmaceutical, dye and textile	220	8370	7300	3691									[57]

Note: All parameters are in mg/L, unless otherwise state; pH is dimensionless.

^a kg/d.

^b A.

3. Treatment of mixed industrial effluents and domestic sewage

Technologies available for mixed wastewater treatment include physical and chemical processes, AOPs, biological processes, and integrated systems. Each treatment method offers different pros and cons, thereby leading to a trade-off between efficiency and cost. AOPs have shown good reputation in treating complex wastewater mixtures, but they are both chemical and energy intensive, and as a result, they are economically unattractive. Biological treatments especially the SGBPs are the most preferred and widely used wastewater treatments because they are resource-efficient, robust and cost-effective at least compared with chemical methods [6].

The SGBP can be installed in different variants or application modes, such as sequencing batch reactors (SBRs), continuous stirred-tank reactor (CSTR), step-feed reactors, granular sludge reactors, etc. The mechanism involves the cultivation of diverse microbial populations in aerated and continuously mixed reactors, which, in turn, utilize the organic compounds and pollutants present in wastewater as a source of carbon for growth and cell maintenance; and afterwards are physically separated from the treated wastewater by filtration or clarification. In general, the microorganisms transform both biodegradable and non-biodegradable (by hydrolysis reaction) organic matters as well as nutrients into low energy level and stable by-products, including carbon dioxide, nitrogen, water, and new biomass. By virtue of aeration and mixing, the oxygen requirements for microbial respiration and growth are supplied, adequate dispersion of food is maintained, and organisms are kept in suspension [58]. A schematic illustration of SGBP, employed for wastewater treatment, is presented in Figure 1. Although the SGBP was originally established and operated for the removal of carbonaceous compounds and nutrients in domestic and municipal wastewaters [59], the system has equally shown great potential for the removal of persistent and recalcitrant industrial pollutants by metabolic and co-metabolic degradation pathways [60,61] and adequate biomass acclimation [7]. However, with the very few literature available, there is gap in knowledge regarding the effectiveness of SGBPs for the treatment of mixture of industrial effluents with domestic sewage. To better understand the process conditions favourable for the efficient removal of recalcitrant industrial compounds and critically appraise the SGBPs’ potential for the treatment of wastewater mixtures intended for reuse in agriculture, the performance of SGBPs in treating the individual industrial effluents, including petroleum refinery wastewater, textile mill wastewater and pharmaceutical effluents (for which extensive studies have been conducted) and domestic sewage are critically reviewed and discussed.

Figure 1. A schematic illustration of suspended growth biological systems for wastewater treatment.

3.1. Application of SGBPs in petroleum refinery wastewater treatment

Biological degradation of hydrocarbon compounds in petroleum wastewater has been particularly successful by the synergistic actions of diverse microbial populations. A specific example was a mixed bacterial strains belonging to Pseudomonas, Flavobacterium, Comamonas, Cytophaga, Acidovorax, Sphingomonas, Bacillus and Acinetobacter genera which jointly degraded hydrocarbons in real petroleum wastewater up to 80 and 89% for total hydrocarbons and COD removal, respectively [6]. The record-high degradation efficiencies could be accredited to joint enzymatic activities of the different bacterial cells involved in the process. Biodegradation of hydrocarbons and other substances, in petroleum refinery wastewater with different microbial strains, has been extensively investigated and Table 3 provides a summary of the findings from few selected studies on biological treatments of petroleum refinery wastewater containing diesel fuel.

Table 3. Performance of SGBPs with specific microbial strains and operating conditions for the treatment of Industrial (petroleum refinery, textile and pharmaceutical) effluents.

Wastewater type/pollutant	Treatment type/Scale/ Bacteria Strain	Operation conditions	Efficiency (%)	Summary of results	References
Petroleum refinery wastewater
Synthetic diesel fuel contaminated wastewater	Lab-scale aerobic fluidize bed reactor. Inoculant: Biomass sourced from rotating biological contactor treating diesel fuel wastewater.	Lava rock particles used as support media for microbial growth. 3 months microbial adaptation. pH: 6.7–7.3; HRT: 4-h.	COD: 97 Diesel: 100	(1) After the three-month cultivation period, the biomass became well enriched with microbial populations with diesel degrading genetic traits, achieving 100% diesel oil removal at lower hydraulic retention times, and at influent diesel oil concentration of 200 mg/L.	[62]
Synthetic diesel oil wastewater	Lab-scale experiment performed in Erlenmeyer flasks. Microbial strain: Acinetobacter venetianus	Incubated in a dark shaking incubator at 150 rpm. Incubation period: 12-h. Temp: 30^oC	COD: 57 Diesel: 78	(1) Acinetobacter venetianus was capable of degrading most aromatic hydrocarbons into lower molecular weight compounds, thus, requiring further treatment for mineralization. (2) Biodegradation of diesel oil correlated with the growth of Acinetobacter venetianus. (3) The degradation capacity of the bacterial strain increased with contact time, highest at 96-h.	[63]
Synthetic diesel fuel contaminated wastewater	Lab-scale experiments conducted in Duran Schott glass bottles. Microbial strains: activated sludge consortium (Pseudomonas aeruginosa, Alcaligenes sp., Pseudomonas sp., Shewanella putrefaciens, Morganella morganii ssp. Morganii)	Diesel concentration: 120 mg/L. Incubation temperature: 30^oC. Incubation period: 7 days. Shake at 120 rpm. Surfactants: rhamnolipids, saponins and triton X-100.	Diesel oil hydrocarbons: 75%	(1) Adaptation of activated sludge biomass to diesel oil allowed for the dominance of Pseudomonas genus in the microbial populations. (2) The biodegradation capacity of the consortium for diesel oil and hydrocarbons was improved from 47 to 75% in the presence of surfactants such as rhamnolipids. (3) The surfactants modify the surface charge of activated sludge biomass allowing for greater interactions between bacterial cells and hydrocarbon compounds.	[64]
Synthetic diesel oil	Lab-scale Conical flask reactor experiments. Bacterial cultures: Exiguobacterium aurantiacum and Burkholderia cepacian.		Diesel oil: 66–69	(1) Addition of surfactant (Triton X-100) enhanced the degradation capacity of Exiguobacterium aurantiacum and Burkholderia cepacian for diesel n-alkanes and branched alkanes known to possess recalcitrant properties. (2) The surfactant allowed emulsification of hydrophobic diesel compound, thereby enhancing its bioavailability for uptake by the bacterial strains.	[65]
Synthetic diesel fuel wastewater	Bench-scale MBR inoculated with activated sludge biomass	Plant operation period: 200-d HRT: 27-h	COD: 80 Aromatic hydrocarbons: 92 TPH: 85 Nitrification: 70	(1) Biomass adaptation to hydrocarbons aided for high carbon degradations and nitrification activity. (2) Gradual exposure of microbials to hydrocarbons is a key for enhanced biomass acclimation and biodegradation of hydrocarbon compounds in petroleum wastewater. (3) Pre-treatments for oil and grease removal are required prior to biological treatments.	[66]
Municipal wastewater contaminated with diesel fuel and salt	Lab-scale MBR inoculated with activated sludge biomass	HRT: 16-h	COD: 90 TPH: 88 NH4-N: 33–83	(1) High salinity levels affected the activity of heterotrophic organisms. (2) Gradual and adequate biomass acclimation is critical for the development of formidable microbial consortia capable of degrading of hydrocarbon compounds. (3) Autotrophic species were strongly affected at high salinity levels, increase in salt resulted in a decrease in nitrification activity.	[67]
Real heavy oil wastewater	Field-scale pilot conventional activated sludge system (CAS) with immobilized biological filter (I-BAF) inoculated with seed activated sludge	HRT: 18-h	COD: 64	(1) Phenolics, alkenes, aldehydes and organic acid compounds were completely removed in conventional activated sludge treatment. (2) Alkane compounds were removed in I-BAF but higher molecular weights alkanes (C15-C23) were resistant to treatments (3) Bacteria predominant in CAS operation belonged to Pseudomonas and Planococcus groups, whereas Agrococcus and Acinetobacter groups dominated the I-BAF system.	[68]
Textile mill wastewater
Synthetic wastewater containing Methylene blue (MB) dye	Lab-scale Sequencing batch reactor (SBR) 4 L. Activated sludge from municipal wastewater treatment plant.	Temp: 25; HRT: 8-h; Biomass acclimation: 35-d; Aerobic granulation: 106-d Experiment period: 173-d; 4hr cycle; Anoxic: 30-min; Aeration: 172-200-min; FC: 4-10 mg/L; MLSS: 7800 mg/L	MB: 56 COD: 93	(1) Activated sludge sourced aerobic granules have the potential for degradation of MB dye. (2) Biodegradation was the principal pathway for the removal of MB dye in wastewater. (3) Acetate, soluble starch and glucose are good support substrates for the formation of aerobic granules with bacterial microcolonies with the potential for degradation of cationic dyes.	[69]
Synthesized MB dye wastewater	Lab-scale Sequential anaerobic/aerobic bioreactor. Inoculant: anaerobic and aerobic sludge from sewage treatment plant.	Biomass acclimation period: 23-d at HRT: 24-h Reactor HRT: 18-h; pH: 7.5–8.6;	COD: 94 MB: 90	(1) The sequencing anaerobic–aerobic bioreactor was able to biodegrade MB dye up to 90% at dye concentration of 70 mg/L. (2) The prolonged biomass acclimation allowed for dominancy of microbial populations capable of secreting enzymes required for the degradation of methylene blue dye.	[70]
Real textile wastewater containing MB dye	Lab-scale Rotating biological contactors using Irepex lacteus fungus biomass		Complete decolourisation of MB dye.	(1) The Irepex lacteus fungus biomass was capable of complete decolourization of MB dye and detoxification of recalcitrant compounds in textile dyeing wastewater after 70 days. (2) The fungal biomass degraded MB dye with no formation of toxic intermediates or end products confirmed by acute toxicity tests performed using bacterial luminescence, aquatic plant growth and seed germination.	[71]
Real textile wastewater containing MB dye	Lab-scale experiments using bacteria (Acinetobacter pittii) isolated from dye house effluent	Static anoxic condition. pH: 7.5; Temperature: 38 ^oC Dadbestfood1	Decolourization: 73 MB: 85	(1) The isolated bacterial strain, Acinetobacter pittii had the capacity to degrade MB dye into metabolites that are non-toxic and decolourize the textile wastewater at optimum pH and temperature conditions of 7.5 and 38^oC, respectively. (2) The bacterial strain has wide application for detoxification of textile dye effluents for reuse in agriculture as the phytotoxicity test performed on germination of zea may showed no inhibitory effects on the crop.	[72]
Pharmaceutical wastewater
Real Pharmaceutical wastewater containing Penicillin G (PEN G.)	Full-scale WWTP (anaerobic, hydrolysis and aerobic processes)	Feed Concentration PEN G: 153 µg/L HRT: 30-h	PEN G: 92.7	(1) Aerobic treatment was efficient for the removal of PEN G. (2) Removal of PEN G was accredited to b-Lactamase enzymes secreted by resistant bacteria. (3) hydrolyzation and bio-transformation was the major removal pathway.	[73]
Real wastewater containing antibiotics including penicillin G	Full-scale activated sludge wastewater treatment plants (2nos)	HRT: > 15-h	20–65 19–89	(1) The removal of antibiotics in secondary treatment processes ranged from 19 to 89%. (2) Antibiotics removal efficiencies depend on the chemical properties and structures of the target compound. (3) Antibiotics that readily adsorb onto particulate matter exhibited comparatively higher removal efficiencies. (4) Penicillin G possesses chemically reactive β-lactam rings that are easily hydrolysed by bacterial β-lactamases.	[74]
Real pharmaceutical wastewater containing carbamazepine (CBZ) and diclofenac (DCF)	Full and lab-scale activated sludge-based processes with activated sludge biomass		CBZ: <10 DCF: mainly 21–40	(1) The removal efficiencies of carbamazepine in activated sludge-based processes are generally below 10% while those of diclofenac vary from 0 to 80%. (2) CBZ possesses a unique bio-recalcitrant property especially at low concentrations. (3) The two compounds are classified as non-biodegradable to poorly biodegradable, but certain operational conditions, such as acidic pH, anoxic-oxic ratio, favour the degradation of diclofenac.	[75]
Real municipal treated wastewater spiked with CBZ	Lab-scale plate reactor experiments with white rot fungus (Phanerochaete chrysosporium)	pH: 7.0 DO@ 0.1 mg/L External carbon (glucose) and nutrient (nitrogen) supplements	CBZ: 40–80	(1) The white rot fungus was capable of degrading persistent CBZ in the presence of sufficient carbon and nitrogen substrates.	[76]
Synthetic wastewater spiked with known concentrations of diclofenac (DCF) and carbamazepine (CBZ)	Lab-scale conventional activated sludge process	MLVSS: 2000mg/L; Treatment period: 513-d Temp: 20–22 ^oC pH: 7.5; DO: > 3 mg/L HRT: 18-h; SRT: 70–110-d Feed concentration: DCF – 10 ppb; CBZ: 20 ppb	COD: 90; Nitrification: 87 DCF: <10 CBZ: <10	(1) DCF and CBZ showed low biodegradation kinetic constants of 2.0 and 0.2 kbiol (L/gss day), respectively at 12-h HRT, hence, the poor biodegradation of the target compounds. (2) Sorptivity of the target compounds unto sludge was insignificant at 12-h HRT. (3) Biodegradation correlated with nitrification activity.	[77]
Synthetic wastewater containing pharmaceuticals including diclofenac and carbamazepine	Lab-scale bioreactor inoculated with activated sludge biomass from wastewater treatment plant	Temp: 20.3-27^oC; HRT: 24-48-h; SRT: 1, 5, 13 & 26-d; Intermittent aeration allowing for nitrification and denitrification	DCF: 20–40 CBZ: < 20	(1) Biodegradation of Pharmaceuticals was in synch with sludge age, SRT above 10-d provided optimal conditions. (2) CBZ was persistent to biodegradation and DCF showed wide variations in treatment.	[78]
Real and synthetic wastewater containing pharmaceuticals including DCF & CBZ	Full-scale WWTP including ASP and Lab-scale SBR inoculated with activated sludge biomass	SBR: Reaction time: 5.8-h; Settle: 1.25-h; SRT: 10-12-d; Temp: 12°C; OLR: 0.17 & 0.33;	SBR: Nitrification: 70 DCF: 68–90	(1) Diclofenac was not biodegraded significantly in full scale ASP and the biodegradation rate constant (kbiol) was in the range classified as hardly biodegradable. (2) Degradation of diclofenac varied between 68 and 90% in SBRs and removal correlated with nitrite concentrations and nitrifying bacteria activity. (3) Carbamazepine remained non-biodegradable in all the experiments with kbiol of 0.2 l gSS^−1 d^−1.	[79]

