Coagulation and Flocculation - Public Water Systems - page 291

coagulation range from 5to 10 millivolts, depending upon the nature of the ma-
terial to be removed.
Although zeta potential measurements are useful as a research tool, it is generally
agreed that this method is not easily adapted to the typical treatment plant because of
the considerable degree of skill and patience required to make the measurements and
the amount of interpretation required to make the data useful. It has been the authors'
experience in visiting plants that have purchased zeta meter equipment for routine
operation that generally it will be found stored and inoperative. Other observers also
have reported a recent decline in the popularity of zeta potential as a control tech-
nique. 64 It is subject to the same shortcomings as any other batch test in that sudden
changes in raw-water conditions may not be detected until a poor-quality finished water
is produced in the full-scale plant.

Streaming Current Detector A streaming current detector may be used to provide
a continuous measure of the relative charges of coagulated particles. This technique
involves placing a sample in a special cylinder containing electronic sensing electrodes
at the top and the bottom. A loose-fitting piston is then partly submerged into the
sample and is reciprocated along its axis to produce an alternating current between
the electrodes when the cylinder contains moving charges. A synchronous motor drives
the piston and synchronous rectifier switch, by means of which the alternating cur-
rent generated by the alternating fluid motion is made to register on a dc meter (Fig.
10-15). An amplifier with adjustable negative feedback is used to provide an output
proportional to the current collected by the electrodes. Readout may be by a microam-
meter, with calibration in arbitrary units. The alternating current is analyzed and related

Synchronous Motor

Loose Fitting Piston
of Insulating Plastic

Sample Reservoir

Silver
Electrodes

AMPL RFB

Readout

Cutaway of Cell Body and Pump
Bore of Insulating Plastic

Synchronous Rectifier

Fig. 10-15. Simplified diagram of a streaming current detector ( Source: Reference 71; used with

permission)