Treatment of wastewater containing phosphorus compounds

Claim 1

In a process for treating wastewater wherein phosphate-containing recycled activated biomass is first admixed in an anaerobic zone with an influent containing phosphate and BOD to form a mixed liquor and treated under anaerobic conditions such as to be substantially free of oxidizing agents and at a dissolved oxygen concentration in said mixed liquor of less than 0.7 ppm and BOD in the influent is sorbed by the biomass with release of phosphorus from the biomass: and wherein the anaerobically treated mixed liquor is subjected to contact with oxygen-containing gas under oxic conditions followed by settling of the oxically treated mixed liquor to separate out a dense sludge layer comprising activated biomass, a portion of which dense sludge is recycled to provide the biomass first admixed with said wastewater influent: the improvement which comprises introducing at least a portion of said recycled sludge, prior to its admixture with the wastewater influent, to a holding zone in which it is held under non-aeration conditions for a period of time at least sufficient to reduce any nitrates and/or nitrites contained therein, such that no significant amount of such nitrates and/or nitrites are introduced into said anaerobic zone.