Death and lysis during aerobic thermophilic sludge treatment: Characterization of recalcitrant products

The nature and fate of the organic fraction released during the aerobic thermophilic biodegradation of microbial cells were examined in a laboratory-scale treatment system. There was strong evidence for exoproteolytic activity during the early phase of the biodegradation process. The lysis products could be separated into two broad fractions, a low molecular weight C 2–C 5 carboxylic acid fraction and a pooled fraction of the remaining dissolved organic carbon (DOC). The former was produced as a result of fermentative metabolism of the aerobic thermophilic process culture growing under oxygen limited conditions. The production of these carboxylic acids occurred simultaneously with their utilization, as evidenced by the decrease in radioactivity in the DOC as a result of pulsing the culture with [ 14C]acetate. However, a fraction of this [ 14C]acetate pulse was metabolized into recalcitrant or slowly biodegradable DOC. The non-carboxylic acid fraction of the DOC in the lysis products was either slowly biodegradable or recalcitrant. The nature of the residual DOC in the supernatant after aerobic thermophilic treatment could not be further characterized but is likely to represent humic substances.