| With the improvement of sewage treatment capacity in China,the production of excess sludge has increased year by year,and its safe disposal is urgent.Land use of sludge after aerobic composting is the main method to realize sludge resource recovery.However,conventional sludge aerobic composting often has low treatment efficiency,and heavy metals,organic pollutants in sludge lead to certain environmental health risks for land use.Therefore,this study focusced on sludge aerobic composting.Through sampling of a certain full-scale sludge aerobic composting,the maturation process of the actual sludge aerobic composting process and its relationship with heavy metals,AOX and other organic pollutants were investigated.Furthermore,a pilot study of aerobic composting for treating deeply dewatered sewage sludge was carried out,and the effects of initial moisture content,dewatering reagent type,composting excipients and return mixture,biochar on the maturation of aerobic composting,heavy metal sepeciation,AOX and other organic pollution reduction were investigated.The specific research results were as follows:(1)Under the conditions of adding 14% of auxiliary materials(calculated as wet sludge)and 200% of back-mixed materials(calculated as wet sludge),conventional sludge aerobic composting in summer for 25 days showed a relatively obvious decompostion effect,and maintaining high temperature for 13 days(>55℃),the dissolved organic matter had the obvious trend of humification.At the same time,Ni,As,and Pb showed a "concentration effect",and their contents increased by 16.5%,18.6% and 26.5% respectively after composting.During aerobic composting,the speciation of heavy metals such as Zn,Cd,and Hg were changed,and the residual form of Zn had a significant positive correlation with the formation of humic acid.The proportion of Hg in the residual state increased from 64.25% to 74.66% during aerobic composting.For example,methylmercury might undergo demethylation and transformation in the early stage of aerobic composting.Aerobic composting had no obvious reduction effect on AOX and mineral oil organic pollutants.(2)The results of the pilot study showed that the aerobic composting with the deeply dewatered sewage sludge(moisture content 65%～70%)under the condition of mixed with the compound hydroxyaluminum agent(39 kg/t absolutely dry sludge),no auxiliary materials were added,and 100% backmixing added,the decompostion effect was obvious,which was observed as rapid temperature rise,long compost high temperature period(over 55 °C and maintained for 5 days),rapid decompostion of organic matter,and also the electrical conductivity did not increase with the addition of dewatering agent.In the dewatering stage,5% biochar(based on sludge wet base)added further increased the high temperature of the compost(more than 55 °C and maintained for 8 days).Therefore,aerobic composting after deep dewatering of sludge could significantly improve the efficiency of sludge treatment(2.13 times of the efficiency of traditional sludge composting)and reduce the operation costs such as the addition of auxiliary materials(23.15% less than the daily operation costs of traditional sludge composting).(3)The addition of biochar in the dewatering process strengthened the passivation effect of Hg in the compost process,and the proportion of residual Hg increased from 29.94% at the beginning of composting to 75.67% at the end of composting(the control group,the proportions before and after composting were from49.28% to 50.26%).On the one hand,it might be due to the chelation of humic acids to Hg in the composting process because of the promotion of decomposing and maturation of biochar.At the same time,it might be due to the direct adsorption of Hg by biochar.Similar to conventional aerobic composting,AOX,mineral oil and benzopyrene did not show significant degradation in the aerobic composting process of deeply dewatered sewage sludge. |
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