EFFECTS OF SULFIDE ADDITION ON COPPER INHIBITION IN METHANOGENIC SYSTEMS

Addition of sulfide can play a major role in recovery from heavy metal inhibition in anaerobic systems. Whether adding sulfide as a preventive means before heavy metal inhibition is more beneficial than adding sulfide after exposure to a heavy metal, is an important question. The objective of this research was to address this point by studying the effects of sulfide addition before and after copper inhibition in acetate-fed, methanogenic systems. Anaerobic toxicity assays and batch kinetic studies were performed using serum bottles with an anaerobic acetate enrichment culture. The results showed that recovery is possible with a feed copper to volatile suspended solids ratio (Cu2+/VSS) of 0.011–0.022 up to 15 mg l−1 of added copper. But the system failed with 20 mg l−1 of copper added with a Cu2+/VSS of 0.015. Adding sulfide after copper spiking can take as long as 50 days for recovery from copper (20 mg l−1) inhibition with a high Cu2+/VSS of 0.055. Interestingly, with sulfide addition before 20 mg l−1 of copper spiking, the methanogenic systems recovered rather quickly (within 4–7 days) even though the Cu2+/VSS was in the range of 0.054 - 0.058. Hence, the sulfide addition before copper spiking was much more beneficial than the same amount of sulfide addition after copper spiking. The best-fit inhibition coefficient, KI (competitive), for copper alone was 0.038–0.068 mg l−1. The KI (competitive) for sulfide addition after copper spiking was 1.94 mg l−1. The inhibition with sulfide addition before copper spiking followed the uncompetitive inhibition model with the best-fit KI of 256 mg l−1.