Comparison of the effiency of aluminium and ferric sulphate in the removal of natural organic matter during drinking water treatment process

The removal of natural organic matter by coagulation in the drinking water treatment train was studied for a period of two years' In the middle of the study, the coagulation/flotation process was modified by replacing the aluminium sulphate by ferric sulphate. At the same time, the filtration unit was enhanced by adding a sand filter unit before the activated carbon filtration and by changing new carbons on to the activated carbon filters. A special aim was to compare the efficiency of the aluminium and ferric coagulants in the organic matter removal by several methods, including high-performance size-exclusion chromatography. A comparison of quantity and characteristics of organic matter in treated water before and after treatment process modification provided an insight into process performance. Approximately 95% of high molar mass organic substances were removed in the process with both coagulation agents. The greatest difference between the coagulants occurred in the removal of organic matter having molar masses of 1000-4000 g mol(-1). These intermediate molar mass organic compounds were removed 25 % more efficiently with iron-based coagulant than with aluminium-based. Low molar mass material was poorly removed regardless of the coagulant. On average, only 10 % of this fraction was removed. According to different measuring techniques used in the study, the ferric sulphate coagulation was 10 % more efficient in the overall organic matter removal compared to the aluminium sulphate coagulation. Turbidity removal was more efficient with aluminium than with ferric sulphate. Turbidity even increased during winter in ferric sulphate coagulation. However, turbidity was effectively removed in filtrations.