Leachate decontamination through biological processes coupled to advanced oxidation: A review

Figures & data

Figure 1. Published articles associated with leachate treatment (Scopus).

Table 1. Comparison of leachate treatment alternatives.

Treatment	Alternative’s name	Test and operational conditions	COD Removal Rate	Advantages	Disadvantages	Costs	Reference
Aerobic Biologic	Activated Sludge	Sludge HRT = 12 h – 24 h. Dissolved oxygen = 0.4– 5.2 mg L-1 Suspends solids = 1500– 3500 mg L-1 Leachate BOD5 = 1.449 mg L-1 COD = 3151 mg L-1	80%	It does not emit bad odors. The easiest adaptation to the real size.	The input should be high biodegradability leachate. The highest energetic consumption. It requires a process of sludge acclimatization.	Between 1.3 USD per m3 and 7 USD per m3	(de Albuquerque et al. 2018; de Carluccio, Fiorentino, and Rizzo 2020; Kattel et al. 2016; Song et al. 2020)
	Membrane Bioreactor	COD = 2000– 4000 mg L-1 pH = 8.56 NH4 = 400– 1000 mg L-1 HTR = 18– 44 h	65–90%	The highest ammonia remove. Allows the recovery of products from wastewater.	Low efficiency with highly toxic leachate. High cost. The main cost is electricity consuptiom It requires a process of sludge acclimatization.	Between 1 USD per m3 and 5 USD per m3.	(Ahmed and Lan 2012; Chen et al. 2021; Dereli, Clifford, and Casey 2020; Saleem et al. 2018; Zolfaghari et al. 2016)
	Aerated lagoons	pH = 9.23– 12.17 Opertation time = 48 month Electrical conductivity = 64– 1120 uS cm-1 Plants used = Phragmite and Vetiver	70%	Low cost.	Temperature dependency Area requerided	For reducing BOD5 and COD = 0,00026 USD per m3 For reducing a nitrogen Kg = 2.15 USD	(Afzal et al. 2019; Carney Almroth et al. 2021; Parde et al. 2021; Saeed et al. 2021; Vymazal, Zhao, and Mander 2021)
Anaerobic – Biologic	UASB Reactor	COD = 3000– 4000 mg L-1 TSS = 1742.5 mg L-1 NH4 – N = 1181.1 mg L-1 Volume tank reactor = 10.8 L Inoculum sludge TS = 22.3 mg L-1 Volume inoculate sludge = 6 L	34.2– 65%	Low sludge production.	Susceptible to changes in ph and heavy metals. It emits bad odors.	Between 10 and 12 USD per m3 d	(Ismail et al. 2020; Kamyab, Zilouei, and Rahmanian 2019; Li et al. 2020; Tsui et al. 2020; Wang et al. 2016)
Physicochemical	Coagulation – Flocculation	Leachate conditions pH = 8.23 Color (Pt-Co) = 2930 TDS = 3200 mg L-1 Coagulant used 0.1 g FeCl3 g AC 0.1 g Ca(OH)2	37– 65%	Easier use in actual size.	Increase of iron and aluminum ions in the treated effluent. Requires prior optimization for reagent selection	Between 1.4 USD per m3 and 2 USD per m3	(Chaouki et al. 2021; Liu, Liu, and Nie 2017; Nazia et al. 2021; Ren et al. 2019; Zhao et al. 2021)
	Adsorption	Raw leachate COD = 1272 NH4-N = 250– 1150 mg L-1 BOD5/COD = 0.12 Adsorptio process Shake time = 1,2,8 h Granular Activated Carbon (GAC) = 10– 20 mg L-1 T = 22.5°C	43– 70%	Offers the possibility to be coupled with another treatment. High PO4 efficiency remove.	Regeneration of the material used for the adsorption of pollutants. High consumption of resources.	1.18 USD per kg of GAC Operation cost per year = 23,215 USD	(Ferraz and Yuan 2020; Franco et al. 2020; Mira Anuar and Chan 2020; Mohammad-pajooh et al. 2018; Reshadi, Bazargan, and McKay 2020; Zielińska, Kulikowska, and Stańczak 2020)
	Advanced oxidation processes	COD = 1500– 4000 mg L-1 Ammonia = 400– 1400 mg L-1 pH = 5– 8	60– 95%	The high output of OH radicals. It could be used for treating toxic wastewater. Amazing performance in the presence of sunlight. Treated effluents with high biodegradability Effluents are available for coupling with another kind of treatment.	Low bibliographic information specifically in heterogeneous photocatalysis employing sunlight. The highest cost when they are used as the only alternative. Better results in acid pH.	The costs can fluctuate according to the leachate age and the goal of treatment. Fenton: 4 USD per m3 – 9 USD per m3 Ozonation: 3.5– 6 USD per m3 Photocatalyisis: 0.8 USD per m3 – 4 USD per m3	(Aghbashlo et al. 2018; Feng et al. 2021; Gautam, Kumar, and Lokhandwala 2019; Giannakis, Lin, and Ghanbari 2021; Han et al. 2020; Haranaka-Funai et al. 2017; Hassan, Zhao, and Xie 2016; Tejera et al. 2021; Wu et al. 2021)

