| The problem of eutrophication, which is mainly caused by excessive discharges of nitrogen and phosphorus, is growing bigger and bigger. Therefore, the wastewater techniques focus on how to remove COD, nitrogen and phosphorus simultaneously. Domestic wastewater, as the mainly resource of water pollution, is of low ratio of carbon to nitrogen (C/N), which restricts the treatment quality. Thus, how to remove nutrient ideally at low cost is spotted to be the point of low C/N wastewater treatment. Orbal oxidation ditch is wildly used in China, because of its simplicity and convenienience, while its improvement is badly needed. This research defined to deal with the problems which have been just mentioned.Solo-nitrogen removal and simultaneous nitrogen and phosphorus removal was investigated using domestic wastewater with low C/N in a pilot scale Orbal oxidation ditch, assisting batch assay experiments. Analysis of dissolved oxygen (DO) and pH variation was also involved in this dissertation. All this is to provide instruction of optimized operation to wastewater treatment plant (WWTP).Using pilot scale Orbal system, we get the result as follow: CODCr in effluent lower than 50mg/L, ammonia in effluent lower than 5mg-N/L, total nitrogen(TN) in effluent around 20～33mg-N/L without enhanced bio-phosphorus removal(EBPR). Strategy of DO concentration controlled at 0.3mg/L, 0.5mg/L and 2.0mg/L in out channel, mid channel and inner channel respectively was promoted to enhance the effect of nitrogen removal. Simultaneous nitrification and denitrification (SND) was occurred in this system. Compared to non-control operation, TN removal efficiency and SND percentage was improved 13.7% and 24.3% respectively. Influence of novel microbes on SND was investigated, and approved to be of little importance. In practical, WWTP was affected by the variation of seasons and temperature.Low temperature is of negative influence on wastewater treatment and sludge settlement. Higher sludge retention time (SRT) is helpful to revise the problem, and pre-anaerobic selector can avoid sludge bulking which was caused by low temperature. Hydraulic retention time (HRT) is one of the most important parameters on bioreactor. HRT of 14.8h and 17.2h were suitable for the pilot scale Orbal system. EBPR and nitrite accumulation had been taking place in this pilot scale Orbal oxidation ditch. Thus, to achieve simultaneous nitrogen and phosphorus removal, the strategy of adjusting C/N was proposed and the best C/N and dosing carbon species were investigated. The most favored CODCr /TN is 5.0, when phosphate removal efficiency is highest, with 1mg-P/L and 15mg-N/L TN in effluent, which goes well with the first level of GB18918-2002. According to the research, carbon species is of little importance on nitrogen removal efficiency. The impact of acetate sodium and glucose on phosphate removal is quite the same, while starch is of great negative effect. The rate and amount of phosphate release is lowest while dosing starch, which is the main reason of phosphate removal deterioration. Inner recirculation was studied in this dissertation, and found to be an important factor on nutrient removal. Preferable treatment result was achieved when CODCr /TN is 5.0 at suitable recirculation rate. Phosphate removal will deteriorate while the concentration of TN in effluent is very low, because of phosphate release in clarifier. Methods were promoted to revise the situation. Denitrifying phosphate-accumulating organisms (DNPAOs), of which the specific phosphorus uptake rate is 13.2mg P/(g VSS·h), were found to be existing in the system.Nitrite accumulation with high rate was found in Orbal oxidation ditch. Temperature and ammonia load is not the main reason of this phenomenon. Shorter SRT, variation of pH and increase of free ammonia, which induced NOB washed out of the system, is the key to nitrite accumulation. Therefore, short cut nitrification in Orbal system treating domestic wastewater could be achieved by controlling SRT, DO and pH value.The variation of online pH value at situations of simultaneous nitrogen/phosphorus removal and solo-nitrogen removal in Orbal system was investigated. The end point of phosphorus release, mass phosphorus uptake period and nitrification closure point make great influence on pH value. However where the phosphorus release finishes or whether phosphorus removal happens, the peculiar pH point will appear according to nitrification ending up in mid channel, which can be a hint of excessive aeration. More parameters are needed to Orbal system control, for simple pH online control is not enough.
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