The streaming current monitor (SCM) uses an electric sensor to determine when charge neutralization has been reached in a suspension. The theory of operation is similar to that of the zeta-meter in that the charge-measuring device is based on the zeta potential. This device measures the net ionic and surface charges of colloids in suspension between two electrodes. A piston moves the water back and forth in the chamber and positive and negative charges are moved downstream to the electrodes, producing a streaming current. The streaming current amplitude and polarity is a function of the sampling location and the type of coagulant used.
If the correct coagulant dose is used (the correct amount of Fe+3 or Al+3 or other coagulant with positive valence), to neutralize the number of negative ions in the water, the SCM will read"0.0" indicating that charge neutralization has occurred. The SCM is not typical of most in-line instruments in that it is not "plug and play" and must be calibrated to a baseline. This baseline will vary from one raw water to another and from one treatment facility to another. To establish a baseline, tests should be conducted to establish a range of SCM readings when the plant is operating at its optimal coagulant dosage. Test results should include the effects of seasonal water quality changes. It is preferable to perform jar tests to acquire optimal coagulant dosage and record the SCM reading immediately after coagulation.
Through testing of the SCM in variable qualities of water, it appears that pH and alkalinity have the greatest effect on the instrument's usefulness. During high pH coagulation, the SCMs response time slows dramatically and at pH values above 8, the unit may be unresponsive to dosage changes. However, the intent of the SCM was to monitor water when attempting to coagulate by charge neutralization. Generally, at pH values of 8 or higher, sweep floc method is used, as it is nearly impossible to coagulate with charge neutralization. Although pH and alkalinity slow response time and require more frequent cleaning and de-calcification of the electrodes, the unit may still prove useful.
Normally, in cold, clear water, assuming the SCM has proven valuable, the operator will attempt to keep the SCM reading slightly below zero and as the temperature and organic particulates increase, raise to the reading through adjustment of coagulant feed to a point slightly higher than zero. Of course, these numbers will vary among different facilities. Periodically re-confirming the baseline will help validate the reliability of the instrument.