Effect of the two-stage thermal disintegration and anaerobic digestion of sewage sludge on the COD fractions

The research presents the changes in chemical oxygen demand (COD) fractions during the two-stage thermal disintegration and anaerobic digestion (AD) of sewage sludge in municipal wastewater treatment plant (WWTP). Four COD fractions have been separated taking into account the solubility of substrates and their susceptibility to biodegradation: inert soluble organic matter SI, readily biodegradable substrate SS, slowly biodegradable substrates XS and inert particulate organic material XI. The results showed that readily biodegradable substrates SS (46.8% of total COD) and slowly biodegradable substrates XS (36.1% of total COD) were dominant in the raw sludge effluents. In sewage effluents after two-stage thermal disintegration, the percentage of SS fraction increased to 90% of total COD and percentage of XS fraction decreased to 8% of total COD. After AD, percentage of SS fraction in total COD decreased to 64%, whereas the percentage of other fractions in effluents increased.