Abstract:
The
removal of natural organic matter
(NOM)
at water treatment works
(WTW) is
essential in
order to prevent toxic compounds forming during
subsequent disinfection.
Coagulation
and
flocculation
processes remain the most common way of removing
NOM. The
properties of the resulting
flocs that form
are
fundamental to the efficient
removal of organic material.
Periods
of elevated NOM loads
at
WTW
can
lead to
operational problems as a result of the deterioration in floc
structural quality.
Assessment of
floc
physical characteristics can therefore be
a crucial tool in
order to
determine and predict solid-liquid removal performance at
WTW. Here the growth,
size,
breakage,
strength, re-growth,
fractal dimension
and settling velocity were
measured for flocs formed from
a
NOM
rich water source. NOM floc
structural
characteristics were measured and evaluated over a one year period
in
order to
monitor the seasonal variation
in floc
structure. The
results showed that a significant
improvement in floc
size and strength was seen during
autumn and summer months. It
was subsequently shown that as the organic
fraction in the floc increases the floc
size,
settling velocity and
fractal dimension
all
decrease. A
model was proposed showing
how these changes were dependent upon the adsorption of
NOM
onto primary particle
surfaces. A
range of
different
chemical coagulant treatment options were applied
for
NOM removal and the resulting
floc
structure compared.
Considering both floc
structure and optimum
NOM
removal the treatment systems were of the following
order (best to worst):
MIEX® + Fe > Fe > Fe + polymer > Al > polyDADMAC.
NOM
floc
re-growth was shown to be limited for
all the treatment systems investigated. The
practical
implications
of the results were:
(1) The requirement for
careful coagulant dosing or order to achieve optimum floc
characteristics.
(2) The
use of a pre-treatment anionic
ion-exchange
stage prior
to coagulation.
(3) A comparison of alum and
ferric based
coagulants suggested the ferric
coagulants gave better floc
structure and improved NOM
removal rates.
