Review on COD and ammoniacal nitrogen removal from landfill leachate using low-cost adsorbent

ABSTRACT

The rapid generation rate of solid waste is due to the increasing population and industrialization. Nowadays, solid waste has been a major concerning problem in handling and disposal thus adsorption treatment process has been introduced which is an effective and low-cost method in removing organic and inorganic compounds from leachates such as chemical oxygen demand (COD) and ammoniacal nitrogen (NH₃-N). A most commonly adsorbent used for the removal of organic and inorganic compounds is activated carbon (AC), yet the main disadvantage is being too expensive in cost. Many researchers tried to use low-cost adsorbent waste materials, such as peat soil, limestone etc. This review article reveals a list of low-cost adsorbent and their capacity of adsorption for the removal of COD and NH₃-N. Furthermore, the preparation of these low-cost adsorbents as well as their removal efficiencies, relative cost, and limitation are discussed. The most efficient, cost-effective, and environment-friendly adsorbent can be used for the removal of COD and NH₃-N thus can be provided for commercial usage or water treatment plant.

Implications: The concentration of organic constituents (COD) and ammonia nitrogen in stabilized landfill leachate has significant strong influences of human health and environmental. This review article shows the list of low-cost adsorbent (i.e., Activated carbon, Peat soil, Zeolite, Limestone, and cockle shell and their capacity of adsorption for the removal of COD and ammonia nitrogen. This would be greatly applicable in future research era as well as conventionally minimizing high-cost materials use and thereby lowering the operating cost of leachate wastewater treatment.

Introduction

The rate of the population growth is rapidly increasing, as well as industrialization and economic growth; the generation rate of solid waste is also increasing day by day around the world. Solid waste management turned out to be a main concerned issue due to the huge amount of wastes are generated and significantly difficult to manage (Alam and Ahmade 2013). This rapid generation rate of solid waste, creates a negative impact on the environment as well as pollutes the water, soil, and air (Moeler 2005). Waste material produces from different sources such as waste from industries, municipal solid waste, institutional waste, as well as building waste. It is stated from the sources of waste, few methods are applied to control the waste material disposal system such as open dumping, incineration, compaction, sanitary landfilling, reduction, composting, and anaerobic digestion (Aziz et al. 2009; Zin et al. 2012). Among all these methods, the landfilling is the most popular and a longtime applied method to reduce the waste disposal (Akinbile and Yusoff 2011). Landfilling is the simplest, lower in cost, and effective method to dispose solid waste.

Leachate contains high substances of organic matter and inorganic matter, consist of chemical substances, heavy metal such as ammonia, sodium, calcium, sulfur, iron, nickel, cadmium, lead and other (Tchobanoglous, Theisen, and Vigil 1993).

The leachate treatment technology is divided into two basic methods of treatment, for example, biology and physical or chemical (Raghab, El-Meguid, and Hegazi 2013). Leachate treatment is very complicated, expensive, and generally requires various application processes (Bashir et al. 2012). The biological treatment method is effective in removing the organic compounds from young leachate where its biodegradability ratio (BOD₅/COD) ranges between 0.5 and 1.0 yet is less suitable to remove ammoniacal nitrogen in stable leachate (BOD₅/COD <0.1) (Yao 2017).

Ammoniacal nitrogen (NH₃-N) and chemical oxygen demand (COD) found in stable leachate are two parameters of refractory organic compounds that are difficult to get rid of through biological treatment (Nurazim, Hanidi, and Mohd 2017). Effectiveness of biological treatment processes become less effective and treatment method physically chemically such as activated carbon (AC) adsorption and chemical precipitation are considered to be the ideal choice in treating soluble solids containing low content of organic compounds such as in stable diffuse solvents.

Yang et al. (2019), Landfills municipal solid waste (MSW) apparently heterogeneous and clogging of drainage system easily happened through landfilling operations. The above two main variables heavily affect the landfill leachate distribution pool. At the same time, the permeability of waste distribution in spatial and temporal domain has been analyzed in this research study. The finding result indicates that waste saturated permeability distribution follows vertical direction logarithmic relationship, and the fresh waste saturated permeability distribution follows time-based polynomial relationship. The drainage system of landfill often encountered the experience of clogging problem after operating for a few years, and the permeability decreasing to one to five magnitude orders. Moreover, numerical modeling, simulation findings result of permeability distributions in spatial and temporal domain have been found to be suitable.

Yang, Xu, and Chai (2018), in this study the author addressing that now a days the landfilling method widely used for managing municipal solid waste (MSW) in many developing countries worldwide. In this research article, the author describes the physical compositions of MSW, coefficient of permeability of landfill, and unit weight, in numerous countries that were discussed and reviewed. Landfill MSW has strong spatial and temporal characterizations. In developing countries, landfill MSW has a higher concentration of organic matter content which varies from 75% to 97%. In most developed region, the organic matter content relatively very small. The coefficient of permeability varied from 3.5 × 10⁻² to 5 × 10⁻⁸ cm/sec in 60 minutes and unit weight varied from 4.9 to 17.8 kN/m³, respectively. Thus, it is important and significant to strengthen classification of waste, recycling, and MSW recovery significant for constructions, operations, and landfill management.

Yang et al. (2016), in this study author addressing Jiangcungou landfill which is situated in Shaanxi China has been investigated in this research article and slope stability has been analyzed. The laboratory experiment were carried out to obtained permeability coefficient of municipal solid waste (MSW). Based upon the finding result, leachate distributions and landfill stability were measured and investigated. The finding result shows that: permeability coefficient ranged from 1 × 10^–7 to 6 × 10^–3 cmsec⁻¹ based on lab experiment tests and also some related landfill parameter. In addition, wastes were covered with the leachate topmost layer and leachate scale ranged was 2 to 5 m depth under the surface of wastes in other layer.

