Separate treatment of sludge liquor (reject water) is a promising alternative for conventional biological extension of sewage plants with sludge digestion. Treatment of this nitrogen-rich water may significantly save in cost as well as in area demand.

This article evaluates pilot plant investigations of five treatment options: the biofilm airlift suspension reactor, the membrane bioreactor, the bioreactor without sludge retention, stripping of ammonia (both airstripping and streamstripping) and the MAP/CAFR-process. The choice of the optimal treatment method is dependent on site-specific factors. This is demonstrated by two case studies.