FC: feed concentration; OLR: Organic loading rate (kg BOD₇ m⁻³d⁻¹).

The effectiveness of most biological treatment processes, in degrading/removing pollutants in petroleum refinery wastewater, depends on the ability of the microorganisms to withstand both hydraulic and toxic shocks of bio-refractory compounds, such as phenols, benzene, toluene, ethylbenzene, xylenes (BTEX) and other organic and inorganic constituents. Microbial response to shocks was studied using chromium (VI) and low HRT to simulate toxic load and hydraulic shock, respectively, and the results showed a drastic reduction in microbial activity which was accompanied by poor effluent quality [80]. The reduced number of microbial populations was a manifestation of toxicity effect of chromium compound on bacterial cells which suppressed their activity. Secondly, at short hydraulic retention times wash-out of active microbial cells was almost inevitable.

The low biodegradability to non-biodegradability of petroleum hydrocarbons has to do with properties such as solubility and hydrophobicity. Hydrocarbons are generally hydrophobic and less soluble in water, and as a result, they are poorly available for biodegradation by microorganisms [81]. Different approaches amongst them include the use of surfactants that have been employed to increase hydrocarbon-bacteria interactions which could ultimately lead to the availability of more substrates and biodegradation enhancement. Surfactants, commonly cited in the literature for their potency to increase the solubility and biodegradability of total petroleum hydrocarbons (TPH) in wastewater, include rhamnolipid [82], triton X – 100 [65], Igepal CO-630 [81] and saponins [64]. It was added that surfactants have both hydrophobic and hydrophilic properties and are capable of solubilizing hydrophobic compounds and enhance their biodegradability. In a laboratory-scale study by [82], bio-surfactant rhamnolipid was found to enhance the microbial degradation of diesel fuel hydrocarbons. In like manner, triton X–100 not only enhanced the biodegradability of diesel oil, but also catalyzed the biodegradation rate by 10 days earlier, achieving nearly 70% removal against 53% obtained in triton X–100 free conditions [65]. While the authors presumed the surfactants were active emulsifying agents with the characteristics of producing polymeric substances which have the capacity to entrap biomass and enlarge, and subsequently settle rapidly under gravity, surfactants could also reduce the surface tension on hydrocarbons and increase their solubility for possible degradation by microorganisms.

In aerobic biological degradation of hydrocarbons in petroleum wastewater, different factors, such as microbial species and acclimation, available nutrients, dissolved oxygen level, pH, temperature, salinity, surfactants, concentration of contaminant [83], and HRT [84], have been identified to play a crucial role. In the investigation by [84] high HRT resulted in low microbial growth rate and accumulation of old cells, thus, HRT of 20-h was recommended as optimum for biodegradation of petroleum hydrocarbons in wastewater.

3.2. Application of SGBPs in textile dyes’ wastewater treatment

Microbial degradation process is an attractive and resource-efficient alternative for decolourization and breakdown of dyes and recalcitrant compound in textile wastewater, achieved through two principal mechanisms: enzymatic activity and microbial biosorption. Enzymatic degradation has high industrial application for oxidation of a wide range of industrial pollutants and this has led to research to elucidate their capabilities for degradation of dyes and bio-recalcitrant substances in textile wastewater. A variety of active extracellular and intracellular enzymes, produced by diverse categories of microorganisms under favourable environmental conditions, have shown great potential to decolourize and degrade the textile dyes and those commonly cited in the literature include azoreductases, laccases, peroxidases [36,37]. Biosorption on the other hand, which involves uptake of pollutants by microorganisms and inactive biomass, has been explored extensively using biological and microbial systems, chiefly bacteria, fungi and microalgal for the removal of dyes in industrial effluents, and promising results have been reported [85]. In-depth studies on the potential of biological processes, for degradation of dyes and organic constituents in textile wastewater using specific microbial strains, have been conducted and findings from selected literature, especially on methylene blue dye textile wastewater degradation, are summarized in Table 3.

Biological treatment of textile effluent has been challenging particularly with wastewater, containing azo dyes known to possess highly stable complex structures, which are resistant to microbial degradation and have toxic properties. While biodegradation of textile dyes depends on their molecular structures, microbial populations and the secreted enzymes as well as other factors, the biodegradation process generally involves two phases: anaerobic and aerobic reaction stages, in sequential order. In the first phase, under anaerobic conditions, cleavage and decolourization of azo dye is realized through reduction and pyrolysis reactions mainly by azo-reductase enzymes, but toxic aromatic amines are produced [86,87]. Under aerobic conditions, the intermediate aromatic amines, which are resistant to anaerobic biodegradation, are further mineralized either completely or partially into inorganic molecules, such as CO₂, H₂O and NH₃, principally by oxidase enzymes secreted by aerobic microbes [86,87].

Partial/incomplete degradation of aromatic amines, under aerobic conditions observed in some studies [88,89], was due to lack of adequate microbial populations with the capability to mineralize intermediate aromatic amines [90]. Besides, considering the toxicity properties of aromatic amines, as confirmed by [89], the activities of aerobic bacteria could be suppressed considerably under the influence of these azo dye intermediates, thus leading to their partial mineralization. However, complete degradation of aromatic amines could be realized by the use of well-acclimated aerobic biomass [88].