In recent years, adsorption gain greater importance in removing contaminates compounds from waterbodies due to its low-cost effectiveness, environmentally friendly, local availability, design simplicity, and ease of operation ability to remove contaminants at low concentration.

Composite materials have been produced for many purposes to improve the properties of adsorption as well as to produce low-cost adsorbents.

Leachate generation at landfills

Leachate production is a concern of the environment that needs to be taken seriously, and many components become the focus of this study. After being disposed of at the landfill, solid waste undergoes various changes such as physical, chemical, and biological responses (Toretta et al. 2016). Water that infiltrates into solid waste, compacted, and brings together extractable chemicals into wastewater known as leachate. Water disruption occurs when the magnitude of gravity overrides the holding force, where the high moisture exceeds the absorption capacity of the solid waste layer (El-Fadel et al. 2002). Landfill resurfacing production is the result of precipitation, infiltration, surface runoff, evaporation, storage capacity, and biochemical processes.

There are various factors affecting the generation of soluble solids such as solid waste compositions, stabilization, and pollutant dissipation by water permeability (Adhikari and Dan Khanal 2015; Şchiopu and Ghinea 2013). This also includes solid waste properties, oxygen presence, waste depth, content moisture, environmental temperature, solid waste processing, and landfill age (Nor Nazrieza et al. 2015). The amount of leachate generation can be determined by water balance. Water balance Equation (1)(1) $\mathrm{L}=\mathrm{P}-\mathrm{R}-{\mathrm{D}}_{\mathrm{u}\mathrm{s}}-\mathrm{E}\mathrm{T}-{\mathrm{D}}_{\mathrm{u}\mathrm{w}}$(1) takes into account all incoming and outgoing water (Diaz, Savage, and Eggerth 2007; Kulikowska and Klimiuk 2008) as follows:

(1) $\mathrm{L}=\mathrm{P}-\mathrm{R}-{\mathrm{D}}_{\mathrm{u}\mathrm{s}}-\mathrm{E}\mathrm{T}-{\mathrm{D}}_{\mathrm{u}\mathrm{w}}$(1)

Where L is leachate production, P is precipitation, R is a surface runoff, D_us is changed to soil moisture, ET is true evaporation loss, and D_uw is changed to the moisture content of solid waste components. This also includes water used in biochemical processes and water that evaporates through solid waste disposal gas (Diaz, Savage, and Eggerth 2007; Kulikowska and Klimiuk 2008).

Leachate properties

A study was conducted by Öman and Junestedt (2008), and Kjeldsen et al. (2002) which stipulates that the Municipal Solid Waste (MSW) composition of solid waste can be categorized into four main compounds. Another way to characterize leachate at the landfill is through a BOD₅/COD ratio indicator. This shows how organic matter in leachate can be avoided. However, no standard value is specified for the indicator. Lim et al. (2016) and Nor Nazrieza et al. (2015) used BOD₅/COD ratio for leachate characterization at the landfill where it is divided into three leachate categories, as shown in Table 1.

Table 1. Landfill stability according to BOD₅/COD ratio (Lim et al. 2016; Nor Nazrieza et al. 2015)

Soluble types	Young	Mid	Stable
Age of landfill (years)	<1	1–5	˃5
pH	<6.5	6.5–7.5	˃7.5
COD (mg/L)	˃15,000	3000–15,000	<3000
BOD5/COD	0.5–1.0	0.1–0.55	<0.1
TOC/COD	<0.3	0.3–0.5	<0.5
NH3-N (mg/L)	<400	400	˃400
Nitrogen ammonia, (mg/L)	100–2000	T.D	T.D
Heavy metal (mg/L)	˃2	<2	<2
Organic compounds	80% VFA	5 – 30% VFA + HA + FA	HA + FA only

Adsorbents used for the removal of chemical oxygen demand (COD) & ammoniacal nitrogen (NH₃-N) from leachate wastewater

There are various types of adsorbents used for removing the contaminant particles from landfill leachate like COD and NH₃-N.

Activated carbon adsorbent

Adsorption by activated carbon has gained considerable attention as its properties of the latter have a greater surface area, high adsorption capacity, better thermal stability (Kamaruddin et al. 2015) and were effective for adsorption of nonpolar contaminants (Kalmykova et al. 2014). Additionally, with the presence of carbon-oxygen surface groups, polar compounds, and metal cations can also be absorbed. With that application, activated carbon is widely used as an adsorbent in the process of water and air purification (Leboda 1992, 1993).

Since then, much focus has been on non-conventional detectors which promised benefits for future trading purposes. Some of the activated carbon is provided from the raw materials and also agricultural waste (Chowdhury, Hamid, and Zain 2015). The activated carbon has been proven to be a good adsorbents for the removal of organic pollutants such as COD and NH₃-N from wastewater; but it is too slow to absorb and was widely reported in the literature.

Daud et al. (2020) used a mixture of granular activated carbon and green mussel for the removal of COD and NH₃-N from stabilized landfill leachate at Simpang Renggam municipal landfill in Johor, which possessed high concentrations of COD (1829 mg/L), and NH₃-N (406.68 mg/L). Its BOD₅/COD ratio is 0.08. The optimum conditions for removal of COD and NH₃-N were determined at 120 min with 200 rpm at pH 7. The optimum ratio of activated carbon and green mussel shell is 2.5:1.5. The values of removal percentage of COD and NH₃-N, are 83%, and 63% respectively. The adsorption result revealed that the Langmuir isotherm was best fitted to its experimental data as compared to Freundlich isotherm.

