Study On Nitrogen Conversion Characteristics And Process Optimization During Composting In Reactors

As a new composting technology,reactor composting has many characteristics such as covering small areas,short fermentation time,high degree of automation control,less bulking agents and good effects of odor control.It is suitable for in-situ treatment of organic wastes such as municipal sludge or animal manure.Silo reactor composting is a typical reactor composting technology,which has been vigorously promoted in China's environmental protection industry.However,its process is still unclear.So,the relevant research work is urgently needed to support the development of technology and equipment,at the same time,giving guidance for practical production.Firstly,the effects of aeration,moisture,temperature and raw material composition on nitrogen transformation and microbial changes during sludge composting were studied in laboratory reactors,and the optimum technological parameters under the condition of laboratory reactors were determined.Based on that,the effects of different aeration,moisture,temperature and raw material composition on nitrogen transformation and microbial changes during sludge composting in a large silo reactor were studied,and the composting process in silo reactor was optimized.The main conclusions are as follows:(1)In laboratory reactor composting,the recommended value of continuous aeration ventilation rate is 0.05 m3·min-1·m-3 of organic matter,the recommended value of intermittent aeration ventilation rate is 0.2 m3·min-1·m-3 of organic matter.In silo reactor composting,Ihe recommended ventilation rate is 0.1 13·min-1·m-3 of organic matter.The optimum aeration mode in a laboratory reactor with a volume of 100 L is to start the fan for 5 minutes and stop it for 15 minutes,the ventilation rate is 1200 L/h.The optimum aeration mode in a silo composting reactor with a volume of 86 m3 is 24 h continuous aeration,the ventilation rate is 525 m3/h.(2)In laboratory composting reactors,low ventilation rate is beneficial to the formation of nitrate nitrogen,ammonium nitrogen and amide nitrogen are mainly produced in high temperature fermentation stage.Intermittent aeration mode could accelerate the temperature rising,and promote the formation of nitrate nitrogen,ammonium nitrogen and amide nitrogen,and is helpful for the preservation of nitrogen.In silo reactor composting,with the increase of aeration time from intermittent mode to continuous mode,the nitrate nitrogen content in the middle and lower layers of the reactor increases gradually,the ammonium nitrogen content in each layer of the reactor increases gradually,the amide nitrogen content increases first and then decreases,and the total nitrogen content increases gradually.(3)When the sludge is composted directly in a laboratory reactor,the moisture content should not be higher than 73%,and when the moisture content is reached 85%,the sludge can not be fermented and the temperature will not rise.In silo reactor composting,when the upper layer moisture content is controlled at about 63%,the middle layer moisture content is controlled at about 57%,and the lower layer moisture content is controlled at about 45%,the temperature and nitrogen conversion in the reactor can reach the optimum state.(4)Exogenous heating can increase reactor temperature and prolong high temperature duration,but it can lead to nitrogen loss.When the heating temperature was higher than 80 ?,the transformation of nutrients and the activity of microorganisms in compost were inhibited.With the increase of temperature in the silo reactor,the contents of total nitrogen and ammonium nitrogen in the upper layer of the reactor increased gradually,the contents of nitrate nitrogen in the upper layer of the reactor increased obviously,and the contents of amide nitrogen in each layer of the reactor increased gradually.(5)The lower C/N ratio is beneficial to the increase of reactor temperature and the increase of total nitrogen and amide nitrogen content.Higher C/N ratio is beneficial to the increase of nitrate nitrogen and ammonium nitrogen,and the degradation of organic nitrogen is greater.In silo reactor composting,adding a certain proportion of rice bran is beneficial to the increase of ammonium nitrogen content.Compared with the composting of sludge with rice bran,the contents of total nitrogen,nitrate nitrogen,amide nitrogen and organic nitrogen in each layer of the compost were higher when the sludge was composted directly.(6)In different volumes of reactor,sludge composting directly without bulking agents is feasible.The maximum moisture content of raw material should not be higher than the maximum water holding capacity of the material to ensure the permeability of the pile.(7)Firmicutes,Actinobacteria,Proteobacteria and Cleroflexi were the dominant bacterial phyla detected in the process of sludge composting in reactors.Ureibacillus was the dominant genus.(8)Ventilation is the most important control parameter in reactor composting.The ventilation mode varies with the volume of reactor,the type and nature of raw materials.The optimum composting technology can be determined by optimizing temperature-aeration-moisture-raw material parameters for reactor composting with different raw materials and volumes.