Study On The Performance Of Pretreatment Technology For COD Reduction In Large-scale Wastewater Treatment Plant

Shaoxing wastewater treatment plant is a municipal sewage treatment plant which mainly treats dyeing wastewater. The main process contents physico-chemical process+A/O process (Ⅰ) and physico-chemical process + oxidation ditch process(ⅡⅢ). Combining with technical requirements of standards upgrading and the main problems of operating system, the study focused on the technological performances of pretreatment processes, on the basis of quality and quantity of wastewater analysis. The results were shown as follows.1,The water quantity and quality characteristics of Shaoxing comprehensive wastewater was systematically analyzed. The volume increased year by year and showed seasonal variation, and the maximum amount of water to 980,000 m3/d, in which, the proportion of dyeing wastewater was more than 80%, the proportion of acidic wastewater was from 2% to 3%, and the proportion of the sewage was 10%. The COD of Shaoxing Comprehensive Wastewater was from 1200 to 2000mg/L, BOD was from 400 to 600mg/L, pH was from 7 to 11, the chromaticity was 250～400 times, B/C was ranged from 0.25 to 0.50, SS was from 400 to 600mg/L, NH4-N was from 40-50mg/L, TP was from 4 to 7mg/L, and the temperature was from 20 to 40℃. The high pollutant concentration, volatile pH, high color, low B/C and the increasing of COD and SS year by year, as well as the decreasing of B/C made the treatment of comprehensive wastewater more difficult.2,The control parameters of physico-chemical pretreatment process was optimized. The results revealed that:(1) pH was the main factor affecting the performance of physico-chemical pretreatment. When the pH ranged from 9 to 9.4, the chromaticity is reduced obviously, and B/C increases from 0.35～0.45 to 0.45～0.55; (2) In order to improve the performance of physico-chemical pretreatment unit, the appropriate dosage of ferrous sulfate forⅠandⅢunit was 500～550mg/L, while the suitable dosage forⅡunit was 600～700mg/L; (3) lppm anionic PAM could make SS less than 250mg/L after physical and chemical pretreatment.3,The performance of ABR process was investigated. On the condition of HRT≥11h,pH≤9,the temperature 25～35℃,SS≤200mg/L,MLSS 20～40g/L for effluent of physico-chemical process, the COD removal rate was 40%～50%, the B/C promotion rate was 19%, the chromaticity reduced from 261 times to 246 times, and SS decreased from 292mg/L to 199mg/L. GC-MS analysis showed that after anaerobic treatment, types of pollutants in wastewater reduced from 84 to 44 species. The following the anaerobic treatment, the COD of aerobic treatment effluent could reduce to 100mg/L. At the same time, there also existed currents short circuits, dead zone and the loss of sludge in ABR process, and they should be solved in engineering application.Based on water quantity and quality characteristics of Shaoxing Wastewater Treatment, this study result showed the important of enhancing the physico-chemical pre-treatment and anaerobic processes to improve the B/C value and the effective transformation and degradation of refractory pollutants. The results can not only provide technical basis for Improvement Project of Standards upgrading in Shaoxing Wastewater Treatment, but also provide reference for similar sewage treatment plants for upgrading.