Avoid pitfalls by determining contaminants first.
by: Bob Thomas
For iron removal, contact time between the water to be treated and chlorine needs to be 20 minutes. For bacteria removal, the US Environmental Protection Agency (EPA) recommends 30 minutes.
To facilitate contact time with chemical feed pumps in a chlorination system, mixing tanks or retention tanks typically are used after the pressure tank.
The major questions to ask when sizing chemical feed pumps:
• How much chlorine should be used?
• Are you primarily removing volatile organics (bacteria, viruses, protozoa, worms or fungi) or minerals such as iron or hydrogen sulfide?
• What is the water flow rate in gallons per minute that will be treated?
• What is the customer’s budget?
• How much maintenance time will the customer be willing to commit to maintain the system
(none or a little) or would the customer be willing to pay someone do it?
Sizing chlorinator systems
Certain contaminants that can be treated by the use of chlorine injection. The chart shows the solution strength and the calculation factor that is used in formulas. Formula A is used to calculate the solution strength of the chemical to be injected into the water. The chemical feed pump can be sized after the solution strength has been calculated using Formula B.
Sizing the chemical feed pump requires that the solution strength be calculated first. There are four things required to calculate the solution strength of the chemical to be injected into the water supply.
1. The contaminant concentration. This can be found from a water analysis and must be expressed in parts per million (ppm) or grains per gallon (gpg) as indicated in the second column of the chemical treatment chart.
2. The treated water usage between refills of the chemical reservoir must be known. If the reservoir is to be filled every day, then the gallons of treated water required per day must be known. If the reservoir is to be filled once a week, then the weekly usage must be known.
3. The calculation factor from the chemical treatment chart is required. Find the contaminant in the first column, then choose the treatment chemical in the third column and follow the row across to the last column which is the Calculation Factor.
4. Choose the reservoir capacity in gallons of the chemical tank to be used for mixing the treatment chemical. The size of the tank in gallons is required.
Enter the values of the four items into “Formula A” to obtain the solution strength that must be present in the chemical reservoir for the chemical feed pump. Do not exceed the maximum solution strength shown in the chemical treatment chart.
Solution strength = (Contaminant concentration x water usage between refills )/(calculation factor x reservoir capacity in gallons)
Sizing the chemical feed pump
Chemical feed pumps are usually specified by the maximum output of the pump in the form of gallons per day. Use “FORMULA B” to obtain the feed pump setting in gallons per day, this will be used to size the pump.
The treated water flow rate in gallons per minute (gpm) must be used in the equation. This is the well pump capacity that can be calculated. The contaminant concentration, solution strength and calculation factor from “Formula A” must also be used in “Formula B.”
(Flow rate in gpm x contaminant concentration x 1440)/(solution strength X calculation factor) = feed pump setting
The feed pump can now be sized. Chemical feed pumps have a range in gallons per day that they can be set at. The proper size chemical feed pump will have the setting that was just calculated in the middle of its range. The solution strength should be modified if needed to allow the chemical pump to operate in its mid-range.
Calculating chemical concentrations in ppm
The chemical concentration in ppm may also be used to size the chemical feed pump. If the perecentage of the concentration of the chemical is known or the ounces per gallon of concentration is known, then the two conversion factors below can be used to convert to ppm.
1. percent of solution x 10,000 = ppm
2. ounces / gallon x 7,812.5 = ppm
The chemical feed pump can then be sized by filling in Formula C below.
Formula C will give the required pumping rate in gallons per day just as Formula B did. It should be noted that the chlorine residual or the polyphosphate residual must be added to the contaminant concentration level when calculating the feed pump setting.
(Flow rate in gpm x ppm of contaminant x 1440)/(solution strength in ppm) = feed pump setting. Feed pump setting is given as gallons per day.
Bob Thomas is owner of Good Water Co., ADA, OK.