Other factors, identified to influence the degradation and decolourization of textile dyes in biological treatment processes, include salinity level, pH values, oxygen transfer rate, dye concentration, structure of dye and solution temperature [37]. High concentration of dyes inhibits microbial activity, enzymatic functions, and biomass growth. The pH and temperature of aqueous solution play a vital role in the affixation of dyes onto textile fibres, thus, understanding their influence in biodegradation of dyes is crucial for efficient decolouration and degradation of dye molecules and recalcitrant chemicals in textile wastewater. Effects of pH and temperature on microbial decolourization of textile dyes have been well examined and the pH range of 3–10 [37] and temperature range of 30–40°C [37,91] have been proposed for efficient bio-decolourization of dyes.

3.3. Application of SGBPs in pharmaceutical wastewater treatment

Removal of pharmaceutical compounds, from wastewater in activated sludge-based processes, can be done by biodegradation, sorption and volatilization. Removal by air stripping or volatilization is almost negligible since the Henry constant for most pharmaceuticals is less than 0.005 [75,92]. The removal pathway is largely a function of the polarity of the compound, implying that highly polar pharmaceuticals are preferentially removed through biodegradation mechanisms (transformation and mineralization) [93], while sorption is the dominant pathway for the removal of non-polar pharmaceutical compounds [92]. Hydrophobic compounds, such as diclofenac, triclocarban, and carbamazepine, have high sorption potential toward the microorganisms, hence their removal in SGBPs is primarily by biosorption, whereas hydrophilic compounds are governed by biodegradation [94]. Several studies have been conducted to assess the potential of SGBPs for the degradation of micropollutants in pharmaceutical wastewater and promising results have been reported especially with respect to compounds, such as analgesic ibuprofen and naproxen, antibiotic Penicillin G, lipid regulator bezafibrate [77,78]. Table 3 provides a summary of selected literature on the removal efficiency of PPCPs (diclofenac, carbamazepine and penicillin G) in activated sludge-based processes and the operating conditions for their removal.

Biodegradation is the most plausible mechanism for the removal of a vast majority of micropollutants and their metabolites that does not constitute a secondary pollution [95]. Microbial degradation process can lead to complete or partial mineralization of pharmaceutical compounds depending on the microbial communities present, availability of substrates, redox and operating conditions of the treatment system and characteristics of target compounds [96]. Due to the recalcitrant properties of some pharmaceutical compounds, direct metabolization may be very limited and in such conditions, microorganisms tend to utilize the co-metabolization mechanisms in the premise of growth substrates to degrade the persistent trace organic compounds [97]. For instance, bio-persistent compounds, such as carbamazepine and diclofenac, have shown a substantial degree of biodegradability when supplemented with organic substrates [97] such as acetate and glucose. Operating conditions of activated sludge-based processes found to have an important role in biodegradation of micropollutants, including sludge retention time, hydraulic retention time, biomass concentration, redox conditions, pH and temperature of medium [92,95].

Sorption mechanism relates directly to hydrophobicity of the compound which is a measure of octanol water partition coefficient (K_ow) having three classifications, namely; Log K_ow < 2.5 implies low sorption potential, Log K_ow > 2.5 < 4.0 indicates medium sorption potential, and Log K_ow > 4.0 represents a high sorption propensity [96]. Similarly, sorption is achieved by electrostatic interactions between the positively charged compounds and the negatively charged surfaces of the biomass [60,92]. Other useful properties for prediction of sorptivity of organic micropollutants in activated sludge-based processes include acid dissociation constant (pK_a) and solid-water distribution coefficient (K_d).

Sorptivity of compounds with Log K_d < 2.48 is relatively insignificant, while compounds having Log K_d > 3.2 are readily available for sorption onto biomass [96]. Acidic pharmaceuticals that are negatively charged at neutral pH or that have chlorine atoms like diclofenac and compounds with K_d less than 500 L/kg [92] are not potential candidates for sorption; therefore, biodegradation could be the major removal pathway. Understanding the sorption mechanisms of micropollutants in SGBPs and factors influencing their behaviour is imperative for optimization of wastewater treatment systems.

3.4. Application of SGBPs in domestic/municipal wastewater treatment

Domestic wastewater is characterized by high levels of organic compounds, nutrients, and solids as well as trace amounts of PPCPs, hence, its discharge into the environment without adequate treatments can lead to serious eutrophication in rivers, high organic pollution in receiving water bodies and water quality impairment, thus limiting downstream water uses. Nutrients and organic matters are the primary pollutants of concern in domestic/municipal wastewater and their removal in activated sludge-based processes has been studied quite extensively.

Removal of organic compounds in wastewater is a task for heterotrophic microbes (mainly bacteria and fungi) which utilize the pollutants as source of carbon for energy production (catabolism) and cell synthesis and growth (anabolism) while decomposing/oxidizing the organic materials into carbon dioxide, water and inorganic nutrients [98]. Mineralization of organic material in activated sludge-based processes assumes two distinct pathways: aerobic (principal mechanism) and anaerobic processes. Aerobic processes are commonly employed where higher wastewater treatment efficiency and effluent quality are required. In aerobic conditions, heterotrophic aerobes utilize the free or dissolved oxygen in the oxidation of organic materials to CO₂, H₂O and biomass while under anaerobic environments (without oxygen) organic pollutants are metabolized into methane, CO₂ and H₂O via hydrolysis, acidogenesis and methanogenesis [99].

Nutrient (nitrogen and phosphorus) removal in wastewater is pivotal to prevent eutrophication in receiving water bodies and minimize nutrients load to downstream water bodies. Nitrification and denitrification are two sequential conventional processes by which nitrogen is removed from wastewater and released into the atmosphere as nitrogen gas, with a fraction settled in bio-solids. In the first step (nitrification), aerobic autotrophs consume soluble ammonia and carbon in the presence of free or dissolved oxygen to generate nitrate and more nitrifying biomass. Occasionally, nitrification involves two metabolic processes: first is ammonium oxidation to nitrite and second is nitrite oxidation to nitrate, and these processes are catalyzed mainly by ammonium-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB), respectively. In the second step (denitrification), the heterotrophic microbes under anoxic conditions utilize the nitrates in the solution as source of oxygen (electron acceptor) to produce extra biomass and nitrogen gas [100].

Alternative to the conventional nitrification–denitrification pathway for biological nitrogen removal is anaerobic ammonium oxidation (anammox) process which converts ammonium and nitrite directly to nitrogen gas by anammox bacteria. There are two-step reactions involved in anammox-based processes: partial oxidation of ammonium to nitrite with low DO by AOB, and subsequent oxidation of nitrite to nitrogen gas under anoxic conditions by anammox bacteria such as Planctomycetales [101]. Anammox process has received considerable attention in recent years as an innovative and resource-efficient process considering particularly its relatively minimal oxygen requirements and less sludge production. The process allows a short-cut for nitrogen removal through partial nitrification to nitrite (nitritation) which is then used for anammox activity. In more specific terms, the stoichiometric values for oxygen and carbon requirements and sludge production in anammox process given as 3.43 g-O₂/g-N, 1.71 g-COD/g-N and 0.73 g-VSS/g-N, respectively are substantially lower than the corresponding values 4.57 g-O₂/g-N, 2.86 g-COD/g-N and 1.19 g-VSS/g-N for conventional nitrification–denitrification pathway [102], adding that anammox process reduces power requirements by 64%, COD demand by 100% and sludge production by 80–90% [102]. For stability and efficiency of anammox process in biological nitrogen removal, conditions that promote nitritation over nitrification, such as enrichment of AOB over NOB, suppression of nitrite oxidation, etc. need to be applied. Like the traditional nitrification–denitrification process, the effectiveness of anammox process depends on operational factors, such as dissolved oxygen level, organic carbon content, substrate concentration, nitrogen loading, temperature and pH condition and sludge retention time [103,104]. Although the conventional biological nitrogen removal was originally developed to take place in two-stage reactors, studies have shown that both the nitrification–denitrification process and anammox process could be operated efficiently in a single-stage reactor by the proper management of dissolve oxygen concentration and selection of appropriate operational conditions [101,105].

Biological phosphorus removal (BPR) is undertaken by bacteria that accumulate phosphorus in excess of their normal metabolic requirements, and these bacteria are generally referred to as polyphosphate accumulating organisms (PAOs). Cycling of microbial biomass and influent wastewater between anaerobic and aerobic reactor zones allows for the proliferation of PAOs with the capacity to accumulate polyphosphates intracellularly in aerobic conditions [106]. Under anaerobic conditions, PAOs take up short chain volatile fatty acids (VFAs) like acetates and store them intracellularly as carbon polymer such as poly-B-hydroxybutyrates (PHAs) to be synthesized in aerobic conditions, and by cleavage of the stored polyphosphate the energy required for the biotransformation is generated and phosphate is released into the bulk liquor [106]. Under aerobic conditions, the PHA is metabolized to drive biomass growth and provide the energy required for uptake of all available orthophosphate and storage of polyphosphate, and excess phosphorus is removed from the bulk liquid by wasting polyphosphate-rich sludge [106,107]. Outside the anaerobic–aerobic operation that serves as the major pathway for the removal of phosphorus in enhanced biological phosphorus removal (EBPR), some PAOs are capable of utilizing nitrate or nitrite as electron acceptor instead of oxygen for uptake of phosphorus from the bulk liquid, thus allowing for simultaneous phosphorus removal and denitrification [108]. Like the nitrogen, phosphorus removal can be achieved in a single-stage reactor by providing the enabling environment for efficient operation of specialized microbial populations [106,108].

3.5. Treatment of mixed industrial effluents and domestic sewage by SGBPs

Treatment of mixture of industrial effluents and domestic sewage is an interesting area with high research need, yet it has remained sparsely studied over the years. Therefore, there is relatively very little literature available on the treatment of mixed wastewater by SGBPs. While there are challenges with mixed wastewater treatment practice, some studies have reported considerable breakthrough.

