Operation Of Pilot-scale Composite CAST Treating Domestic Sewage With A Low Organic Carbon Content

Nowadays, domestic sewage of many towns in China presents a very loworganic level, leading to unstable operation of existing wastewater treatment plants,and it is hard to meet dischage standards in terms of TN. CAST process, amodification of SBR process, has been widely used in China due to severaladvantages, such as a strong ability of removing nitrogen and phosphorus, a smallarea coverage, a high resistance to influent load impact, and a low possibility ofsludge expansion. However, CAST is relatively unsuitable for processingwastewater with a low carbon source, for lack of carbon source would makedenitrification efficiency decline. In fact, there are few reports on treatment ofwastewater with a low organic content like less than150mg/L COD. So, a new typeof CAST called composite CAST was developed in my study, which ischaracterized by two forms of sludge growth: suspended growth and fixed growth,for the aim of upgrading a treatment plant which receives domestic wastewater witha low concentration of organic compounds. This study will provide a good examplefor efficient disposal of low carbon level wastewater through upgrading CAST.A pilot-scale composite CAST was investigated in this study using realdomestic wastewater with a low organic carbon from a small city of China asexperimental sewage. The objectives include startup of CAST, optimization ofoperational parameters and treatment efficiency, the main conclusions are asfollows:For the startup of the composite CAST, a certain amount of activated sludgewas first innoculated, and it took30days to complete the startup. At this point, thesludge concentration was1600mg/L and the biofilm was2mm thick on thesupporters. The effluent quality was almost45mg/L COD,1.5mg/L NH4+-N,8.5mg/L TN, and0.4mg/L TP.The best cycle for the composite CAST was filling1.0h, aeration1.5h,settling1.0h, and drawing0.5h. The optimal return ratio of sludge was20%. Theaeration process was also optimized, that is, the DO concentration was controlled at2to3mg/L for the first30min,1to2mg/L for the second30min, and <0.5mg/L for the last30min.The good treatment ability of the composite CAST under the predeterminedoptimal operational conditions could make the effluent concentrations of COD,NH4+-N, TN, TP and SS meet standard1A of GB18918-2002during the heatingand nonheating periods. The composite CAST showed a powerful ability ofnitrogen removal which was ascribed to classical nitrification and denitrificationand simultaneous nitrification and denitrification, and simultaneous nitrificationand denitrification was the dominant mechanism. Furthermore, the compositeCAST was a cost-effective way to upgrade an exising CAST for efficient treatmentof wastewater with a low carbon source.