Treatment Of Leachate From Msw Landfill And Incineration Plant By Combination Of Physicochemical With Biological Process

In China, municipal solid waste (MSW) is being generated at an annual increasing rate around 10%, and its disposal is becoming a huge problem for the communities involved. Solid waste comprises of a broad range of materials. Currently, the primary MSW disposal methods are incineration and landfilling. After landfilling, MSW undergoes physicochemical and biological changes. Consequently, the degradation of the organic fraction of the wastes in combination with percolating rainwater leads to the generation of a highly contaminated liquid called"leachate". Whereas MSW incineration plants are designed to hold MSW in waste pits for a few days to allow drainage of leachate before combustion because of MSW in China contains a large fraction of food waste with high water content. The common features of the two types of leachates, whether that generated from landfill or incineration plant, are high strengths of organic concentration (chemical oxygen demand COD), ammonia (NH3–N), volatile fatty acids (VFA), heavy metals and other hazardous pollutants. Therefore, leachate has high concentration of the pollutants and great changes in quality and quantity, which can pollute the surrounding environment and should be treated to meet relevant standards before discharge into sewers or direct disposal into surface water.In general, the appropriate leachate treatment methods are mainly based on specific characteristics of leachate under examination. The leachate treatment methods are physical, chemical, and biological. Bio–refractory contaminants, contained mainly in older leachates, are not amenable to conventional biological processes, whereas the high ammonia content might also be inhibitory to activated sludge micro–organisms. In more recent decades, a combination of physical, chemical and biological methods are often required for efficient treatment of leachate to meet local discharge standards. However, it is difficult to obtain satisfactory treatment efficiency by any one of these methods alone.The amount of MSW production in Chongqing reaches to about 7500 ton/day, a majority of which are disposed at two modern MSW disposal plants which are Changshengqiao (CSQ) MSW Landfill Site and Tongxing (TX) MSW Incineration Plant. CSQ landfill site and TX incineration plant has an average leachate generation of 500 m3/day and 200 m3/day, respectively. The two kinds of leachate were characterized and submitted to a combined process in this study. Optimum operating conditions for each process were identified. The performance of the treatment was assessed by monitoring the removal of organic matter (COD) and ammonia (NH3–N).CSQ leachate samples were collected and analyzed to investigate the main parameters of leachate characteristics. It was observed that this leachate presents a relatively high value of pH ranging from 7.90 to 8.47. It can be deduced that leachate corresponded to the methanogenic phase of decomposition. The COD, BOD5, NH3–N, TOC, SS and VFA concentration values were 4000–5000, 374–1436, 1330–1500, 831–946, 812–979 and 384–782 mg/L, respectively. So, this leachate displayed high concentrations of contaminants. The BOD5/COD ratio of the leachate was ranged from 0.09 to 0.22, which indicated the raw leachate was low biodegradability. As well as, raw CSQ leachate was a significant composed of large molecules with MW >100 kDa accounted for 24.5%, which tends to have high concentrations of humic substances (refractory organics). Therefore, CSQ leachate has been a characteristic as old leachate and could not be biologically treated. So, physical–chemical treatment should be used before biological treatment to remove the humic substance and enhance biodegradability.In the present study, a combination of physicochemical and biological processes was investigated to remove the crucial pollutants (COD and ammonia) from the CSQ leachate so that the water quality of the final effluent could be meet the Chinese landfill pollution control standard (GB16889–2008). Therefore, a laboratory scale of air stripping, Fenton, SBR, coagulation and nanofiltration (NF) process was designed and operated to treat the raw CSQ leachate with initial concentration of 4560 mg/L COD, 1500 mg/L NH3–Nand 933 mg/L BOD5. It has been confirmed that air stripping (at pH 10.5, NaOH dose of 17500 mg/L, air flow rate of 12.5 L/min, and aeration time 23 h) effectively removed 96.7% of the ammonia. Air stripping is act to crumbling the high MW >100 kDa to smaller MW 10–50 kDa and 50–100 kDa with rate is over 75%. Moreover, Air stripping is effective to remove the organics of MW 4–10 kDa with COD removal rate is over 60% due to adhesion with air bubbles. The Fenton process was investigated under optimum conditions (pH 4.0, Fe2+ of 0.08 mol/L, H2O2/ Fe2+ of 10 and reaction time of 20 min), COD removal of up to 65.1% was achieved. Biodegradability (BOD5/COD ratio) increased from 0.18 to 0.38. Fenton is effective mainly on the organics of MW 2–4 kDa, 10–50 kDa, 50–100 kDa and > 100 kDa with COD removal rate is over 75%. Therefore, Fenton can effectively remove the soluble organics of medium and high MW. Thereafter the Fenton effluent was mixed with sewage at dilutions to a ratio of 1:3 before it was subjected to the SBR reactor. Up to 82.4% BOD5 removal and 61.2% COD removal were achieved by the SBR under the optimum parameters of 11 hr aeration time and 4 mg/L DO. SBR is effective mainly on the organics of MW <2 kDa with COD removal rate is over 70%. Therefore, SBR can effectively remove the soluble organics of very low MW. In the coagulation process, The optimum coagulant (Fe2(SO4)3) was a dosage of 800 mg/L at pH of 5.0, which reduced COD to an amount