MICROTSCM (07/09)
Page 4
REV 2.4
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Additive Chemicals
Contaminant Particles
1. Charge Neutralization 2. Bridging
Figure 1: Effects of Chemicals
3.1 Treatment of Water for Clarification
Most water treatment chemicals consist of a cationic (positively charged) chemical e.g.
aluminum salts, ferric salts, polyamines or cationic polyacrylamides, some of which have
the cationic part tagged on to a long polymer chain. As stated earlier, raw water entering
the WTP is an energetically stable system of suspended particles with a net negative
charge. Cationic chemicals are added to bring the charge to neutral.
Before the development of Streaming Current technology, the best way to determine the
optimum dosage has been the jar test method. The jar test involves taking a representative
sample of the water being treated and placing it in several jars. Different amounts of
clarifying chemicals are added to each jar; stirred and comparing the clarity of the water in
the different jars. Jar tests are time consuming and it is difficult to reproduce the
conditions of the WTP in a jar. The tests can take several hours rendering them useless
when plant personnel are really responding to rapid changes in water quality. A typical
curve of Turbidity vs. Chemical dosage is shown in Figure 3.
Some considerations when treating the water are the rate of floc formation, the size of the
floc formed, how fast the floc settles, and the clarity of the final settled and filtered water.
Other techniques exist, such as a dosing curve, which indicates a recommended dosage for
a given water turbidity. This is generally built up over years of dosing experience with the
water, but has the disadvantage that turbidity caused by extremely small particles requires
a higher dosage than that caused by larger particles, and therefore can only be adapted for
use with known type turbidity on any given water.
