Study On The Transformation Of Nitrogenous Compounds In Sludge During High Solid Anaerobic Digestion

The treatment of sewage sludge has become a bottleneck restricting the healthy development of municipal sewage treatment industry.Today,more and more attention has been paid to the treatment of high solid anaerobic digestion?AD?because it provided a relatively environment-friendly and sustainable method.At present,the studies are mainly concentrated on the AD treatment of sludge under low solid content?3%⁵%?condition,and few studies on AD of high solid content sludge after dewatering.For high solid AD,the biodegradation efficiency of organics,especially the nitrogenous compounds was low.In addition,in China,the content of nitrogenous compounds in sludge is relatively high,and the decomposition of these compounds produces high content of ammonia which may cause inhibition effect on AD process to some extent.Therefore,it is necessary to further investigate the transformation mechanism of nitrogenous compounds and the corresponding measures for improving their digestibility.In this study,the dewatered sludge was collected and used for AD treatment.And used physical chemistry technology,molecular biology,proteomics,etc.,combined the importance of solid content in high solid AD and nitrogen conversion in sludge,took into account the forward-looking of scientific research,and strengthen the high solid sludge AD efficiency strategy was investigated to provide a powerful theoretical basis and technical support for improving the efficiency of high solid sludge AD.The main results are as follows:?1?When the solid content of the sludge increased from 8%to 16%,the methane production and degradation rate of volatile solid?VS?increased firstly and then decreased with the increase of solid content.The optimal solid content was 12%under which the methane yield and VS degradation rate were 107 mL·g VS^-1 and 27.71%,respectively.The concentration of volatile fatty acid increased with the increase of the sludge solid content.The rapid acidification of macromolecule organics lasted for 12 days and at the 9th day,the peak content of VFA was attained(195 mg·L^-1549 mg·L^-1).When the solid content was changed between 8%and 16%,the total polysaccharide decomposition efficiency was varied between31.94%and 35.66%,and the total protein degradation efficiency was varied between 27.09%and 32.40%.There was no significant difference between the experimental groups,but the total protein decomposition efficiency was lower than the total polysaccharide.The low decomposition efficiency of total protein in the sludge restricted the degradation rate of VS in the sludge.?2?The nitrogenous compounds and their conversion products in sludge was analyzed,and the results showed that in the AD system with a solid content of 12%,the main inorganic nitrogen in the sludge was ammonia nitrogen.The increase of ammonia was very fast within the first 12 days.The ammonia nitrogen concentration reached 1200 mg·L^-11 after AD process.The concentration of nitrate nitrogen and nitrite nitrogen were all at a very low level,and ultimately decreased to 2.23 mg·L^-1and 0.52 mg·L^-1,respectively.The main organic nitrogen was protein,and it gradually transformed from bound state to dissolved state,and a large amount of ammonia nitrogen was formed at the same time.The high ammonia nitrogen concentration showed a certain inhibitory effect on the AD system,especially on the degradation of the protein.The heterocyclic nitrogen transformation was different from that of the protein,and its decomposition rate was gradually accelerated,eventually reduced to0.29%.Part of the organic nitrogen converted to humic acid-like and fulvic acid-like substances,which was more difficult to decompose.?3?After AD,the molecular weight of the protein in sludge was decreased and the macromolecules converted to small molecules.Most of the proteins left in the digested sludge came from the micro-organisms.These proteins could not be further decomposed by the microbes because of the decreasing microbial metabolic capacity and enzyme activity at the end of AD process or lack of specific enzymes for the hydrolysis of these proteins,and ultimately resulted in the low decomposition efficiency of total protein in sludge.Then it limited the decomposition efficiency of the sludge protein.?4?After AD,the trypsin was added exogenously into the digested sludge.It was found that after the trypsin addition,the ability to decompose protein in the system was restored,and it was also proved that the low protein degradation efficiency of the sludge was related to the loss or decrease of the protease activity during the AD.The optimal addition amount of trypsin(50000 U·g^-1)was 23.3 mg·g VS^-1.Compared with the group without trypsin,the VS degradation rate,protein degradation rate and the amount of methane production were increased by 21.5%,23.5%and 43.93%,respectively.