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Physico-chemical treatments have been found to be suitable not only for the removal of refractory substances from stabilized leachate, but also as a refining step for biologically treated leachate. Prior to discharge, an additional effluent refining using physico-chemical treatments, such as chemical precipitation, activated carbon adsorption and ion exchange, can be carried out on-site.
Numerous research studies on the treatment of stabilized leachate using different types of physico-chemical treatments.
The treatment of stabilized leachate was conducted by two combinative set processes of coagulation-flocculation-activated carbon and Fenton's oxidation activated carbon has been examined by Ramirez et al. (Ramirez Zamora et al. 2000). Fenton oxidation pre-treatment was found to improve the adsorption capacity of the GAC for COD removal (0.443 g/g of the GAC) at pH 4.0, compared to coagulation (0.193 g/g of the GAC) at pH 5.7. This might be due to the transformation of organic compounds into oxidation byproducts that had smaller molecules than the starting compounds.
A combination of coagulation and photo-oxidation (UV-vis) for the treatment of stabilized leachate from the Qingshan landfill (China) was explored (Wang et al. 2002). This combined treatment with 500 mg/L FeCl3 as the coagulant was able to remove 64% COD with an initial concentration of 5800 mg/L. However, only 31% COD removal was achieved by UV-vis irradiation alone at the wavelength (λ) of 313 nm under the same COD concentration. This result suggested that the combined treatment is more effective than separate process.
Stabilized leachate from the Badazos landfill (Spain) has been treated by integrating ozone and GAC adsorption (Rivas et al. 2003). Approximate 90% COD removal has been accomplished with an initial concentration of 4970mg/L. Using adsorption after the ozonation step allowed the formation of smaller molecules during ozonation, which are more suitable for adsorption than the big molecules present in the leachate. In addition, adsorption is capable of removing the remaining organic compounds and metal species in the leachate (Oh et al. 2004).
Rivas et al. (Rivas et al. 2004) combined a sequential coagulation and Fenton oxidation process in their study. The aim of employing this combined process is to remove the colloidal particles present in the leachate. About 90% COD removal was achieved with an initial concentration of 7400 mg/L at pH 8.5 by using 800 mg/L of FeCl3.
In the Bordo Poniente landfill (Mexico), the transformation of recalcitrant organic compounds from stabilized leachate using a coagulation-ozonation coupling was investigated. The use of Fe2(SO4)3 (iron (III) sulphate or ferric sulfate), as the coagulant during pre-treatment was found to be effective. At pH 4-5, the two-step treatment with ozonation gave 78% COD removal with an initial concentration of 5000 mg/L (Monje-Ramirez and Velasquez 2004).
The treatment efficiency of stabilized leachate from the Gramacho landfill (Brazil) by combining coagulation and ozonation followed by ammonium stripping for 96 hr was evaluated (Silva et al. 2004). This combination was found to be effective for complete removal of NH3-N with an initial concentration of 800 mg/L. When 3 mg/L of ozone was employed, ozonation was able to achieve 48% COD removal with an initial concentration of 3460 mg/L.
In Malaysia, a comparative study for the removal of ammonium nitrogen has been undertaken by Aziz et al. (Aziz et al. 2004) using granular activated carbons and limestones in the Burung Island landfill. Approximately 40% of ammonium nitrogen with an initial concentration of 1909 mg/L was eliminated with 42 g/L of GAC while 19% removal was achieved using 56 g/L of limestone under the same concentration.
Biodegradability enhancement of landfill leachate using air stripping followed by coagulation/ultrafiltration (UF) processes was introduced by (Pi et al. 2009). The air stripping process obtained a removal efficiency of 88.6% for ammonia nitrogen (NH4-N) at air-to-liquid ratio of 3500 (pH 11) for stripping 18 h. The single coagulation process increased BOD/COD ratio of 0.089 with the FeCl3 dosage of 570 mgl−1 at pH 7.0, and the single UF process increased the BOD/COD ratio to 0.311 from 0.049. However, the combined process of coagulation/UF increased the BOD/COD ratio from 0.049 to 0.43, and the final effluent concentration of BOD, COD, NH4-N and colour of leachate were 1223.6mgl−1, 2845.5mgl−1, 145.1mgl−1 and 2056.8, respectively.
