Process options for nitrogen and phosphorus removal in domestic wastewater

Abstract

The introduction of biological nitrogen removal at many sewage treatment plants in Sweden, particularly those located at coastal regions up to Stockholm archipelagio in the east, is now proceeding primarily by plant trial operation and construction works to be finalised in 1995/96 at the latest. The most characteristic problems with biological nitrogen removal in Sweden are the cold winter climate and the deficit of an easily biodegradable carbon source for anoxic denitrification.This report includes some R&D works from the sewage treatment plant in Halmstad on the west coast, including the concluding results of 2-years' trial operation on nitrogen removal. The main problems have been to evaluate two different process alternatives during different climate conditions as well as different external carbon sources and compare such a carbon source with an internal carbon source generated by anaerobic enzymatic hydrolysis of primary or mixed raw sludge.Methanol has been evaluated to be the best external carbon source with the lowest biological sludge production and little detrimental influence on the sludge characteristics. The results of experimental works and cost calculations on the acid phase of anaerobic sludge digestion under formation of volatile organic acids (acetic acid etc.) showed that the costs for this carbon source with a modified sludge treatment scheme is competitive with methanol from both a technical and economical point of view. Dosage of primary sludge as carbon source for anoxic predenitrification has also given satisfactory results during the warmer period of the year (June - November). This method is not applicable in Sweden during the colder period of the year, because the nitrification will be depressed due to a decrease of the sludge age.The two different process alternatives for biological nitrogen removal were conventional pre-denitrification and a combined pre- and post- denitrification respectively. The last-mentioned system is probably the best process alternative with better operation flexibility and possibilities to a more far-reaching nitrogen removal. Both alternatives are recommended for practical operation in order to make it possible to switch over from one alternative to the other with respect to e.g. the water temperature.Problems in connection with an expected, more stringent phosphorus limit of 0.3 mg P/l in the effluent are also discussed. A polishing step in addition to chemical pre-precipitation is required.