Transformation Of C,N,P And Heavy Metal During The Proecess Of High Temperature Aerobic Composting Of Crop Straws And Sewage Sludge

A large number of sewage sludge generated every year is one of serious environmental problems in China, sewage sludge composting with good economic and environmental benefits is an important method for sludge treatment and disposal. High temperature aerobic composting is widely used for its low cost, high efficiency and small secondary pollution. Currently, the main disadvantages of sewage sludge composting are low nutrient content and high heavy metal content that become key restrict factor of its application in farmland. Choosing conditioner is particularly important for the quality of compost products, mostly used conditioner at present are crop straw, garbage, sawdust, etc. Crop straw in China is rich in resource but low in efficiency because most is abandoned and burned. Crop straw with abundant nutrients like nitrogen, phosphorus, potassium and organic matter is ideal conditioner for sludge composting. This study selected four different straws (rape straw, wheat straw, corn straw, rice straw) as conditioner mixed with sewage sludge to proceed high temperature aerobic composting, the impact of different straws on high temperature aerobic composting is discussed for providing the basis on improving utilization of crop straw and sewage sludge.The mixed composting of four straws and sewage sludge were proceeded and the results are as follows:(1) Composting process were successfully completed with four different straws added into sludge, temperature rising rate order of piled body was rice straw> wheat straw> corn straw> rape straw, cooling rate order was rice straw> corn straw> wheat straw> rape straw; high temperature holding time order of piled body was wheat straw> rice straw and corn straw> rape straw. In comparison, adding rice straw was more conducive to speed up the compost into high temperature period and maturation period, and promote the process of sewage sludge composting. Different conditioners had unconspicuous effect on pH and water content of piled body. (2) The content of total organic carbon and dissolved organic carbon both declined in four treatments. Total organic carbon decreased by 22% to 27% and the minimum degradation exists in treatments adding rape straws. Dissolved organic carbon decreased by 56%～67%. and the minimum degradation exists in treatments adding corn straws.(3) The variation of cellulose, hemicellulose and lignin content had same trend in four treatments, the degradation difficulty order was hemicellulose> cellulose> lignin. The degradation of cellulose and hemicellulose reached maximum by adding rice straws and the increase of lignin reached maximum by adding wheat straw.(4) The formation of humus in sludge composting had closely associated with cellulose, hemicellulose and lignin in straw. FA showed a downward trend. HM, HA, and HA/FA showed a upward trend. Comparatively, adding corn straws was more conducive to raise HM, HA content during sludge composting and increase the degree of humification.(5) The variation of ammonium nitrogen in four treatments both increased at first and then decreased later, the content of ammonium nitrogen had a significant correlation with temperature and pH of piled body. The ammonium nitrogen content reached maximum in treatments adding wheat straws and corr straws. The content of nitrate nitrogen gradually increased, it nad significantly negatively correlation with temperature and water content, and reached maximum by adding rape straws.(6) The variation of total nitrogen and acid hydrolyzed nitrogen had same trend in four treatments, their contents increased obviously at the end of composting. The content of total nitrogen and acid hydrolyzed nitrogen separately increased by 8.4%～20.5% and 14.2%～31.1%. During sewage sludge composting process, the main component of nitrogen was organic nitrogen that took up 57%～92% of total nitrogen. Adding corn straws was more conducive to increase total nitrogen and adding rice straws was more conducive to increase acid hydrolyzed nitrogen.(7) Amidonitrogen accounted for about 13% to 25% of hydrolyzed N, its content was increasing and reached maximum by adding corn straws. Amino sugar N which accounted for only a small fraction of hydrolyzed N increased in temperature rising and high temperature periods but decreased in later composting period, but the overall trend was increasing. The content of amino acid nitrogen increased at first and then decreased that showed a similar trend with acid hydrolyzed nitrogen, it accounted for about 30% to 40% of acid hydrolyzed nitrogen and the proportion changed little at the end of composting. The unidentified nitrogen ratio of acid hydrolyzed nitrogen increased by 4% to 7% and the increment reached maximum in treatment added rice straws.(8) Nitrogen loss between 7.2%～28.4% existed in all treatments applied with four kinds of straws and the composting was quite ideal. The nitrogen loss reached maximum and minimum separately by adding wheat straws and rape straws.(9) The content total phosphorus and organic phosphorus both increased evidently after adding four kinds of straws for the reason of "concentration effect", but the variation was not significant. The available phosphorus was affected significantly by application of straws during sludge composting and adding corn straws was conducive to enhance the content of available phosphorus.NH4C1-P, Al-P, O-P were main components of inorganic phosphorus in composting process, and their proportion was 79.5%～81.1% in the end of composting. Adding rice straws was conducive to improve NH4C1-P content and rape straw was conducive to improve Al-P and O-P content.(10) The concentration order of heavy metals in sewage sludge was Cr> Pb> Cd, and their concentrations both increased after adding straws, it indicated that straw conditioners had no function for reduction of heavy metals during sewage sludge composting.The organic combination was main combination pattern of Cr, and Cd existed with convertible and carbonate combination, the forms of Pb distributed evenly. Adding coin straws reduced content of convertible Cd, increased residual and carbonate Cd; the inrement of Cr in organic combination reached maximum in treatments added rape straws and convertible Cr increased least in treatments added rice straws. The increment of Pb in organic combination reached minimum in treatments added corn straws and residual Pb increased mostly in treatments added rice straws.