Figure 2. Correlation map on landfill leachate treatment created in VOSVIEWER.

Table 2. Publications in which AOP and biological treatment are used together in leachate.

First treatment	Second treatment coupled	Operating conditions tested	Chemical leachate composition	Landfill age or type of leachate	Type of residues disposed	Highlights	Problems and lack of investigation.	References
suspended grow bioreactor (Biologic treatment)	Fenton oxidation process	Biologic treatment: Bioreactors of 2 L capacity Air blower with a flow rate = 0.175 L min-1 Treatment time = 60 days. Fenton process: batch system at ambient temperature. pH was adjusted to the desired values in the range from 2 to 12. the mixture was stirred under 200 rpm during 60 min of reaction Treated leachate was neutralized with NaOH or/and H2SO4.	COD (mg O2 L-1) = 95,700 BOD5 (mg O2 L-1) = 42,000 Nitrate-N (mg L-1) = 8.79 Nitrite-N (mg L-1) = 24.1 Sulfate (mg L-1) = 2541 Cr (mg L-1) = 2.18 Cu (mg L-1) = 0.21 Fe (mg L-1) = 2.78 Mn (mg L-1) = 9.19 Zn (mg L-1) = 5.38	Young leachate	Household waste	The coupled had 30% more efficiency than the first treatment.	Amount of chemicals used in the process to pH neutralization There is a lack of economical information and then the treatment cannot implement in a real process.	(El Mrabet et al. 2020)
Activated sludge process and Zahn-Wellens test	Fenton – Ozone	Biologic treatment: The biodegradability was assessed through Zahn-Wellens test. Batch experiments in a reactor of 1 L. Hydraulic retention: 1– 2 days. Air pumps and diffusors was located at the bottom of the reactor. Fenton It was developed in batch conditions in a Cylindrical glass reactor. pH was increased until 9 with NaOH. Reagent dose ratio of COD/H2O2/Fe = 1/4/0.8 Ozonation: Reactor volume: 2.6 L Concentration: 10– 40 mg L^−1 Treatment time 240 min	COD ((mg O2 L-1) = 5920 BOD5 (mg O2 L-1) = 3160 BOD5/COD = 0.53 NH4-N (mg L-1) = 710 F (mg L-1) = 24 SO4 (mg L-1) = 114 NO3 (mg L-1) = 64 Cl (mg L-1) = 2280	Medium-aged landfill	Municipal non-hazardous waste	The researchers carried out several combinations between AOP and biologic treatments. The biodegradability had significant variation when they used AOP as the first treatment COD and BOD7 removal efficiency over 93%	It did not analyze heavy metals remove. The scope of this investigation could be broader. It did not carry out financial analysis.	(Klauson et al. 2015)
Photo-Electron-Fenton (PEF)	Aerated biological oxidation	AOP: PEF developed in batch system in presence of two Hg lamps with λ = 254 nm Iron plates were used as cathode and anode. Effective contact area: 155.22 cm^2 pH adjusted to initial value. Electrolysis time: 0–90 min. Biologic treatment: Based on the addition of a mixed consortium of bacteria, fungi, protozoa, organic load and O2 Automated bioreactor Reactor volume: 5 L Samples were taken each 3 hours until 24 hours.	COD ((mg O2 L-1) = 2441 BOD5 (mg O2 L-1) = 451 BOD5/COD = 0.18 TN (mg N L-1) = 1403 TDI (mg Fe L-1) = 11.29 SS (mg L-1) = 437 TC (mg C L-1) = 2225 Turbidity (NTU) = 81	Mature leachate	Organic waste	They proved that coupled treatment gets better efficiencies than alone one. The use of RSM could be an amazing alternative for futures computational processes of both alternatives in the leachate treatment field.	Applying AOP as the first treatment notoriously rise biodegradability, however, they did not analyze the cost impact of the AOP used related to energy consumption. Research carried out in acid pH. They did not make a financial analysis. High reagents consumption.	(Seibert et al. 2019)
