DUAL-MEDIA FILTRATION OF SECONDARY WASTEWATER EFFLUENT WITH COAGULATION BY ALUMINUM SULFATE, POLY ALUMINUM CHLORIDE AND FERRIC CHLORIDE

Experimental studies were carried out to investigate and compare coagulation and filtration characteristics of secondary wastewater effluent with aluminum sulfate (alum), poly aluminum chloride (PAC) and ferric chloride. Coagulation properties of secondary wastewater effluent, such as optimum coagulation pH, coagulation efficiency for turbidity and color and size distribution of floc particles, were almost the same when alum and PAC were used as coagulants. The optimum pH range for coagulation of turbidity and color with aluminum coagulants were 4.5-6.0, and with ferric chloride 3.5-4.5. Ferric iron coagulant had the same effects on coagulation of turbidity and floc formation as aluminum coagulants , but had higher effect on coagulation of color. The over-all removal efficiency for turbidity as well as color and the run length of the filter were almost the same and decreased with an increase in filtration rate when aluminum coagulants were used. The other hand, the efficiency did not change with filtration rate and the decrease in filter run length with filtration rate was smaller when ferric iron coagulant was used. The efficiency for color removal was better and the filter run length was longer in filtration with coagulation by ferric iron coagulant than aluminum coagulants