Industrial wastewaters, especially those from chemical-based operations, contain compounds that are toxic and inhibitory to microbial activity [8], and amongst them include chromium, sulphides, phenolics, PAHs, cyanides, etc. Mixing industrial effluents together creates a more complex composition that can easily hamper the oxidation process of biological treatment units. Industrial contaminants interact in complex ways that could drastically change the pH of wastewater to extreme acidic or alkaline conditions making it incompatible with sewage treatment [109,110]. To ascertain treatment compatibility and avoid undue upset of sewage treatment plants, a comprehensive analysis of the industrial wastewater is strongly required. However, of particular concern is the temporal and spatial variability of industrial wastewater, as regards to flow and chemical composition, due to changes in manufacturing products, transitory operation of industrial plants, cleaning and washing, etc., which cause great fluctuations in the characteristics of inlet (influent) wastewater to sewage treatment unit, and consequently may result in abrupt failure of biological process [111]. The high degree of uncertainties, pH fluctuation and variabilities in industrial wastewater characteristics can potentially lead to loss of genetic ability of competent microbial communities already developed through biomass acclimation [112], thus, leading to inefficient biodegradation. However, unit operations, such as equalization and neutralization, are of huge practical importance in industrial wastewater treatment by biological process. The toxicants in industrial wastewaters have negative impact on biomass population, particularly the nitrifying bacteria, which are naturally slow growers that require a longer solids retention time (SRT) over their heterotrophic counterparts [8]. Depending on the origin, industrial effluents unleash acute toxicity and/or lethal effects on microbial communities, which can potentially lead to loss microbial diversity, loss of nitrification capacity, and impair the functioning of biological treatment process.

However, studies have shown that biomass, including nitrifying bacteria, could be engineered/nurtured to develop resistance to toxic substances through microbial adaptations, and consolidate their oxidation reactions/activities for the degradation of recalcitrant organic compounds and nitrification [7,8,109]. Specifically, in the recent study by Zou and co-workers, it emerged that after acclimation to phenol, the biomass became enriched with bacterial populations having phenol-degrading traits and good nitrification activity, thus, achieving high phenol removal and nitrification capacity when utilized in phenol biodegradation studies [7]. It was further highlighted that the adaptation mechanism not only enhanced phenol degradation/removal, but also increased the rate of degradation with total phenol removal in 1.2-h, and by this strategy, nitrifying bacteria were shielded from toxic inhibition [7]. Noteworthy, phenol degradation by the acclimatized biomass was accredited to Burkholderiaceae, Anaerolineaceae, Rhodocyclaceae and Cyanobacteria bacterial strains [7]. In addition to direct phenol impact on nitrifying biomass, [109] highlighted that full-scale nitrification can be inhibited through indirect mechanism of oxygen depletion due to the fast growth of heterotrophic bacteria with phenol degrading traits. Similarly, the crucial role of biomass adaptation in industrial/complex wastewater treatment by biological process was also confirmed in the work by [8] where pharmaceutical/chemical wastewater mixed with municipal sewage was treated effectively with stable nitrification activity up to 40% industrial contribution to treatment plant inflow. Increasing the industrial flow share resulted in nitrification breakdown and which, however, was recovered after prolonged adaptation of nitrifying bacteria. Furthermore, in a recent study by [113] diversified bacteria populations, sourced from municipal wastewater treatment plant, were assessed for the potential to treat simulated mixed industrial and domestic wastewater. Findings indicate that the mechanism of long-term biomass adaptation played a crucial role in development and multiplication of competent bacteria populations and acquisition of new genetic properties capable of degrading complex organic compounds of petroleum, textile and pharmaceutical origins in the mixed wastewater up to 92.1% reduction measured as COD, while maintaining stable nitrification (about 65%) [114]. Considering the high degree of treatment efficiencies achieved through the adoption of certain process conditions that enhance the metabolization of degradation-resistant organics and huge economic benefits with biological treatment technologies, SGBPs could serve as potential treatments for the mixture of industrial effluents and domestic sewage.

In other studies, SGBPs showed promise for the treatment of various complex wastewater mixtures including mixture of industrial effluents and domestic sewage. In the mid-1980s, Nesaratnam and co-worker investigated the effectiveness of activated sludge system for the treatment of combined industrial effluents originating from oil refinery and petrochemical plants and domestic wastewater [55]. According to the study, activated sludge system has a great potential for simultaneous carbon and ammonium removal as well as removal of the industrial pollutants such as chromium, hydrogen sulphide, phenol, and cyanide [55]. It is important to note the toxic compounds only exerted temporal inhibition on nitrifying biomass, but over time the microbial populations recovered and became fully adapted to the pollutants and consequently, a stable nitrification activity was sustained. That is not all, the authors also cited a number of studies conducted before the mid-1980s where activated sludge process was successfully employed for the treatment of mixed industrial and domestic wastewater through strict control of the proportion of toxic compounds discharged into the sewers [55]. SBR, operated under periodic exposure of biomass to set process conditions, was used effectively to treat complex mixture of wastewater from industries producing a variety of chemicals, drugs, pharmaceuticals and pesticides [111]. Authors laid particular emphasis on the loading rate of industrial complexes as a key factor upon which the biological oxidation process depends, and that biodegradation performance was actively maintained at organic loading of 1.7 kgCOD/m³/day but became inhibited at 3.5 kgCOD/m³/day loading of complex organics [111]. The results clearly indicate that microbial populations once fully adapted to set environmental conditions they develop a sort of immunity for the selected pressures and become capable of metabolizing the bio-resistant compounds already adapted to providing their stress history is not exceeded considerably.

Addition of domestic effluents and/or wastes from food manufacturing to industrial wastewaters provides the heterotrophic organisms a background rich in assimilable carbon sources to sustain and support their growth and metabolic functions and which, in extension, enhances the biodegradability of recalcitrant and/or inhibitory organic pollutants in industrial waste streams [115–117]. Industrial effluents, in particular, those originating from chemical-based industries are rich in degradation-resistant organics, and their mixture with domestic (carbonaceous-rich) wastewater for treatment by activated sludge-based processes is a promising approach to efficiently and economically treat complex industrial liquid discharges that may otherwise attract huge treatment costs [8,116]. However, emphasis was made on volume ratio of the mixture as it plays a crucial role in toxic compound assimilation in domestic sewage [54,110]. The appropriate volume ratio of industrial and domestic wastewater in a single mixture for efficient biodegradation varies, mainly due to the characteristics and composition of the industrial streams. There are a number of cases where this treatment practice has been carried out with successful outcomes. In Singapore, mixed sewage at a volume ratio of 60% industrial waste to 40% domestic waste was treated efficiently by activated sludge process [56]. Similarly, in Pistoia, Italy, a municipal wastewater treatment plant with configuration: primary grit and oil removal, chemically enhanced primary sedimentation and a conventional nitrification–denitrification activated-sludge process collects and efficiently treats mixed industrial and domestic wastewater at a ratio of 70% textile wastewater to 30% domestic sewage (Gori et al., [118]). A volume ratio of 1:2 petroleum wastewater to brewery wastewater was revealed to provide the perfect mixture for efficient treatment in fluidized bed reactor [116]. A volume mixture of 40% industrial (pharmaceutical and chemical) effluent to 60% municipal wastewater was efficiently treated using a pilot-scale MBR with good nitrification performance [8]. The complex organic constituents of industrial wastewater mixture from petrochemical and cosmetic manufacturing industries with >80% microbial inhibition were effectively degraded in SBR utilizing acclimated sludge biomass (activated sludge) at a dilution ratio of 1:5 to urban wastewater [119]. With the integration of hydrolysis acidification process to SBR, a higher proportion (>60%) of mixed industrial effluents from papermaking, tanning, printing and dyeing industries was efficiently treated in a mixture with domestic sewage [42]. The hydrolysis acidification process provided conditions for anaerobic digestion of inhibitory organics which improved the biodegradation kinetics of bio-recalcitrant compounds in the industrial effluents [42]. From the various literature reviewed, treatment of complex industrial wastewater mixtures is practically feasible by adopting the mechanisms of biomass adaptation and dilution with urban sewage at a ratio that allows for good assimilation for microbial degradation. The dilution ratio is majorly dependent on the composition and concentration of toxicants in industrial wastewater.

Given that removal of bio-recalcitrant pollutants in biological treatments is not necessarily an indicative of toxicity abatement [112], the performance of biological treatments needs to be assessed on the basis of possible generation of harmful by-products during treatment by performing relevant toxicity tests tailored to treatment goal. In this view, a genotoxicity test, conducted on Salmonella using raw and biologically treated mixed complex wastewater (hospital, industrial and domestic), revealed that as much as the genotoxicity risks confirmed in the raw wastewater, none was identified under the treated wastewater assay [120]. While the efficient treatment performance may not represent various wastewater conditions, the biomass acclimation mechanism has proved to be an integral operating condition in the biological treatment of complex wastewaters. The few available literature, specifically on biological treatment of real mixed industrial effluents and domestic/nutrients-rich, wastewater are presented in Table 4. Based on the treatment outcomes, it could be affirmed that prior acclimation of biomass to bio-recalcitrant pollutants in complex wastewater mixtures presents promising process condition and strategy for biodegradation of toxic and refractory organic pollutants in industrial and complex wastewater mixtures [7]. From both the specific applications of biological processes in mixed industrial and domestic wastewater as well as the individual industrial effluents, there is positive indication that SGBPs might have the potential and capability to improve the quality of complex mixed wastewater considerably. Meanwhile, considering the relatively short-term biomass acclimation conditions and a rather rapid biodegradation assay is commonly used/adopted for the estimation of treatment capacity of suspended growth processes, there is a likelihood of underestimation of the real potential of biological processes for the treatment of industrial effluents. Hence, longer biomass adaptation is most appropriate for activated sludge-based processes receiving industrial wastewater for efficient degradation of bio-recalcitrant industrial compounds.

Table 4. Performance of SGBPs for the treatment of real mixed industrial and domestic wastewater.

