[Transformation of Protein in Sludge During High Solids Anaerobic Digestion]

Dewatered waste sludge with a total solid (TS) concentration of 12% was used for mesophilic (37℃) anaerobic digestion (AD). The biotransformation mechanism of protein and the reason for the low conversion efficiency of protein under high solids AD was investigated by analyzing the variation of protein composition in the sludge before and after AD. The results showed that the conversion rate of protein in the sludge was 34.26% after 45 days of AD. The reason for the low efficiency of protein conversion was the poor mass transfer efficiency under the condition of high solids content and the large amount of ammonia nitrogen produced with the hydrolysis. After 45 days of AD, the total ammonia nitrogen (TAN) concentration reached 1201 mg·L^-1, which resulted in the inhibition of the AD process, especially the decomposition of protein. Some of the protein converted to humic acid-like and fulvic acid-like substances, which are more difficult to decompose based on the three-dimensional fluorescence spectroscopy (3D-EEM) analysis. Two-dimensional electrophoresis (2-DE)- mass spectrometry (MS) was adopted for identifying the composition of protein in sludge before and after AD. It showed that the relative molecular weight and the isoelectric point (pI) of the protein in the sludge decreased after AD and most of the proteins left in the digested sludge came from the micro-organisms. These proteins cannot be further decomposed by the microbes because of the decreased microbial metabolic capacity at the end of the AD process or lack of specific enzymes for the hydrolysis of these proteins. This ultimately resulted in the low decomposition efficiency of the total protein in the sludge.