Study On Operation Control Of Treatment Process Of CASS-Coagulation Sedimentation For Municipal Wastewater Treatment

In this project, the overall aim of this research was to study the "CASS- coagulation sedimentation" process employed in a sewage treatment plant in Fengning county.The capability of the CASS-coagulation process employed in the sewage treatment plant was designed at 8,000m3/d. The sludge load rate of the CASS tank was 0.056kgBOD/kgMLSS·d, hydraulic retention time(HRT) was 17 hours. Tubular static mixer was installed in the grid-style coagulation tank. The treated sewage water met the Level A discharge standard shown in Table 1 of《urban sewage treatment plant emission standards》。The indirect(inoculation) method was employed in the cultivation of the activated sludge in the CASS process. After 30 days’ operation, the quality of the treated sewage had become stable, which indicate the microorganisms in the activated sludge had adapted the environment and been capable to absorb and decompose the organic pollutants in the sewage, efficiently. According to the literature review and survey analysis, the affecting factors of the treatment performance of a CASS process include Dissolved Oxygen(DO) level, the characteristics of seeding sludge and CASS process. The NH3-N and TN levels reached the required discharge standard by adjusting the operation factors of the CASS process.The operation data of the CASS process shows that, the levels of effluent COD、BOD、SS、NH3-N and TP were in the ranges of 45.6~57.9mg·L-1, 6.2~7.8mg·L-1, 30.4~51.8mg·L-1, 6.8~8.2 mg·L-1 and 0.6~0.8 mg·L-1, respectively; when the levels of influent COD, BOD, SS, NH3-N and TP were 190~389 mg·L-1, 160~180 mg·L-1, 150~200 mg·L-1, 25~30 mg·L-1, 1.5~1.8 mg·L-1, respectively. Besides the SS and TP levels were not in the range required in the discharge standard and the COD level was also out of the required standards.The outcomes of the coagulation experiments aiming to investigate the optimal conditions show that, using the polymerization aluminum chloride(PAC), the optimal operation conditions in this case include: the dosage at 50 mg·L-1, pH at 7.5, temperature at 30℃, stirring intensity at 120 r·min-1, the coagulation time in 10 min, precipitation time in 50 minutes. The average levels of COD, BOD, SS, TN, NH3-N and TP in the effluent were 40.9 mg·L-1, 5.9 mg·L-1, 7.7 mg·L-1, 8.3 mg·L-1, 2.4 mg·L-1, 0.21 mg·L-1 and 6.9~7.0. The system operation was stable and water quality of the effluent met the standard requirements.Seleting CASS- coagulation sedimentation process for the treatment of urban sewage was reasonable and accountable, the operation efficiency was great and stable. The efficient and steady operation of the process was ensured by employing advanced operation control. The total investment of the sewage treatment project was 39,711,200 yuan, and treatment cost was at 1.9 yuan·m-3.