Study On The Operating Characteristic And Optimizing Control Of Low DO Concentration In SBR Process

Currently, the activated sludge process is the most widely used method among the municipal sewage treatment technologies. However, there are some problems such as high energy consumption of aeration and low treatment efficiency during real operation. At the current stage, how to save the energy consumption of aeration, improve the treatment efficiency and control sludge bulking have become a heated topic and a difficult point of both domestic and foreign studies. This research adopts the SBR technology, use the real domestic sewage as the object of treatment under the condition of low dissolved oxygen (DO) and study systematically the initiating and maintaining of low DO conditions, factors that affect the stable operation of the system, the realization of SND and short-cut nitrification as well as the derivation of the real-time control and energy conservation theory.During the initiating process of the experiment, we adopt the way of gradually decreasing DO and manage to keep the final average DO between 0.5 mg/L-0.7 mg/L, and keep the removal rate of COD maintaining at 85% from the beginning to the end. The SVI value rises from 80 mL/g during the early period of initiating process and stabilizes at about 137 mL/g and the effluent SS maintains between 1 mg/L-2 mg/L. Although slight sludge bulking phenomenon occurs in the system, it does not affect the overall setteability and effluent effect of the system and the effluent is clear.Under the condition of low DO, we study the factors that affect the stable operation of the system and work out that every effluent index of the anoxic/aerobic(A/O) operation method is better than that of the aerobic/anoxic(O/A) operation method, and the aerobic/anoxic(O/A) operation method is more complicated and trivial in the operational process and it is not good to better control the effluent index; Meanwhile, we also explore the influences of the different F/M on the treatment effects of the system. We also study the treatment effects of the system when the F/M is 0.05～0.06 kgCOD/kgMLSS·d,0.2～0.25 kgCOD/kgMLSS·d and 0.4～0.5 kgCOD/kgMLSS·d respectively and find that all effluent index is good when F/M is 0.2～0.25 kgCOD/kgMLSS·d, no serious sludge bulking phenomenon occurs, the stable operation of the system can be realized and the effluent can reach discharge standards. The other two F/M can hardly realize the stable operation of the system and will finally result in serious sludge bulking phenomena; the experiment also studies the influences of sudden temperature changes on the stable operation of the system and finds out that drastic drop in temperature can trigger deterioration of the sludge settleability and the SVI rises significantly. When it resumes to normal temperature, the SVI value decreases to a certain extent, but still not resume to the normal range before the temperature drop, and sludge bulking phenomenon of certain degree occurs. The drastic temperature drop has greater influence on the nitration effect of the system-the removal rate of NH4+-N declines to around 20%. The nitration effect can resume when resuming normal temperature and drastic temperature drop has less impact on the removal effects of PO43-P and COD.The calculation of nitrogen balance proves that obvious SND phenomenon occurs under the condition of sludge micro-sludge of SBR process. About 23.11% of the total nitrogen was removed through SND phenomenon. When DO concentration was 0.5mg/L, the nitrate nitrogen yields-to-the ammonia nitrogen decrement ratio was 0.454, and the nitration ratio balanced the denitrification ratio. At this moment, the average partical size of the sludge zoogloea particles was 5.02μm～6.83μm, indicating that SND is not only the result of "microenvironment effect".The quick start of the short range nitration and denitrification can be realized with the synergetic effect of DO and the real time control of PH as well as temperature and low DO. The sludge which realizes short range nitration preliminarily and over-aeration have great negative effect on the accumulation of nitro nitrogen. We allocated aeration time properly, adopted the real time control strategy and stopped aeration when or before ammonia nitrogen finished its oxidation. The cumulative percentage of nitrite was very high, and this not only make sure that the ammonia nitrogen was completely oxidated, but also prevent the further oxidation of nitrite. The real time control both realizes the short range nitration and can maintain the steady operation of short range nitration.Under the condition of low DO, we adopt the online monitoring methods of pH, DO and ORP to take real time control of the system and find that two inflection points occur to the pH value during the system reaction process. These two inflection points marks the complete phosphorous release and complete phosphorous uptake, and the inflection point of complete phosphorous release can be taken as the characteristic point of real time control for complete phosphorous removal. DO has only one relatively obvious change point and at that moment NH4+-N is just removed completely. Thus, this point can be taken as the real time control point for the complete removal of NH4+-N; the variation condition of ORP and the variation condition of the COD concentration in the system have certain laws, but we find that this point is not very stable through many experiments. Therefore, this point cannot be taken as the real time control point for the complete removal of COD, but can be taken as the reference point for the removal status of organics in the system.Take an urban sewage treatment plant, the sewage treatment capacity of which is 10000 m3/d as an example. It will relatively save 17% aeration when DO in the aeration tank is maintained at 0.5 mg/L compared with the condition when DO is maintained at 2.0 mg/L. To a real sewage treatment plant, it will save a considerable sum of operating expense. In the reactor that was tested in this experiment, low DO saved more aeration than high DO. It saves about half aeration when average DO is 0.5 mg/L compared with the condition when average DO is 2 mg/L.