Detho et al. (2021) used a mixture of carbon mineral composite adsorbent for the removal of COD and NH₃-N from a stabilized landfill leachate at Simpang Renggam municipal landfill in Johor, which possessed high concentrations of COD (1829 mg/L), and NH₃-N (406.68 mg/L). Its BOD₅/COD ratio is 0.08. The adsorption result revealed that the Langmuir isotherm was best fitted to its experimental data as compared to Freundlich isotherm. Langmuir and Freundlich isotherm adsorption capacity for COD and ammoniacal nitrogen were 0.9971 and 0.9914, respectively.

Daud et al. (2017a) used a mixture of granular activated carbon and cockle shell for the removal of COD and NH₃-N from a stabilized landfill leachate at Simpang Renggam municipal landfill in Johor, which possessed high concentrations of COD (1763 mg/L), and NH₃-N (573 mg/L). Its BOD₅/COD ratio is 0.09. The adsorption result revealed that the Langmuir isotherm was best fitted to its experimental data as compared to Freundlich isotherm.

Kaur, Mor, and Ravindra (2016) used an active bones ash to treat stable leachate at Dadumajra Disposal Site, Chandigarh, India and had managed to get rid of 79% COD at his early research (2088 mg/L).

Kaur, Mor, and Ravindra (2016) were also used an activated cow dung ash (ACA) and cow dung ash (CA) to treat wastewater for the removal of COD by use of batch mode experiment. According to the batch experiment result in the removal of COD, ACA had managed to get rid of COD at 79% whereas CA removed 66% of COD at the same working rate.

Azmi et al. (2016) used chemically activated sugar cane (SCAC) for the removal of COD and NH₃-N from stable leachate at Saponic Labs (anaerobic type), Kampar, Perak, Malaysia. It was found that the usage of SCAC as an adsorption attempts, nailed to get rid of 46.65% NH₃-N and 83.61% COD with an adsorption capacity of 14.62 mg/g and 126.58 mg/g, respectively.

Motling, Mukherjee, and Dutta (2014) used commercially activated carbon for the removal of COD from leachate at Dhapa Disposal Site, Kolkata, West Bengal, India. It was found that the usage of commercially activated carbon successfully get rid of 92% of COD with a 180 minutes touch time approaching 5520 mg/L.

The adsorption study using activated carbon granules provided from the oven heating was performed by Foo, Lee, and Hameed (2013) on a stable diffusion from Pulau Burung Disposal Site (TPSB), Penang, Malaysia. It was found that about 79.93% of COD was eliminated at 2.67 hours with 64.93 mg/g adsorption capacity.

Foo, Lee, and Hameed (2013) in his study performed the adsorption treatment of semi-aerobic landfill leachate using tamarind fruit seed to derive granular activated carbon, known as tamarind seed activated carbon (TSAC) prepared using microwave heating. Batch experiment result revealed that by increasing the adsorption dosage and contact time, the removal of adsorption color and COD were of 91.23% and 79.93%, respectively.

The comparison between COD and NH₃-N from stable leachate at TPSB was performed by Halim et al. (2010). It was found that the activated carbon adsorption capacity of coconut lead carbon-mineral composite media for COD is 37.88 mg/g compared to NH₃-N, 22.99 mg/g. Ammoniacal nitrogen (NH₃-N) adsorption capacity by activated carbon is lower than media carbon-mineral composites.

Aziz et al. (2010) used activated carbon from coconut found that COD adsorption capacity was 256.41 mg/g, higher than the activated carbon and limestone (166.67 mg/g). While for ammoniacal nitrogen (NH₃-N), its adsorption capacity of 14.71 mg/g was recorded by carbon, higher than the activated carbon mixture and limestone (10.83 mg/g).

Kulkarni (2013) used activated carbon from coconut fiber to treat domestic wastewater and found that COD was eliminated at 75% to 80% by group experiments and experimental columns. Table 2 shows a summary of previous studies for wastewater treatment and leachate using activated carbon.

Table 2. Summary of wastewater treatment and leachate using peat soils by previous researchers

Researchers	Sample	Parameter	Working range					Removal efficiency
			pH	Contact time	Dosage	Speed	Particle size	COD	NH3-N	Heavy metals	Others
Rosli et al. (2017b)	Leachate	Color & Iron			0:4						74.4% & 79.6%.
Champagne and Khalekuzzaman (2014)	Leachate	COD	5.88––7.43	2–5 day	-	-	-	80%	86%	-	-
		NH3-N
Kalmykova et al. (2014)	Leachate	PHCs	-	20–30 min	-	-	-	-	-	-	35%
		PAHs									63%
Heavey (2003)	Leachate	COD	-	-	-	-	-	88.1%	99.9%	-	-
		NH3-N
Bartczak et al. (2015)	Water	Nikel	1, 3, 5, 7 and 9 (5–7)	1, 3, 5, 10, 15, 30, 45, (60), 90, 120, 180 and 240 min	(0.5), 1, 2, 3, 4 and 5 g	-	-	-	-	61.27 mg/g	-
		Lead								82.31 mg/g
Zehra, Lim, and Priyantha (2015)	Water	Lead	3.1, 4, 5.1 and (6.1)	30, 60, 90, (120), 150, 180, 210 and 240 min	0.05 g	250 rpm	-	-	-	15 mg/g	-
Na et al. (2016)	wastewater	Phosphorus	7–8	0.5, 1, 1.5, 4, 8, 12 and 24	6 g	150 rpm	0.5, 2 and 4 mm	-	89.23–97.08%	-	88.04–95.32%
		NH3-N
Mohamed et al. (2014)	Grey water	BOD	3.8–6.1	7, 14, 28 day	-	-	-	-	-	-	52–74%

However, the usage of activated carbon is not appropriate in developing countries due to its high-cost requirement concerning production and the need for the regenerative process of re-activating activated carbon columns (Toretta et al. 2016). There are various treatment processes available through ion exchange which are considered as cost-effective if low-cost ion exchangers such as zeolites are used (Mohammadizaroun and Yusoff 2014).

