Metabolic activity. Microbial populations capable of utilizing formate, acetate, propionate, and glucose were detected in the granular sludge samples. The specific activities of these substrates (millimoles per gram of volatile suspended solids per day, presented as the means of triplicate samples ± standard deviations) were as follows: glucose, 37.74 + 10.54; propionate, 1.97 + 0.56; acetate, 5.00 ± 0.76; and formate, 47.30 ± 5.37. Since the granular sludge had been
washed three times prior to testing, organisms present in the reactor medium, as well as those not firmly attached to the granules, were removed and therefore were not responsible for the observed activity. The four substrates tested varied in chemical complexity and thus represented four steps in the degradation of complex substrates into simple products during anaerobic digestion.