Studies Of The Cellular Materials Released From Waste Activated Sludge And Phosphorus Recovery During Sludge Reduction Process

Reduction and reuse of waste activated sludge (WAS) are popular topics in environmental research. Biological utilization of the cellular materials released from sludge disintegration is an attractive approach to achieve the desired degree of sludge reduction. A full understanding of the release of many organics, nitrogen (N) and phosphorus (P) species during WAS treatment, biodegradability of supernatant of the treated WAS and P recovery will help to construct a cost effective program of WAS reduction and reuse.Batch ultrasound treatment (sonication) runs were performed on WAS of A/O, An/O and A2/O units to identify the important factors affecting sludge disintegration and releases of the cellular materials and their effects. The effects of sonication on the releases of COD, total N (TN) and total P (TP) were first investigated; the results suggested that the 4:1 pulse ratio sonication was the desired sonication pattern for the tests. Under the sonication conditions of this research (ultrasound intensity of 0.167-0.500 W/mL and treatment time of 10-60 min), the releases of organic, N and P species increased with the treatment time; under 60 min in sonication at 0.500 W/mL, COD release efficiences of 50.8%,54.1%and 56.9%, TN release efficiences of 52.1%,47.2%and 70.4%, and TP release efficiences of 45.3%,68.1%and 88.2%were observed for the A/O, An/O and A2/O WAS samples respectively. The ultrasound intensity was a less important factor than the treatment time; the critical ultrasound intensity was estimated at 0.330-0.500 W/mL.After sonication, the supernatants of the treated WAS samples were more biodegradable as evidenced by the higher BOD5/COD of 0.33-0.68 and the predominance of organic compounds of MW≤2 kDa, which are called as smaller molecular. Organic N species were the predominant in TN released, followed by ammonia N and negligible amounts of nitrified N species. A major part of the TN released can be biologically removed since these supernatants all had a high value of COD/TKN, which are more than 9.Different amounts of TP were released from the three kinds of WAS due to their different TP contents. For the A/O WAS with low TP, about the same TP release rates were observed under sonication at 0.167-0.500 W/mL; P recovery would not be practical given its small PO43--P content of 13.0%-18.2%. For the An/O and A2/O WAS with more TP (TP was 46.8%-55.7%and 57.2%-76.2%of the dry weight for the An/O and A2/O WAS respectively), much more TP species were released under sonication; the greater release of TP species of higher%of PO43--P (the values of PO43--P/TP were 73.6%-89.2%and 89.9%-96.5%for the An/O and A2/O WAS respectively) could make P recovery by chemical precipitation more attractive.Thermal treatment and acid-alkalia adjustment were also studied and compared with sonication for sludge reduction effectiveness. Results of thermal treatment of WAS have demonstrated the importance of temperature during the process; more release of dissolved cellular materials and greater sludge reduction were observed at a higher temperature and that the treatment efficiences and change patterns in the process were temperature dependent. At a relatively low temperature of 50℃, although the thermal treatment efficiencies improved with the treatment time, the max observed sludge reduction and material releases were all less than 20%. At a higher temperature of 70 and 90℃, the rates of sludge reduction and material releases were fast initially and then slow down gradually. The same pattern was observed in P releases during the thermal treatment at the three temperatures mentioned above. Composition of TN released by thermal treatment was about the same as that by sonication. So high temperature treatment in short duration would therefore be the most effective mode for sludge reduction. WAS reduction by pH adjustment was only marginally effective at a very high initial pH of 12. Considering the treatment efficiences and cost, sonication was clearly the best method of the three studied for sludge reduction.So that recycling of the sonicated WAS would not increase the P loading to the existing wastewater treatment system, P recovery by chemical precipitation was studied. Series of experiments were performed on the supernatant of the sonicated An/O WAS (60 min at 0.330 W/mL) to identify the best operation conditions for P recovery employing MAP and HAP methods. With the initial pH of 9.7, mixed contact time of 10 min and Mg/P of 1.5,96.3%of soluble PO43--P in the MAP precipitation was recovered, and the initial pH of 9.0, mixed contact time of 5 min and Ca/P of 3.87,94.0%of soluble PO43--P in the HAP precipitation was recovered. Incorporating P recovery in the sonication WAS reduction system, about 50% of TP may be recovered, which was an important benefit due to the current shortage of P resources.Batch biodegradability tests were performed on the supernatants of sonicated WAS of the A/O, An/O, A2/O units. The supernatants of the sonicated A/O WAS exhibited good results to aerobic degradability for organics and N by nitrification and denitrification. Aerobic COD reduction and anaerobic TP release of the supernatant of the sonicated An/O WAS+P recovery by MAP were better than that by HAP. Supernatant of the sonicated A2/O WAS+P recovery by MAP was less efficience than the raw domestic wastewater in terms of COD reduction, nitrification, denitrification and P release. Given the proper mixture of raw wastewater and the supernatants of all sonicated WAS had similar biodegradability as the raw wastewater, the supernatants of sonicated WAS (from A/O, An/O, A2/O units)+P recovery may be recycled to a biological treatment systems (of organic removal, nitrification, denitrification and P release) without any harm.The results of this study do demonstrate that the combined sonication of WAS and P recovery by chemical precipitation is capable of achieving 50%reduction in the amount of the wastewater treatment plant sludge and recovering 50%soluble TP of the sonicated WAS. Furthmore the remaining dissolved cellular materials of the recycle sonicated WAS would have no adverse effects on the stable performance in raw domestic biological treatment system.