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POOL FILTERS

"Pool Filters"
By Darren Maher

A good filtration system is one that keeps your pool water healthy and clean while maintaining a high flow rate and good water turnover. Perhaps most important to the health of your pool is to choose a filter whose size and flow rate are adequate for your amount of pool water. In order to properly size a filter, one must calculate the pools’ volume, capacity and flow rate. Firstly, it is important to calculate the pools’ volume in order to understand how much water there is to be filtered. Pool volume can be calculated as follows:

 

Rectangular Pool:	Length X Width X Average Depth = Volume (cubic ft.)
Circular/Oval Pool:	Radius X Radius X 3.14 (pi) X Average Depth = Volume (cubic ft.)

The Capacity of your pool refers to the gallons of water that the pool will contain (not to be confused with volume, which is a spatial measurement in cubic feet). The capacity of your pool can be measured as follows:

Capacity: Pool Volume X 7.48 = Capacity (Gallons)

Flow Rate is the volume of water that flows past one point over a specified time. Flow rate is measured in gallons-per-minute and/or gallons-per-hour. Determining the flow rate of the pool will measure how long it will take to “turn-over” or circulate the entire body of water one time. Flow Rate is calculated as follows:

Flow Rate: Capacity (divided by) / turnover time (hours) = Flow Rate/hr.

Example:

Let’s say that you have a 16x32 rectangular pool, and that you are trying to figure out which filter will work for you. As well, you want a turnover time of 8 hours (i.e. it will take eight hours to circulate the whole body of water). Start with calculating volume:

Step 1: Calculate the Volume

16 x 32 x 5 (avg. depth) = 2560 cubic feet

Step 2: Calculate the Capacity

2560 x 7.48 = 19148.80 Gallons

Step 3: Calculate the Flow Rate*

19148.80 / 8 = 2393.60 Gallons per Hour OR

2393.60 / 60 = 39.9 Gallons per Minute

As you can see, a filter system that allows for a minimum flow of 40 Gallons per Minute (GPM) or more will be adequate filtration for this particular pool. The best advice is to oversize the filter. This will allow for slower wear of the media. You will also find that as filters collect dirt, the flow rate per-square-foot of filter media will be decreased. Oversizing allows the filter to collect more dirt at the highest efficiency level. Most filter manufacturers provide GPM ratings for each of their models. If you need assistance finding out which filter best suits your needs, please visit the Ashton Pools showroom and talk to one of our pool professionals.

Types of Filters

The most commonly used filters employ three different media types: sand, cartridges and diatomaceous
earth. Let’s take a look at each of these types and outline their advantages and disadvantages.

Sand Filters

A sand filter forces water through graded layers of sand to pull out debris and contaminants. This type is filter is commonly used in both aboveground and inground pools. The media used in a sand filter is #20 silica sand (nepheline syenite), which is very coarse. The main benefit of employing a sand filter is that they are economical to purchase and easy to maintain. A high-rate sand filter that is cleaned regularly (through backwashing) will go many years before you will need to replace the sand. In comparison, DE or cartridge filters need regular cleaning and their media need replacing on a more regular basis. A high-rate sand filter filled with new sand can remove dirt particles 25 microns in size. After time and use, the sand will begin to wear down and become rounded, thus allowing particles up to 60 microns in size to pass through. A Micron is a unit of measure for small grains of material. To give you an idea of how efficient 25-micron filtration is, a grain of table salt is typically 100 microns in size. Therefore, sand filtration can remove particles that are one quarter the size of a grain of table salt. Backwashing is the most important part of sand filter maintenance, and is usually performed once per week. Backwashing simply means to reverse the flow of water through the filter to force debris and contaminants out of the sand that has trapped them, and then to send the contaminants out the waste line.

When to Backwash: Cloudy water, increased pressure and poor circulation are all indicators that your filter is dirty. You can use the pressure gauge to determine if the filter needs cleaning. If your filter has an inlet pressure gauge, mark the pressure with a grease pencil right after a good backwash. Once the pressure has risen 8-10 psi from this base-line reading, it is time to backwash.

How to Backwash:
1. Turn off your pump and turn the control valve on the filter to the “backwash” setting. Make sure your backwash hose is attached, so as to drain the dirty water away from the pool.
2. Restart the pump. Let the water drain for about 2-3 minutes, or as directed by the filter manufacturer. Turn the pump off again when done.
3. Turn the filter to the “rinse” position and run the pump for about 30-60 seconds.
4. Turn the pump off, turn the valve back to “filter” and restart the pump.

