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Filtration Operation: Back to Basics

By John Watt

When it comes to keeping pool water clean and clear, the swimming pool industry knows that the pool filter is key. Chemicals help keep pool water clean but even chemical manufacturers agree that if the filter isn't working properly, chemicals alone can't provide clean and clear water.

It's important to get back to basics to truly understand how sand, DE and cartridge filters work. But to truly understand how filters work, pool professionals must understand how the pool pump feeds water into these filters. Understanding the relationship between the pool pump to the pool filter gets to the heart of obtaining clean and clear water. Pool professionals who understand this relationship can then incorporate filter cleaning, chemicals, variable frequency drives and automatic controllers to obtain pool water nirvana.

Pump & Filter Sizing

Filtration systems, regardless of the media being used, are designed to filter water within a particular range of water flow. One of the most common mistakes service techs see in the field is a filtration system equipped with an oversized pump. Why is an oversized pump a problem? A pool pump that pushes water through the filter too fast impedes the filter's ability to perform its job correctly. The faster you move the water through filtration media, the worse job it does. The slower the velocity the better job the filter does separating debris from the water. It is therefore extremely important to properly size the filtration system to the required pump flow to achieve efficient water filtration.

Here are the current standards for filtration:

  • High-rate sand filtration: 15 gpm per square foot of surface area
  • DE: 1 to 2 gpm per square foot of surface area
  • Cartridge filtration: .375 gpm per square foot of surface area

To size a filtration system correctly, it is necessary to determine the required turnover rate for the pool as well as verify the proper filtration rate. A typical commercial pool of 150,000 gallons needs a 6-hour turnover rate and would therefore require 416 gpm (gallons per minute) flow rate. The challenge comes in when you encompass the clean and dirty flow rates. When the filter is dirty, the resistance to flow goes up and flow rate drops. When the filter is clean, the resistance to flow goes down and the flow rate goes up.

For example, a pump might will give you 275 gpm at 83 feet of head and 400 gpm at 60 feet of head (23 TDH change between clean and dirty filter mode). That being said, the filtration will now have to handle 400 gpm even if the needed flow rate for turnover is only 275 gpm.

There are three things that need to be taken into consideration when looking at the system.

  • Clean filter flow rate: Making sure that the system will handle the extra flow when the filter is clean.
  • Dirty filter flow rate: Making sure that the minimum turnover flow rate is met when the filter is dirty.
  • Backwash flow rate: Ensuring that the flow rate meets the manufacturer's requirement for sand and DE filtration where backwashing is applicable. (This pertains to filter size, pump size as well as backwash plumbing size.)

So, how do we know what the system is currently doing and what it is capable of?

  • Backwash the filter as prescribed by manufacturer.
  • Attach vacuum and pressure gauges to the pump.
  • Convert vacuum and pressure readings to TDH (vacuum X 1.13 pressure X 2.31).
  • Apply TDH to the manufacturer's curve for the existing pump. (This will give you the flow rate at its maximum with a clean filter.) Make sure that the maximum flow rate does not exceed the maximum flow rate for your filtration system.
  • Add 23 feet of head to your TDH and that will give you the dirty filter flow rate. (Make sure that this flow does not drop below the minimum flow rate to meet your turnover time as well as the minimum required flow for the other equipment on the pool such as heaters, skimmers, sanitizing system, etc.)

What are your options?

  • Change the pump to one that better fits the flow requirements.
  • Add a VFD with flow control that will allow you to hold a constant flow rate to compensate for TDH changes during the clean to dirty filter cycle.
  • Add additional filtration (additional filter or larger single filter).

Note that of these three options, the addition of a VFD is the only option that will actually pay for itself over time in energy savings.

However, keep in mind that an oversized filter with a smaller HP pump can improve filtration but cause other problems. A typical example of inadequate backwash time or flow rate will show up when the system is put back into filtration mode. When a sand filter is not properly backwashed a small amount of debris (silt) will come out of the returns for the first few minutes after returning the system to normal filtration mode. Some of the silt will also remain in the filtration system working its way deeper into the sand bed. This can lead to costly service, repairs or even premature replacement of the filter.

Filter Pressure & Filter Cleaning

Once the filter gets dirty, it needs to be cleaned to continue to operate properly. Filter pressure is one of the best ways to determine if a filter is dirty and needs to be cleaned. The change in pressure differential (pounds per square inch, or psi) can help a service professional determine if a filter is dirty or needs to be backwashed. In order for this type of information to be most effective, it is helpful to keep a log of the filter pressure for each pool. Many service technicians will do this at the time of pool opening and leave the information posted near the filter in the pump room or will log it into the customer's pool chart. One of the most important figures to note is the initial psi on the gauge at the top of the filter tank. If a pool is having trouble with water turbidity or with the heater cycling while trying to maintain the proper water temperature, service techs can start their diagnosis by looking at the filter pressure reading. If, for example, the pressure reading at pool opening was 18 psi but now shows 30 psi when the trouble is occurring, the first thing to consider is a dirty filter. Begin by backwashing the filter to see if that fixes the problem. Restoring the water flow allows the filter to function properly, which should allow the water to clear and might even fix the problems with the pool water heater. As mentioned earlier, when the filter pressure goes up the water flow rate goes down. A lack of water flow through the heat exchanger allows the water to absorb too much heat and shut off on the high limit safety circuit built into the heater.

However, a newer challenge has presented itself now that variable speed pumps have become extremely prevalent. In the past, the pump was either on or off and service technicians took the psi measurement when the pump was on. But with variable speeds pumps (VSPs) they do not run constantly at full speed, making it harder to obtain an accurate psi reading. In the case of VSPs the flow increases and the pressure also increases but not necessarily because the filter is dirty. To overcome this issue, many service techs today turn up the pump to the highest preset circulation speed (not the highest speed on the pump) and take a psi reading during their weekly service. Those that aren't doing a weekly service often ask their customers to perform this task and tell them to call the pool service company if the psi increases by more than 10 pounds. Running the pump at the highest preset circulation speed and checking psi is a part of the weekly service along with cleaning the filter baskets, skimmers, etc. Cleaning the filters when there is a 10-pound increase in pressure is especially important with cartridge and DE filters as it can prolong the life of the filter. Following this procedure will help avoid compression and compaction of dirt and debris on the face of the grids and or cartridges making them last longer and easier to clean.