June 8, 2019
By John Watt
Why filter? When it comes to keeping pool water clean and clear, the pool industry knows filtration is imperative. Chemicals help keep water clean, but even chemical manufacturers agree if the filter is not working properly, chemicals alone cannot provide clean and clear water. That said, it is important to get back to basics to truly understand how sand, diatomaceous earth (DE), and cartridge filters work.
However, to truly understand filter operation, pool professionals must know how the pump feeds water into these devices. Understanding the relationship between the pump and filter gets to the heart of obtaining clean and clear water. Pool professionals who recognize this relationship can then incorporate filter cleaning, chemicals, variable-frequency drives (VFDs) and automatic controllers to obtain pool water nirvana.
Filtration systems, regardless of the media being used, are designed to filter water within a particular flow range. One of the most common mistakes service techs see in the field is a filtration system equipped with an oversized pump. This is problematic because a pool pump that pushes water through a filter too fast impedes its ability to perform properly. The faster water is moved through filtration media, the worse it performs. The slower the velocity, the better a filter will perform at separating debris from the water. Therefore, it is extremely important the filter equipment is properly sized for the required pump flow to achieve efficient water filtration.
The current standards for filtration include:
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 containing 567,812 L (150,000 gal) needs a six-hour turnover rate and would therefore require a 1575 lpm (416 gpm) flow rate. The challenge occurs when factoring in clean and dirty flowrates. When a filter is dirty, the resistance to flow goes up and the flowrate drops. When the filter is clean, the resistance to flow goes down and, in turn, the flowrate increases.
For example, a pump that will provide 1041 lpm (275 gpm) at 25.3 metre of head (83 feet head) and 1514 lpm (400 gpm) at 18.3 metre of head (60 feet head) (23 total dynamic head [TDH] change between clean and dirty filter mode). That said, the filtration will now have to handle 1514 lpm (400 gpm) even if the required flowrate for turnover is only 1041 lpm (275 gpm).
Three things need to be taken into consideration when looking at the system.
A technician can determine what a system is currently doing (and what it is capable of) using the following procedures.
Note: Of these three options, the addition of a VFD is the only one that will actually pay for itself over time in energy savings.
Keep in mind, an oversized filter with a smaller horsepower pump can improve filtration but cause other problems. A typical example of inadequate backwash time or flowrate 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.
Like any filter, once it gets dirty, it needs to be cleaned to continue to operate properly. Filter pressure is one of the best ways to determine if the media is dirty and needs to be cleaned. The change in pressure differential (pounds per square inch [psi]) can help a service professional determine if a filter is dirty or needs to be backwashed. For this type of information to be effective, it is helpful to keep a filter pressure log for each pool. Many service technicians do this when opening the pool, leaving the information posted near the filter in the pump room or by logging it into the customer’s pool chart.
One of the most important figures to note is the initial pressure 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 124 kPa (18 psi), but now reads 207 kPa (30 psi) when the trouble is occurring, the first thing a tech should consider is a dirty filter. In this case, the filter should be backwashed to see if it fixes the problem. By restoring the water flow, it allows the filter to function properly, which should allow the water to clear and might even fix any problems with the pool heater. As mentioned earlier, when the filter pressure goes up the flowrate goes down. A lack of water flowing through the heat exchanger allows the water to absorb too much heat and, as a result, it triggers the shut off on the high limit safety circuit built into the heater.
However, a newer challenge has presented itself now that variable-speed pumps (VSPs) have become extremely prevalent on today’s pools. In the past, the pump was either on or off and service technicians took the pressure measurement when the pump was on. That said, VSPs do not constantly run at full speed, which makes it harder to obtain an accurate reading. In the case of VSPs, the flow and pressure increases, but not necessarily because the filter is dirty. To overcome this issue, many service techs today turn up the pump to the highest pre-set circulation speed (not the highest speed on the pump) and take a pressure reading during their weekly service. Those that are not performing a weekly service often ask their customers to look at the reading and call the pool service company if the pressure increases by more than 69 kPa (10 psi).
Running the pump at the highest pre-set circulation speed and checking filter pressure is part of the weekly service routine, along with cleaning the filter baskets, skimmers, etc. Cleaning the filters when there is a
69 kPa (10 psi) increase is especially important with cartridge and DE filters as it can prolong the equipment’s life. 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.
There are several ways to clean filters. The most common method of cleaning is backwashing. The following are detailed best practices and tips to perform this procedure for each type of filter.
First, service techs must determine the proper backwash flowrate (56.7 to 75.7 lpm [15 to 20 gpm] per square foot). For example, a 0.29 m2 (3.1 sf) sand filter needs no less than 176 lpm (46.5 gpm) and no more than 235 lpm (62 gpm). Using too much water flow can potentially lift the sand bed and pass it to waste. In a worst case scenario, the laterals and or filter tank can become damaged from the sand blasting effect as the water is introduced at the bottom of the tank at a high velocity.
If there is not enough water flow, the debris from the sand bed will not be completely removed. As a result, any debris remaining in the filter works its way deeper into the media. When this happens, the dirt in the bottom of the filter can turn to caliche (a hardened natural cement of calcium carbonate that binds other materials such as gravel, sand, clay, and silt). Should this occur, the filter must be replaced; in most cases this buildup cannot be removed because of its size and difficulty to break apart.
