Process Optimization For Greenhouse Gas Emissions And Nitrogen Loss In Sewage Sludge Composting

Along with the development of the urbanization,the amount of sewage sludge has increased dramatically.Due to the high contents of organic matter,moisture and pathogenic bacteria,80% of the sludge has not been effectively disposed,sewage sludge disposal becomes a difficult problem.Composting is an effective measure to despose the sludge because of low operation cost,simple process and easy operation.,Aerobic composting with short cycle is commonly used in engineering.However,aerobic composting brings a lot of evaporation of ammonia which causes severe nitrogen loss during the composting.In addition,there are significant greenhouse gases generated such as carbon dioxide,methane and nitrous oxide,which reinforce the greenhouse-warming effect.In order to meet the requirements of environmental protection and realize the resource utilization of sludge,this paper studied the greenhouse gases and nitrogen loss in the process of composting through aerobic composting experiments.This paper studies the effects of the bulking agents and nitrogen fixing agents on the composting process,especially on organic matter degradation,greenhouse gases and nitrogen loss.The nitrogen transformation and nitrogen source of nitrous oxide are studied by adding three different types of nitrogen substance to the composts.There are many shortages during aerobic composting of bioleaching sludge in Wenchang sewage treatment plant,such as slow heating and the large consumption of bulking agents.Adding no bulking agents to composts is proposed and is evidenced practical through the test.Therefore,the best initial moisture content and aeration rate are further to determine,thus providing theoretical basis for the reducdtion of nitrogen and greenhouse gas emissions in engineering application.In the experiment,wood chip,wheat straw and rice husk are chosed as bulking agents,and the results showed that the characteristics of bulking agents can significantly influence the greenhouse gas emissions and nitrogen loss.Wood chips have less degradation in the composting process and produce a little CO2,but it can significantly inhibit the generation of CH4(29.37 mg?kg^-1)because it has the largest aeration porosity and total porosity.Wheat straw isn’t easy to release moisture due to the strongest water-holding capacity and largest water-holding porosity.Thus it inhibits ventilation and lead to more CH4(317.57 mg?kg^-1)and N2O(5.90 mg?kg^-1)emissions.And it contains lots of soluble organic matter to generate a greatest deal of CO2(86.36g?kg^-1).Although the wheat straw can significantly reduce the NH3 release rate in the early period,the release rate at the later stage is higher than others,therefore it produces the maximum NH3(1474.79mg?kg^-1).Rice husk has the weakest water-holding capacity and largest aeration porosity to promote ventilation and produce less CH4(217.73mg?kg^-1),and N2O(5.32mg?kg^-1).Adding nitrogen-fixing agents into composts can reduce the amount of volatile organic matter degradation but can promote the transformation of organic matter to the soluble organic matter which does not affect the generation of heat and CO2.The nitrogen-fixing agents change the oxidation of the heap and destroy the conditions of methane production,thus reducing the production of CH4.The nitrogen-fixing agents significantly reduce the p H value of the heap to result in the accumulation of dissolved total nitrogen and ammonia,which reduce the volatility of NH3 and the nitrogen loss.Compared with the control,the cumulative amount of ammonia volatilization of the No.1 and No.2 decreased by 58.84% and 86.34% respectively,and the inhibitory effect of No.2 is stronger than ferrous sulfate.Ammonium sulfate,sodium nitrate and sodium nitrite were added to the heap to study the nitrogen transformation and the source of nitrogen in nitrous oxide.After adding sodium nitrate,the nitrate reduced quickly in the composts,nitrate nitrogen,nitrate content and nitrous oxide is extremely low in the whole composting process,which eliminate the possibility that the nitrogen in nitrious oxide directly comes from the nitrate nitrogen by denitrification.The addition of ammonium sulfate promoted the nitrification process,which resulted in an increase in nitrite nitrogen content and greatly increased the production rate of nitrous oxide.The addition of sodium nitrite to the compost on day 0 and day 30 can also significantly boost the production of nitrous oxide.In conclusion,nitrite is the direct result of nitrogen in nitrous oxide.No adding bulking agents can prolong the thermophilic period,control high temperature and decrease the amount of bulking agents.Initial moisture content of 60% enhance vigorous microbial activities and produces the large amount of CO2 and higher temperature.CH4 production is relatively small and the NH3 release more when the initial moisture content is 60%.Although the NH3 release is low,but the organic matter degrades slowly,which is not good for the stability of the compost product when the initial moisture content is 70%.Although the high aeration rate can help to degrade organic matters,low aeration rate extends thermophilic period,kills bacteria increase and increases water loss to meet the requirements of the engineering.