Peat soil adsorbent

A study conducted by Champagne and Khalekuzzaman (2014) used a peat soil bio-filter system to get rid of almost 80% COD and 86% NH₃-N with a hydraulic loading rate of 3.4 m³/day at Ottawa Trail Road Disposal Site, Canada. Heavey (2003) used a dry-air peat soil with 60 mm/day hydraulic loading rate to treat leachate successfully eliminating almost 88.1% COD and 99.9% NH₃-N. Peat soils have an extensive capacity to absorb heavy metals (Bartczak et al. 2015; Zehra, Lim, and Priyantha 2015), phosphorus and NH₃-N (Na et al. 2016), BOD (Mohamed et al. 2014) organic pollutants such as petroleum hydrocarbons (PHCs) and aromatic hydrocarbons of phthalates and polycyclic (PAHs) (Kalmykova et al. 2014). A study conducted by Bartczak et al. (2015) on the removal of nickel and lead found that their maximum adsorption capacity is 61.27 mg/g and 82.31 mg/g at the optimum pH range of 5 to 7 and reached a balance time of 60 minutes.

The adsorption of lead using peat soil from an aqueous solution with a maximum adsorption capacity of 15 mg/g reaches a 120-min balance time (Zehra, Lim, and Priyantha 2015). Na et al. (2016) used peat soils to see the impact on phosphorus and NH₃-N in wastewater with the percentage of separation between 88.04% to 95.32% and 89.23 to 97.08%, respectively.

A study conducted by Kalmykova et al. (2014) from leachate at the Brudaremossen Dump Site, Sweden found that the percentage of removal of PHCs and PAHs were 35% and 63%, respectively, using peat soil filters. The peat soil media filter has been directly conducted by Mohamed et al. (2014) proved to be able to get rid of BOD from 7th to 28th day with a percentage of 52% to 74% with an increase of pH from 3.8 to 6.1.

The advantages of using peat soils as absorbers cannot be denied because peat soils can be chewable and as a renewable natural source (Kalmykova et al. 2014). Since recent years many studies have been conducted focusing on adsorbs from nature that does not generate further pollution. The use of peat soils for leachate treatment of removal of organic pollutants measured as COD and NH₃-N is rarely reported in the literatures. Based on previous studies and subsequently due to the availability of peat soils in Malaysia, with this option peat soil was selected as an alternative to minimize conventional adsorbent material to treat leachate and is the appropriate choice for this study. Table 3 shows a summary of previous studies for wastewater treatment and leachate using peat soils.

Table 3. Summary of wastewater treatment and leachate using activated carbon by previous researchers

Adsorbent	Sample	Researcher	Parameter	Working range					Removal efficiency
				pH	Contact time	Dosage	Speed	Particle size	COD	NH3-N
Granular Activated Carbon and green mussel shell	Stabilized Leachate	Daud et al. (2020)	COD NH3-N	7	120 minutes	2.5:1.5 g	120 rpm	75 to 150 μm	83%	63%
Granular Activated Carbon and Cockle Shells	Stabilized Leachate	Daud et al. (2017a)	COD NH3-N	1–10	50, 100, 150, 200, 250, 300, 350 and 400 min	0, 10, 20, 30, 40, 50 and 60 g	40, 60, 80, 100, 120, 140, 160, 180, 200 and 220 rpm	2.00–3.35 mm	-	-
Activated cow dung ash	Stablized leachate	Kaur, Mor, and Ravindra (2016)	COD	2, 4, (6), and 8	60, (120), 150, 180 and 210 min	0.5, (1), 2 and 5 g	-	45 mm	79%	-
Activated carbon of sugar cane (SCAC)	Stablized leachate	Azmi et al. (2016)	COD NH3-N	3, 4, 5, 6, (7), 8, 9, 10 and 11	30, 60, 90, 120, 150, (180), 210, 240, 270, 300, 330 and 360 min	1, 2, 3, 4, 5, 6 and (7) g	50, 100, 150, (200), 250 and 300 rpm	-	83.61%/126.58 mg/g	46.65%/14.62 mg/g
Trade activated carbon	Leachate	Motling, Mukherjee, and Dutta (2014)	COD	2, 3, 4, 5, 6, 7, (8) and 9	10, 30, 60, 90, 120, 150, (180) and 210 min	0.1, 0.2, 0.3, (0.4) and 0.5 g/L	50, 100, 150, 200, 250 and (300) rpm	-	92%	-
Activated carbon granular (oven heating)	Stablize leachate	Foo, Lee, and Hameed (2013)	COD Color	2, 4, (6), 8 and 10	0.2, 0.5, 1, 1.5, 2, 2.5 and (2.67) min	2, 4, (6), 8, 10, 12 and 14 g	120 rpm	1–2 mm	-	79.93% 91.23%
Activated carbon (coconut)	Stablize leachate	Halim et al. (2010)	COD NH3-N	4, 5, 6, (7), 8, 9 and 10	5, 15, 30, 45, 60, 90, (105) and 120 min	3, 5, 8, 12, 17, 23, 30, 38, 47, 57 g	200 rpm	1.18–2.36 mm	37.88 mg/g	6.0790 mg/g
Activated carbon (coconut)	Stablized leachate	Aziz et al. (2010)	COD NH3-N	2–12	0.5, 1, 1.5, 2, 2.5, 3, 4, 6, 12 and 24 min	0–40 cm^3	50, 100, 150, 200, 250 and 300, (350) and 400 rpm	2.00–3.35 mm	256.41 mg/g	14.71 mg/g
Activated carbon (coconut)	Sewage wastewater	Kulkarni (2013)	COD	3, 4, 5, (6), 7, 8, 9, 10 and 11	30, 60, (90), 120, 150 and 180 min	0.5, 1, (2), 3, 4 and 5 g	200 rpm	-	75–80%	-
Activated carbon & limestone	Leachate	Aziz et al. (2004)	NH3-N	5, 5.5,6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5 & 10	-	5,10,15,20, 25,30,35 & 40	-	-	-	40%

Rosli et al. (2017b) in their study said that the removal of the pollutant from the aqueous solution adsorption is the most commonly used method. In these cost-effectiveness days, the availability of adsorbent should be economical and cheaply available in large quantity and it should be required negligible amount discharge in the waterbodies. The removal percentage of color and iron (Fe) were 74.4% and 79.6%, respectively. This indicates that peat can be used as a cost-effective medium to replace the AC partially for the removal of color and iron considerably lower cost adsorbent.

