Changes of NOM Fractions during Optimization of Coagulation Process on the Drinking Water Treatment Process in Riga, Latvia

The water treatment plant in Riga (Latvia) employs conventional chemical coagulation and precipitation process for removal of turbidity from humic rich waters. Removal of NOM during coagulation is relatively effective, from 16.93±5.56 to 4.46±1.11 mg-C/l, however the final concentration of NOM in drinking water is still too high to ensure low level of disinfection by-products. The possibility to optimize NOM removal by adjusting pH and alum dose was investigated in Jar test experiments. To understand which fractions are removed in the process the changes of hydrophobic and hydrophilic fractions of NOM were analyzed. Very hydrophobic fractions (VHA) were removed by 60% at ambient pH whereas lowering of pH to 6 increased removal efficacy of NOM only by 10%. Adjustments of pH in the range of 5 to 7 and alum dose in the range of 5 to 20 mg Al/l did not significantly enhance further removal of either hydrophobic fractions or hydrophilic fractions of NOM. The study showed that enhanced coagulation is not necessarily an effective means of the removal of NOM from water containing high levels of humic substances