Coagulation-filtration of a secondary effluent by means of pre-hydrolyzed coagulants

Jar tests were applied to a secondary effluent in order to determine optimal conditions for coagulation. The metal salts Al-2(SO4)(3), AlCl3 and FeCl3 were used as coagulants, and their efficiency under various degrees of pre-hydrolysis was examined. The variables under examination were the dose of the coagulant, the degree of pre-hydrolysis, B = [OH-]/[Me(+3)], and the pH at two different levels (pH = 5.5 and pH = 7.5). The efficiency of the various coagulants was evaluated for the removal of turbidity, phosphorous, COD and UV absorbance at 254 nm. The results showed that as far as turbidity removals are concerned, pre-hydrolysis of the aluminum and ferric chloride coagulants at values of B equal to 0.5 to 1 gave optimal results. However, for phosphorous removal, the higher the degree of prehydrolysis, the lower the removal of phosphorous. No remarkable differences were observed for the removal of organic matter measured as GOD, while alum gave the best results for the reduction in UV absorbance at 254 nm. Direct filtration of the effluent showed that the coagulant dose required was an order of magnitude lower than that required for coagulation. 35 cm of filter bed were sufficient for effective filtration. All coagulants performed well, increasing the degree of turbidity removal as compared to the absence of coagulants. The lowest headless rate was observed when alum was used, while the highest headless rate, almost three times as much, when the aluminum chloride coagulants were used (with and without pre-hydrolysis). Ferric coagulants, with or without pre-hydrolysis, demonstrated intermediate headless rates. Copyright (C) 1996 IAWQ.