Microbial activity in aerobic sludge digestion

Abstract Laboratory experiments were performed to study variations in microbial activity during aerobic sludge digestion. The main objective was a critical evaluation of the most common measures of activity, these being: ATP, plate counts, dissolved oxygen uptake rate and activity of dehydrogenase enzymes. Their variation with digestion time was monitored for batch and semi-continuous flow regimes at three different operating temperatures: 10, 20 and 30°C. The relation between these measures and the mass content of the sludge was also investigated. It was found that ATP offers the best estimate of activity, while plate counts can result in erroneous and misleading interpretations of the metabolic state of the mixed liquor population. ATP was found to parallel the decrease in biodegradable suspended solids (BVSS) during batch digestion, when corrected for solids reductions from cell lysis. It was concluded that the decrease in ATP does reflect a decrease in active biomass from the endogenous metabolism of the bacterial population, and that the ATP content per active cell mass remains constant with detention time thus allowing the estimation of active fraction of the sludge.