Electro Fenton (EF)	Membrane Bioreactor (MBR)	MBR Volume reactor = 6 L Oxygen rate = 5 L min −1 Pump capacity = 0.7 Lpm Pore size = 0.1 um MLSS concentration = 10 g L −1 EF Volume reactor = 1 L Specific electrode surface = 0.069 cm^2 cm^−3 Independent variables = pH, H2O2 dosage, reaction time, current density. Optimum conditions: pH = 2.85 Current density = 129.5 A m^−2 H2O2 dosage = 55.75% Reaction time = 40 min.	COD ((mg O2 L-1) = 23,200 BOD5 (mg O2 L-1) = 4872 Ammonia nitrogen (mg L-1) = 2196 TDS (mg L^−1) = 32,760 TSS (mg L-1) = 498 Chloride (mg L-1) = 2670 Sulfate (mg L-1) = 587 Phosphate (mg L-1) = 185	Mature leachate	Municipal solid waste	The include the treatment costs. They got a second order polynomial model for making predictions. Chloride removal over 83%. Coupling AOP – MBR achieve the highest efficiencies.	They just include one sample leachate. Futures researchers could carry out several inlet compositions.	(Pieus Thanikkal and Pooppana Antony 2021)
Photocatalysis TiO2/Ag	fermentation using C. tropicalis	Photocatalysis: They used Response Surface Methodology for getting better results. Optimum operating value: pH→4,31 TiO2/Ag (g dm-3)→0,937 Reaction time(min)→138,6 Response variable: COD and NH4 removal efficiencies. Biological treatment: C. tropicalis strain incubated at 30°C for 48 h with 150 rpm agitation speed. microbial suspensions contained →7.5* 10^6 CFU mL-1. Biodegradation process was performed in batch (Erlenmeyer flasks of 250 mL).	COD (g dm-3) = 24,000 BOD5 (g dm-3) = 2160 BOD5/COD (g dm-3) = 0.09 TOC (g dm-3) = 21 TDS (g dm-3) = 3.4 NH4 (mg dm-3) = 346.5 NO3 (mg dm-3) = 15.84 SO4 (mg dm-3) = 114.13 Mg (mg dm-3) = 32 Ca (mg dm-3) =15 Cu (mg dm-3) = 1.42 Zn (mg*dm^−3) = 1.53 Fe (mg dm-3) = 24.2	Mature leachate	domestic solid waste	Reduction of heavy metals over 90%. TiO2/Ag shows high photocatalytic activity and recyclability. The combination of AOP and Biological treatment remove toxic compounds.	The main problem was the use of chemical products to pH fit. The work was developed just under acid pH. Although COD removal rate was 70% after the coupled, they should use a polish treatment.	(Yasmin et al. 2020)
Membrane Bioreactor	Electrochemical Oxidation (EO)	Samples of leachate was stored at 4°C until use. Lab scale design basis in a vessel Biolafitte stirred tank reactor Biologic treatment with a sludge recycling system Volume reactor of 4 L Membrane features: 400 mm length, 30 mm diameter, 004 m^2 surface and 0,2 um pore size. Membrane maintenance time: every 20–25 days. Temperature biologic treatment = 25°C AOP: Glass reactor of 0,25 L EO used platinized titanium grid as anode and Stainless steel as cathode. Surface area of anodes: 0,2 dm^2 pH worked: 9.	pH = 8,12 EC (ms cm-1) = 27,6 COD (mg L-1) = 8000 BOD5 (mg L-1) = 1400 BOD5/COD = 0,18 TSS (g L-1) = 500 VSS (mg L-1) = 270 NH4*N (mg L-1) = 100 Greases (mg L-1) = 600 Total Phenols (mg L-1) = 700 Total VFA (mg L) = 1500	Mature leachate	Municipal waste	For the MBR treatment, the analysis was developed in pH values close to the real one. The researchers include the cost of each treatment in the investigation. Using MBR before AOP reduced the amount of energy required.	They did not carry out analysis in AOP with real pH using chemical products. This kind of AOP increases the final cost treatment due to the amount of energy required.	(Aloui et al. 2009)
Photo-Fenton	Activated sludge (AS)	AS Sludge age = 9,45 d MLSS = 4279 mg L-1 Hydraulic retention time = 11,3 d SST sludge = 3250 mg L-1 SSV sludge = 2670 mg L-1 SSF = 580 mg L-1 Fenton process Sulfuric acid (Pronolab, 98%, 1.84 g cm-3) sodium hydroxide (Merck) Hydrogen peroxide solution (30% (v/v), 1.10 g cm-3, Quimitécnica, SA) Photo-Fenton Reactor Wooden box (80 cm × 80 cm x 50 cm) pH range = 2.4– 4.4 H2O2 concentration range = 1284– 12,216 mg L-1 Fe range = 16–184 mg L-1 Irradiation time = 5– 180 min	TC (mg L-1) = 1785 COD (mg L-1) = 12,797 BOD5(mg L-1) = 4251 BOD5/COD = 0,33 ammoniacal nitrogen (mg L-1) = 1416 total phosphorous(mg L-1) = 13,5	Middle age	Municipal waste	The coupled enhanced the leachate biodegradability Treatment efficiency over 96% in terms of COD and BOD5.	They did not include the cost for using that combination in a real treatment. They did not analyze a possible Fe residual which can require a polish treatment.	(Colombo et al. 2019)