Wastewater mixture	Treatment	Removal efficiency (%)	Summary and findings	References
Mixed industrial and domestic (industrial sources not specified)	Biological treatment using cyanobacteria strains (Anabaena ory- zae, Anabaena variabilis and Tolypothrix ceytonica)	Organics–COD: 89; BOD5: 74; FOG: 94 Metals – Zn: 86; Cu: 95	Study indicates that the use of acclimated cyanobacteria species is promising for the treatment of complex wastewater. While organics and FOG removal followed biodegradation route, biosorption was the pathway for removal of metals in mixed wastewater.	[43]
Oil refinery, petrochemical and domestic wastewater.	Activated sludge process	Nitrification: 72 (average) COD: 80–90 (average) BOD5: > 90	Pollutants, such as phenol, CN, H2S and Cr, exerted a temporal inhibition on biomass especially the nitrifying cells and suppressed nitrification activity. However, after biomass adaptation to the pollutants, nitrification resumed appreciably, and pollutants were removed. High nitrification and carbon removal were attained at 7-h HRT as against 12-h HRT.	[55]
Soup and food industry, municipal wastewater	Activated sludge process	COD: 93; BOD5: 94; Oil & grease: 94	It was found that volume ratio of industrial and municipal wastewater is a key factor in the treatment of mixed industrial and domestic wastewater. A volume ratio of 50:50 was identified as suitable for good assimilation of industrial waste streams into nutrients-rich municipal wastewater.	[54]
Petroleum and brewery wastewater	Fluidized bed reactor	COD: 90	Nutrients-rich wastewater improved the biodegradability of recalcitrant compounds in industrial wastewater (petroleum waste stream). Toxic compounds in petroleum wastewater unleashed hazardous effects on microorganisms with population and biomass reduction as consequences. Higher organic removal (90%) at 24-h HRT.	[116]

FOG: Fat, oil and grease.

4. Wastewater reuse in agriculture

Water scarcity is a widespread problem that attracts great concern. Agriculture is identified as the major culprit of this problem as it accounts for 70% of all freshwater withdrawals globally [121]. According to [121], agricultural operations have increased by more than 100% in the past three decades in response to economic and demographic growths, and about 60% increase in food demand is predicted by 2050 with 10% increase in freshwater demand for irrigation. Moreover, with the estimated 5 cubic metres of water required in production of per head daily food coupled with the current pace in population growth and industrial expansion, freshwater scarcity is bound to attain alarming thresholds, especially in regions with low annual rainfall [1]. Wastewater reuse in agriculture is a water resources management measure which is not limited to addressing water scarcity and quality deterioration problems but serves as supplemental nutrient source for increased and sustainable food production for the teeming population at reduced costs.

For the safe use of wastewater in agriculture, guidelines have been established at international, regional and national levels to ensure proper management and administration of reclaimed water and gain public acceptance of the produce. While some of the water reuse criteria like the California guidelines are stricter, the United States Environmental Protection Agency (US EPA) and the World Health Organisation (WHO) standards were established to accommodate possible water reuse in low-income countries, and are used as references by many countries. In general, these criteria, especially the WHO guidelines, focused mainly on the microbial risks of the reuse water, distinguishing between different classes of water reuse in agriculture with particular reference to crop types, effluent quality, applied treatment technology and irrigation methods [122]. The WHO water reuse guidelines have been adapted to specific regions with local epidemiological, sociocultural and environmental factors taken into account. The most commonly used guidelines, for water reuse in agriculture and water quality for irrigation, are presented in Table 5.

Table 5. Agricultural water reuse criteria and guidelines for the interpretation of water quality for irrigation.

Water reuse criteria					Suitability of water quality for irrigation
Parameters	Reuse conditions	USEPA [123]	WHO [122]	EU [124]	FAO [123]
					Indicators	Degree of restriction on irrigation			Trace metals
pH		6.0–9.0				None	Slight to Moderate	Severe	Trace metals	RMC (mg/L)	Trace metals	RMC (mg/L)
BOD5 (mg/L)	Irrigation of crops likely to be eaten uncooked	≤ 10		≤ 10	ECw (dS/m)	< 0.7	0.7-3.0	>3.0	Al	5.0	Li	2.5
TSS (mg/L)		5		≤ 10	TDS (mg/L)	< 450	450–2000	>2000	As	0.10	Mn	0.20
Turbidity (NTU)		≤ 2		≤ 5	Na (SAR) – Surface irrigation	< 3.0	3.0–9.0	>9.0	Be	0.10	Mo	0.01
TC (CFU/100 mL)				2	Na (mg/L) -sprinkler irrigation	< 69.0	>69.0	-	B	0.75	Ni	0.20
FC (CFU/100 mL)		BDL	<1000	E. coli ≤ 10 or BDL	Chloride (mg/L) -surface irrigation	< 142.0	142–355	>355.0	Cd	0.10	Pb	5.0
Intestinal nematode			<1	≤ 1	Chloride (mg/L) -sprinkler irrigation	< 106.5	>106.5	-	Co	0.05	Se	0.02
Legionella spp. (CFU/L)				≤ 1000^a	Boron (mg/L)	< 0.7	0.7–3.0	>3.0	Cr	0.10	Sn & Ti	-
					NO3-N (mg/L)	<5.0	5.0–30.0	>30.0	Cu	0.20	V	0.10
					Bicarbonate (mg/L)	< 9.5	91.5–518.5	>518.5	F	1.0	W	-
					pH	6.5–8.4			Fe	5.0	Zn	2.0

BDL: Below detection limit; ^awhen there is a risk of aerosolization; RMC: Recommended maximum concentration.

Wastewater reuse in agriculture is gaining significant traction and the well-established guidelines that exist particularly in developed countries are critical for regulation of the reuse practice. The non-existence of guidelines or lax regulations on water reuse in most developing countries hampers the key objectives of water reuse. However, to the best of current knowledge, there are no guidelines for emerging contaminants such as endocrine disrupting compounds (EDCs), PPCPs, PAHs, etc. With the current global water scarcity crises coupled with freshwater resources pollution, extensive reuse of both domestic and industrial wastewater in agriculture could serve as a sustainable solution to the freshwater problems. The mixing of industrial liquid wastes with municipal effluents which is prevailing in many cities especially in developing countries would amount to definite application of industrial wastewater in crop irrigation as water reuse in agriculture is implemented across the globe. Therefore, future integration of industrial wastewater into water reuse in agriculture is an area with huge interest, and for safe, extensive and sustainable water reuse practice in agriculture, future inclusion of emerging contaminants in water reuse criteria is highly recommended.

5. Potential of industrial wastewater for reuse in irrigated agriculture

Wastewater reuse in agricultural irrigation (especially of food crops) merits high consideration due to the increasing water demand from the sector. Agriculture accounts for nearly 70% of all freshwater withdrawals worldwide [121], and 15% additional increase is projected by 2050 [125]. Moreover, the presence of beneficial nutrients and elements in wastewater makes its reuse for crop irrigation lucrative and attractive. However, the majority of the wastewater used in irrigated agriculture is sourced from domestic and municipal waste streams, and a very rare consideration is paid to industrial effluents whose chemical origin always overshadows its nutrient-rich and water-volume potential. Consequently, the possibility of applying industrial wastewaters in agriculture has not been extensively studied and only a few literature are available on this topic. To ascertain the reuse potential of this resource in irrigated agriculture, the experiences and outcomes of irrigation with industrial effluents in various case studies need to be critically evaluated. Table 6 presents laboratory, greenhouse and field-scale experiments on the use of mixtures of industrial effluents and domestic sewage (where indicated) for irrigation, and provides crucial information relating to the wastewater treatment type applied and quality of irrigation water used. The information in Table 6 further helps to identify specific challenges faced when industrial wastewater is used for irrigation, best management practices (BMPs) to be adopted for successful reuse practice and assess the potential of biological treatments in improving the quality of mixed industrial and domestic sewage for agriculture.

Table 6. Cases of irrigation of crops with mixed and individual industrial effluents and in a mixture with domestic sewage.

