Abstract:
Increased regulatory pressure to meet THM standards with the possible introduction
of HAA regulations means that organic matter, the predominant DBP precursor needs
to be removed effectively from site whilst coping with increased economic and
environmental drivers for change.
Organics are typically removed via coagulation / flocculation which can be optimised
by operating in the correct zeta potential range (-10mV and +8mV) and the correct
DOC:FE coagulant dose ratio (1:1). However, due to spatial and temporal changes in
bulk NOM and its fractional character there are no standard operating conditions,
consequently sites have to respond to changing conditions.
Seasonal trends in DOC and THMFP levels were measured under optimised and
current operational conditions. Results indicated that UV254 could be used to predict
coagulant dose and zeta potential could be used to indicate optimum coagulation
conditions for DBP precursor removal across 16 sites. Under optimised condition
improved organics removal and floc properties was observed across all source waters
however the degree to which this occurred depended on the source water.
Understanding of the fractional make up is thus important to assess the potential for
optimisation of the coagulation process. Providing the electrostatic forces are
minimised, increasing the NOM content of the floc matrix will lead to an increase in
floc strength. One of the implication of optimised coagulation conditions were that
flocs could be created at lower retention time then quoted by best practice guidelines.
As such a 66% decrease in flocculation energy could be achieved without any
detrimental impact to floc quality or removal efficiency.
One of the important findings is that due to the inherent difference in source waters,
optimisation for a site can vary in terms of cost and clarified water quality, hence each
site needs to be evaluated before schemes are carried out.
