Phosphorus precipitation is ususally achieved by addition of chemicals like calcium hydroxide, ferrous or ferric chloride, or alum, either in the primary or the final settling tank.
Alum is more expensive and generates more hydroxide, which creates extra sludge, that is difficult to dewater. Use of lime results in an increase of approximately 50% in surplus sludge, but the sludge is reported to have good dewatering properties. When using iron salts, a molar ratio of 1.0:1.4 of iron to phosphorus is reported to give 91-96% removal of total phosphorus using ferrous chloride dosed directly beneath the aerator.
Chemical addition prior to biological treatment is feasible if a primary settling tank exists as in the case of the conventional activated sludge process. The dose requirement then increases, but chemical precipitation also improves organic removal, thus reducing BOD load on the biological treatment. For extended aeration plants there is no primary settling; chemical addition has to be done in the final settling tank.