removal of biological nutrients from a wastewater yielding a low phosphorous output includes providing a serial multistage bioreactor containing activated sludge having in hydraulic series an anaerobic zone and a downstream aerobic zone , with each zone having an upstream inlet and a downstream outlet . the wastewater is provided to the anaerobic zone inlet . a quantity of chemical sufficient to precipitate soluble and particulate phosphorous is added to the downstream aerobic zone in an amount sufficient to yield a low phosphorous output . treated water is separated from the activated sludge and precipitated phosphorous and a return activated sludge separated from the treated water is recycled to the anaerobic zone .