Acetoclastic methanogenesis: A key to anaerobic digester stability.

摘要

Understanding anaerobic digester stability is important for many wastewater treatment plants interested in increasing loading rates. An assay was developed to measure an important factor in digester stability by measuring the maximum acetoclastic methane production rate (Vmax,ac). The ratio of the plant acetoclastic methane production rate (Vplant,ac ) to the Vmax,ac is a measurement of the fraction of the digestion capacity used and on average equaled 0.6, suggesting very little excess capacity for acetate use in digestion.;The Vmax,ac test was used to determine whether oxygen exposure from recuperative thickening of digester sludge caused an activity loss. Gravity belt thickening was found to have no detrimental effect on the acetoclastic methanogenic activity; dissolved air flotation (DAF) thickening was found to cause around a 20% loss in Vmax,ac, suggesting that DAF recuperative thickening may lead to greater instability.;Two recognized genera of acetoclastic methanogens transform acetate to methane. Methanosarcina's higher growth rate allows this acetoclast to accommodate transient loadings more efficiently and enables more stable digestion; however in conventional digestion, Methanosaeta typically dominates. Two reactors were established at a 17 day hydraulic retention time (HRT) to determine whether feeding frequency alters the acetoclastic dominance. Hourly feeding enriched for Methanosaeta, while once-daily feeding enriched for Methanosarcina. Monod kinetic constants were measured for both reactors. Both reactors were stressed by increasing the reactor feed concentration. The Methanosarcina enriched reactor absorbed the increased load better than the Methanosaeta enriched reactor.;Feed solids degradation kinetics affect the acetate concentration and thus the acetoclastic methanogen dominance. A portion of the feed sludge was found to be degraded more rapidly (with a first order constant of 4 day -1) than the baseline rate (0.09 day-1). These findings were used to develop a simple model describing the degradation of feed solids and acetoclastic methanogen competition. The model was used to simulate a Methanosarcina selector (staged digestion with recycle flow from the second digester back to the first). The model results indicate that the Methanosarcina selector could develop a dominant Methanosarcina population at a combined HRT of 20 days with HRTs of the first stage ranging from 5--7 days.