Room for Advancement

Advanced biological nutrient removal plants did not reduce phosphorus concentrations significantly but aimed at nitrogen reduction in a volume that was typically half that of older nutrient removal plants. This approach was finding favour for treating large flows at works where space was important. It employed multiple anoxic zones obtained by restricting aeration and by the step feeding of settled sewage. Most plants used 4 tanks in series to achieve this arrangement. Little or not mixed liquor recirculation was required; alkalinity supplements were usually unnecessary for sewages of low alkalinity. More carbon could be added if necessary to stimulate denitrification. If phosphorus removal was required it was attained by chemical addition. Two operating plants are briefly described.