Treatment of Coloured Surface Water by Coagulation/Direct Filtration: Effect of Water Quality, Type of Coagulant and Filter Aids

Abstract

Jar test data from own investigations and from the literature were evaluated in order to determine the effect of initial humus concentration, dosage and pH on humus coagulation by aluminium sulphate (alum). An empirical correlation was derived that may be applied, for example, for dosage control when raw water concentrations vary. Here the correlation is used to estimate coagulant demands for different types of humus as a function of initial humus concentration. The results are compared with such characteristic humus parameters as apparent molecular weight, colloidal charge and specific colour values.

Coagulation/direct filtration was investigated in a pilot-plant with one type of natural humus and two different coagulants, alum and polyaluminium chloride (PAC), respectively. Two types of gravity filters were used, single-medium sand and dual media hydroanthracite on top of sand. Total filter depth was 0.8 m. Head loss distribution within the filter bed was monitored using pressure transducers located at 10 cm intervals along the filter column. Effluent quality was continuously monitored in terms of turbidity and UV-absorbance (A254). Anionic, cationic as well as nonionic polymers were tested as filter aids. The dosing point was shifted from the mixing tank to the flocculation unit and finally to the filter column.

Type of primary coagulant, coagulant dose, polymers and pH were evaluated in terms of effluent quality, filter ripening, head loss and net water production. For process design purposes, an empirical model was developed in order to predict the length of a filter run and the net water production under different operational conditions.