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When I was teaching filtration at technical schools and seminars throughout the country, I kept hearing the same question: “Which type of filter is best?” Attendees were not asking which manufacturer made the best filter, but rather which of the three types — DE, sand, and cartridge — was most effective

To answer that question, I got involved in some in-depth testing of all of the pool and spa filters that were available at the time. The tests were commissioned in the 1980s by a manufacturer that was preparing to introduce a new line of filters.

During the testing process, we discovered some of the reasons that pool and spa professionals gave for using their filter of choice. Here are a few:

“This is the filter we have always used.”

“We like the salesperson from that company.”

“Backyards are getting smaller, and this filter takes up less room.”

“If we buy 100 of these filters, we get a free trip to Hawaii.”

“This filter is very easy to install and maintain.”

“I don’t know why we use this filter.”

Some of these answers may sound silly, but if you think about them, they are good reasons. For example, if you have used one type of filter for a long time, it’s probably because it does the job for you. Or, you may like the way the salesperson takes care of you and any problems that arise. And, of course, a filter that is easy to install and maintain saves you money and is good for your customer.

But if you don’t know why you are using a certain filter, maybe you should ask yourself that question. Experimenting with different filter brands can be beneficial. For example, during the testing process, we found that filters of the same type from different manufacturers performed differently due to design variations. This means that Filter A might need to be cleaned more often than Filter B, even though they are the same size, have the same flow rate (gallons per minute), and have the same dirt-loading capacity. 

It is also beneficial to explore the different types of filters. Each of the filtration types will work in most applications. However, there are some applications where one type of filter will work better than the others.

Filtration trade-offs

The choice of which filter to use is affected by a variety of factors. For example, the sand filter may be a good choice for desert areas because it is easy to backwash. However, in drought areas of the country, the use of sand filtration is discouraged because of the water wasted during the backwashing process. If the pump is delivering 60 gpm, and the filter requires 5 minutes of backwashing every month, that is 3,600 gallons of water a year. On top of that, the chemicals, and sometimes even the heat, need to be replaced. 

The same problem is associated with DE filters, which also require backwashing. However, DE filtration can be used in areas where water is scarce if the filter system contains a separation tank. This is a capture tank with a bag inside that catches the DE and the dirt, allowing clean water to return to the swimming pool or spa during backwashing. The trade-off is that the cost to add the separation tank is probably more than the cost of the filter. Also, the equipment pad will need to be enlarged to accommodate the tank and the extra plumbing.

Where water is at a premium, one successful sales approach is to recommend a very large cartridge filter for the swimming pool. This filter is big enough that it only needs to be cleaned once or twice during a swimming season. This saves water and also makes the pool easier to maintain.

A smaller filter may be just the right size and fit the pocketbook, but the trade-off is that the filtration quality will not be as good. A smaller filter also will become an ongoing maintenance problem, which means it will need to be replaced much sooner than a larger one. Some of the new “mega” cartridge filters may go as long as 6 or 7 years before the filter elements need to be replaced.

The best fit

Some filtration types work better for specific applications than others. For example, DE filtration works best for pools that require a high degree of clarity, since  this type of filter can remove very small particles. In fact, by the time you are ready to clean the filter, it is pulling out particles that are close to 1 micron in size.

The sand filter works best for commercial pools because the backwashing process can be automated. This means that it can be programmed to shut itself down, reverse all of the valves, backwash the filter, and then reverse the valves again and go back into operation — all without any human interaction.

For hot tubs and spas that contain smaller water volumes (e.g., 400 gallons), the cartridge filter is particularly effective because it is so large. A DE filter would work in a hot tub or a spa, but the problem is that you are dealing with hot water that contains a lot of body oils and suntan lotions that end up coating the filter. The cartridge filter is larger, so it can handle a lot more of this type of coating before it needs to be cleaned. A sand filter used in a spa at a resort, for instance, would need to be backwashed almost daily.

When deciding on the best filter for your project, you need to consider several factors. Make sure the filter fits your budget. It should also be manufactured by a company that stands by its products and has a good salesperson that takes care of your company. Remember: It is a good idea to experiment with different types and brands of filters. There is always a new idea that may become the next best filter design.

The optimum flow rate

To maximize the effectiveness of a filter, make sure that the water flow is at the optimum level [see graphic]. The most important finding from the tests we performed was the effect that high-velocity water flow had on the quality of filtration and the cycle time, which is the time it takes for a filter to accumulate enough debris to require cleaning.

High velocities of water flow will cause filters to short-cycle and be harder to clean. “High velocity” means exceeding a certain speed of water flow through the piping as measured in feet per second (fps). This includes the piping inside the filter. A measurement of 10 fps is considered high velocity. This is approximately 70 gpm through a 1-inch PVC pipe (a modern 1-hp pump) and 100 gpm for a 2-inch PVC pipe (a 1.5-hp or higher pump).

Some manufacturers once claimed that the higher the velocity, the better the filtration — but this is the opposite of what really happens in a filter. But, if you take a filter and move very high-velocity water flow through it, you can practically pass a bowling ball through the filter. If you slow it down, you are actually pulling microbes and bacteria out of the water. The problem with velocity is that it just drives the particles right through the filter.

Here are the negative effects of high-velocity water flow through the different types of filters:

High-rate sand filters. High-velocity water flow will drive the dirt particles deeper into the sand bed than the filter was designed for, requiring a longer backwash cycle to release the dirt particles so they can exit the filter to waste. In some cases, high-velocity flows can drive the dirt completely through the sand bed and back into the pool or spa. This is called channeling. When this happens, there is little or no filtration taking place. High-velocity flows will wear down the rough sand particles to the shape of smooth marbles. At that point, the sand will need to be replaced. 

Diatomaceous earth filters (DE). High-velocity flows can drive the dirt particles into and through the fabric covering the filter grid. The weave of the cloth becomes clogged and cannot be backwashed out to waste. The clogged weave reduces the amount of filtering area, causing short filter cycles. Also, this makes the filter difficult to clean. You will need to take the filter apart and soak and clean each grid manually. This condition may require the replacement of the filter grids as well. 

Cartridge filters. The cartridge filter has a more difficult time handling excessive flows than the other two types of filters. When this filter was introduced in the 1960s, there was great excitement. With a lot of filter area in a small tank, the industry saw many benefits to using it. Unfortunately, this was also a time when high-horsepower pumps were being aggressively sold. The high-velocity water flows just drove the dirt particles into the filter cloth and, in most cases, completely through the filter. It was so bad that many of the health departments banned the use of cartridge filters for commercial pools and spas.

These bans are no longer in place, thanks to larger cartridge filters with improved plumbing. When cartridge filters first came out, the water flow rate was fast because the piping was smaller (1 inch). Now the plumbing is up to around 2 inches — or in the case of one filter, there is hardly any plumbing in it at all. This means we can now move higher flows through cartridge filters than previously. Thanks to these advancements — and the convenience of easy maintenance — the cartridge filter has become the predominantly used filter in the pool and spa industry.

For best results, keep your water flow rate as low as possible. Variable frequency drive pumps are a new energy-efficient way of pushing water through the system. With the rising costs of electricity, these pumps are very popular today, and provide the unique advantage of allowing you to control water flow with the push of a button.

Fred Hare is an aquatic consultant based in Roseville, Calif. He retired from Pentair Pool Products in 2002 after 44 years in the industry.