Selection and Dosage Determination

Chemical selection depends on many factors. These factors include the treatment objective, the quality and variability of the untreated water, chemical availability, delivery time, suitable storage, feed equipment and accuracy, safety considerations, cost, and employee training. The treatment objective is perhaps the most important factor in chemical selection. A treatment objective might include reduction of lead levels in customers homes with a corrosion inhibitor, elimination of taste and odor compounds with ozone, or maintenance of a chlorine residual at the farthest end of the system. 

Untreated water variability may favor one coagulant over another. 

Spring snowmelt may impact alkalinity in cold weather regions, and rain events may cause turbidity spikes in river supplies. Limited chemical availability may make some chemicals too expensive to consider. Delivery time may be critical to utilities with limited storage. Storage tank material and piping may not be compatible with some chemicals. For example, stainless steel fittings are not recommended for use with sodium hypochlorite. 

Feed equipment may not be compatible with some chemicals. Chemical pump size may also dictate chemical selection. For example, corrosion inhibitors are often purchased in varying concentrations. Ideally, the selected chemical concentration will allow the pump to operate in its most efficient and accurate range. Safety considerations are often significant. For example, if proper storage for chlorine gas use is not available, sodium hypochlorite use might be considered. Cost is always a factor in chemical selection. Chemical selection may depend on the complexity of employee training or ease of use. 

Dosage determination is based on treatment objective. For example, one objective for chlorine dose may be to achieve a target residual at the end of the distribution system. The objective for hydro-fiuosilicic acid addition may be to achieve a required concentration. The objective for coagulant dose may be to achieve some minimum turbidity goal. 

---