Industrial effluents/wastewater mixture	Type of treatment applied	Quality of irrigation water	Crop irrigated	Scale of reuse study	Wastewater reuse outcomes and observations	Reference
Mixed effluent from spinning and weaving, artificial silk making, and from production of pigments, fertilizers, paper, pesticides, plastic and petroleum.	No treatments indicated. Water samples collected from El-Amia drain.	Fe: 12.82; Mn: 8.90 Zn: 6.01; Cu: 3.99 Pb: 2.98; Cd: 0.71 Ni: 1.02; Co: 0.91	Lettuce, turnip and tomato	Soil pots experiments conducted in greenhouses	Reduction in yield and green pigments production, with 51, 49 and 62% inhibition in leaf area of turnip, lettuce and tomato, respectively compared with control plants irrigated with fresh water. Increased accumulation of heavy metals (Fe, Mn, Zn and Cu) in plant tissues, particularly the roots compared with the controls.	[129]
Mixed industrial wastewater mainly from electric and electronics, automotive, information technology and telecommunication	No treatment indicated. Water samples collected from discharge point at Bayan Lepas, Penang, Malaysia	pH: 7.23; EC: 0.52 TDS: 0.027; TSS: 19 COD: 17.5 BOD5: 8.75; P: 0.5 Ca: 90.2; Mg: 20.5 K: 18.3; Fe: 0.9 Mn, Ni, Zn: 0.0021 - 0.0038	Lablab purpureus	Hydroponic cultivation	Chromium ions retarded the growth of Lablab purpureus seedlings, specifically by 68% and 76% inhibitions in root and shoot growth, respectively. There were significant reductions in photosynthetic pigments including chlorophylls a and b, and carotenoids.	[130]
Mixed industrial effluents with domestic sewage	Untreated	pH: 7.3-9.4 EC: 1.1-2.6 TDS: 840–1258 TSS: 330–495 Pb: 0.53-1.39 Ni: 0.30-0.86 Cr: 1.0-19.0 Cd: 0.01-0.05 Zn: 0.02-0.10	Lady finger (Abelmoschus esculentus) and tomatoes (Lycopersicon esculentum)	Field-scale experiments	Long-term irrigation leads to build-up of heavy metals in the soil and transfer of pollutants to the soil and the vegetable crops.	[131]
Agro-industrial wastewater	Activated sludge process, ultrafiltration and UV radiation	pH: 6.8 EC: 2.8 TSS: 26.6 TN: 3.9 PO4-P: 0.4 BOD5: 23.1 COD: 40.6 Surfactants: 0.7 E. coli: 7.0E+04	Tomato and broccoli	Field-scale experiments	Irrigation with treated wastewater, including activated sludge-treated effluent, did not affect the market yield and qualitative traits of tomatoes and broccoli significantly. The dry matter and soluble solid contents of tomato fruits and the dry matter contents and diameter of broccoli heads were not significantly affected by the treated wastewater. However, faecal coliforms and total heterotrophic counts detected in the crops suggest the need for disinfection of biologically treated wastewater prior to reuse for irrigation.	[132]
Electroplating effluent	Treated. (Industrial treatment plant) treatment type not indicated	pH: 2.2; EC: 24750 TS: 4825; TDS: 2228 Cl^-: 3638; SO4^2+:993 COD: 1359 BOD5: 205 Na: 88326 K: 206.4; Ca: 0.1	Wheat (Triticum aestivum) and Barley (Hordeum vulgare)	Laboratory and pot cultured experiments	Seed germination and growth performances were maximum at 25% effluent concentration and decreased as effluent concentration increased. Plumule and radicle lengths were highest at 25% effluent concentration, higher than those under control treatment with tap water.	[133]
Textile effluent	Treated. Treatment type not indicated	pH: 8.2; EC: 7.34 SS: 128 BOD5: 496 COD: 960 TN: 238; PO4-P: 14 Na: 142; K: 7.0 Ca: 267; Cl^-: 692 Alkalinity: 792	Wheat cultivars (WH-147, PBW-343 and PBW-373)	Laboratory and pot-cultured experiments	Dilution improved the quality of wastewater to discharge limits. Shoot and root yields, and photosynthetic pigments of the cultivars at 6.25% effluent concentration were higher than those under control treatment.	[134]
Distilleries and breweries	Dilution	Raw wastewater: pH: 4.1; EC: 25.3 TN: 2000 NH4-N: 125; TP: 240 NO3-N: Trace K: 13,000; Cl^-: 4100	Tomato, chilli, bottle gourd, cucumber and onion	Laboratory experiments	Critical dilution percentage for germination of all crops except tomato and gourd was 25%, while 5% was for tomato and gourd.	[135]
Textile wastewater	Anoxic-aerobic photobioreactor	BOD5: 17.1 COD: 172; TOC: 64 TN: 131; NO3-N: 13 TP: 17	Radish (Raphanus sativus)	Laboratory experiments	Plant growth and yield were higher under treated wastewater irrigation over the freshwater control irrigation by 49, 19.2 and 62% for dry weight, leaf number and leaf area, respectively.	[127]
Mixed textile (50%) and domestic sewage (50%)	Lab-scale UASB Treated and diluted with distilled water	COD: 43.4 BOD5: 27.8 TSS: 56.1; TDS: 37.3 CO3^2+: 12.1 Na: 19.1; K: 18.3 Ca: 10.0; Cl^-: 15.8 Zn: 18.1; Cd: 14.2 Cu: 16.5; Cr: 15.8	Zea mays	Laboratory experiments	Growth performance and yield of Zea mays irrigated with treated wastewater with respect to chlorophylls, carotenoids and leaf number did not differ considerably from those that received control treatment. The concentrations of heavy metals, such as Cd, Cr, Cu and Zn in plants, were within the National environmental quality standards. Dilution of treated wastewater with freshwater further improved the quality and enhanced the productivity of the reused water.	[128]
Mixed textile and domestic sewage	Treated	pH: 7.8; EC: 4.2 NO3-N: 3.9 NO4-N: 4.1 PO4: 8.6 K: 22.4; Ca: 99 Mg: 84.2; Na: 275 Fe: 0.72; Mn: 0.06 Zn: 0.04; Cu: 0.09 Pb: 0.02; Ni: 0.01	Olive (Olea europaea L) cultivars: Nabali,	Greenhouse and field-scale experiments	N, Cu, Mn, Fe, Pb and Na contents of the leaves increased with increasing salinity level of treated wastewater and which was accompanied by a decrease in K and Mg contents of the leaves. P, Na, Cl, Mn, and Pb contents of the roots increased and K decreased as salinity level in treated wastewater increased. Except for Fe, Na and Cu, the elements’ concentrations in plant leaves were within the adequate levels.	[136]
Mixture of textile effluents with urban sewage	Biological oxidation, clariflocculation and ozonation	PAHs: Acenaphthylene: 20.4 Fluorene: 36.2 Phenantrene: 145 Anthracene: 15.2 Pyrene: 5.8 Chrysene: 5.2	Strawberry (cv. Camarosa)	Soil-pot experiments	The translocation and bioaccumulation of residual compounds, such as PAHs, PCBs and Cr(VI) in treated wastewater to strawberries, was studied. The concentrations of the compounds were within the limits set by Italian/European Government. Besides the Cr(VI), all target compounds were not detected in both the strawberries and soil above the matrices with quantitation limits (MQLs).	[137]
Mixed textile (60%) and domestic (30) wastewater	Conventional ASP + sand filtration + disinfection	pH: 8.1; EC: 1.972 TSS: 3.4; COD: 68.2 NH4-N: 0.74 NO3-N: 15.3 PO4-P: 0.1; K: 22.2 Cl^-: 390; Na: 318 Ca: 70.8; Mg: 22.9 Alkalinity: 317 Surfactants: 0.79	Ornamental shrubs (Buxus, Photinia, Pistacia and Viburnum)	Outdoors experiments in soil pots	Comparable growth outcomes between plants that received treated wastewater and control treatment (well water). Drip irrigation was particularly crucial in administering the reclaimed water to the plant safely and avoid the possible negative impact due to contact with pollutants in water.	[118]
Fertilizer industry effluent	Treated	pH: 7.1; EC: 1.3 TDS: 1490; TSS: 120 BOD5: 12.5 COD: 42.8 PO4^+: 15.6 Ca: 184; Mg: 10.1 Cl: 4012; K: 12.4 Na: 234; SAR: 2.97	Wheat crop (Triticum aestivum, var. L. cv. HD-2285)	Field-scale experiments	The crop productivity measured by no. of plants/ m^2, no. of ears/ m^2, length of ear/ m^2, plant height (cm), yield (Q/ ha) and protein content were higher in crops that received treated wastewater in comparison to those under tube well irrigation (control).	[138]
Produced water from oil and gas operations	Dilution with municipal treated wastewater and missed with nutrient solution	pH: 5.81; EC: 2.4; TOC: 200; TN: 259 Alkalinity: 20 B: 3.19; Ba: 0.49 Ca: 39.1; Cu: 1.45 Fe: 3.17; K: 181 Mg: 4.61; Na: 527	Wheat	Soil pot experiments under greenhouse conditions	Crops irrigated with 90 and 50% diluted wastewater displayed some forms of developmental arrest. There were reductions in above and belowground biomass and photosynthetic production. Grain yield reduced by 70–100% in produced water irrigation compared to the tap water control.	[139]
Mixture of industrial wastewaters from chemical, petrochemical, pesticide, dye, pharmaceutical and agrochemical operations.	Effluent treatment plant	As: 5.9; Cd: 4.3 Cr: 4.6; Cu: 6.7 Fe: 25.1; Mn: 9.2 Ni: 5.3; Pb: 4.9 Zn: 12.6	Spinach, Radish, Tomato, Chili and Cabbage, Cress, Dill, Coriander, Brinjal and Okra.	Field-scale experiments	The adopted industrial wastewater irrigation led to accumulation of metals in the irrigated soils and plants. Among the cultivated crops, Spinach, Radish, Tomato, Chili and Cabbage exhibited higher accumulation and translocation of heavy metals (As, Cd, Cr, Pb and Ni) in their edible tissues. Crops with high resistance to metal contamination are recommended for irrigation with the industrial wastewater.	[140]
Paper mill effluent, Textile and dyeing effluents		pH: 8.9 EC: 0.694; TDS: 347 Salinity: 0.3; Cl^+: 60 NO3=N: 3.08 N: 0.7; PO4-P: 0.37 Mg: 17.0; Ca: 53.6 Cu: 0.36; Mn: 0.6 Cr: 0.02	Green gram (Phaseolus mungo), mustard (Brassica napus) and jute (Corchorus capsularis)	Laboratory experiments	With adequate treatment, selection of suitable crops, industrial effluent could serve as a potential source of water and nutrients for growth of crops. Jute was more sensitive to toxic impact of the wastewaters than green gram and mustard.	[141]
Tannery effluent	hydrolysis-anaerobic-Sequence Batch Reactor integrated with a constructed wetland	pH: 6.53; EC: 2.23 Cl-: 450 Salinity: 2.9* BOD5: 56; COD: 170 NO3-N: 22.8 NH4-N: 17.1; TN: 50 Phenol: 0.71 TC: 1; FC: ND	Vegetable crops (tomato, cabbage and onion, swiss chad, beet root and carrot)	Field and pot scale studies	Treatment unit improved the quality of tannery wastewater to acceptable discharge limits. Plant uptake of Cr was higher in field study than the pot experiments. While the major source of Cr to the plants was the soil, the concentration of metals (Cr, Cd, Pb and Ni) were below the FAO/WHO guidelines. Shoot growths in crops that received treated wastewater except onion were comparable to the control treatment with underground water. Minimal effluent inhibitions of 0, 7.3, −0.36, 15, 4.2 and 2.3% for carrot, tomato, cabbage, onion, swiss chard and beet root, respectively were recorded.	[142]
Mixture of industrial effluents, municipal and household wastewater	Oxidation pond	pH: 7.24; EC: 1.33 TDS: 846 NO3-N: 18.1 PO4-P: 5.2 Ca: 141.8; Mg: 49.3 Na: 156; K: 28.5 Zn: 0.87; Cu: 0.59 Cd: 0.14; Pb: 0.77 Ni: 0.09	Winter wheat (Triticum aestivum L.)	Field study	Higher growth outcomes in treated wastewater irrigated sites compared to the control treatment with freshwater. Measuring parameters: thousand seed weight, grain yield, biological yield and harvest index were by 19.7, 17.8, 12.6 and 9.0%, respectively higher in treated wastewater irrigation than freshwater irrigation. Heavy metals Zn, Cu, Ni, Cd, and Pb accumulated in the plant roots but were poorly translocated to the grains and the hazard quotients were below 1 except for Cd.	[126]
Reactive textile wastewater	Biodegradation using biofilm bioreactor	pH: 8.17; EC: 12.6 COD: 41.5 BOD5: 21.6 Turbidity: 45.1 Alkalinity: 1758 SAR: 5.99 NO3^+: 1220 NH4^+: 4.27 PO4^3+: 10.1; K: 41.2	Wheat	Field study	Treatment improved the quality of effluent to irrigation reuse potential. Grain yield was about 23% higher in treated wastewater irrigation than the groundwater control. The nutritional composition of wheat grains irrigated with the treated wastewater was enhanced by 241% Energy, 212% Carbohydrate and 402% Protein in comparison to the nutritional yields in wheat grains that received untreated wastewater.	[143]
Brewery effluent	Anaerobic digestion and primary facultative pond treatments	pH: 8.98 EC: 3.44 COD: 110.9 NH4-N: 28.9 NO3-N: 2.2 PO4-P: 30.9 Na: 937.4	Swiss chard (Beta vulgaris), saltbush (Atriplex nummularia), Salicornia meyariana and sorghum (Sorghum bicolour)	Field study	Plant growths were not significantly impacted by the treated wastewater. Leafy vegetable (swiss chad) had the best biomass production and chlorophyll concentration index due to the plant’s ability to strive in saline environment. By the adoption of good cropping management (crop rotation), addition of soil amendment (gypsum and Trichoderma) and salt tolerance crops, brewery effluents could serve as potential source of water for irrigated agriculture.	[144]
Pulp and paper industrial wastewater	Sequential anaerobic -aerobic biological treatment.	Temp: 17; pH: 6.9 EC: 2.67; TDS: 102 Turbidity: 90 TSS: 720; CI^-: 252 COD: 532 BOD5: 287 Ca: 449; Mg: 112 Na: 256; K: 72	Mustard (Brassica campestris)	Laboratory study	No phytotoxic effect on plant was observed. Treatment improved the quality of wastewater quite substantially to level that enhanced seed germination (90%), seedling vigour index and seedling growth which were higher than the growth performances under tap water control treatment.	[145]
Mixed petroleum, textile, pharmaceutical and domestic wastewater	Activated sludge-based bioreactor	pH: 7.1; TDS: 493 EC: 0.99; Turbidity: 83.5 TSS: 111; Colour: 302 COD: 209 BOD5: 23.8; NO3-N: 22.8 NH4-N: 14.9; TN: 62.3 PO4-P: 7.5	Lettuce and silverbeet	Laboratory study under hydroponic conditions	Treated mixed wastewater had no adverse impact on germination of the seed crops as comparable results were recorded in crops that received tap water irrigation. Comparable leaf numbers were recorded between treated wastewater and tap water irrigation. Residual organic complexes in treated wastewater had an inhibitory effect on root growth.	[146]