Li, Hua et al (Li et al. 2010) conducted experiments to investigate whether the combined process of coagulation/flocculation and powder activated carbon (PAC) adsorption was an efficient treatment method for stabilized landfill leachate . In coagulation/flocculation experiments, coagulants including Al2(SO4)3, FeCl3, PACl), and PFS were employed to study the optimum conditions for the removal of COD, SS and turbidity by jar tests. The optimum working pH for the tested coagulants was 5.5-6.0. The optimum dosages were 0.6 gAl3+/L for Al2(SO4)3 and PACl, 0.6 g Fe3+/L for FeCl3 and 0.3 g Fe3+/L for PFS, respectively. Among the tested coagulants, PFS showed the highest COD removal efficiency (70%), SS removal efficiency (93%), turbidity removal efficiency (97%), toxicity reduction (74%) and the least sludge volume (32 mL). The adsorption experiments suggested that the dosage of PAC=10 g/L and the contact time=90 min were the appropriate working conditions. Under the optimum condition, the removal efficiencies of COD, Pb, Fe and toxicity of the stabilized landfill leachate were up to 86%, 97.6%, 99.7% and 78%, respectively, by the combined coagulation-flocculation and adsorption process.
Removals of pollutants and toxic organic compounds and reduction in bio-toxicity of leachate along an operating full-scale treatment system utilizing chemical coagulation, sand filtration, microfiltration (MF) and reverse osmosis (RO) membrane were evaluated by Theepharaksapan et al (Theepharaksapan et al. 2011). High pollutant removals were achieved mainly by coagulation and sand filtration. Major toxic organic pollutants, i.e. DEHP, DBP and bisphenol A were removed by 100%, 99.6% and 98.0%, respectively.
A mature landfill leachate was treated using a coagulation/flocculation process followed by a photo-Fenton oxidation treatment by (Vedrenne et al. 2012). The leachate was obtained from a landfill in Tetlama, Morelos (Mexico) during the drought season. The treatment of this effluent consisted of a coagulation-flocculation process using an optimal dose of FeCl3·6H2O of 300 mg/L. The supernatant was treated using a photo-Fenton process mediated with FeCl2·4H2O and H2O2 in a compound parabolic concentrator (CPC) photo-reactor operating in batch mode using an R ratio (R = [H2O2]/[Fe2+]) of 114. The global removal efficiencies after treatment were 56% for the COD, 95% for TC, and 64% for NH4+.
Combination of biological and Physicochemical processes for landfill leachate:
Physicochemical treatment units are placed either as pretreatment to reduce the loading rate for biological processes or as post-treatment to reach a high quality discharge standard. For example, activated carbon (Kargi and Pamukoglu 2004) and ammonia stripping/coagulation have been commonly used as pretreatment of sequencing batch reactor (SBR) (Uygur and Kargi 2004).
Bohdziewicz and Kwarciak (Bohdziewicz and Kwarciak 2008) studied landfill leachate treatment efficiency during fermentation process in UASB reactor and post-treatment in RO process. UASB reactor was operated at a hydraulic retention time (HRT) of 7 days and organic loading rate (OLR) of 0.6 kg COD - m−3d−1. The HRT (3 days) and OLR 1.3 kg COD - m−3d−1 were taken as the optimum fermentation process parameters. Under such designed conditions COD removal achieved value of 76%. Due to poor quality of UASB effluent, wastewater was put into the post-treatment process. In RO process COD, BOD, chloride, ammonia nitrogen parameters were removed in 95.4%, 90.2%, 85.4% and 88.7%, respectively.
Hasar et al. (Hasar et al. 2009) presents a configuration for the complete treatment of landfill leachate with high organic and ammonium concentrations. The complete treatment consisted of ammonia stripping, coagulation/flocculation, Aer/An MBR and reverse osmosis. By this configuration, leachate could be used even for all the reuse applications at the optimal conditions because the final COD value decreased to less than 4mg/l. The flux reduction in reverse osmosis was acceptable for the effluent of Aer/An MBR operated at SRT 30 days, which is an optimal condition for the biological treatment.
