Fate of wastewater organic pollution during activated sludge treatment: nature of residual organic matter

Activated sludge treatment is used to remove wastewater organic matter (OM) and residual OM is found at low levels in treated water. The molecular composition of OM of waters and sludge was determined in order to understand the fate of the organic substances during biological treatment and the nature of residual OM. Proteins, sugars, lipids and polyphenolic compounds were quantified in a municipal wastewater treatment plant via chromatographic analysis after chemical hydrolysis. The concentration of OM was decreased by more than 90% by biological treatment. However, if 63% of the OM were characterized in activated sludge, molecular analysis did not allow us to characterize all the OM in the water samples. Relatively higher amounts of total organic carbon (TOC) were identified in wastewater (50%) than in treated water (20%). Likewise, for total nitrogen, 33% were identified as amino acid nitrogen in wastewater, and only 10% in treated water. Analytical pyrolysis suggested that some of the difficulties in identifying OM of treated water with common analytical methods are due to the presence of complex structures, refractory to hydrolysis. These structures may also be refractory to microbial degradation since they are concentrated during the biological treatment of wastewater. Suggestions on the possible origin and structure of resistant OM were formulated according to the analytical results.