Inhibition of anaerobic digestion by organic priority pollutants

Anaerobic digestion has been widely used for stabilization of municipal wastewater sludges and industrial wastes, and anaerobic digestion of municipal refuse recently has attracted attention as a renewable energy source. Process instability caused by toxic inhibition is a major problem with anaerobic treatment systems, and there are definite questions concerning the cause and control of inhibition from organic chemicals. Because of these questions and concerns, a study was initiated to study the inhibition of complex anaerobic cultures by organic priority pollutants;Twenty-four compounds considered to be representative of the semivolatile organic priority pollutants were selected for testing. A batch bioassay technique using serum bottles was used to determine inhibitory effects from these compounds. Anaerobic seed was obtained from a laboratory digester maintained on a synthetic substrate resembling municipal refuse. Suppression of gas production provided evidence of inhibition. All inhibition evaluations were performed at 37(DEGREES)C;Of the twenty-four organic chemicals tested, only seven produced significant inhibition at concentrations of 100 mg/L or less. These seven were nitrobenzene, 4-nitrophenol, 2-nitrophenol, 2,4-dichlorophenol, hexachloroethane, hexachlorocyclopentadiene and hexachloro-1,3-butadiene. Reversible inhibition was observed for all compounds except 2,4-dichlorophenol and hexachloro-1,3-butadiene;Recovery of methane production in anaerobic cultures inhibited by the nitro organics was due to biological reduction of the nitro group to an amine group. Aniline was positively identified as the degradation product of nitrobenzene. Since the amine group is relatively nontoxic compared to the nitro group, recovery occurred;Adsorption of the chlorinated hydrocarbons onto the solids in the anaerobic cultures was a major removal mechanism. Recovery of gas production in cultures inhibited by hexachloroethane was due to acclimation of anaerobic microorganisms to the low concentrations of this compound remaining in solution after removal by adsorption. However, gas production did not recover in cultures inhibited by hexachloro-1,3-butadiene even though the concentration was significantly reduced by adsorption. A combination of adsorption and anaerobic dechlorination of hexachlorocyclopentadiene caused the removal of this compound from solution and thus eliminated the inhibition of gas production