Study On Treatment Of Leachate From Municipal Solid Waste Incineration Plant By Biological Process And Its Efficiency

Waste incineration for power generation is an effective technology for itsadvantages in reducing the volume and mass of municipal solid waste （MSW）,detoxification and energy production. However, a considerable amount ofleachate can be generated during the period of MSW stored in the storage bunkerbefore incineration. The leachate contains various contaminants such as organics,refractory compounds, ammonia nitrogen （NH₄⁺-N）, etc. It is a difficulty and afocus of wastewater treatment to treat the leachate from MSW incineration plant. Inthis dissertation, a new anaerobic-aerobic combined process was adopted andoptimized for treating the leachate from an incineration plant in Beijing and thecharacteristics of the organics in the leachate degraded by the combined processwas discussed.The treatment efficiencies of the leachate from MSW incineration plant by theup-flow anaerobic sludge blanket （UASB） reactor and the expanded granular sludgebed （EGSB） reactor were compared. The results showed that the EGSB reactor hadhigher operation loading and higher COD removal efficiency than the UASB reactor.When the influent COD concentration was around72000mg/L, the maximumorganic loading rate （OLR） of the UASB reactor was12.5kgCOD/（m³·d）, and thecorresponding COD removal efficiency and hydraulic retention time （HRT） were82.4%and5.8d, respectively. With HRT of4d and velocity liquid (V_up) of2m/h,OLR of the EGSB reactor reached18.2kgCOD/（m³·d）, and the COD removalefficiency reached90.3%. Furthermore, the leachate could be treated by the EGSBreactor without pH adjustment. When OLR≤18.2kgCOD/（m³·d）,85.7%of theremoved COD was converted to methane in the EGSB reactor. Methanogens in thegranular sludge mainly included Methanosaeta sp., Methanobacterium sp. andMethanosarcina sp. The anaerobic degradation kinetic of the leachate was studied,and the results indicated that the leachate was inhibitive to the anaerobicbiodegradation. The maximal specific degrading rate (q_max) of organics reckoned bythe Haldane model was3.45gCOD/（gVSS·d）, and the half-saturation constant（K_s）and the substrate inhibition coefficient （K_i） were19.265g/L and130.996g/L, respectively.The effluents from EGSB reactor need to be further treated as it contains highconcentrations of ammonia nitrogen. A system comprised of anoxic moving bedbiofilm reactor （MBBR） and two-stage aerobic MBBR was used to treat theanaerobic effluents. NH₄⁺-N removal and total nitrogen （TN） removal efficiencieswere84.1%and69.8%with total HRT of3.75d, influent NH₄⁺-N concentration of850mg/L, reflux ratio of300%and DO of above3mg/L. However, when theinfluent NH₄⁺-N concentration increased to1000mg/L, the NH₄⁺-N removalefficiency declined to around70%even though total HRT was extended to4.8d.Therefore, an anoxic/two-stage aerobic MBBR-MBR system was proposed andadopted to treat the anaerobic effluents of the leachate. The results showed that theanoxic/two-stage aerobic MBBR-MBR system was effective for nitrogen removal.Under the following conditions as influent pH of about7, total HRT of6.8d andtotal reflux ratio of400%, the removal efficiencies of COD, NH₄⁺-N and TNreached around80%,99%and81%, respectively, even though the influent NH₄⁺-Nconcentration increased to around1650mg/L and the influent COD concentrationwas around6500mg/L. However, the influent COD concentration should be <8000mg/L to avoid being detrimental to nitrification. Furthermore, short-cut nitrificationwith about90%and80%of nitrite accumulation efficiency took place in the secondstage of aerobic MBBR and MBR. The most probable number （MPN） measurementshowed that the amount of ammonia-oxidizing bacteria was much more than that ofthe nitrite-oxidizing bacteria in the second stage aerobic MBBR and MBR. And thehigh pH and high concentration of free ammonia in the reactors were critical forshort-cut nitrification. q_maxof denitrification for the sludge in the anoxic MBBRfitted by Haldane model was2062mgNO₂--N/（gVSS·d）, K_sand K_iwere140.2mg/Land836.3mg/L, respectively. And q_maxof nitrification for the sludge in the firststage aerobic MBBR, the second stage aerobic MBBR and MBR were172.8mgNH₄⁺-N/（gVSS·d）,261.8mgNH₄⁺-N/（gVSS·d） and782.6mgNH₄⁺-N/（gVSS·d）, respectively; K_swere57.9mg/L,82.3mg/L and148.9mg/L, respectively; and K_iwere339.7mg/L,600.7mg/L and601.4mg/L,respectively.The organic pollutants in the leachate from MSW incineration plant could be effectively removed by EGSB-anoxic/two-stage aerobic MBBR-MBR combinedprocess. The removal efficiencies of COD, BOD₅, NH₄⁺-N, TN and TP were98.4%,99.7%,98.9%,81.8%and91.8%, respectively. Most organic pollutants wereremoved in EGSB reactor. COD and BOD₅removal by the EGSB reactor took91.5%and95.6%of each of their total removal by the combined process,respectively. NH₄⁺-N and TN were mainly removed in the anoxic/two-stage aerobicMBBR-MBR system as NH₄⁺-N and TN removal by the system took100%and84.3%of each of their total removal by the combined process, respectively. Besides,experimental results showed that after treated by the combined process, thebiological toxicity of the leachate decreased. The dissolved organic matter （DOM）was effectively removed by the combined process. TOC removal efficienciesreached to100%,100%,96.88%,98.90%and98.67%, corresponding to DOM withmolecular weight of>100kDa,50k-100kDa,10k-50kDa,4k-10kDa and <4kDa,respectively. Protein, amino acid, carboxylic acids compounds and aliphaticcompounds were obviously removed. However, fulvic acid substrates was generatedafter the leachate treated by the combined process, which lead to the increase ofaromatic degree.