Remember that these are general guidelines--always follow manufacturers instructions if they are given.
Each year you will want to open up your filter and take a look at the sand, as over time and use it may become hard and channeling may occur. Channeling (sometimes called “charging”) happens when water forces its way through the sand around the areas where it has hardened. To fix this problem, or avoid it altogether, turn the pump off, cut flow off to the filter with diverter valves and start filling the tank up with a garden hose. The water will overflow from the tank (cleaning out dirt) and you can now take a broomstick and gently break up the hard clumps. Caution: Be careful not to hit the laterals, as they are fragile and will break. The laterals are usually located at the bottom of the tank and are what pull the water out of the sand and back to the pool. If you damage a lateral, don’t be surprised to find sand piles on the pool floor.

Once all clumps have been broken up, check the sand level and add more if necessary. Otherwise you can close up the filter, open diverters, turn pump back on and do a quick backwash.

Replacing Sand: Over time, the grains of sand in the filter will be worn down and become more rounded, allowing larger dirt particles to pass through and re-enter the pool. The sand will also need to be backwashed more frequently, and the operating pressure will be higher. This is typical of sand that needs to be replaced.

1. To remove the sand, turn the pump off and divert water flow.
2. After exposing the sand, start scooping it out with your hands or a small scoop, or try your wet/dry vac. Once again, be very careful not to damage the laterals.
3. Fill tank about 1/3 with water to act as a cushion for sand to be added.
4. Slowly pour sand into filter tank until it is about 2/3 filled.
5. Reassemble filter and do a backwash before resuming normal filtration.

Cartridge Filters

Using a cartridge filter on your pool has several advantages over other types of filtration. Cartridge filters do not require backwashing like sand filters, meaning that you won’t be wasting vast amounts of pool water to keep it clean. They will also catch smaller dirt particles (only 5-10 microns in size) than sand filtration, and the cartridge media can last anywhere from 3-10 years before it needs to be replaced.

The biggest benefit to owning a cartridge filter system is that it requires infrequent cleaning. Each season with a cartridge filter should entail one or two thorough washings of the filter cartridge—once halfway through the season and again before winterizing. Some cartridge cleaning solutions allow you to clean the filter without even removing them from the filter casing. Ideally, at the end of the pool season, the cartridges are removed and cleaned, and stored indoors, preferably somewhere clean and dry. You can determine when to clean your filter by the pressure, similar to how you would with a sand filter. Mark the base line pressure on the gauge with a grease pencil right after a cleaning, and then make another mark 8-10 psi higher. Between these marks is the normal operating range, and when the pressure rises above this range it is time to clean the filter.

How to Clean Your Cartridge Filter:
1. Remove cartridge from filter housing as per manufacturer’s instructions. Use a garden hose with a straight flow nozzle to wash between pleats, pointing nozzle downwards at about a 45-degree angle. Rinse until all debris is gone.
2. Soak the cartridge for at least 1 hour (preferably overnight) in a filter cleaning solution to remove the oils from the filter.
3. Rinse the cartridge.
4. Let your filter dry before putting it back into the tank and reassembling the housing.

Allowing the filter to dry will tighten the weave of the fabric, so that it can trap smaller particles of dirt. This helps to extend the life of your filter and give you a consistent level of filtration for years to come. Cartridge filters are definitely one of the items on your pool that will last longer if properly cared for.

Diatomaceous Earth Filters

The least common filter used in Southwestern Ontario just happens to be the most efficient. Diatomaceous earth (or DE as it is frequently called) will filter out dirt particles as small as 1-3 microns, which is smaller than what a sand or cartridge filter can pull out. Better filtration means that the pool water will be kept clear and sparkling; moreover, that the pool will overcome dirt and clarity problems more quickly. The operation of a DE filter is much different than the operation of cartridge or sand filters. A DE filter requires that Diatomaceous Earth (in a white powdery form) be added to its interior grids via the skimmer. When DE gets added to the water, it passes into the filter and forms a cake over a series of filter grids. It is this cake of Diatomaceous Earth that traps dirt and debris and locks it there until the filter is backwashed.

