High-denitrification Control Technology Research Based On Low C/N Urban Sewage In CAST Process

After the completion of Three Gorges Reservoir, the ability of water self- purification is weaken due to water flow become slowly. The excessive emission of nitrogen and phosphorus resulted that eutrophication was more severe in the reservoir, especially, Taihu Lake have appeared cyanobacteria events in Wuxi in the year before past, which gave rise to concern about excess emissions of nitrogen and phosphorus in the Three Gorges Reservoir Area. The deep removal of nitrogen and phosphorus is more and more urgent. CAST Technology (CyclicActivated Sludge Technology), as a new variant of SBR process, has many merits, such as good function of nitrogen and phosphorus removal, smaller area, without primary sedimentation tank and secondary settling tank, low infrastructure and operating costs. so CAST process is adapt to Three Gorges reservoir area which has smaller area and short of funds. At the same time, the bio-selecting tank is set up in CAST process to limit the growth of filamentous bacteria, with high operating automation and low labor intensity. In recent years, CAST is used more and more widely in China,especially in the Three Gorges Reservoir.Although there are many researches of CAST technology, the productive experient is still at a preliminary stage.In this paper, the CAST process treatment highly influent Nitrogen, average 60mg/L, cultivation and domestication of sluge, process test, optimization of operation and microorganism changes in the system were systematically and thoroughly researched. In 2008, there had research the feature on the removal of CODCr, TN, NH3-N and other pollutants in CAST Process and analyze production problems. Through adjusting key parameters affecting CAST process, the different roles of affecting factors were systematically researched to find best operation modes that adapt to Lijiatuo WWTP in low and normal temperature periods. On the base of analysis operation datas in 2008, especially Nitrogen removal rule and affection factors analysis, a new operation mode, adapted to Lijiatuo WWTP that wastewater quantity and Nitrogen removal pressure increased in future, was brought forward. The variation, factors and indicative of ORP during nitrogen removal process and the relationship between microbes and nitrogen were preliminary studied in 2009.During CAST process test, there are five operating modes, A: Influent0.5h-Influent/Aeration1.5h—Settle1h—Decant1h; B(preliminary design pattern ): Influent/Aeration2h—Settle1h—Decant1h; C: Influent/Aeration 2h—Slight Aeration 1h —Settle 1h—Decant 1h; D: Influent 1h—Aeration 2h—Settle 2h—Decant 1h; E: Influent/Aeration 3h—Settle 2h—Decant1h.According to the actual operation of the past year, C was selected in low temperature period: Influent/Aeration 2h—Slight Aeration 1h—Settle 1h—Decant1h and DO, MLSS and other operating parameters were optimized. At the conditions that DO was controlled at 3.0 ~ 4.0mg/L in main exposure stage, below 0.5mg/L in micro-exposure stage, the sludge age was 15 ~ 20d, MLSS value was 4000 ~ 6000mg/L, the sludge recycle ratio was about 30%, the system can achieved the best nitrogen removal, about 75%. B(design pattern) was selected in normal temperature period: Influent/Aeration2h—Settle1h—Decant1h; DO, MLSS and other operating parameters were determined .At the conditions that DO was controlled at 3.0 ~ 4.0mg/L, MLSS value was 4000 ~ 5000mg/L, the sludge age was 14~16d, the sludge recycle ratio was about 20% , the system can achieved the best nitrogen removal.Base on WWTP actual operating parameters and quality of influent and effluent in 2008 ~ 2009, the following conclusions were obtained: the nitrogen removal in CAST process were significantly different at different modes of operation; in CAST process operation, it was should to compared with water quality of design and actual, attention to different operating modes with different quality to avoid nitrogen pressures caused by high percentage of industrial wastewater and high influent nitrogen concentration. In the actual design, in order to obtain better nitrogen removal, the nitrogen loading values should not be higher than 0.04gN/gMLSS.d and sludge age is about 15d.Currently, the influent is less and load is not high in Lijiatuo WWTP, increased water quantity in future will bring serious pressure of denitrification. Therefore, this topic studied on strengthening the simultaneous nitrification and denitrification,and raised the concept of DO Step-Control. Under the premise of removing pollutants such CODCr,BOD5 and so on, the results showed that nitrogen removal in DO Step-Control was more stable and effective than ordinary whether in low or normal temperature operating conditions. Denitrification rate increased by 10% ~ 15%, the effluent TN concentrations was mostly below15mg/L, TN effluent was 11mg / L or so.ORP as one of control parameters, more show its role in the nitrogen removal process. In 2009, When carry out the pilot study for researching the role of ORP making during the Nitrogen removal process, the study emphases is put on the relationship between ORP, CODCr, NH3-N and NOX-N during denitrification process in anaerobic tank and the main reaction tank . Research found that the different influent wastewater quantity, ORP values is significantly different changes. And the results indicate that influent wastewater quality makes an important role in affecting ORP. Linearity pertinency between ORP and logarithm of CODCr,NH3-N concentration product and NOX-N concentration ratio is preferable, R2=0.9146;Linearity pertinency between ORP and logarithm of DO in aeration phase of the main reaction tank, R2=0.8708.A variety of bacteria are mainly play the role of sewage purification in Sewage treatment plant, there are interdependent functional relationship between Micro- organisms and bacteria. Community structure of Microfauna Performances bacterial community structure to a large extent, so Micro-organisms can indicate good or bad effects of sewage treatment. In the CAST Process, micro-organisms, especially ciliates, Pearson correlation analysis while changes of ciliates type and quantity and the corresponding physical and chemical parameters of Drainage have been shown that : Aspidisca costata,Litonotus carinatus stokes and Vorticella microstoma Ehrenberg are dominant species of micro-organisms. here are interdependent functional relationship between Dominant species of Ciliates and physical and chemical parameters of Drainage, the different species have different degree of correlation.