Phosphorus removal by chemical precipitation in a biological aerated filter

Enhanced concentrations of phosphorus entering the aquatic systems have been linked with eutrophication and its associated problems. The EC Urban Wastewater Treatment Directive, adopted in May 1991, included effluent standards for phosphorus in the U.K. for the first time. The need for an efficient and cost-effective phosphorus removal process has now become apparent. Two pilot-scale columns, an aerated filter packed with seeded expanded shale and an anoxic filter packed with plastic media, were operated using settled sewage feed. Iron (II) sulphate heptahydrate solution was dosed onto the top of the aerated filter at varying ratios of total phosphorus to iron. The performance of the plant with respect to BOD, COD, TKN and suspended solids removal was unaffected by chemical dosing. A significant reduction of nitrification occurred; however, this may be linked to increased BOD and ammonia loading. The run time of the biological aerated filter was not significantly affected by chemical addition. The choice of chemical dose used depends on the stability of process required, which increases with increasing dose, capital available and the EC limits imposed on the effluent to be discharged. To meet EC requirements, the most favourable chemical dosing ratio was 1:1.25 (weight ratio P:Fe).