of 381 mg/L. Coagulation is effective mainly on the organics of MW > 4kDa with COD removal rate is over 45%. Therefore, Coagulation can effectively remove the soluble organics from water and prevent membrane from fouling. NF membrane was used as a final step of the leachate treatment to refine the effluent. It was achieve removal efficiency 81.1% of COD and 79.8% of BOD5 under optimum pH of 8, pressure of 0.5 MPa and temperature 20oC. NF has an effect mainly on the organics of MW <2 kDa, 2–4 kDa, 50–100 kDa and >100 kDa with COD removal rate is over 75%. NF has a good effect on the removal of soluble organics. The water quality of final effluent, COD≤72 mg/L, BOD5≤23 mg/L, NH3–N≤10 mg/L, was meet to the Chinese landfill pollution control standard (GB16889–2008). The operation cost of this combined treatment method (air stripping–Fenton–SBR–coagulation–NF) was estimated to be $ 23.63 /ton.TX leachate samples were collected and analyzed to investigate the main parameters of leachate characteristics. It was observed that this leachate presents a relatively low value of pH ranging from 5.59 to 6.10. It can be deduced that leachate corresponded to the acidic phase of decomposition. The COD, BOD5, NH3–N, TOC, SS and VFA concentration values were 54600–67600, 27800–32400, 1230–1770, 14800–16100, 6820–7930 and 12100–15700 mg/L, respectively. So, this leachate displayed very high concentrations of contaminants. The BOD5/COD ratio of the leachate was ranged from 0.51 to 0.54, which indicated the raw leachate has good biodegradability. Therefore, TX leachate has been a characteristic as young (fresh) leachate and could be using biological treatment as primary treatment. Raw leachate was diluted with sewage in order to reduce the concentration of inhibitory compounds. The biological process operating with an optimal leachate to sewage ratio 1:6. In this way, the characteristics of the influent became the following: pH = 6.5–8.0, COD = 9870–12340 mg/L, BOD5 = 5210–6130 mg/L, NH3–N = 540–685 mg/L. This diluted leachate was mainly consists of the organics of organic matters with MW <2 kDa and has insignificant composed of large molecules (> 100 kDa). In the present study, a combination of physicochemical and biological processes was investigated to remove the crucial pollutants (COD and ammonia) from the TX leachate so that the water quality of the final effluent could be meet the Chinese integrated wastewater discharge standard (GB8978–1996). Therefore, a laboratory scale of anaerobic, aerobic, coagulation and NF process was designed and operated to treat TX leachate (diluted) with initial concentration of 10180 mg/L COD, 654 mg/L NH3–N and 5395 mg/L BOD5. The anaerobic reactor was more effective at removing organic matter. COD, BOD5 and NH3–N removal were 62.0%, 49.2%, and 49.5%, respectively, at an optimal reaction time of 20 hr and HRT of 4 day. Anaerobic is effective mainly on the organics of MW 2–10 kDa with COD removal rate is over 79%, while the concentration of the organics with an MW >50 kDa have been increased due to accumulated some of the low MW with others to form larger MW. The aerobic reactor was more effective at removing organic matter and ammonia. COD, BOD5 and NH3–N removal were 94.0%, 96.9%, and 89.4%, respectively, at an optimal aeration time of 14 hr and DO level of 4 mg/L. Aerobic was achieved a good COD removal efficiency in the all fractions of DOM. The highest removal efficiency was 98.2% in the DOM fraction of MW < 2 kDa, while the lowest removal was 76.3% in the DOM fraction of MW 2–4 kDa. Chemical coagulation can be beneficially used to remove colloidal particles. An optimal pH of 5 and an optimal Fe2(SO4)3 dose of 600 mg/L were obtained for chemical coagulation to yield 40.0% COD and 37.1% NH3–N removal. Coagulation is effective on the organics for all fractions with the exception of those of MW < 2 kDa with COD removal rate is over 45%. Therefore, Coagulation can effectively remove the soluble organics from water and prevent membrane from fouling. NF membrane was used as an advance step of the leachate treatment to refine the effluent. It was achieve removal efficiency 72.5% of COD and 55.6% of BOD5 under optimum pH of 6, pressure of 0.4 MPa and temperature 20oC. NF has an effect mainly on the organics of MW <2 kDa, 50–100 kDa and >100 kDa with COD removal rate is over 75%, whereas the organics of MW 4–10 kDa and 10–50 kDa have been poor removal (<15%). Nanofiltration has a good effect on the removal of soluble organics. The water quality of final effluent, COD≤38 mg/L, BOD5≤12 mg/L, NH3–N≤8 mg/L, was comply with first–class of the Chinese integrated wastewater discharge standard (GB8978–1996). The operation cost of this combined treatment method (anaerobic–aerobic–coagulation–NF) was estimated to be $ 17.72 /ton. Consequently, these two combined processes were successfully employed and very effectively decreased pollutants loading. The final effluent of the combined treatment could be directly discharged into waterway without effects on the health of aquatic ecosystems or considered for non–potable use. It is observed that these two combinations of leachate treatment are demonstrated outstanding treatment performances compared with other similar combined treatment in different selected countries. It is also concluded that the two treatment processes have perfect treatment effects and the expenses are relatively low comparing to other leachate treatment systems that have been applied in China, making these methods are a competitive and an attractive economic alternative treatment methods. So the selected treatment processes for the two leachates have a worth generalizing and a positive guidance reference meaning for the practical treatment project of leachate from landfill site and incineration plant.