Zeolite adsorbent

Zeolites are tetrahedral molecules crystalline in nature having the composition of alumina (AlO₄) and silicates (SiO₄) (Perego et al. 2013). The adsorption of heavy metals using zeolites through their ion exchange capacities makes the use of the adsorbent in the treatment of wastewater very high (Al-Jubouri, Curry, and Holmes 2016). Zeolite has a porous structure that provides the binding surface for the adsorption of metals. Some of the mineral zeolites commonly use are chabazite, clinoptilolite, and analcime. Clinoptilolite is the most common and most abundant in nature, having a pore size of 5A to 10A (Malamis and Katsou 2013). Clinoptilolite has very high adsorption qualities for the removal of heavy metals such as lead (Pb). Zeolite (clinoptilolite) has shown to have a higher removal capacity for Pb and cadmium (Cd) (Delkash, Bakhshayesh, and Kazemian 2015). Also Erdem, Karapinar, and Donat (2004) had conducted a study to observe the behavior of clinoptilolite zeolite for the removal of cobalt (II) ion (Co²⁺), cuprus ion (Cu²⁺), zinc ion (Zn²⁺), and manganese ion (Mn²⁺) in wastewater. It was observed that the adsorption ratios of clinoptilolite metal cations fitted in with Langmuir, Freundlich, and Dubinin-Kaganer-Radushkevich (DKR) isotherms and according to the isotherm studies, the sequence of adsorption were Co² > Cu²⁺ > Zn²⁺ > Mn²⁺. The deduction was that natural zeolite holds great adsorption characteristics for the removal of cationic heavy metals from industrial wastewater. Iskander, Khald, and Sheta (2011) reported that the adsorption of zinc and manganese on zeolite and bentonite showed the binding energy of zeolite was greater than bentonite.

In many industries, coal is utilized as a fuel. Coal is one of the sources which generates fly ash as a by-product as well as the main cause of air pollution and disposal problem. Because of its low-cost, it can be utilized as the formation of zeolite by utilizing the hydrothermal process (Hui, Chao, and Kot 2005). Javadian et al. (2015) conducted a study on the removal of cadmium using fly ash into amorphous aluminosilicate adsorbent and reported an 84% removal at an optimum condition pH of 5 and an adsorption capacity of 26.246 mg/g.

Table 4 shows a summary of previous studies for wastewater treatment and leachate using zeolite.

Table 4. Summary of wastewater treatment and leachate using zeolite by previous researchers

Researcher	Wastewater	Parameter	Working Range				Efficiency
			pH	Contact time (min)	Agitation speed (rpm)	Dose (g)
(Rida, Bouraoui, and Hadnine 2013) Algeria	Synthetic wastewater	Methylene blue (MB),	2, 3, 4, 5, 6, 7, 8, 9, 10, 11 (5)	5, 10, 15, 20, 30, 40, 50, 65, 70, 80, 90, 110, 120 (110)	200, 800	0, 0.5, 1, 1.5, 2, 2.5, 3 (1)	95% dye was absorb
(Kehinde and Aziz 2015) Penang, Malaysia	Textile wastewater	Color, SS, COD	2, 2.5, 3, 4, 5, 6, 7, 8, 9, 10, 11 (2–3)	5, 10, 15, 20, 25, 30, 35, 40, 60, 80, 100, 120 (100)	50, 100, 150, 200, 250, 300, 350, 400 (150)	1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 (8)	Removal of 89% of color, 90% of SS 79% of COD
(Huang et al. 2010) Guangzhou, China	Synthetic wastewater	NH3-N	4, 5, 6, 7, 8, 9(8)	10 −300 (180)	-	8, 16, 24, 32, 40, 48, 56, 64 (64)	Remove 96% NH3-N
(Kehinde and Aziz 2015) Penang, Malaysia	Textile wastewater	Color, SS, COD	3	100	150	16	91, 96, and 62% removal of color, SS and COD
(Aydm and Kuleyin 2011) Turkey	Leachate	NH3-N	-	0–1400 (60)	-	30, 40, 60, 80,100 150 (100)	48%, removal NH3-N
(Lakdawala and Patel 2015) Gujarat, India	Sugar industry wastewater	BOD, COD	-	-	-	5, 10, 40, 80, 120, 160, 200 (160)	30.77% and 26.68% BOD and COD
(Aydm Temel and Kuleyin 2016) Giresun, Turkey	Leachate	NH3-N	2, 3, 4, 5, 6, 7, 8 (7)	5–1440 (60)	100, 200, and 300 (200)	30, 40, 60, 80,100 150 (100)	Adsorption capacity of 9.07 mg/g
(Zhao et al. 2016) Beijing, China	Synthetic wastewater	NH3-N	2, 3, 4, 5, 6, 7, 8, 9, 10, 11 (6)	-	200	0.5 to 25.0 (5)	Adsorption capacity 17.5 mg/g
(Wijesinghe et al. 2016) Victoria, Australia	Synthetic wastewater	NH3-N	3, 4, 5, 6, 7, 8, 9, 10, 11 (6)	15, 30, 60, 120, 180, 240, 300, 360, 420, 480, 540, 600, 660, 720, 840, 960, 1080, 1200, 1320, 1440 (60)	100	2	44% removal of NH3-N
(Mashal et al. 2014) Belfast, UK	Synthetic wastewater	NH3-N	5.5, 7, 9 (7)	-	-	-	Adsorption capacity 32 mg/g
(Khosravi, Esmhosseini, and Khezri 2014) Tehran, Iran	Synthetic wastewater	NH3-N	4, 5, 6, 7, 8, 9 (7)	1 −120 (45)	150	1, 2, 3, 4, 5, 6, 7, 8, 9, 10 (6)	Adsorption capacity 43.47 mg/g

Notes. The optimum value in bracket.

