The Digestibility of Waste Activated Sludges

Abstract

Laboratory digestion studies using waste activated sludges (WAS) were conducted to compare the digestion performance between anaerobic and aerobic processes. Nine samples of WAS from seven wastewater treatment plants were collected and batch‐digested under both anaerobic and aerobic conditions for 30 days at 25 °C. The cation content of wastewater (both floc and solution phases) and solution biopolymer (protein and polysaccharide) was measured before and after digestion and compared with volatile solids destruction data. The study revealed that each digestion process was associated with a distinct biopolymer fraction, which accounted for differences in volatile solids reduction under anaerobic and aerobic conditions. The anaerobic digestion data showed strong correlations between soluble protein generation, ammonium production, percent volatile solids reduction, and floc iron (Fe). These data suggest that the amount of volatile solids destroyed by anaerobic digestion depends on the Fe content of floc. In aerobic digestion, polysaccharide accumulated in solution along with calcium and magnesium. For aerobic digestion, correlations between divalent cation release and the production of inorganic nitrogen were found. This implies that divalent cation‐bound biopolymer, thought to be lectin‐like protein, was the primary organic fraction degraded under aerobic conditions. The results of the study show that the cation content in wastewater is an important indicator of the material that will digest under anaerobic or aerobic conditions and that some of the volatile solids will digest only under either anaerobic or aerobic conditions. Water Environ. Res.,

Physical and biochemical changes in sludge uponTubifex tubifexpredation

“…It is not known whether such mechanism is of importance during WP. The calcium concentrations remain in sharp contrast to what other authors found with regard to the release of soluble Ca 2+ [66,68]. A possible explanation for the relatively stable Ca 2+ concentrations, when comparing WAS and ER, is the difference in batch duration, which was 30 days in the aforementioned studies versus 4 days in our present study.…”

“…A similar trend was observed for humic-like substances. The release of soluble carbohydrates and proteins, upon aerobic treatment of WAS, has been reported also by other authors [66,68]. Results are from a 3-day batch experiment with VS reduction of 63% ± 3 and 13% ± 3 for WP and ER, respectively, using a worm/VS ratio of 15 g/g.…”

“…When the findings of Park et al [66] and Tamis et al [21] and the results presented here are considered, it seems that the 45-50% of the initial VS, which remains undigested during anaerobic digestion, is digested by additional aerobic (worm) treatment. Furthermore based on the similar VS reduction levels between WP and ER-30, it seems that the worms specifically target a fraction of the sludge that is predominantly biodegradable under aerobic conditions, yet at significantly higher degradation rates when compared to the endogenous decay of WAS.…”

“…Besides the aerobic removal of proteins and the concomitant release of soluble compounds, Park et al [66] also showed that WAS conversion, using sequenced aerobic and anaerobic (or vice versa) conditions, reaches the same level of VS reduction of the combined processes, reaching about 63%. Roughly 45-50% of the initial VS is removed in the first stage of either aerobic or anaerobic treatment after 30 days.…”

“…Unfortunately the CST of ER-30 was not measured. However, CST deterioration was reported by Park et al [66] who found that the CST increased from 50 to 517 seconds with an extended aeration time of 30 days for sludge stabilization.…”