Another problem can occur if the sand filter is backwashed too frequently. In some cases, silt can pass through the filter even when it is clean. As a result, service centres will likely receive calls from pool owners explaining they have sand in their pool and additional sand comes out of the pool returns when they vacuum. To begin diagnosing this situation, service techs should ask the client if the ‘sand’ makes a pile or a cloud when they brush their pool. If the former occurs, there could be an issue with the filter. If the latter happens, it is not sand, it is silt. If silt is the problem, it means the sand filter is too clean and, therefore, should not be backwashed. When a sand filter is clean, it allows larger particulate to pass through. This is a sign to stop backwashing the filter and, instead, allow it to load up so it will begin to trap the silt. A clarifier or flocking agent can also be added to help with this process.
Determining how long to backwash a sand filter should not be determined by looking at the water flowing out of the backwash line. Sand filter backwashing should be completed in three minutes. Service techs should use a stopwatch when performing this task rather than guessing.
Unlike sand filters, frequent backwashing of DE filters does not affect the filtration rate. However, it does create extra work and expense for service techs. Similarly, the backwash process should only take three minutes; however, the cycle should be broken down into a one-minute backwash, then a one-minute filtration for three separate cycles (making sure to shut the pump off each time when switching between backwash and filtration).
This procedure helps to separate the DE coating from the grids so it can be removed from the filter. These filters should be backwashed at the same 69 kPa (10 psi) increase over normal operating pressure as any other filtration media. The filter should be torn down, cleaned, and degreased at least twice a year, too. Body lotions and sunscreen collect on the grid material and cannot be removed via backwashing. A degreasing agent should be used to complete the filter maintenance process. It is important to note, muriatic acid will permanently lock the oils and lotions into the grid material and, is therefore, not recommended.
When recharging a DE filter, the manufacturer’s recommendation should be followed as to the amount of media to use to charge the filter. The proper method is to use a 19-L (5-gal) bucket to mix the DE and water together to create a slurry. Once mixed, it should be poured slowly into the skimmer while the pump is running. This will ensure the grids or elements are properly coated from top to bottom.
If a service tech finds a backwash valve on a cartridge filter, something has gone awry. This equipment is not designed to withstand water flow in reverse direction; therefore, it must be taken apart to clean. That said, these filters are cleaned in the same manner as a DE filter teardown. Hosing the surface debris off the cartridge is only the first step. Body oils and lotions are the number one contributor to a plugged cartridge filter. Therefore, getting the oils and lotions off the cartridge surface is the difference between frequent media replacement and cartridges that last longer between cleanings.
The oily buildup on the surface plugs up the pores on the cartridge, which restricts water flow and creates a very sticky surface. These filters trap debris on the surface; when the filter is turned off, it falls to the bottom of the tank to free up more surface area for the next cycle. If the surface of the cartridge is sticky, the debris does not fall away, which causes the filter to plug up rather quickly.
Many pool service professionals now offer cartridge filter cleaning services. In addition to hosing off the filter, the media is soaked in muriatic acid along with a cleaning agent. Then, it is re-soaked in a liquid chlorine bath to brighten and lighten the filter (making them look nicer when they are returned to the client). Finally, the filters need to be air dried before they can be used again.
Allowing them to dry completely gives the cartridge fibres time to fluff back up, which is important because they need to expand for the filter to be effective. If they can be pushed down easily, it reduces the filter cycle.
Many service companies suggest to their clients to have two sets of cartridges so they can continue to use their pool while the other set is being cleaned. The most effective system is for the service tech to take the filter media when they close the pool for the season, clean it at their shop, then return the filters ready-to-use in the spring when the pool is re-opened. Since pools are typically at their dirtiest during spring opening, those cartridges are filtering more debris than usual. In fact, some service professionals change the filters after starting up the pool. Once the water is clear, they install new filters to provide a more productive filtration process over the course of the summer. This also extends the lifespan of the new filter cartridges.
On large commercial pools, service technicians should perform weekly filter inspections. An automatic controller can be used to set a backwash schedule based on the pressure reading. This can be an invaluable addition to a pool’s filtration system. With an automatic backwash controller, the need for a service technician to be present to perform this maintenance procedure—although recommended—is not necessary because the parameters set in the controller will automate the backwashing process. Most backwash cycles occur when a 69 to 103 kPa (10 to 15 psi) increase occurs after starting with a clean filter pressure reading. Controllers can also be set to backwash on specific days of the week or time of day.
Understanding the relationship between the pool’s pump and filter is integral to obtaining clean and clear water. Using filter pressure readings and understanding how VSPs can affect these measurements are also key to knowing how the filter is performing and when it needs to be cleaned. Once the filters and pumps are sized correctly to work in tandem, regular filter cleaning will help ensure the cleanest, clearest pool water.
John “MacGyver” Watt has spent the last 25 years working for Pentair in a variety of positions, including field service technician, regional service manager, national trainer, and product specialist. He currently works for Pentair’s application engineering and new product development for the United States, Canada, and Latin America. The nickname “MacGyver” was given to him early in his career with the company because of his ability to overcome challenges with whatever materials he had on hand. He can be reached via e-mail at email@example.com.
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