Civil Engineering Reference
In-Depth Information
to zeta potential, which can be used to control coagulant dosage. A flow-through
cylinder and recording ammeter may be built into the system to permit continuous
monitoring. Almost all of the successful applications of the instrument have involved
the titration of charge-influencing materials, either batchwise or continuously. In these
applications, uncertainty as to what is measured is of little consequence as long as the
addition of anionic and cationic materials changes the reading in a reproducible man-
ner. Titrations are carried out in the same fashion as acid-base titrations with a pH
meter. Continuous readings are obtained while one material is added to the other.
The limitations of using streaming current for continuous coagulation control are
similar to that of zeta potential approach. For example, since this approach uses par-
ticles' electrical charge as the only parameter to evaluate coagulation performance, it
may not be suitable when anionic polymers are used as coagulation aid to provide
bridging effect. The readout of streaming current for such systems may indicate that
the coagulation is poor (because particles are even more negatively charged compared
to that in original raw water), whereas the actual performance may be satisfactory due
to bridging effect. As with zeta potential, the data must be correlated with the usual
indices of plan performance, as there may not be any consistent relationship between
charge and filtered water clarity even at a given plant for various seasons of the year.
However, the continuous nature of the streaming current detector may make it attractive
in some instances where cationic coagulants are used in a water that shows little
variation from season to season.
Colloid Titration
A simple titration technique using an indicator that changes colors
when a solution is titrated to electric neutrality has been developed.
65,66
An excess
amount of positively charged polymer is added to the naturally negatively charged
water. It is then back-titrated with a standard negatively charged polymer. An empirical
correlation is used to relate the volume of titrant to the proper coagulant dosage, in a
manner analogous to zeta potential measurement.
Like the streaming current detector, colloid titration has the advantage of using a
larger, more representative sample than that used in zeta potential measurements. Since
its conclusions are based on the electrokinetic properties of a suspension, its results
are subject to the same limitations given for the streaming current detector and the
zeta potential devices.
Continuous Filtration Techniques
Pilot Filters
The goal of the water treatment plant should be to produce the minimum
possible filter effluent turbidity at the minimum chemical cost. The best measure of
the efficiency of the coagulation-filtration steps would be the direct continuous mea-
surement of the turbidity of coagulated water that has passed directly through a pilot
granular filter. The application of continuous turbidity monitoring equipment to the
effluent of the plant-scale filters should be a must for monitoring plant performance.
However, a real-time turbidity meter also has limited value as a control technique
because of the substantial lag time between the point of chemical coagulant addition
and the point of filtrate turbidity monitoring. For example, improper coagulation of
the incoming raw water may not be immediately apparent at the discharge of the plant
filters until the water has flow through the clarifier, which usually has a hydraulic
detention time of 1
1
⁄
2
to 3 hours.
The pilot filter technique is applied by sampling plant-treated coagulated water from
the discharge of the plant rapid mix basin before it enters to flocculation basin and
