Jar tests were applied to a secondary effluent in order to determine optimal conditions for coagulation. The metal salts Al2(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 pre-hydrolysis, the lower the removal of phosphorous. No remarkable differences were observed for the removal of organic matter measured as COD, 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 headloss rate was observed when alum was used, while the highest headloss 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 headloss rates.

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The effect of five operating variables on the removal of fulvic acid from water by means of aluminum and polymers was studied. These variables were the pH, the calcium concentration in the water, and the polymer dosage, molar mass and degree of hydrolysis. The study followed a 25−1 resolution V fractional factorial experimental design. Based on a statistical analysis, the most significant effects were the pH, the calcium concentration, the polymer molar mass and the interaction of pH with calcium concentration. A calcium concentration of 40 mg/L, a pH of 7 and a polymer with a molar mass of 5 to 6 million were found to be most effective based on fulvic acid removal by filtration. The effect of calcium was larger at pH 8 than at pH 7. The type of polymer did not have any significant effect.