The Zetasizer Nano particle characterization system from Malvern
Instruments has become an essential tool at a water treatment production
facility in the US. Here it is being used to monitor the zeta potential of raw
water in order to optimise the concentration of alum added as a flocculating
agent. Zeta potential values are key to monitoring and maintaining optimum conditions
in the plant, and measurements using the Zetasizer Nano have proved to be both
easy to make and operator independent.
Impurities in waste water are primarily anionic, and cationic additives have
been developed for their neutralisation. By optimising the addition of additive,
the amount of additive used and hence the cost of treatment, can be reduced.
The appearance of turbid water is due to the presence of suspended particles.
By controlling the effective charge of these contaminating particles the efficiency
of removal can be improved. The effective charge can be quantified through the
measurement of zeta potential. At close-to-neutral charge, particles flocculate
and sediment leaving the water clear. Zeta potential measurements provide a
tool for quantifying the optimum concentration of additive required.
Studies of additive concentration versus zeta potential and turbidity have
shown that flocculant addition beyond a certain point is counter productive.
For example in one system, when the zeta potential of the water mixture is close
to zero, so too is turbidity. At low doses of flocculating agent the zeta potential
is approximately -16mV and turbidity is high. At +2mV turbidity it is at its
minimum and beyond this point the further addition of flocculating agent reverses
and increases the charge of the contaminating material, restabilising it in
the water, increasing the turbidity.
The goal of the US facility is to maintain a zeta potential of around zero.
Only by measuring and monitoring the zeta potential on a control chart can operators
make rational and optimal adjustments in the amount of alum added.
The Zetasizer Nano series combines dynamic, static and electrophoretic light
scattering, enabling the measurement of particle size and zeta potential in
a single easy to use system. A unique cell for zeta potential measurement consists
of an electrode and folded capillary moulded into a single chamber, that can
be disposed of after a number of measurements instead of cleaning. Importantly
for many water treatment applications, the addition of the MPT-2 Autotitrator
allows zeta potential to be studied as a function of additive. www.malvern.com