Limestone adsorbent

Limestone is a sedimentary rock formed from calcium carbonate (CaCO₃) in the form of calcite minerals from the buildup of shells, corals, algae, and fragments of organisms (Ataman et al. 2016). The limestone has been used in the industry primarily in the process of decoloration due to its effectiveness and low cost (Halim and Ahmad 2013; Kanawade 2016).

The study conducted by Kanawade (2016) has been using activated limestone and activated carbon to remove NH₃-N from synthetic residual water. The result showed that 58% of NH₃-N was eliminated with an optimum media ratio of 25 limestones against 15 activated carbon. This shows that the use of activated carbon can be reduced by combining media with limestone.

The use of limestone in the preparation of composite media to remove boron from aqueous solutions has also been reported (Alias, Halim, and Wahab 2011). Alias, Halim, and Wahab (2011) used a low-cost composite media which is a combination of four types of adsorption which were activated carbon, zeolite, limestone, and pellet residues. The results show that its removal can reached up to 50.49% with boron adsorption optimally at pH 5 with a touch time of 120 minutes at a dosage of 280 g/L.

Halim and Ahmad (2013) used limestone as a low-cost absorber to remove boron from synthetic wastewater showing boron adsorption occurred optimally in the range of pH 6 to 8, the touch time of 90 minutes and 240 g/L limestone dosage at which the removal was 40%.

A study conducted by Hussain et al. (2007) was on the removal of NH₃-N from synthetic residual water using a mixture of limestone and activated carbon as low-cost adsorbents. The results show that the mixed ratio of the activated carbon method (25:15) has eliminated 58% NH₃-N. The results show that limestone has the potential to replace the activated carbon that can get rid of NH₃-N if it is calculated in terms of its low cost.

Limestone has also been combined with activated carbon to remove NH₃-N from leachate at Pulau Burung Island, Penang, Malaysia (Aziz et al. 2004). The results showed that 40% NH₃-N could be removed at an initial concentration of more than 1000 mg/L using a ratio of 5:35 (activated carbon mixture and limestone).

Foul et al. (2009) had been using activated limestone and carbon to remove NH₃-N and COD from dissolved solids at the same disposal site with a mixture ratio of 25 (limestone): 15 (activated carbon) by volume. It was found that NH₃-N and COD were eliminated up to 48% and 86%, respectively. Table 4 summarizes the removal parameters using limestone.

Table 5 shows a summary of previous studies for wastewater treatment and leachate using limestone.

Table 5. Summary of wastewater treatment and leachate using limestone by previous researchers

Adsorbent	Sample	Researchers	Parameter	Working range					Removal efficiency
				pH	Contact time	Dosage	Speed	Particle size	NH3-N	COD	Others
Limestone and activated carbon	Synthetic residual water	Kanawade (2016)	NH3-N	7	30, 60, 90, 120, 150, 180, 210, 240 and 270 min	0–40 cm^3	350 rpm	2.36–4.75 mm (BK), 0.65–2.36 mm (KT)	58%	-	-
Activated carbon, zeolite, limestone and rice husk	Water	Alias, Halim, and Wahab (2011)	Boron	2, 3, 4, (5), 6, 7, 8, 9, 10, 11, 12 and 13	5, 10, 15, 30, 45, 60, 90, (120) and 150 min	0.5, 1, 2, 5, 6, 8, 10, 12 and 14 g (280 g/L)	200 rpm	-	-	-	50.49%/1.8985 mg/g
Limestone	Synthetic residual water	Halim and Ahmad (2013)	Boron	2, 4, 6, 8, 10, 12 and 14 (6–8)	5–120 min (90)	2–14 g (240 g/L)	-	-	-	-	40%
Limestone and activated carbon	Synthetic residual water	Hussain et al. (2007)	NH3-N	2, 5, (7), 11 and 13	30, 60, 90, 120, 150, 180, 210, 240 and 270 min	0–40 cm^3	350 rpm	0.65–1.14, (1.14–2.0) and 2.36–4.75 mm	58%	-	-
Limestone and activated carbon	Leachate	Aziz et al. (2004)	NH3-N	2–13	60 min	0–40 cm^3	350 rpm	2.36–4.75 mm	40%	-	-
Limestone and activated carbon	Leachate	Foul et al. (2009)	NH3-N		0.0, 0.5, 1.0, 1.5, 2.0, 2.5, (3.0), 4.0, 6.0, 12.0 and 24.0 j	0–40 cm^3	50–400 rpm (350)	2.0–3.35 mm	48%/10.83 mg/g	86%/370 mg/g	-
			COD
Limestone and activated carbon	Leachate	Aziz et al. (2011)	Color	-	10–120 min (60)	0–40 cm^3	50–350 rpm (350)	-	-	-	88%

Cockleshell

Daud et al. (2017a) used the mixture of cockle shell (CS) and granular activated carbon (GAC) as adsorbent for the removal of COD and NH₃-N from the stabilized landfill leachate. The adsorption condition occurred optimally in the range of mixing ratio 20:20, shaking time of 120 min, shaking speed of 150 rpm, dosage of 32 g, and pH level of 6. The adsorption isotherm study revealed that Langmuir isotherm was the best fitted for experimental work as compared to Freundlich isotherm. The mixing ratio of CS and GAC can be used as a good and economical adsorption for the treatment of stabilized landfill leachate.