Though the DE filter has proven to be the most efficient residential filter by far, it is arguably the filter that requires the most regular maintenance. DE filter owners are often given the task of adding the proper amount of earth every few days to ensure the filter has adequate filtering capabilities. Furthermore, the DE filter is the only one requiring the filter to be opened up and cleaned by hand regularly. Backwashing can clean some debris and used-up DE out of the filter, but will not remove all of the cake. Hand cleaning entails the removing of any leftover cake from the grids, and then dismantling the grids themselves and soaking them in an acid-water mixture. This somewhat high level of maintenance is probably the reason why DE is not more commonly used in pools in Ontario, although those who do use it will tell you that it is worth it.

To Recap:

TYPE ADVANTAGES DISADVANTAGES

TYPE	ADVANTAGES	DISADVANTAGES
Sand	Most economical Low level of weekly maintenance	Lowest efficiency of the three types
Cartridge	No backwashing Easy and infrequent cleaning High efficiency	High cost to replace filter media (but they aren’t changed often).
Diatomaceous Earth	Most efficient filtration Cleaner water and quicker recovery time when pool clouds.	Requires frequent maintenance More complicated than other systems

 

Troubleshooting – Filters

1. I have Sand or Diatomaceous Earth entering into my pool.

In sand filters, there are two possible causes. One is that a lateral is broken (laterals are the ‘fingers’ on the inside bottom of the filter responsible for collecting clean water and returning it to the pool). A hairline crack in any lateral will allow for sand to pass through. Secondly, the sand could be old and worn. New silica sand is very coarse, but through use becomes smaller and finer. Small sand can pass through the openings in a lateral and enter the pool. Replacement is necessary to remedy either of these causes.

In DE filters, the most obvious culprit is a worn out septa (A septa refers to a grid section with nylon mesh coating that the DE binds with in order to form the cake). Even a small tear in the nylon meshing will allow for DE to pass through. Thoroughly inspect the grid system for any tears or unraveling of the mesh.

2. I have dirt particles coming out of my returns, making the water cloudy.

The most common mistake for new pool owners is to have the multi-port valve of their filter in the wrong position. Make sure to follow manufacturer’s instructions on how to operate the filter’s multi-port valve. Having your filter in the ‘recirculate’ position versus the ‘filter’ position means that you are simply bypassing the filter and circulating water back through the pool.

A second and more serious possibility could be that the filter media is damaged or worn and needs replacing. Tears in filter cartridges will allow for water to pass right through. Damage to the grids/septa in your DE filter will allow for dirt and DE to pass into the pool. In a sand filter, it is best to visually inspect the interior to see if the sand is worn or if there is a broken lateral.

3. I have a very short cycle between backwashes.

First and foremost, an increase in the amount of dirt, oils and algae in the pool water will mean an increase in the amount of backwashing you will have to do. High bather loads will also make for more frequent cleaning of your filter.

Secondly, look at the filter itself. A sand filter can suffer from increased pressure due to calcified sand inside the filter. This calcified sand forms hard layers, which impedes flow and increases pressure. Otherwise, some pool products (especially coagulants and clarifiers) when added too fast have the ability to clog the sand and therefore increase pressure. Make sure to introduce these chemicals slowly so that they have the chance to dissolve. If you think this is the cause, you will need to clean your filter media.

4. There is little filter action, and little pressure change.

Cartridge Filters with poor filtration and little change in pressure usually indicates that the filter cartridge is worn out or torn, thereby allowing for water to pass through uninhibited. Visually inspect the cartridge and replace it if necessary.

Sand Filters can suffer from a condition called “charging” where the sand has either been improperly replaced or installed, or installed in insufficient amounts. This allows for water to channel itself through the sand and collect at the bottom with little to no filtration. Visually inspect the filter and replace and/or top up the sand as necessary. Another cause for poor filtration is the hardening of the filter sand through calcification. Depending on the pH and alkalinity of the water, calcium has the ability to come out of solution and form scale on pool surfaces— including the inside of your pool filter. Considering that calcium, water and sand are the basis of concrete, it is not hard to understand the potential for the filter sand to harden over the years. Look for evidence of calcification, charging or channeling in the sand filter and replace as necessary.

DE Filters, when inadequately charged with DE, will not have adequate filtering area to keep the pool clean. Be sure to add enough DE to coat all of the grids and septa. Secondly, Diatomaceous Earth has the ability to coagulate and solidify on the grid surface so be sure to backwash regularly and open the filter and remove any solidified DE.

*Formulas taken from “The Ultimate Tech Manual.” Pool and Spa News, Hanley-Wood, Washington 2002. Pgs. 20-22.

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