Study On Treatment Of Digested Nightsoil Wastewater

With the expansion of city scale and the rising of the population density, the amount of nightsoil clean-up is on the rise. Detoxification, resources and reduction of urban nightsoil has already begun. The amount of national urban disposed nightsoil was up to 21.41 million tons, the quantity of harmless treated nightsoil was only 8.461 million tons, and the rate of harmless treatment of nightsoil was only 39.52% in 2009.The urban nightsoil maintaining high level moisture content which commonly surpasses above 98%, it can produces a large number of sewage with high concentration of suspended solid after harmless treatment. Such sewage with high concentration of organic matter, nitrogen and phosphorus, if directly discharged without any treatment, will be very harmful to the environment.Wuhua city nightsoil integrated treatment plant utilized nightsoil of public lavatorys and the bottom sediment of septic tank, which mainly comes from community septic tank rather than septic tank of hospitals, schools and factories, to be anaerobic digested. After the treatment of anaerobic digestion, the solid produced by liquid-solid separation of nightsoil can be made into solid organic fertilizer,and the liquid can be emitted as digested nightsoil wastewater. The concentrations of SS, CODcr, NH3-N and TP in the sewage are very high, which can be up to 15000 mg/L, 10000-40000mg/L,1000-4000 mg/L and 80-600 mg/L respectively.All the anaerobic digested wastewater has been stored in the settling tank since the nightsoil integrated treatment plant began trial operation in November 2008. Through the survey of the present state of all equipments and constructions, we found that the original wastewater treatment process were just composed of settling tank and SBR reactor, there were no pipes between constructions, which leads to high-concentration nightsoil digestion wastewater could not be well treated. The suspended matter in the sewage settled down and a thick layer of nightsoil sludge covered in the bottom, other suspended matter floated to the near surface of settling tank and a 10 cm layer of nightsoil sludge formed on the wall. The SBR reactor had not been brought into service since completion.Based on the existing constructions and equipments, considering the quality of sewage and reconstruction demands and other limited factors, the sludge drying field-triple Anoxic -anaerobic-improved ICEAS craft was adopted. Sludge desiccation field was built to treat nightsoil anaerobic digestion wastewater to reduce the content of SS and the quantity of sludge. The settling tank was transformed as anoxic pond and anaerobic pond. A part of organic matter, nitrogen and phosphorus was removed in the sewage by adjusting the volume of inflow and surplus sludge backflow. Meanwhile, the SBR pond was transformed into an ICEAS pond, and the organic matter, nitrogen and phosphorus in the sewage could be further removed.The intermittent inoculation was used to cultivate the activated sludge in the ICEAS pond, which could make the sludge well adapt to the digested nightsoil wastewater. The active sludge, in which the biofacies were very plentiful in the later period of cultivation, was provided with strong impact resistance capacity, high stability and high treatment rate.The improved ICEAS craft has a better ability in treating the digested nightsoil wastewater than similar wastewater treatment crafts. The remove rates of SS, CODCr, BOD5, NH3-N and TP could reach 84.32%,92.32%,87.24%,52.74% and 67.29% respectively, and the concentrations of SS, CODCr, BOD5, NH3-N and TP in effluent were 143.22 mg/L,236.18 mg/L,82.68 mg/L,1469.34 mg/L and 21.37mg/L respectively. Improved ICEAS craft has some advantages compared with other methods.