In 2018a, Daud et al., also experimented with the ability of the mixing ratio of granular feldspar and cockle shell which is used for the removal of COD and NH₃-N from the landfill leachate. The mixing ratio revealed that the optimum range of mixing ratio of felspar and cockle shell was 20:20, pH level of 6, shaking speed of 150, agitation time of 120 minutes, with a dosage of 30 g. The adsorption isotherm study revealed that Langmuir isotherm was best fitted for the experimental analysis compared to the Freundlich isotherm. The mixing media used produces encouraging results thus potentially becomes a good and economical adsorbent.

Back in 2017, Daud et al., in his study used cockle shell as an adsorbent media for the removal of COD from the stabilized landfill leachate. The ideal dosage, shaking speed, and pH for the removal of COD is determined by the particle size ranging from 2 to 3.35 mm. Leachate characterizations were then determined. Results revealed a high concentration of COD and (NH₃-N) 1763 mg/L and 573 mg/L, respectively. The optimum shaking speed condition for COD removal was determined at 150 rpm. The optimum pH and dosage were 5.5 and 35 g/L.

Daud et al., in his recent study in 2018b, used CS to analyze the optimal condition for the reduction of COD and NH₃-N from stabilized landfill effluent. The impact of two parameters (pH and dosage) was analyzed through applications called response surface methodology (RSM) and central composite design (CCD). The optimum removal of COD and NH₃-N was at 65.6% and 53.6%, respectively, and were obtained at pH of 6.34, a dosage of 20.21 g with 0.888 desirability value.

Mohd Fauzi (2006) said in their study that sanitary landfill leachate needs to be treated very effectively before discharging into the environment. Waste material like cockle shell is used in this study to remove the BOD, COD, VSS, SS, pH, conductivity, and temperature. The treatment process is operated under semi-anaerobic. The obtained data will examine the quality of leachate before and after the treatment of leachate effectively. The removal percentage of BOD, SS, and VSS are better in 35 days of retention time. The cockle shell is a waste material, thus making this as the best way to reuse the waste for the treatment of landfill leachate plus it is economically low in cost. As for human similarly, reduce, reuse and recycle waste can be practiced.

Crushed cockle shell was used in treating polluted river water by Moideen et al. (2016). A jar experiment was performed by varying the crushed cockle shell dosage ranging from 1 to 4 g. The most extreme reduction of COD was at 38.8% and was found with a dosage of 3 g/L of crushed cockle shell. Unfortunately, the increment of crushed cockle shell dosage did not indicate the maximum level of reduction. Also, in the light of adsorption isotherm, Freundlich (R² = 0.9798) was best fitted as compared to Langmuir isotherm (R² = 0.5737).

Mojiri et al. (2017a), in his study, used a new composite adsorbent BAZLSC for the removal of heavy metals, such as vanadium. BAZLSC includes of bentonite, zeolite, cement, activated carbon, limestone, and cockle shell. RSM and CCD were used to analyze the experimental data in order to get its removal percentage. Independent factors were identified such as pH, dosage, and initial concentration. The removal percentage of vanadium was 86.36% based on the RSM, the optimum ratio of the parameters was at a pH of 3.49, a dosage of 1.71 g/L, and an initial concentration of 52.69 mg/L. Adsorption isotherm revealed that the Freundlich isotherm was a better fitted compared to Langmuir isotherm.

In this study also, bentonite and powdered cockle shell were used to remove Molybdenum (VI) from aqueous solution. Based on the removal percentage analyzed using RSM and CCD, independent factors were identified such as pH and initial concentration. The removal percentage of Molybdenum (VI) increased as pH increased until 3.5. Meanwhile, the removal percentage was also increased by the increasing of the initial concentration by 30 or 40 mg/L. Meanwhile, the increment of ratio had made the removal efficacy decreased. RSM and CCD were applied to determine the optimum removal effectiveness. Independent factors were also considered such as pH, initial concentration (mg/L) and shaking time (min). The removal percentage of bentonite is more effective than the shell. The optimum condition of pH is 5.8, with an initial concentration M_o of 39.2 mg/L and shaking time of 38.6 minutes, bentonite itself removed 81.3% of Molybdenum (VI).

Impact of factors affecting on removal efficiency

There are several factors that impacted the removal efficiency of COD and NH₃-N from leachate. These factors are pH, contact time, shaking speed, temperature, adsorbent dosage, and initial concentration. The optimum condition of COD and NH₃-N removal in leachate is by varying the pH, contact time, shaking speed, temperature, adsorbent dosage, and initial concentration (Sahu, Acharya, and Meikap 2009). The optimum removal of COD and NH₃-N was obtained at different pH values. The impact of pH and temperature shows a reduction in efficiency. The increasing temperature from 15 to 35°C, lowers the efficiency from 93% to 87%. Similarly, increasing the value of pH from 5.7 to 9 influences efficiency reduction from 87% to 96%, respectively, Ghanizadeh and Sarrafpour (2001). The impact of optimum shaking speed and contact time play a vital role in the removal of COD and NH₃-N. The maximum removal of COD and NH₃-N were obtained at the speed of 200 rpm with a time interval of 20 min. However, for the COD removal, the time interval is about 30 min. The impact of dosage adsorbent was assessed with a speed of 200 rpm, pH of 6 and dosage ranges from 3 to 23 g. Initially, the percentage removal increased for both COD (65%) and NH₃-N (63%) with the increment of adsorbent dosage. However, by increasing the adsorbent dosage the removal percentage begins to decrease.

