Removal of organic xenobiotics in activated sludges under aerobic conditions and anaerobic digestion of the adsorbed species

The abiotic and biotic fate in activated sludge plants of a mixture of aromatic pollutants including polycyclic aromatic hydrocarbons, polychlorinated biphenyls, chlorinated and non-chlorinated benzenes, phenols and surfactants was studied. To this aim, an activated sludge collected from a municipal plant was artificially spiked with the target compounds. Under aerobic conditions, adsorption and biodegradation were distinguished as possible mechanisms of target compound removal through comparison of tests with and without oxygen. With the exception of phenol, which was completely biodegraded (at an average rate of 2.6mg phenol g -1 volatile suspended solids h -1 ), the pollutants were completely or significantly removed from the liquid phase through adsorption. In the tested conditions the amount of substance removed through adsorption ranged from 75-80% for 4-dodecylbenzenesulfonic acid and pentachlorophenol to more than 90% for the other compounds. Kinetic and equilibrium parameters describing the adsorption of the different xenobiotics to the sludge were also determined. The role of anaerobic digestion in the final decontamination of the xenobiotic-containing sludge was evaluated under mesophilic and thermophilic conditions. A significant average depletion of the chlorinated pollutants was observed in mesophilic conditions (24.6%). The addition of yeast extract stimulated the process by allowing a higher chlorinated pollutant average depletion (49.7%) and complete disinfection of the sludge.