All parameters are in mg/L except pH & SAR (sodium adsorption ratio): dimensionless; Temperature: ^oC; EC: mS/cm, Turbidity: NTU; Colour: Pt-Co; TC (total coliforms) & FC (faecal coliforms): CFU/100 mL; ND: not detected; *percentage

Exploring the possibility and extent of utilization of industrial effluents in agriculture has led to the utilization of different water conditions including untreated, diluted and treated wastewaters for studies on both food and non-food crops. Overall, the studies revealed that with adequate treatments, industrial effluents could serve as alternative source of irrigation water and potential source of plant nutrient (in irrigated agriculture) for sustainable food production, while preserving the environment and water sources. Looking into the specific studies on reuse potential of industrial wastewater for irrigation, [126] found that winter wheat (Triticum aestivum L.) irrigated with biologically treated mixed industrial, municipal and household effluents had better morphological development and produced greater yields than counterparts grown under freshwater irrigation. This is clearly due to the quality of water utilized, which according to the available data, met the FAO water quality criteria for irrigation except for trace metal Cu (Table 5). The better growth performance and high yields in crops that received the treated wastewater over the freshwater irrigation could be a function of nutrients’ availability in the reused water which supported the plants for better growth and development. Similarly, textile mill effluent, treated by anoxic-aerobic photobioreactor, was used for irrigation of radish and the results indicate higher dry weight, leaf number and leaf area by 49.0, 19.2 and 62.0%, respectively compared to those that received freshwater irrigation [127]. Again, water quality and nutrient content of the treated wastewater are responsible for the higher biomass yields and growth outcomes in radish. In the study by [128], it was revealed that the growth and yield parameters (such as above and belowground biomass, photosynthetic pigments, leaf area, plant height, leaf number, etc.) in Zea mays irrigated with biologically (UASB – upflow anaerobic sludge blanket) treated mixed textile and domestic wastewater were comparable to the outcomes in control treatment irrigated with freshwater. The authors added that dilution of the treated mixed wastewater further enhanced its productivity for the growth of maize crop.

Dilution of industrial wastewaters, intended for reuse in irrigated agriculture with freshwater, is identified as a good water resource management practice to reduce the prohibitive costs of treatment and health risks in wastewater reuse for irrigation. Blending industrial effluent with freshwater dilutes the concentrations of hazardous substances, heavy metals, salts and decreases their phytotoxic effects to plants [135,147].

Sequential anaerobic and aerobic biological process showed to be the effective treatment for pulp and paper industrial effluent to meet agricultural reuse quality [145].

The reuse potential of the treated wastewater was confirmed following a phytotoxicity assessment performed on mustard plant which had higher germination rate (90%) and produced longer shoot and root elongations under treated wastewater irrigation over tap water treatments [145]. Similarly, it was affirmed in another experimental study [118] that mixed industrial (textile) and domestic could serve as a suitable source of irrigation water for ornament plants following the plants’ excellent growth performance and outcomes, although the reuse effluent was further treated by sand filtration and disinfection. Recently, biologically treated reactive textile wastewater was applied for irrigation of wheat crop and the yield and overall outcomes was excellent and high nutritional values with respect to protein, carbohydrates, and energy were recorded in the grains [143]. Paper and pulp industrial wastewater, known to contain organic halides and chlorinated organic compounds, served as a source of irrigation after biological pre-treatment in Jordan [13]. In the review conducted by [148], it was gathered that wastewater from food processing industries was reused for crop irrigation with no compromise on crop quality, higher yields were obtained, and no accumulation of hazardous elements was found in the soil. At 25% effluent concentration, electroplating wastewater was successfully reused for the growth of Hordeum vulgare, and could serve as a potential source of micro-nutrients that are beneficial to plants [133]. Overall, the reuse outcomes of the respective studies were largely dependent on the irrigation water quality, plant tolerance to substances in water and micro- and macro-nutrients availability in the reused water. Based on the available information, the treated mixed wastewater did meet certain criteria in terms of parameters stipulated by the FAO (Table 5) for the interpretation of water quality for irrigation.

Majority of the literature consulted indicate there is indeed positive indication for the reuse of treated industrial/mixed wastewater in agriculture; however, issues, such as salinity problems, plant uptake of heavy metals and toxicity, were reported in some cases. Salinity is a common problem in irrigated agriculture, especially where wastewater is reused. Depending on the source, industrial effluents contain certain salts in certain amounts. Irrigation (long term) with water containing considerable amount of salts can lead to soil salinity and sodicity to levels that are detrimental to crop and soil health, even on a short term [149]. Similarly, industrial wastewaters contain varying concentrations of heavy metals which accumulate in the soil and eventually migrate into plants and could potentially end up in the food chain, posing a threat to public health. Like salts, heavy metals are not biodegradable, and as a result, biological treatments are not suitable for their removal in wastewater, hence, specialized treatments are required to get rid of them. Growing of crops with high resistance to metal contamination and salt tolerance are recommended BMPs to address the specific problems of salinity and heavy metal accumulation in industrial wastewater irrigated agriculture [144].

The risk of pathogenic contamination of farm products is another troubling concern in wastewater irrigated agriculture. The detection of faecal coliforms, faecal enterococci and total heterotrophic bacteria counts in tomatoes and broccoli irrigated with biologically treated agro-industrial effluents [132] is a clear indication of serious food contamination with the likelihood of human exposure to the risk of pathogenic diseases, thus highlighting the need for inactivation of pathogenic bacteria and helminths in biologically treated wastewater prior to irrigation. For this particular reason, WHO recommends a high degree of disinfection be applied after efficient biological stabilization of wastewater to inactivate faecal coliforms, pathogenic bacteria and enteric viruses [150]. However, the proposed treatment is originally applicable to domestic/municipal sewage presumed to be free of organic and inorganic chemicals, recalcitrant organic compounds and heavy metals that are potentially hazardous to human health and the environment. The mode of water application is critical for the safe reuse of wastewater in agriculture, and drip irrigation method is recommended to ensure proper administration of the water to crops while avoiding its direct contact with the edible parts of the crop to eliminate possible route of microbial contamination [132].

The overall assessment of treatment and reuse potential of mixed industrial effluents and domestic sewage treated by activated sludge-based processes is presented in Figure 2.

Figure 2. Potential of suspended growth biological processes (SGBPs) for the treatment of individual wastewaters for reuse in irrigated agriculture. TPH: total petroleum hydrocarbon; PPCPs: pharmaceuticals and personal care products; MB: methylene blue; LR: low range; PAHs: polycyclic aromatic hydrocarbons.

