Research On Cyclic Activated Sludge Combined Biofilm System For Domestic Sewage Treatment In Small Towns

The sewage discharge increased dramatically by the fast increase of towns in China, and water environment faces big problems and challenges in small towns. In contrast to big cities, small towns has less population, so its quality and quantity of wastewater varies much more widely; the retention time in drainage pipeline is shorter in small towns, which results in the lower rotting degree. In addition, economic and technical conditions are often limited in samll towns, and so far there are still very few suitable technologies for wastewater treatment in samll towns. Therefore, it is necessary to develop new wastewater treatment techniques which could remove nitrogen and phosphorus effectively and have good anti-shock loading capability. Furthermore, this technology must be controled easily. So a new wastewater technology named Cyclic Activated Sludge combined Biofilm System (CASBS) has been developped for the wastewater treatment in samll towns. This new technology has advantages of phostrip, Sequencing Batch Reactor and Membrane Bio-reactor, thus it removes nitrogen and phosphorus effectively, achieving good anti-shock loading capability and making running operation easily. This dissertation has studied influence of flow pattern characteristics, microbial activities and categories, the concentration of dissolved oxygen (DO) and fluent velocity, on the pollution removel efficiency and mechanisms. Also, the dissertation has studied the nitrogen and phosphorus removal efficiency of CASBS when the influent quality varies dramatically. The results of the research are showed as following.â‘ This cyclic design of CASBS has characteristic of both mixed flow and plug flow pattern, which could improve its anti-shock loading capability. There are numerous categories of microorganisms in CASBS, such as Vorticella, Epistylis, Zoothamniumarbuscula in Protozoa and Rotifer in Metazoan and so on, which could also improve the anti-shock loading capability of CASBS.â‘¡The concentration of DO could affect the removal efficiency of nitrogen and phosphorus of CASBS. When DO in the end of the aeration stage in outer loop are 3.5-4.5mg/L, 2-3mg/L, 0.6-1.5mg/L, respectively, the average removal efficiency of chemical oxygen demand (COD) are 93.1%, 97.7%, 95%, respectively; the average removal efficiency of NH₄⁺-N are 93.1%, 97.7%, 95%, respectively; the average removal efficiency of total nitrogen (TN) are 67%, 72.1%, 78.6%, respectively; the average removal efficiency of total phosphorus (TP) are 94.2%, 94.6%, 93%, respectively. So due to different DO, the removal efficiencies of COD and NH₄⁺-N are always good, but the removal efficiencies of TN change largely. The removal efficiency of TN is better in low DO condition. CASBS could remove phosphorus effectively. The phosphorus concentration effluent can reach the (B level) requirements of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB 18918-2002) steadily, but cannot steadily reach the (A level) requirements of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB 18918-2002) in long time.â‘¢Velocity in outter loop would affect the pollution removal efficiency of CASBS. When the average velocities in outter loop of CASBS are 0.35m/s, 0.15m/s, 0.02m/s, respectively, the average removal efficiency of COD are 93.2%, 95.2%, 85.8%, respectively; the average removal efficiency of NH₄⁺-N are 98%, 93.8%, 72.9%, respectively; the average removal efficiency of TN are 67.8%, 80.4%, 67.8%, respectively; the average removal efficiency of TP are 91.7%, 93.7%, 63.7%, respectively. When the average velocity is 0.02m/s,the pollution disposing efficiency of CASBS is extremely low; When the average velocity is 0.35m/s, the active sludge clumps is smaller than clumps in CASBS with other velocities. In that case, the microbial activity becomes worse, so the removal efficiency of TN is not good; When the average velocity is 0.15m/s, the pollution removal efficiency of the CASBS is excellent.â‘£The anti-shock loading capability of CASBS is excellent. When the influent quality varies dramatically: the concentration of COD, NH₄⁺-N, TN, TP are 106⁶84mg/L, 15.2⁶4mg/L, 23.9⁷5.2mg/L, 1.39¹2.57mg/L, respectively, the average concentration of COD, NH₄⁺-N, TN, TP in effluent are 17.2mg/L, 1.05mg/L, 8.36mg/L and 0.25mg/L, respectively. The average removal efficiency of COD, NH₄⁺-N, TN, TP are 95.2%, 97.1%, 80.1% and 94.6%, respectively. The concentration of COD, NH₄⁺-N, TN in effluent can reach the (A level) requirements of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB 18918-2002), and TP can reach the (B level) requirements of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB 18918-2002).⑤CASBS removes TN effectively because of the effect of traditional biological nitrogen removal technology and Simultaneous Nitrification and Denitrification (SND). In addition, the removal efficiency of TP of CASBS is high, for it could remove phosphorus by discharging water and excess sludge rich in phosphorus and by denitrifying phosphorus removal.