Li et al. (Li et al. 2009) carried out advanced treatment of mature landfill leachate from a municipal landfill located in southern China (Jiangmen) in a full-scale plant using a new process. The combined process has SBR serving as the primary treatment, with PFS coagulation coupled with a Fenton system as secondary treatment, and a pair of up flow biological aerated filters (UBAFs) in parallel as tertiary treatment. The overall removal efficiency of COD in this process was 97.3%, with an effluent COD less than 100 mg/L. Up to 99% ammonia (NH3-N) removal efficiency was achieved in the SBR, with an effluent of less than 3 mg/L. The total phosphorus and suspended solids in the final effluent were reduced to less than 1 mg/L and 10 mg/L, respectively.
Guo et al. (Guo et al. 2010) characterized and submitted theleachatee fromthe Changshengqiao landfilll (Chongqing, China) to a combined process of air stripping, Fenton, SBR, and coagulation. The overall removal of COD and NH3-N were 93.3% and 98.3%, respectively.
A combination process was developed including sequence batch reactor (SBR), coagulation, Fenton oxidation, and biological aerated filtering (BAF) in series. A total reduction of COD (98.4%), BOD5 (99.1%), NH4-N (99.3%), TP (99.3%), SS (91.8%), turbidity (99.2%) and color (99.6%) achieved in the final BAF effluent (Wu et al. 2011).
Cassano et al (Cassano et al. 2011), investigated the treatment of medium age landfill leachate by employing several set ups including a sequencing batch biofilter granular reactor (SBBGR) step, with or without ozone enhancement, followed or not by a polishing stage with solar photo-Fenton (SphF).
The objectives of the investigation were to compare different treatment strategies in order to achieve the lowest operating cost and to reduce the toxicity of the final effluent, evaluated by three different tests (respirometry, Vibrio fischeri and Lepidium sativum phytotoxicity). These objectives were addressed for two different target COD values, namely 160 and 500 mg/L, to be met in the final effluent for disposing of to water bodies and to sewers, respectively, requested by Italian environmental regulation.
Zgajnar et al. (Zgajnar Gotvajn et al. 2011) compare Fenton's oxidation with existing biological SBR employed for treatment of theleachatee, generated inthe local municipal landfilll. Fenton's oxidation, accomplished at different molar ratios of reagents (M(Fe2+)/M(H2O2) was 1/1; 1/3.3; 1/6.6 and 1/13.3) and various temperatures (20-45 °C), assured good removal of organic compounds (80% as COD) as well as other pollutants and slightly reduced toxicity. They concluded that Fenton's oxidation is not appropriate for polishing already treated leachate in SBR, but it could be a viable option for pretreatment of landfill leachate.
Aziz et al (Aziz et al. 2011) treated landfill leachate by using the SBR process. Two types of the SBR, namely non-powdered activated carbon and powdered activated carbon (PAC-SBR) were used. Response surface methodology (RSM) was applied for experimental design, analysis and optimization. Based on the results, the PAC-SBR displayed superior performance in term of removal efficiencies when compared to SBR. At the optimum conditions of aeration rate of 1 L/min and contact time of 5.5 h the PAC-SBR achieved 64.1%, 71.2%, 81.4%, and 1.33% removal of COD, colour, NH3-N, and TDS, respectively.
A solar photo-Fenton process, without iron addition, is proposed for the decontamination of a landfill
leachate in a pilot plant with CPCs, after a preliminary pre-treatment in aerated and non-aerated lagoons by Vilar et al (Vilar et al. 2011). The solar photo-Fenton reaction leads to 60% mineralization (DOCfinal = 1200mgL-1) and 90% reduction of aromatic content of the leachate after 5 sunny clear days (165 kJUV L-1), consuming 275mM of H2O2.
Claudio et al (Di Iaconi et al. 2011) developed process based on ozone enhanced biological degradation, carried out in an aerobic granular biomass system (SBBGR) was tested at lab scale for treating a typical medium age landfill leachate. The results have shown that ozonation greatly improves the biological treatment's effectiveness. A strong synergy between chemical and biological oxidation was observed with an O3 consumed / COD removed ratio as low as 0.24.
As a whole, a combination of two treatment process to be more efficient and effective than individual treatment. This could be due to the fact that a two-step treatment has the ability to synergize the advantages of individual treatments, while overcoming their respective limitations. A combined treatment is indeed capable of improving the effluent quality and minimizing the residue generated at a lower treatment cost than an individual treatment.