Lesson 6 - Filtration

    Filtration goes hand in hand with disinfection to keep your pool water sparkling clean and clear. One is not more important than the other, and neither by itself will produce perfect results. Let's say they are two sides of the same coin. Yet they are not the same thing, so we'll cover them separately. Lesson 5 - Disinfection should be read as well.

    To filter the water, we pass it through what is termed filter media which is inside of a filter tank. So if we want to describe what type of filter you have we describe two variables - type of media and size of tank. The bigger the tank, the more filter area it has and the more filter media it can hold. There are three types of filter media - 1)sand, 2)paper cartridge, and 3)diatomaceous earth (or D.E. for short).

    While it seems obvious that a large filter tank is will do a better job filtering water than a smaller tank, the type of filter media you use will depend on several factors. Let's look at each type of media and determine the benefits and costs of each type.

1) Sand Filter

    The most common type of filter we see in the Memphis area (which includes three states: Tennessee, Mississippi and Arkansas) is a sand filter. They are popular because they are very easy to service, they are usually not expensive, and they do a good job of filtering water. 

    Here's how a sand filter works. The filter is filled about half way up with sand. The water comes into the top of the tank, gets pushed down through the sand, trapping any particles or debris in the sand. The water passes through the sand and out the bottom of the filter, on it's way back to the pool. That's all there is to it. 

    As the filter gets full of dirt, the sand gets compressed. The filter builds up pressure because it gets harder and harder for the pump to push the water through the sand. That's why you see a rise on the filter pressure gauge. Speaking of pressure gauges, if the gauge is not working properly, replace it with a new one. They only cost 10 bucks, and the pressure gauge is the only instrument you have to tell you what is going on inside the filter. 

    Most filters normally run at a pressure of 10 pounds per square inch (psi) to 20 psi, but I've seen them as low as 5 psi and as high as 50 psi when the filter needs to be backwashed. Since most filters have a maximum pressure rating of 50 psi, you don't want to let it get that high. (It also explains why most pressure gauges go up to 60 psi.) Don't rely solely on the pressure gauge to tell you when it's time to clean or backwash the filter. It should be done at least once a week, and maybe more often if you're trying to clear a cloudy pool.

    When it's time to clean the filter, we simply run the water in reverse, sending the water from the bottom of the tank, up through the sand, lifting the dirt as it goes. Usually the city code mandates that the waste water is sent into the closed sewer system of the house, but sometimes people who live in the country just send the water out to a field or pond. This process is called backwashing and takes just a few minutes. That's part of what makes sand filters so popular.

    While were on the topic of backwashing, I often need to show new pool owners the proper way to backwash the filter. I'll do that for you right now as if you had called me to your house to give you a lesson. I'll assume you are standing in front of your sand filter while the pump is running. Note the filter pressure on the filter gauge. 

    There is a valve with a handle that rotates 360 degrees called a multi-port valve, because it has multiple ports and it basically tells the water where to go. The valve can be on top of the filter (top mount) or off to the side (side mount). Usually it stays in the 'filter' position, but when we backwash the handle gets pushed down and rotated 180 degrees to the 'backwash' position.

    {Note: It's important to turn the pump off before you push the handle down and rotate it. I like to joke that if you push the handle down when the pump is on, the water gets confused and doesn't know where to go. But actually the truth is that the water tries to go in every direction at the same time. Now this may or may not cause a problem, depending on how your system is configured. I've seen gaskets and o-rings spring a leak, because the shear force of the water starting and stopping quickly puts too much pressure on the pipes. On the other hand, I once knew a fountain maintenance man who told me he never shuts the pump off and has been doing it that way for years with no bad consequences. Most filter manufacturers advise you to turn the pump off before moving the handle and that's good enough for me.}

    So, turn the pump off, verify that the filter pressure is at zero, push the handle down all the way, and rotate it 180 degrees to the backwash position. (If the valve manufacturer only wants you to move the handle clockwise or counter-clockwise, they will indicate it on the valve face. Otherwise, either direction is ok.) Make sure there is either no valve on the backwash line or that any valve is open. Turn the pump back on and watch the filter pressure gauge come up from zero to about ten pounds.

    Many multi-port valves have a little sight glass where you can observe the water as it makes it's way out the backwash line. Sometimes there is a sight glass on the backwash line itself, or the top of the filter has a clear lid. If you're lucky, the system will have all three which (along with the pressure gauge) allows you to see exactly what is happening. If you have none of these visual aids, then I recommend asking your pool professional to install a sight glass.

    There is no strict time limit for backwashing a filter... 3 to 5 minutes is normal. If I'm in a big hurry and the filter is not very dirty, 2 minutes may be enough. If the filter is really dirty I might need to let it go for 10 minutes. That's why it's nice to have a sight glass to see what the water looks like. Keep in mind that while you are backwashing you are taking water out of the pool at a rate of 50 to 90 gallons per minute. So make sure there is extra water in the pool before you begin.

    Okay, we've backwashed for say 4 minutes and taken 1/2 inch of water out of the pool. Now we turn the pump off again, push the handle down, and rotate it to the 'rinse' position. Turn the pump back on and we'll rinse for 1 to 2 minutes or just long enough for me to tell you this story.

    Back in the old days most filters had no rinse cycle. There was just a push-pull handle that was either up or down - filter or backwash. Problem was you had to backwash for as long as 15 minutes which took a lot of water out of the pool. Then when you moved the handle back to filter, often dirty water blew back into the pool, clouding up your clean pool. So they added a rinse cycle to wash out the top of the filter, thus shortening the backwash cycle and eliminating the blowback problem. End of story.

