Phosphate Treatment FAQs – Part 2

How are phosphate dosages controlled?

Since phosphates do not change basic water chemistry, measuring phosphate residuals in the raw and finished water is necessary to monitor the dosage rate. Orthophosphate ion (PO4) is the most common species used to measure the initial and total quantity of phosphate in the water. Orthophosphate can be measured on a cold-water sample, while the total phosphate requires a digestion step to break down all other forms of phosphate to the ortho form. Simple field or lab test kits can be used to monitor all forms. Subtracting the initial orthophosphate quantity from the total phosphate yields the quantity of polyphosphate present in the finished water (Total-Ortho=Poly). Phosphorus, reactive (Hach method 8048), also called orthophosphate Phos Ver 3 (Ascorbic Acid) method is used widely to determine orthophosphate residual in water. Phosphorus, Acid Hydrolyzable (Hach method 8180) is used to determine total phosphate concentration in water.

What happens if phosphate is overfed?

An overdose of phosphate is difficult to detect immediately unless orthophosphate is being monitored in the finished water. Too much orthophosphate typically will not result in a water quality problem unless calcium hardness reacting with the phosphate begins to form a slight turbidity at higher water pH. Excessive polyphosphate dosage may result in an accelerated cleaning of scale and tuberculation from the pipe surface, resulting in colored water, turbidity or suspended solids.

Is phosphate expensive to use?

Compared to other technologies of corrosion inhibition or iron/manganese removal, application of orthophosphate, polyphosphate, or blended phosphates is considered at least comparable in cost. Phosphates may range in cost from $0.30/lb for generic orthophosphates to more than $4.00/lb for specialty phosphate blends. At a dosage rate of 1-5 mg/L, the chemical cost may range from less than 1 cent to more than 10 cents per thousand gallons of water treated, depending on the treatment criteria and water quality. In many cases, the cost of the phosphate is partially offset by the direct O&M savings or benefits to the utility and consumer. Not all products are equivalent, yet may appear chemically similar on paper. Each type of phosphate chemical offers specific advantages (dry or liquid form, specialty or generic ingredients, quality, grade, etc.) that contribute to the overall cost of the product.

Working with Spon Water Consulting takes the guesswork out of your phosphate treatment program. We evaluate your water quality, treatment process, chemicals used / ingredients, identify your options, generate a cost analysis, and many times during the process, save you significant revenue.

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