UPGRADING FOR NITROGEN REMOVAL UNDER SEVERE SITE RESTRICTIONS

The Rya WWTP in Goteborg, Sweden is a highly loaded activated sludge p lant which has been upgraded to remove nitrogen and to increase the bi ologically treated flow by 33% to reduce bypassing. Severe site restri ctions made it difficult to increase the plant area. This was solved u sing a compact process based on tertiary nitrification in trickling fi lters and recirculation to a highly loaded activated sludge unit for d enitrification. The necessary volumes were achieved by expanding the p lant upwards, thus making it possible to place the trickling filters a in part of the area occupied by the former aeration basins. The recir culation of trickling filter effluent made it necessary to double the secondary settler capacity. This was solved by retrofitting the existi ng settlers as stacked settlers with a second tray on top of the origi nal basin. The considerations behind process selection and plant desig n and costs are described. (C) 1998 IAWQ. Published by Elsevier Scienc e Ltd