    Now let's turn the pump off, push the handle down, rotate it back to filter, and you're ready to go. Turn the pump on and watch the pressure gauge. Let's say it rises and stays at 12 psi. That is your start up pressure reading. The sand is nice and fluffy and ready to capture more dirt. As stated elsewhere, the filter should be backwashed at least once a week.    

    The water doesn't get completely filtered on it's first pass through the sand, but rather by constantly re-filtering the water eventually gets 'polished' until it's crystal clear. Since sand is a fairly coarse grain material, some smaller particles will never get trapped. Paint, small bits of algae, and so forth may need to be coagulated into larger particles so they can be trapped. That's basically what water clarifiers do, thus aiding the filter in doing it's job.

    Occasionally the sand in the filter needs to be changed. For example, if a lot of oil is used by swimmers, then the sand can clump up and become less efficient. Or if calcium based chlorine is used frequently, the sand can get crusty on the top, and again the efficiency suffers. A classic symptom of a filter with poor sand is that you backwash, then a day later the filter pressure goes way up and you need to backwash again. Then the same thing happens every day or two, and you notice that when you backwash, the water is not very dirty at all. Solve this problem by changing the filter sand. Most professionals advise to change the filter sand every 5 to 7 years as a general rule. 

2) Paper Cartridge Filter

    I spent extra time covering sand filters so that I could introduce some new terms like 'multi-port valve' and 'filter pressure gauge'. The next two types of filters, paper and d.e., will go more quickly since we have a basis on which to build. 

    Paper cartridge filters have been routinely used on portable hot tubs and spas for many years, and I use them exclusively on fountains because of their low cost and good quality. In the past decade we've seen an increasing number installed on swimming pools by builders. They are much larger in size than those I just mentioned, but they work the same way. Paper cartridge filters have two distinct advantages over sand filters, but they have two disadvantages as well. 

    On the plus side, paper cartridges are able to filter out more particles of a smaller size. That's because they are made of a tough but finely woven paper (almost cardboard) material which is folded in an accordion fashion to increase the filter area. While a sand filter has only 2 to 4 square feet of filter area representing the surface of the sand in the filter tank, a cartridge filter can have 25, 50, 75 or even 100 square feet of filter area. Plus it filters out finer particles. The water will get cleaner faster than the sand filter can ever achieve.

    The second advantage is that the paper cartridge filter does not need a sewer connection since it does not get backwashed. This can lower the cost of a new installation for the pool builder, or it can solve a problem where a backwash line cannot be installed in an already existing situation. It's a great way to upgrade a filter system, even when the backwash line cannot be used.

    Now for the drawbacks. These filters are more costly to operate and more work to clean. The filter elements themselves can be washed out many times before requiring replacement. But the size of a filter element required to clean a pool can get rather large and can be quite heavy. Some elements are so large that two strong people are needed to hoist it up, then take to a car wash to pressure clean it. Of course they are not always so large, but even the small ones require more work than a simple 5 minute backwash. (One manufacturer decided to solve the size problem by having 20 small filter elements. I always dreaded cleaning that filter because there were just so many of them.)

    Then the cost of the elements are a factor. A medium sized element can cost over $100 with the large ones costing more. Oil and algae can shorten the life of a cartridge element, but taken care of properly, they can last as long as 2 or 3 years. 

3) D.E. Filter

    Diatomaceous earth (d.e.) is a naturally occurring material that makes a great filter medium. It's much smaller than sand, so it filters down to smaller particles. Inside the filter tank there are wing-like louvers covered with fabric. These filter elements are arranged for maximum surface area, so the effective filter area might be 36, 48 or 60 square feet. (Compared to a typical sand filter which may have 2 to 4 square feet.)

    When d.e. powder is introduced down the skimmer, it travels to the filter tank and 'flocks' onto the wing-like elements. The filter is backwashed just like a sand filter, but it needs it only once a month instead of weekly. A filter that backwashes like a sand filter and does as good a job as a paper cartridge filter. Sounds great, and they are great. They have drawbacks, though. 

    First, they cost 2 to 3 times what a sand filter does. Some are made of stainless steel tanks, with many louvered elements inside, and are quite expensive. But the good news is that the costs are coming down, as the tanks are made of plastic composite material. The d.e. powder itself is fairly inexpensive so that is not a big issue. Second, it's a little known fact that when a d.e. filter is backwashed, not all of the d.e. powder is removed. That means the filter must be opened and rinsed out with a garden hose at least twice a year. Some people do this themselves, many hire a professional to do it.

    Every 10 years or so the filter elements themselves will need to be replaced. They may get holes in them, allowing some d.e. powder to pass through to the pool. Or they may become impregnated with small particles and not let the water pass through. There may be 5 to 10 elements costing $20 to $40 apiece.

Conclusion

    Regardless of the type of filter you may have, they all work the same way. Water passes through a filter medium (i.e., sand, paper, d.e. powder) which traps the dirt and allows the (cleaner) water to go back to the pool. A question a lot of people ask is, "How many hours a day should I run the filter?" Good question, one that if you ask 5 pool guys you'll get 5 different answers. Here's mine.

    If it was my pool, I'd run the filter 24 hours a day. I'd want the water as clean as possible. The system is designed to run continuously, so that's what I'd do. When the pool is off, it is vulnerable. It can't filter the water, the skimmers can't skim the surface, the chlorinator can't disinfect, the pool cleaner can't run. Thus anytime the pool is off, the water quality is steadily degrading.

    Of course it's not my pool, so you may have a reason for not running the filter all of the time. I once had a customer who just didn't like the idea of the filter running while the family was asleep at night. So they had a timer set to run the pool from 8 am to 10 pm. That's fine with me if that's what the customer wants to do. 

    The key criterion is how clean and clear is the water? If the filter only runs 12 hours a day and the water clarity is great, then 12 hours is fine. But if the water is not staying clear, the time running should increase. 

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