NITROGEN REMOVAL IN A SINGLE SLUDGE EBPR PROCESS / Kväveavskiljning i en bio-P process
Studies on enhanced biological phosphorus removal (EBPR) in activated sludge processes have been conducted on a pilot plant at the Sjolunda wastewater treatment plant in Maimo, Sweden, since 1986. The plant receives wastewater from a combined sewer system. The aim of this study was to investigate the consequences of combining EBPR with biological nitrogen removal, especially with regard to denitrification and phosphate release as well as uptake under anaerobic and anoxic conditions. The long term effect of having nitrate in the anaero bic reactor in an EBPR process was studied during 1991. Furthermore, the effect of the variations in the wastewater quality on the combined EBPR and biological nitrogen removal process was studied for three years, 1992-1995. The results showed that high removal ratios of both nitrogen and phosphorus are possible during long periods, ‘the nitrogen removal was stable during the whole three year period, whereas the phosphorus removal was unstable during prolonged periods with low concentrations of COD in the influent wastewater. The combined biological phosphorus and nitrogen removal process implies that during these periods the risk of recirculating nitrate to the anaerobic reactor increases. Such a recirculation both stabilises the nitrogen removal and withdraws some ol the readily degradable organic material from the bio-P bacteria. This study showed that a phosphorus limited EBPR process can cope with the day to day variations, but it also showed that, occasionally, measures have to be taken if the demands for phosphorus removal are stringent. The evaluation of the denitrification and phosphorus uptake in the anoxic and the aerobic reactors showed that a substantial part of the phosphate uptake took place in the first anoxic reactor, i. e. some of the bio-P bacteria can denitrify. The fact that the phosphate uptake rate decreased in the second anoxic reactor and then increased significantly in the first aerobic reactor suggests that some of the bio-P bacteria can not denitrify.