Studies on recovering phosphorus from municipal sewage by precipitation

Filtrates of the digested sewage from a municipal sewage treatment plant were analyzed for phosphate P, Mg, ammonia N, K, suspensions and pH and then treated with MgCl2 or MgO to ppt. P as struvite crystals under lab. conditions (conical reactor of a vol. 10 L). The effect of pH, molar Mg/P ratio and hydraulic retention time on the phosphate removal efficiency was studied. The degree of the P removal with MgCl2 (excess 100%) was 90% after 30 min and 95% after 60 min. The removal with MgO (excess 100%) let to recover 80% of P after 120 min. The struvite crystal suspension was easily sepd. by sedimentation. The content of heavy metals in the resulting struvite was lower than the resp. limits for fertilizers.