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Before you purchase a chemical feed pump, you should know essentially how many gallons per day it will need to supply. To determine this, there are few things you need to know.
1. The Output Rate of your well pump.
2. The Dosage Requirement for the chemical(s) you are going to feed into the water.
3. The Strength of the Solution you are feeding.
To determine the output rate of the well, use this simple system:
Be sure all water is turned off and stays off during your test. Select a tap in a location where you can observe the operation of the well pump, open the tap, and let the water run until the well pump comes on. Immediately close the tap and time how long the well pump runs until it shuts off. When the pump is off, use a bottle or bucket to measure the amount of water that can be let out of the faucet before the pump turns on again. Divide the amount of water, in gallons, by the time, in minutes, that it took to refill the tank. The result is the well's output in gallons per minute.
Example: It took the tank 2.5 minutes to refill and you counted 22 gallons drawn off before the pump restarted. Divide the gallons by the minutes and you find that you well's output capacity is 8.8 gallons per minute.
To determine the dosage requirement, you calculate what is needed to achieve your treatment goal.
As an example, we'll say that your water has 2 parts per million iron and two parts per million hydrogen sulfide. It takes one part per million chlorine to treat each part per million of iron and 3 parts per million chlorine to treat each part per million of hydrogen sulfide. Plus, you should leave a one part per million residual. Therefore, your dosage requirement would be 9 parts per million.
(Note: We're using 1 ppm chlorine requirement to treat iron as a convenience. Standard charts can vary from 0.6 to 1.0 parts per million in their recommendation. See, for example, this chart on our site. Sizing and setting pumps is a trial and error process. If your estimate is a little off, you'll be able to correct it.)
Go here for requirements for feeding polyphosphate to sequester iron and hardness.
To determine the Solution Strength, you need to consider the strength in parts per million of the original substance and how much—if any—you are going to dilute it.
In our example, we'll use household laundry bleach as the source of chlorine. Straight household bleach is 5.25% chlorine. This means that bleach contains 52,500 parts per million chlorine. If you dilute a gallon of bleach with a gallon of water, the solution strength will be 26,250 ppm chlorine. To solve our example problem, we'll assume you're using full strength bleach.
To select a pump, try to choose one that will produce the needed gallons per day output at approximately the medium setting on the pump.
Please check highlighted fields. Enter your well output rate in gallons per minute, required dosage in parts per million, and solution strength in parts per million.
Enter your well output rate in gallons per minute, required dosage in parts per million, and solution strength in parts
per million, then hit "calculate."
Here's the formula:
Well pump output rate in gallons per minute, multiplied by
Required dosage in parts per million, multiplied by
1440—the number of minutes in a day—divided by
Solution Strength in parts per million, which equals
Needed Metering Pump Output in gallons per day (GPD).
So, which pump do you buy? Here's the rule again:
To select a pump, try to choose one that will produce the needed gallon per day output at approximately the medium setting on the pump.
In our example, this would mean that ideally you would want a 4 or 5 gallon per day pump. But, we don't sell a pump in this size (and you certainly want to buy a pump from us—right?). So, what we would suggest is buy the 10 gallon per day pump and dilute the feed solution with a gallon of water. This will give you a solution demand of about 4 or 4.5 gallons per day. This will allow you to set the pump's one-to-ten pump rate at 4 or 5. It allows room to adjust in either way to increase or decrease the chlorine dosage.
Another tip to keep in mind when sizing a chorine pump. This is from a Water Technology technical article: “The stronger the chlorine solution, the faster it will lose its strength. A 2-parts-water-to-1-part-chlorine solution will lose strength much faster than will a 5-parts-water-to-1-part-chlorine solution.” Therefore, you get a more predictable result if you'll buy a larger output pump and mix your solution thinner.
If you need help in selecting a pump size (and most customers do), don't hesitate to call or email.
(whole house & well units)