ADVANCED STEADY-STATE MODEL FOR THE FATE OF HYDROPHOBIC AND VOLATILE COMPOUNDS IN ACTIVATED-SLUDGE

A steady-state, advanced, general fate model developed to study the fa te of organic compounds in primary and activated-sludge systems. This model considers adsorption, biodegradation from the dissolved and adso rbed phases, bubble volatilization, and surface volatilization as remo val mechanisms. A series of modeling experiments was performed to iden tify the key trends of these removal mechanisms for compounds with a r ange of molecular properties. With typical municipal wastewater treatm ent conditions, the results from the modeling experiments show that co -metabolic and primary utilization mechanisms give very different tren ds in biodegradation for the compounds tested. For co-metabolism, the effluent concentration increases when the influent concentration incre ases, while the effluent concentration remains unchanged when primary utilization occurs, For a highly hydrophobic compound (partition coeff icient K-d > 0.01 m(3)/g VSS), the fraction of compound removed from a dsorption onto primary sludge can be very important, and the direct bi odegradation of compound sorbed to the activated sludge greatly increa ses its biodegradation and reduces its discharge with the waste activa ted sludge. Volatilization from the surface of the primary and seconda ry systems is important for compounds with moderate to high volatiliti es (Henry's law constant H-c = 0.001 to 0.1 m(3) water/m(3) air), espe cially when these compounds are not biodegradable. Finally, bubble vol atilization can be a major removal mechanism for highly volatile compo unds (H-c > 0.8 m(3) water/m(3) air), even when they are highly biodeg radable.