Abstract:
A long term, large scale pilot study was performed to assess the use of a novel
process based on suspended ion exchange (SIX®) and in-line coagulation
(ILCA®) pretreatment for ceramic membrane filtration (CeraMac®), for treating
three variable quality UK surface waters.
SIX was shown to remove similar quantities of dissolved organic carbon (DOC)
to coagulation for low to moderate DOC source waters. However, during
periods of high DOC concentrations and high specific UV absorbance, the
removal of organic compounds was reduced. The long term DOC removal data
for the SIX process indicated good performance, which was in line with
previously reported results from studies using other suspended ion exchange
processes.
Organic characterisation using liquid chromatography–organic carbon detection
(LC-OCD) revealed the differing selectivities of the SIX and ILCA processes, for
low and high molecular weight organic fractions respectively. When these
processes were used in combination, a broad range of organic compounds
were removed, leading to a 50% reduction in DOC concentration in comparison
with an existing full scale conventional treatment process. Subsequently,
disinfection by-product (DBP) formation was significantly reduced (62% vs. the
conventional process) due to the lower DOC concentration, reduced specific
reactivity of the residual organic compounds and reduced formation of
brominated DBPs.
Removal of high molecular weight organic compounds (biopolymers) was
shown to be critical for stable operation of ceramic membranes at high flux.
Optimised in-line coagulation (ILCA) pretreatment (which flocculated the
biopolymers) led to negligible membrane adsorption of organic compounds, as
low molecular weight (LMW) fractions (which are recalcitrant to coagulation)
were shown not to be retained by the membrane. Due to this, when using
optimised ILCA, additional removal of LMW organic fractions by using SIX in combination with ILCA provided no measureable benefit with regards to
membrane fouling suppression.
Automation of coagulant dosing was achieved for the high SUVA waters tested,
using simple feed forward control based on the UV transmittance of the feed
water. The application of this automated system led to very low membrane
fouling rates (0.24kPa/day), despite highly challenging operating conditions of
elevated fluxes (185 L m-
2
h-
1
) and highly variable feed water dissolved organic
carbon concentrations (1-10mg/l).
