The Investigation Of Sludge Minimization In Airlift-Reflux Combined Reactor

Nowadays, China has vast rural and remote areas, where the problem of decentralized swage pollution is more and more serious. In order to control the decentralized wastewater, there are four popular processes: constructed wetland, membrane bioreactor, purification tank and conventional activated sludge process. The characteristic of constructed wetland is simple operation, the low total cost, covering a large area, vulnerable to the effects of temperature and the treatment effect of poor stability; the characteristic of membrane bioreactor and purification tank are covering a small area, a good treatment effect, running complex, high maintenance requirements and higher operation cost; the characteristic of conventional activated sludge process is covering a small area, good treatment effect, large volume excess production and high operation cost. Our task group developed Airlift-Reflux Integrated Reactor, which could remove organic substances, remove nitrogen nutrient and reduce sludge production simultaneously, was developed to solve the high production of excess sludge, the high maintenance frequency and the equipment complex of decentralized sewage system. It was mainly used to treat decentralized sewage. This test observed the removal efficiency of contaminants in pilot-scale reactor under the optimum operation condition. Meanwhile, this paper investigated the mechanism and performance of sludge minimization.In order to determine the sludge discharge cycle and the best proportions of different volume for the reactor. At last, it could be beneficial for the promotion and application of actual project. The capacity of polit-scale Airlift-reflux Combined Reactor is 10.31m3. The experiment is conducted with the domestic sewage and investigate the removal efficiency of CODcr、 NH₄⁺-N、TN and SS. During the 12-month period the following results were concluded:（1） The reactor treats 12.24 t wastewater every day. when the average influent concentration of CODcr、NH₄⁺-N、TN and SS are 404 mg/L、44.9 mg/L、62 mg/L and 93mg/L respectively, the effluent concentration of CODcr、NH₄⁺-N、TN and SS are 45mg/L、2.5mg/L、 19.7mg/L and 6.62mg/L respectively. The average removal efficiency of CODcr、NH₄⁺-N、TN and SS are 88.9%、94.4%、68% and 92.9% respectively. It shows that the reactor has a good performance in term of removal contaminants.（2） During the stable operational period, the observed sludge coefficient（Yobs） was 0.139 g TSS/g COD in the system. It exhibits that Airlift-reflux Combined Reactor had a good performance for sludge minimization. Using Yobs of the conventional activated sludge system, the sludge production of Airlift-reflux Combined Reactor（under short sludge retention time） would 162 kg TSS in theory. According to the sludge production from Steady state—ASM3 model, the sludge production of Airlift-reflux Combined Reactor（under long sludge retention time） would 142 kg TSS in theory. So there was about 20 kg TSS sludge reduction through maintenance metabolism and endogenous respiration. However, the actual sludge production in the reactor were about 82.5kg TSS. So there was approximately 59.5 kg TSS sludge reduction through lysis-cryptic growth and microbial predation. Through material balance analysis for ammonium,there was about 46.3kg TSS of the total minimization of sludge production through cell lysis-cryptic growth; According to material balance, there was approximately 13.2kg TSS biomass reduction through microbial predation. So that, Airlift-reflux Combined Reactor could reduce79.5kg TSS in the period. There were 25.5%、57.7% and 16.8% of the total minimization of sludge production through maintenance metabolism and endogenous respiration 、 lysis-cryptic growth and microbial predation respectively. The system has the rate of sludge autolysis is 0.589 kg TSS/d.（3） Experiment founded that when the effluent of the reactor（get rid of phosphorus） still met standard level B, floating sludge increased from the settling zone which could not met discharged standard. Therefore, Using surface load of the secondary tank and the equation of vesilind, the maximum sludge concentration could be confirmed in the settling zone of the reactor which was 10012mg/L. Although this system was aerobic combined settling tank, the sludge in the settling zone could quickly recycled to the aerobic tank. So the sludge concentration of the settling zone was approximately equal to the one in the aerobic tank. Therefore, the filled mass sludge of Airlift-reflux Combined Reactor was 121.84 kg TSS. According to the observed sludge coefficient and the started mass sludge, the period of discharged sludge was about 134 d in theory. Experiment simulated sludge under aerobic and anoxic. Through the dynamic equation confirmed that the net rate of sludge autolysis under aerobic and anoxic was +4.38 g TSS/m3/h and-4.22 g TSS/m3/h respectively. When the net increased sludge under the anoxic volume was equal to the net decreased sludge under aerobic volume, the reactor had the optimum volume proportion. Therefore, theoretical calculation showed that Vaerobic zone:Vanoxic zone=1:1 was the optimum volume proportion.