- Modification of small activated sludge plants to recycled systems is proposed as a means to improve nitrogen removal and control of settling properties. The modification process involves separation of the aeration basin to anoxic and aerobic zones and addition of internal recycle of mixed liquor from the aerobic to the anoxic zone. This mode of operation may prevent problems of floating sludge in the final clarifier caused by uncontrolled denitrification. In the recycled system, part of the organic carbon is removed under anoxic conditions. Therefore, it may assist in the selection of microbial population with better settling characteristics, since most filamentous microbes have been reported to lack denitrifying ability. The modification considerations and guidelines are discussed based on experimental results obtained from operation of bench-scale recycled units. Influent COD/ammonia ratio, aerobic volume fraction, and recycle rate are shown to be critical parameters in the modification and operation of such systems.