In activated sludge-type wastewater treatment in which phosphate is taken up in the aeration zone, solids are concentrated in a separate zone to form phosphate-enriched sludge and phosphate is resolubilized in an anoxic zone, a major part of the latter is returned to aeration without intervening phosphate removal and phosphate wasting is solely from sludge.

http://www.freepatentsonline.com/4183808.html