Abstract:
Drinking water processes that utilise surface water require the removal of natural
organic matter (NOM), primarily to reduce the formation of harmful disinfection
by-products. This is particularly important given that NOM concentrations in
source waters have risen over the past decades in many parts of the world. An
effective technology to remove NOM is ion exchange (IEX), which has reportedly
offered high final water quality (low DOC, down to 0.25 mg L⁻¹) when used prior
to coagulation. However, the performance of IEX varies depending on the water
source characteristics and this is currently difficult to estimate. To better
understand the relationship between physicochemical properties of NOM and
their removal, the individual and combined characteristics charge, hydrophobicity
and molecular weight were explored at pilot and bench scale. It was shown that
charge plays a major role in the removal of NOM. Model compounds were
increasingly removed with increasing charge density. While this emphasised the
importance of charge, the results also showed that hydrophobicity became more
important at low charge densities. Molecular weight distribution analysis revealed
that medium sized organic compounds were dominant in the investigated waters
and were well removed. However, high concentrations of this fraction were seen
to limit removal. Size exclusion of high molecular weight compounds resulted in
low removal of this NOM fraction. The most treatable water by IEX was identified
to contain a high charge, a low concentration of high molecular weight
compounds and a large proportion of hydrophobic organic matter. For these
water types removal of 90% can be expected. Resin properties influenced the
removal efficiency, not only by their state (virgin or used) but also by their
properties. The use of three different resins showed that larger pore sizes
improved the removal of NOM. Resin made of polystyrene was seen to provide
higher removal of aromatic compounds due to the contribution of non-
electrostatic interactions. The combination of IEX and coagulation showed
consistently high treated water quality for varying source water through selective
removal of NOM by IEX, which facilitated an improvement in the coagulation
process, resulting in higher dissolved organic carbon and reactive disinfection by-
product precursor removal.
