Ion exchange in combination with coagulation for natural organic matter removal in surface water treatment

Natural organic matter (NOM) removal is essential in drinking water treatment, since the presence of NOM results in a yellow water color, has a detrimental impact on the biological stability of drinking water, is a precursor for disinfection by-product formation, and it has a negative impact on most downstream treatment processes. For the latter reason, NOM removal preferably takes place as early as possible in the water treatment train, and it is in this respect that De Watergroep investigated the combination of ion exchange (IEX) followed by coagulation for NOM removal of surface water at WTW De Blankaart in Belgium. This study falls within the Flemish-Dutch water research program (VNWKO), part of the Joint Research Program (Bedrijfstak Onderzoek,BTO).
This report summarizes a study performed in 2016 – 2019 on innovative technologies for removal of natural organic matter from surface water. The investigated technologies are ion exchange and enhanced coagulation/flotation, and pilot research was conducted on a 50 – 58 m³/h semi-industrial pilot plant consisting of a high rate fluidized bed IEX system with on-site spent brine treatment, followed by a coagulation/flotation unit. On- site IEX coagulation/flocculation treatment was compared to standalone coagulation of the full-scale installation at WTW De Blankaart.
The high rate fluidized bed IEX system, the subsequent coagulation/flotation system and on-site treatment system for the spent IEX brine were all judged to be reliable. Only during algal bloom events a decrease in NOM removal by the IEX occurred which was compensated for by the coagulation/flotation system. At low temperatures below 3°C, precipitation of Na2SO4 occurred, blocking pipes of the spent brine treatment system which could be mitigated by heating.
At WTW De Blankaart, the NOM removal obtained by the IEX – coagulation/flotation treatment scenario was low compared to standalone coagulation. This is due to (i) the optimal coagulation conditions applied in the standalone coagulation process, resulting in very high NOM removal, and (ii) the high sulphate content of the raw reservoir water. Furthermore, 42% less coagulant was used in the IEX – coagulation/flotation treatment compared to standalone coagulation which resulted also in 35% less sludge production. In IEX – coagulation/flotation, coagulation is performed at a higher pH resulting in a lower SO42- content of the treated water compared to the standalone coagulation process.