Study On Municipal Solid Waste Rapid Degradation And Leachate Ammonium In Situ Removal Based On Bioreactor Landfill

With the development of national economy and the expansion of municipal population,Municipal Solid Waste(MSW)production is increasing.Sanitary landfilling is a primary method of MSW treatment in China.However,problems of environmental pollution caused by landfill leachate and biogas are widely concerned by society.On one hand,efficient MSW management technology is needed for minimizing the MSW and recovering of reeyelable materials.On the other hand, advanced technologies for accelerating MSW stabilization,recovering biogas and controlling leachate pollution,in particular for leachate ammonium removal are also needed.In this dissertation,methods for the rapid bio-decompostion of MSW and in situ removal of leachate ammonium were investigated.Results showed that leachate recirculation could improve MSW body settlement.However,it would result in the retardation of MSW degradation.By studying the decomposition of MSW under oversaturated condition in comparison with leachate recirculation,a novel technology for accelerating MSW decomposition was proposed.MSW classification could accelerate MSW decomposition and the advent of methane-producing stage. Inhibition of high concentration of free ammonia in landfilled waste to nitrification was the main reason resulting in the low rate of in situ ammonium removal in forced air-supply MSW reactor.A considerably higher activity of nitrate reduction was found in landfilled waste with the increasing of nitrate load in recirculated leachate,and an enhanced nitrate reduction rate was observed.In addition,vertical differences in the denitrification capacity of landiflled waste was found.Organic compounds played an important role in the reduction of nitrate in bioreactor landfill.Some principle experimental results of the study are summaried as follows:(1)Municipal Solid Waste(MSW)decomposition under oversaturated condition in comparison with leachate recirculation was investigated in two simulated reactors (A and B).Reactor B was operated with leachate recirculation while in reactor A,an oversaturated condition was maintained without leachate recirculation.Results showed that MSW decomposition in reactor A was much faster than that in reactor B. The final CODcr concentration and pH value of leachates in reactor A were around 1000 mg·L^-1and 7.0,respectively,in comparison to 18000 mg·L^-1and below 6.5 in reactor B after 420 days of incubation.The total biogas and methane yields were 2800 L and 1330 L in reactor A while only 1470 L and 600 L in reactor B.These results indicated that the relative retardation of MSW degradation in reactor B was primarily resulted from the increased resistance of mass transfer and the accumulating of biogas caused by leachate recirculation and the resultant acid inhibition.In addition,biogas production was positively related to ambient temperature.Bioreactor landfill operated under liquid oversaturated and MSW loose condition could be a promising approach for accelerating municipal solid waste degradation.(2)Effect of municipal solid waste classification on material change in landfill unit at its early stage was investigated as compared with that in the conventional mixed landfill unit.Results showed that approximate 0.573 m³ could be saved from per ton of waste due to classification and recycle.Among eighteen leachate indexes, eight of them including Cu²⁺,Pb²⁺,Chroma,Total-P,SS,Cd²⁺,Zn²⁺and Cl^- in the unit filled with classified waste were over over 1.5 times higher than those in the conventional unit.Five of them involving NH₄⁺,Total-N,Mn,BOD₅ and CODer were slightly higher in the classified unit.pH,TCr and EC were equivalent in both units.However,Fe and DO were lower in the classified unit.In addition,methane concentration in landfill gas of classified unit was much higher in comparison with that of the conventional unit over the whole experimental period.All these results obtained at the early stage of landfill indicate that waste classification waste classification could be an effective method in saving landfill storage capacity, centralizing leachate pollutants,accelerating the decomposition rate of organic waste, and promoting the emergence of methane-producing stage of landfill.(3)Two simulated municipal solid waste bioreactors being of different degrees of stabilization were used in this experiment,to investigate the effect of aeration and activated sludge inoculation on the removal of ammonium nitrogen.The results showed that when the initial ammonium-N concentration was 500-1200 mg L^-1the removal rate ranged within 38%-90%.However,the addition of activated sludge did not produce apparent impact on the removal of pollutants.Besides,there were almost no significant increase of nitrate-N or nitrite-N concentration in the MSW bioreactor with the decrease of ammonium-N concentration.Instead,high content of ammonium-N in the gas phase was deteced.These observations suggested that ammonia stripping was the main patyway of ammonium-N removal from leachate. High concentration of free ammonia which inhibited ammonium nitrification was probably the main contributor to the ammonia stripping in the MSW bioreactor.(4)Ex situ nitrification followed by the use of the landfill as an anaerobic bioreactor for denitrification has been recommended as an alternative strategy for leachate ammonia removal.The potential effects of the increasing nitrate load and the decreasing organic carbon content in injected leachate on the denitrifying capacity of municipal solid waste(MSW)were investigated through seven successive tests in a simulated landfill reactor.Results showed that MSW possesses a considerably high denitrification capacity.Nitrate reduction could be initiated within 48 h after the first addition of nitrate.Nitrate reduction rate increased with the increasing nitrate loading concentration.When the NO₃^--N loading concentration increased to 850 mg·L^-1, nitrate reduction rate could reach up to 35 mg·L^-1·h^-1.Nitrate reduction capacity of the up-layer waste was higher than that of the down-layer waste.Transient nitrite accumulation could be found after the addition of nitrate in each test.However,the maximum NO₂^--N accumulation efficiency declined with the increasing nitrate loading concentration.Organic carbon played important role in the redcution of nitrate,and both endogenous and exogenous orgainic materials could act as electron donor of this process.