Control of Phosphorus Discharges: Present Situation and Trends

The dominating sources of phosphorus in municipal wastewaters are excreta, 1.4 g P/(cap.d) and detergents, 0.6 – 2 g P/(cap.d). Detergent phosphorus can be substituted by nitrilotriacetic acid or zeolites, but if a substantial reduction of phosphorus in municipal waters is to be achieved, modifying the treatment process is necessary. Primary, treatment by sedimentation removes only 10–15% and secondary biological treatment 20–30% of the phosphorus in waste water. If chemicals are added to the primary or secondary treatment stage or to a separate chemical stage, phosphorus can be efficiently removed. An effluent level of 0.8–1.5 g P m-3 is easily achieved and with a filtration step it is possible to maintain 0.2 g P m-3 in the effluent. Different process configurations are discussed. As precipitants, ferrous and ferric salts, alum and lime are widely used. By introducing anaerobic zones in the activated sludge process, it is possible to promote the growth of bacteria which enhance biological phosphorus uptake. This makes it possible to achieve high phosphorus removal without or, al least, with very small chemical additions. Several emerging physical, chemical and biological phosphorus removal processes are discussed. The removal of phosphorus to a level of 0.8–1.5 g P m-3 increases cost 10–20% compared with conventional primary secondary treatment. Higher removal efficiencies will rapidly increase the marginal cost per marginal kg P removed.