Nitrogenous organics in sewage sludge: New insights into bottlenecks of anaerobic transformation

Abstract

As the most crucial carrier of carbon and nitrogen in sludge, nitrogenous organics, are essential in governing the efficiency of sludge anaerobic digestion (AD). An in-depth understanding of the anti-degradation mechanism of nitrogenous organics in sludge at various occurrence states is needed. This study classified nitrogenous organics in sludge into extracellular nitrogenous organics (e-NO) and intracellular nitrogenous organics (i-NO). Further, it identified the hindrances of anaerobic degradation of e-NO and i-NO in sludge by comparing the structural and AD performance differences between sludge and other nitrogen source substrates (food waste, straw, chicken manure, soybean, and beef). Experimentally, soybean and beef (with excellent e-NO degradation rates) and chicken manure (with excellent i-ON degradation efficiency) were further compared. We found that the relative abundance of peptidase genes in sludge was generally lower than in soybean and beef. Also, the pyruvate (a volatile fatty acid precursor) metabolism of some amino acids (Gly, Ala, Phe) with high content was inactive. Refractory hydrolysis and incomplete fermentation of nitrogenous organics in sludge are the main reasons for the low e-ON degradation rate. Further experiments showed that the anaerobic degradation rate of i-NO in the sludge was lower than that of e-NO. However, the former was significantly enhanced by extracellular polymeric substances (EPS) removal. Thus, the distribution and structure of EPS in the sludge were compared with that found in the chicken manure. We observed that the proportion of tightly bound EPS (TB-EPS) in sludge was higher, and the refractory glycoconjugates were mainly distributed in TB-EPS. Meanwhile, organic-bound Fe was higher in the sludge EPS. These results indicate that the difficulties of anaerobic degradation of i-NO arise from a tight EPS layer protecting the cell body. Future studies may consider combining enhanced hydrolysis methods (i.e., additional enzymes) and cell-breaking pretreatment techniques to facilitate the degradation of nitrogenous organics in sludge. In addition, refractory amino acids can also be studied to produce functionalized organic matter (such as humic substances) for high-value applications.