Table 3. Doping catalyst applied in toxic wastewater.

Material of TiO2 doping	Methodology applied for doping the TiO2	Wastewater input composition	Removal efficiency	Benefits of the treatments	Optimum conditions	References
Iron (Fe)	Sol–gel method	Azo dye Reactive Black 5 (RB5) = 20 mg L-1 E. Coli = 2.16x10^5CFU/ml	-RB5 > 96% -E. Coli > 95%	This doping may grow the photocatalyst activity by lowering the energy employed for photon generation. It contributes to moving the wavelength until the visible region in the spectral response of the catalyst. - Antibacterial activity.	0.5 mol% Fe – doped TiO2	(Stoyanova et al. 2013)
Silver (Ag)	Liquid impregnation (LI)	Methyl Orange (MO) = 5 mg L-1 solution in 250 mL flasks.	MO = 80%	Better photocatalytic efficiency of TiO2 in the visible light region. The Ag doping inhibit the electron-hole recombination. - Antibacterial activity.	- pH = 7.0 – 1% Ag/TiO2 calcined at 700°C	(Us Saqib, Adnan, and Shah 2017)
Nitrogen (N)	Sol–gel method	COD = 6140 mg L-1 BOD5 = 558 mg L-1 COT = 1890 mg L-1	TOC = 75%	This alternative can remove heavy metals. - It allows the possibility of coupled with biologic treatment.	- pH = 6.0 - Initial COD concentration = 200 mg L-1 - Temp = 37°C	(Hu et al. 2016)
Tungsten (W)	sol–gel method	COD = 63,600 mg L-1 BOD5 = 57,900 mg L-1	COD = 60%	- pH value close to real condition.	- pH = 7.17 - Content = 2.66 wt.%	(Azadi et al. 2020)
Carbon (C)	sol–gel method		COD = 64%	- It requires less amount of doping material for getting the optimum.	- pH = 6.53 - Content = 0.07 wt.%
Activated Carbon (AC)	sol–gel method	COD = 1000 mg L-1	COD = 49% Color = 28.26%	- The color and COD removal efficiency increased when the UVA intensity was higher.	Light intensity = 32 (µW/cm^2) Time = 60 min	(Rojviroon, Rojviroon, and Sirivithayapakorn 2015)
Neodymium (Nd)	Template-free sol-gel method	methyl orange in an aqueous solution	Degradation of methyl orange = 96.5%	It prevents recombination of electron hole pairs. - Better photoactivity in wavelength of 200– 300 nm.	1 mol% Nd-TiO2	(Liang et al. 2020)

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