The Study Of Anoxic-oxic And Single Aerobic Digestion For Waste Activated Sludge

The waste activated sludge is dramatically increased with the increase of amountof municipal wastewater treatment in China. Therefore, the secure and effective disposalof waste activated sludge has been a topic of major interest in the field of biologicalwastewater treatment. In practice, anaerobic digestion is suitable for the disposal ofwaste activated sludge in large scale wastewater treatment plant, while aerobic oranoxic-oxic digestion are more suitable for the small or medium scale wastewatertreatment plants. Unfortunately, in terms of aerobic or anoxic-oxic digestion process, themechanisms of sludge reduction and bacterial metabolisms during sludge digestion andthe key control parameters, such as anoxic/oxic (A/O) ratios and sludge retention time(SRT) and so on, are still not very clear, leading to that the design of aerobic oranoxic-oxic digestion process is more empirical. In concern of sludge reduction andnutrient removal, this study theoretically analysize the nitrogen and alkali balances ofanoxic-oxic digestion process based on the mechanism of anoxic-oxic digestion.According to the anoxic ammonification, nitrification and aerobic ammonification ratesof waste activated sludge, the anoxic/oxic (A/O) ratios are calculated and experimentalverified. In addition, this study investigate the production of SMP in anoxic-oxic andaerobic digestion systems, the role of cell lysis and endogenous respiration in theaerobic sludge digestion, as well as the properties of heterotrophic growth in anoxicstage in anoxic-oxic digestion with nitrate as the sole nitrogen source. The main resultsare as follows:(1) Based on the principal of nitrogen balance and the theoretical analysis ofanoxic-oxic digestion, when use the nitrate as the electron acceptor, the air demand is reduced for17.88%and the energy consumption is also greatly reduced. At the sametime, the system can realize alkali balance and maintain the pH stable. The optimal A/Oratio of batch and continuous anoxic-oxic digestion process are calculated to be1:2and1:1.2, respectively, according to the sludge anoxic ammonification, aerobicammonification and nitrification rates..(2) When the1:2of A/O ratio and20d of SRT were controlled in batchanoxic-oxic sludge digestion, the VSS removal was47-49%, which was as high asaerobic sludge digestion (45-47%).48-50%of total nitrogen and40-42%of TCODwere removed in anoxic-oxic digestion, which is higher than aerobic digestion by40-42%and5-7%under the same operational condition.(3) When the1:1.2of A/O ratio and20d of SRT were controlled in continuousanoxic-oxic sludge digestion, the VSS removal efficiency was53-59%, which washigher than batch test by7-11%. At the same time, the TN and TCOD removalefficiency were as high as55-60%and50-53%, which were much higher than batch test.The sludge reduction, TN and TCOD removal were enhanced in continuous anoxic-oxicdigestion by the A/O digestion strategy.(4) The SMP production of anoxic-oxic digestion is much less than that of aerobicdigestion. When the SRT were10,13and20days, the SMP production of anoxic-oxicdigestion were100,80, and46mg/L, respectively, while those of aerobic digestionwere150,315and89mg/L, respectively. In consideration of reduction of sludge andSMP production in digestion process,20d of digestion time and anoxic-oxic digestionshould be the optimum selection.(5) In aerobic digestion, the contribution of cell lysis and endogenous respiration inthe sludge reduction were86-87%and13-14%, respectively. The contribution of celllysis in the reduction of total bacteria, active bacteria were81-84%and90-93%,respectively, while those of endogenous respiration were16-19%and7-10%,respectively; The results indicated that cell lysis is the main pathway of sludgereduction.(6) Nitrate can be utilized as the sole nitrogen source for activated sludge growth.Compared with the systems with ammonia or organic nitrogen as nitrogen source, theactivated sludge with nitrate as nutrient has lower sludge yield (0.30-0.35mg COD/gCOD) and lower nitrogen content (8-9%), as well as the lower protein content inEPS. Experimental analysis of nitrogen transformation pathway demonstrated thatprotein in cells was synthesized by the process of dissimilatory nitrate reduction toammonium (DNRA).