Future perspective and challenges in removal of COD and ammoniacal nitrogen

In this review article, the usage of low-cost adsorbents has introduced a new alternative for water treatment system, and it also brings more opportunities in producing a better way of management. Therefore, the adsorbent may be a good replacement of commercially available carbon due to its comparable efficiency and a significant low cost. In certain investigations, heavy metal removal efficiency of adsorbents from wastewater has been reported to increase after modification. However, in this era very less amount of works have been carried out. Thus, our future points of view are to expand evacuation efficiency of adsorbents after modification (at least prerequisites of heat, acid, and bases), recovery of adsorbents, and recovery of metal particles and utilization of adsorbents at commercial level. The challenges faced in the removal of heavy metals from wastewater are that it might require a lot of adsorbents and additional synthetic substances to keep up a pH that provides reasonable conditions to adsorption.

Conclusion

This review article shows the capability of COD and NH₃-N removal from leachate using low-cost adsorbents. Many studies have been carried out for the removal of heavy metals from the leachate. The activated carbon has normally been used for adsorption process in removing the organic pollutants measured as COD and NH₃-N, which is costly. To replace the high-cost activated carbon, consequently, a wide array of inexpensive adsorbents has been investigated. With nowadays cost-effectiveness, the availability of adsorbent should be economical and cheaply available in large quantity and it should require negligible amount of discharge in the waterbodies. Heavey (2003) used a dry-air peat soil with a 60 mm/day hydraulic loading rate to treat leachate successfully eliminating almost 88.1% COD and 99.9% NH₃-N. This indicates that peat can be used as a cost-effective medium to replace the AC partially for the removal of color and iron considerably as a lower cost adsorbent. Zeolites are tetrahedral molecules crystalline in nature having a composition of alumina (AlO₄) and silicates (SiO₄). Zeolites have great ion exchange and a hydrophilic property, which make them an appropriate treatment of heavy metals. Modified zeolite gives a higher adsorption removal percentage as compared to natural zeolite. Limestone is a sedimentary rock formed from calcium carbonate (CaCO₃). Using the mixture of limestone and activated carbon has the potential to replace the activated carbon that is able to get rid of NH₃-N and COD up to 48% and 86%. Cockle shell (CS) also was used to analyze the optimal condition for the reduction of COD and NH₃-N from stabilized landfill effluent. The optimum removal of COD and NH₃-N was obtained at 65.6% and 53.6%, respectively. Furthermore, the optimal removal of COD and NH₃-N were also compared to the pH, contact time, shaking speed and dosage. It is concluded that the optimum value of pH is in the range of 6 and 8 for both COD and NH₃-N, the optimum value of contact time is in the range of 60 and 120 min, and the optimum value of shaking speed is in the range of 150 and 350 rpm. Similarly, the optimum dosage is in the range of 5 and 60 g. In general, the adsorption isotherm study has been found best fitted both the Langmuir and Freundlich models for the experimental analysis.

Acknowledgment

The authors would like to express thanks to Ministry of Higher Education, Universiti Tun Hussein Onn Malaysia under Research Fund E15501, Research Management Centre (RMC) UTHM and Quaid-e-Awam University of Engineering, Science & Technology (QUEST) Nawabshah, Sindh, Pakistan.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Notes on contributors

Amir Detho

Amir Detho works as a Lab Engineer at Energy & Environment Engineering Department, Quaid-e-Awam University of Engineering, Science & Technology, Nawabshah, Pakistan. He received his Bachelor of Engineering (Mechanical Engineering) in the year 2010 and Master of Engineering (Energy & Environment Engineering) in the year 2018 from Quaid-e-Awam University of Engineering, Science & Technology Nawabshah, Sindh, Pakistan. He is currently pursuing Ph.D. in Faculty of Civil Engineering and Built Environment, Universiti Tun Hussein Onn Malaysia. His current research interest includes landfill leachate/water and wastewater treatment technology. He is a registered member of the Pakistan Engineering Council (PEC).

Zawawi Daud

Zawawi Daud works as an Associate Professor at Faculty of Civil Engineering and Built Environment, Universiti Tun Hussein Onn Malaysia. Dr. Zawawi received his Ph.D. in Civil Engineering (Environment) from Universiti Sains Malaysia in 2009. Dr. Zawawi’s research focuses on alleviating problems associated with water pollution issues from industrial wastewater and landfill leachate. His latest interest is on natural adsorbent material in water and wastewater treatments.

Mohd Arif Rosli

Mohd Arif Rosli is currently a lecturer in the Faculty of Engineering Technology, Universiti Tun Hussein Onn Malaysia. His current research interest includes landfill leachate/water and wastewater treatment technology, building services and performance; water, air, noise and indoor environmental quality. Mohd Arif holds a Ph.D. degree in Civil Engineering from Universiti Tun Hussein Onn Malaysia. He is a member of The Clean Air Forum Society of Malaysia (MyCAS).

Halizah Awang

Halizah Awang received the Ph.D. in Educational Curriculum from Universiti Sains Malaysia in 2010. Since January 2011, she has been with Faculty of Technical and Vocational Education, Universiti Tun Hussein Onn Malaysia as an Associate Professor. Her research focuses on Curriculum and Instruction in Technical and Vocational Education and Training (TVET), Job and career development in TVET and teaching and learning in TVET.

Mohd Baharudin Bin Ridzuan

Mohd Baharudin Bin Ridzuan works as a senior lecturer at Faculty of Civil Engineering and Built Environment, Universiti Tun Hussien Onn Malaysia. Mohd Baharudin is currently doing Ph.D. in Environmental Engineering field at Universiti Tun Hussein Onn Malaysia. His research focuses on environmental issues including water pollution, wastewater treatment, erosion and sedimentation control and rainwater harvesting.