6. Research gaps and future directions: adapting industrial effluents to agriculture or irrigation

As water scarcity continues to intensify across the globe as a result of accelerated population growth, industrial development, agricultural expansion and climate change, the need for a targeted integrated wastewater reuse in agriculture becomes central. Industrial effluent is a valuable resource for irrigated agriculture considering its high water-volume and nutrients-rich properties which have been unfairly undermined by its chemical origin. As a result, industrial wastewater has not been given the needed consideration for reuse in agriculture; therefore, its potential in irrigated agriculture remains unexplored. Reuse or not, industrial wastewater needs to be treated for safe environmental disposal, therefore, adapting industrial wastewater to agriculture will require a little more additional measures in the form of treatment and management practices.

Amongst other factors, cost of treatment and concerns for public health safety are the major constrains for the implementation of industrial wastewater reuse in agriculture, especially in low-income countries. Biological treatments are more economical and resource efficient than physicochemical alternatives and they have shown a great deal of potential for improving the quality of industrial wastewater mixtures. However, some constituents, such as HMs, salts, pharmaceuticals, PAHs, phenolics, amines in industrial wastewater, are generally not biodegradable, hence, irrigation, especially on a long term with water containing these compounds/elements, can lead to soil salinity and sodicity problems, changes in soil structure, loss of soil fertility, problems of toxicity to crops [123,149,151]. Therefore, in order to use industrial wastewater for irrigation sustainably, the quality has to meet the reuse criteria stipulated by local and international regulating organizations such as USEPA, WHO and FAO.

Blending industrial effluents with domestic/municipal is a direction that can break the gridlock surrounding industrial wastewater reuse in agriculture. Mixing with municipal sewage will bring about a substantial degree of dilution of chemicals and toxic substances in industrial wastewater and equally enhance the biodegradability of recalcitrant organic compounds amenable to biological treatments [115]. Another innovative path to adapting industrial wastewater to irrigation already cited in the literature [128] is by dilution of biologically treated effluents with freshwater to generate a more improved water quality that can be used sustainably for irrigation to maintain agricultural production, while water sources and environmental qualities are preserved. However, considering the high salt loads and concentration of heavy metals, some mixed industrial and domestic wastewater cannot be sustainably used for irrigation, and for such scenario, BMPs, such as cultivation of salt-tolerant plants, crops with low capacity for heavy metal uptake, application of soil amendments, etc. are the principal solutions to addressing the problems [149]. Further treatment with efficient and cost-effective systems is another recommended option to ensure a safe reuse of wastewater in agriculture. AOPs are among the alternative treatment technologies that have been proposed for effective treatment of industrial wastewater intended for reuse. Among the AOPs, great attention has been given to ozone oxidation process. Considering the negative implications of using industrial and mixed wastewater for crop irrigation due to water quality issues already discussed, potential treatment options for possible reclamation and reuse of mixed industrial effluents and domestic sewage in agriculture are conceptualized and presented in Figure 3. The capability of each treatment option to produce effluents that meet water quality parameters categorized into physical, chemical, organic compounds and microbial parameters is illustrated by binary words – yes and no (Figure 3).

Figure 3. Conceptualization of mixed wastewater treatment options with possible water quality attainments and level of reuse applications for irrigation. SGBPs: suspended growth biological processes; Cl: chlorination; AOPs: advanced oxidation processes. (Water quality parameters, as indicated in Table 5: P – physical; C – chemical; O – organic compounds; M – microbial).

7. Conclusions

Wastewater reused in agriculture is sourced mainly from domestic/municipal sewage, while the industrial streams are unfairly disfranchised due to their chemical origin. Although industrial effluents, depending on the source, may contain hazardous substances, they could be resourceful in irrigated agriculture considering their high water-volume and nutrient-rich potential. Cost of reclamation of industrial effluents and public health concerns are the major constrains for the implementation of industrial wastewater reuse in agriculture. Biological treatment processes are generally resource-efficient and relatively cost-effective for wastewater decontamination. If adequately operated, and by adoption of dilution mechanism and full biomass adaptation, SGBPs have the capability to improve the quality of mixed industrial effluents and domestic sewage for adaptation to irrigated agriculture. Integration of industrial effluents into domestic sewage treatments allows some degree of dilution of toxic substances and provides the biomass the assimilable food and enabling environment required for metabolization and co-metabolization of degradation-resistant industrial pollutants. The volume ratio of domestic/municipal sewage to industrial effluents, which depends primarily on characteristics and composition of industrial streams, is particularly critical in the development of blended mixtures that allow assimilation of toxic and recalcitrant compounds for microbial degradation. Biodegradability of refractory industrial pollutants in activated sludge-based treatments is enhanced through biomass acclimation, allowing for the development and multiplication of diverse competent microbial populations with genetic traits capable of degrading target pollutants.

Based on the reuse outcomes in agriculture and quality of the irrigation water summarized in this paper, SGBPs have the potential to improve the quality of mixed industrial effluents and domestic sewage for application in agriculture. Irrigation with biologically treated mixed industrial effluents and domestic sewage presented some problems, such as plant toxicity, accumulation of heavy metals in plants and agricultural soils and soil salinity; however, cultivation of heavy metal and salt-resistant crops is a recommended management practice to ensure safe and sustainable adaptation of industrial effluents to irrigated agriculture. Blending the biologically treated mixed industrial and domestic wastewater with freshwater could potentially remove the bottlenecks for the extended use of industrial wastewater for irrigation. Integration of supplementary treatments, such as AOPs, into activated sludge-based processes will allow the recovery of high-quality water for wider irrigation reuse applications. With the evidence collected in this review, it can be concluded that SGBPs, if operated optimally, could address the problem of wastewater treatment quality for agricultural reuse or in the very least improve the quality of mixed wastewater considerably for further treatment to meet irrigation reuse quality. However, the level of current knowledge and research data are limited’ therefore, further integrated studies are necessary to fully explore this area and understand the sustainability of irrigation with mixed industrial effluents and domestic sewage.

Acknowledgements

The first author sincerely thanks Petroleum Technology Development Fund (PTDF), Nigeria for providing full financial sponsorship for PhD research at the University of Surrey, Surrey, United Kingdom.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Correction Statement

This article has been republished with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

This work was fully supported by Petroleum Technology Development Fund (PTDF), Abuja, Nigeria, under the PTDF Overseas Scholarship Schemes (OSS) for Human Capital Development [Grant number PTDF/ED/OSS/PHD/ POF/1109/17]. Other supports came from InRoot and Water-Food scoping projects at University of Surrey as part of STFC Food Security Network led by the University of Manchester [Grant number ST/P003079/1].

Notes on contributors

Precious N. Egbuikwem

Precious N. Egbuikwem (PhD Candidate) is Lecturer at the department of Agricultural and Bio-Environmental Engineering, Imo State Polytechnic Umuagwo, Nigeria. She is currently a postgraduate researcher in the Centre for Environmental Health and Engineering (CEHE), department of Civil and Environmental Engineering, University of Surrey, United Kingdom. She is a PhD Scholar of the Petroleum Technology Development Fund (PTDF) Nigeria. Her PhD research is in Environmental Engineering with particular focus on mixed industrial and domestic wastewater treatment using biological, chemical and physical processes; sustainable treatment and reclamation of mixed wastewater and reuse of treated wastewater for crop irrigation under hydroponic conditions.

Gregory C. Obiechefu

Dr. Gregory C. Obiechefu is Deputy Rector (Academics) and Dean, School of Engineering Technology, Imo State Polytechnic Umuagwo, Nigeria. He is a faculty member Agricultural and Bio-environmental Engineering of the Polytechnic delivering quality lectures in Soil and Water Conservation specialization. He is the Chairperson Committee of Deans of Engineering of Polytechnics and Colleges of Technology (COMPODET) of Nigeria. His work involves Soil and Water Engineering, Erosion studies, Irrigation, Evapotranspiration and Wastewater Treatment and Management.

Faisal I. Hai

Hai, Faisal I is Professor in the Faculty of Engineering and Information Sciences, University of Wollongong Australia. He is the director of the Strategic Water Infrastructure Lab (SWIL). He carries out a number of research studies, teaching and governance leadership roles across the Faculty of Engineering and Information Sciences (EIS). He is Head of Postgraduate Studies of the School of Civil, Mining and Environmental Engineering (CME) where he coordinates and supports the research of around 80 higher degree research (HDR) students (PhD, Masters). He is the coordinator of the ‘Environmental Engineering and Water Resources’ Powerhouse of the Faculty of EIS. He also coordinates the ‘Water and Bioresource Technologies’ strength of the Simulation, Modelling, Analysis, Research and Teaching (SMART) Research Facility at UOW. As a member of the Faculty Research Committee (FRC) and the Faculty International Committee (FIC) leadership teams, Professor Hai participates in the review and implementation of the Faculty Research and International Partnership/Outreach programmes.

Ma. Catriona E. Devanadera

Ma. Catriona Devanadera is a faculty member and researcher at the Department of Community and Environmental Resources Planning, College of Human Ecology, University of the Philippines Los Banos, Philippines. She is an Environmental Engineer/Specialist in projects related to water and wastewater treatment, constructed wetlands treatment system, watershed management plan, sustainability planning, wetland management and conservation

Devendra P. Saroj

Devendra P. Saroj Dr (PhD, CEnv, MSOE, FHEA) is Senior Lecturer at the department of Civil and Environmental Engineering, University of Surrey United Kingdom. He is Head of Centre for Environmental Health and Engineering (CEHE) and Programme Director – MSc Water and Environmental Engineering. His work covers a wide area of water and wastewater treatment systems and processes. His recent works include nutrient pollution of freshwater bodies, natural wastewater treatment systems (wetlands) membrane systems for organic micro-pollutant removal, membrane-assisted bioreactors for domestic and industrial wastewater treatment and recycling, membrane technology assessment for urban water reuse scenarios, decision support in water supply, reuse and sanitation, adaptation of sewage treatment works for biological nutrient removal, removal of organic micro-pollutants, biofilm biotechnology for advanced wastewater treatment, microbial communities in bioreactors for wastewater treatment, recovery of phosphorus from sewage and anaerobic biodegradation of organic waste materials.