PROCESS OPTIONS FOR NITROGEN AND PHOSPHORUS REMOVAL IN DOMESTIC WASTE-WATER

The introduction of biological nitrogen removal at many sewage treatme nt 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 1995/9 6 at the latest. The most characteristic problems with biological nitr ogen 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 Hal mstad on the west coast, including the concluding results of 2-years' trial operation on nitrogen removal, The main problems have been to ev aluate two different process alternatives during different climate con ditions 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 biolo gical sludge production and little detrimental influence on the sludge characteristics. The results of experimental works and cost calculati ons on the acid phase of anaerobic sludge digestion under formation of volatile organic acids (acetic acid etc.) showed that the costs for t his carbon source with a modified sludge treatment scheme is competiti ve with methanol from both a technical and economical point of view. D osage of primary sludge as carbon source for anoxic predenitrification has also given satisfactory results during the warmer period of the y ear (June - November). This method is not applicable in Sweden during the colder period of the year, because the nitrification will be depre ssed due to a decrease of the sludge age. The two different process al ternatives for biological nitrogen removal were conventional pre-denit rification and a combined pre- and post-denitrification respectively. The last-mentioned system is probably the best process alternative wit h better operation flexibility and possibilities to a more far-reachin g nitrogen removal. Both alternatives are recommended for practical op eration in order to make it possible to switch over from one alternati ve to the other with respect to e.g. the water temperature. Problems i n connection with an expected, more stringent phosphorus limit of 0.3 mg P/l in the effluent are also discussed. A polishing step in additio n to chemical pre-precipitation is required.