whidbeywaterfilters

Aqua Alternatives

Flow Rate

Flow Rate

Determining Flow Rate
Before designing a filtration system, it is important to know the flow rate of the water in gallons per minute. Different media require different flow rates in order to backwash the contaminants from the media; some are thicker and heavier than others and may require more force to blow out the dirt. A quick and simple way to determine the flow rate is to time how many five-gallon buckets can be filled in one minute.

Have two five-gallon buckets handy. For accuracy, make marks on the inside of one of the buckets to designate units in gallons, so that it is easy to see how many gallons the bucket is holding. Turn on the water full blast before placing the bucket in position. Put the bucket under the water when the timer starts and check the water level every ten seconds. When the first bucket is full, move the second marked bucket in place. If the second bucket does not completely fill, note the number of gallons at one minute. Optionally, watch how much water fills one bucket in thirty seconds.

For example, if the rate is 10 gpm, then it will take one minute to fill two buckets or 30 seconds to fill one bucket. Try the test again at a different water source. Another example for 12 gpm would be filling 3 gallons in 15 seconds, or two five-gallon buckets plus two more gallons in one minute. If in thirty seconds the five-gallon bucket is not totally full, like down a gallon, then that would be four gallons in thirty seconds, or 8 gallons per minute. Most water systems require around 10 gpm. The ECOsmarte system functions best at 10-12 gpm.

Flow Rate Requirements
  • A chlorine system requires at least 6.5-9 gpm.
  • An ozone system requires 8 gpm minimum, and preferably 10 gpm.
  • An ECOsmarte system requires 10-12 gpm.
If the Flow Is Too Low

ISSUE

If the flow rate is too low, there will not be enough water pressure to clean out the media during backwash. Gradually the contaminants will build up inside the tank and the water will increasingly foul. When the media in the tank is fouled, the water will smell, it will be colored, and it can stain fixtures.

In contrast, with sufficient flow during back-wash, the media will be cleaned out regularly and the water will remain fresh.

To view visually what is happening, turn off the lights in the room and shine a flashlight from behind the tank. Make a mark on the tank at the top level of the media. Now put the timer into backwash and watch the level of media rise. Ideally, it should rise up at least six inches above the mark made when the media was at rest. 

If the media rises only two or three inches, the tank will foul much more quickly than normal, and the media will have to be changed much more frequently, like every year or two instead of every 3-10 years, depending on the type of media and the quality of the water being filtered.

SOLUTION

If the desired system is an ECOsmarte but the flow rate is too low, there are several options:

  • Add a 300 gallon (or larger) holding tank and a booster pump designed for 10-12 gpm. This is the best option.
    (Average prices: 300-gallon tank, around $1000; booster pump between $1000-$2000; installation, around $400-$600)
    By adding a holding tank with a float switch wired to the well pump, and a booster pump set at 10-12 gpm, you can be assured not only that your water will be cleaned properly, but also in case the power goes out you will have a back-up supply of water.  
    I recommend Dab Pump 727956.
  • If a holding tank and booster pump is too much expense, then select a different system, like an ozone or chlorine system.
  • Another option for a sub-standard flow rate would be to use a lighter media in the filter tank that requires lower gallons per minute. The trade-off is that the lighter the media, the less well the iron and minerals will be removed. In contrast, a thick, heavy media with a high flow rate will produce a strong thorough backwash; however, the heavier the media, the more flow is required.
  • If you happen to be on a chlorinated community water system, the chlorinated water will reduce the amount of minerals to be removed, and you might be able to get by with 8 gpm instead of 10 gpm. Nevertheless, it is wisest to achieve the optimal flow rate of 10-12 gpm.
  • If you choose to do nothing and live with an insufficient flow rate, the media will clog quickly and may need to be changed every one to three years. With very bad water, it may foul within six months. 
If the Flow Is Too High

ISSUE:

If the flow is too high, the filter cannot function. If the water is too strong, it will blast a channel directly through the media and will not be filtered at all. The water must travel slowly enough to have time to be absorbed and filtered by the media. 

SOLUTION:

The solution is to install a pressure reducing valve before the water enters the filter system.

For example, if the water flows at 15 gpm or 25 gpm out of the well, the reducing valve must be adjusted down to 10-12 gpm to allow the filter to operate properly. If high pressure is desired for irrigation, install a tee before the flow reduction valve to route raw water to the yard at higher pressure.

Determining GPM Recovery Rate at the Pressure Tank

Tools needed:

(a) Accurate psi gauge
(b) 5 gal bucket
(c) Stopwatch or clock

Procedure:

  1. Make sure the water system is at full pressure (psi).
  2. Make sure no water is being used during the test.
  3. Turn off well pump.
  4. Drain water into bucket until there is no more pressure: how many gallons did you drain?
  5. Close drain valve.
  6. Turn on pump and time in seconds until the pump turns off: How many seconds did the pump run?

Repeat 2 or 3 times for consistency.
Formula to figure GPM recovery rate:
Gallons divided by seconds  X  60 =  GPM