Performance of modified rapid infiltration system treating domestic sewage using limestone media

The removal of main pollutants from domestic sewage in modified rapid infiltration system, which was constructed by mixed limestone and coarse sand, was investigated. Experimental results showed that ammonia removal rates tended to decrease with increasing hydraulic loading rates (HLR) in common rapid infiltration system. The average removal rate was 87.11% when HLR were below 1.0 m/d compared with 28.94% as HLR were above 2.0 m/d. In modified rapid infiltration system which containing 50% limestone, the average removal rate of ammonia and total phosphorus was 72.38% and 43.32% by enhancing re-aeration and reducing organic pollution load, respectively, the effluent average pH was buffered to 7.32 and the average HLR was 2.25 m/d. In modified system which containing 30% limestone, the average removal rate of ammonia and total phosphorus was 15.65% and 42.96% in the condition of normal water distribution, respectively, the effluent average pH increased to 7.80 and the HLR decreased to 0.76 m/d gradually. Phosphorus adsorption capacity of limestone grain was 78.55 mg/kg. Limestone is able to neutralize acids produced during the nitrification process, increase the ability of ammonia removal by decreasing HLR in the initial period and prevent filter materials clogging as well.