The pros and cons of the main types of pool filters — sand, diatomaceous earth (DE), cartridge — are generally well known in the pool industry. But which is the best filter for a particular job? Which is best for the environment? And is the most environmentally friendly filter necessarily best for every job?

At the fundamental level, sand filters are easiest to maintain because cleaning them requires only turning a backwash valve. Simple enough. Plus, the sand inside a sand filter holds up much longer than the media inside other types of filters. With DE, you get the clearest water because those fossilized exoskeletons of tiny diatoms filter out particles as small as 5 microns, effectively "polishing" the water. Meanwhile, the benefits of cartridge filters, particularly today's larger models, are relatively long filter cycles, more square footage of filtration area, and the fact that cleaning a cartridge filter consumes less water than the backwashing process typically required to clean sand and DE filters. And even large cartridge filters take up less real estate on the pad than a sand filter.

photo of equipment pad for pool using sand and cartridge filters

Another point of comparison between filter types is the amount of resistance each adds to the system. This is particularly relevant for those interested in creating the most hydraulically efficient system. Sand filters add the most resistance to a pool plumbing system because moving water through the media bed requires high pressure. DE filters are next on the resistance comparison line, and cartridge filters add the least. In addition, sand and DE filters typically require backwash valves, which also add resistance because flow through typical backwash valves is inefficient. Because they don't require backwashing, cartridge filters often garner favor from those seeking the most-efficient filter.

The fundamental differences between the filter types seem to suggest that people who want to do the least work possible should opt for sand filters, people with the highest water-clarity standards should choose DE, and people who prioritize such environmental concerns as water conservation and reducing energy consumption should choose cartridge. But it's not that simple.

Some of the complications arise from recent product innovation that claims to have married the clarity of DE with the simplicity of cartridge. This is made possible by new DE grid designs that can be cleaned by rinsing with a hose — the same procedure as cleaning some cartridge filters.

photo of DE filter and gas heater for a pool

In many situations, the answer lies in determining the most significant need a new filter will have to meet — is it water clarity, physical size, ease of maintenance, conserving energy use and costs, or concern for the environment?

When presenting options to home-owners, for both new pools and renovation work, Rick Chafey, co-owner of Red Rock Pools and Spas in Gilbert, Ariz., almost always recommends large cartridge filters. However, on one recent job, he replaced a cartridge filter with a sand filter. Why? Because there was a massive amount of dirt getting into the pool.

The cartridges on this particular job required cleaning more than once a month because of frequent dust storms that the pool was not sheltered from, Chafey explains, adding that the cleaning frequency on this job was far greater than what even the most rugged cartridges are designed to withstand. Therefore, the cartridges were wearing out far sooner than the same cartridges would in a normal dirt-load situation. And the work to tear down the filters, clean the cartridges and reassemble the filters was excessive. When that task is required only once or twice per season, which is normal for large cartridge filters, it is quite manageable, but these cartridges were going from clean to dirty in about three weeks, Chafey recalls.

photo of equipment pad with a large cartridge filter

So Red Rock replaced one of the two cartridge filters on this pool with a sand filter, which now handles the massive dirt load for the pool. The remaining cartridge filter is plumbed in for the laminars, which require cartridge filters, Chafey explains, because sand filters do not remove the smaller particles which could cause tiny blockages in the laminar nozzles and interfere with the smooth effect of these water features.

Another reason that Chafey typically specs cartridge filters is that they perform better at slower speeds than other types of filters, and Chafey uses variable-speed pumps whenever possible.

"When you run the pump longer and slower, you save energy," he explains. "Longer, slower filter cycles negate friction and head loss, so you get better filtration with slower flowing water."

To explain this phenomenon, Chafey describes trying to remove dirt from a T-shirt with both a fire hose and a sink faucet: A fire hose will push the dirt into and through the shirt, ruining it. However, holding a dirt-covered shirt under a sink faucet allows the dirt to rinse off the surface. Imagine the same comparison inside a filter. High pressure, high velocity water can force dirt into and through the media, rather than allowing it to be caught (i.e., filtered) by the media. Also, once it is embedded, it is very difficult to remove the dirt.

On a related note, the greater square footage of filtration area found in cartridge filters (compared with sand and DE) is one design feature that improves hydraulic efficiency. When you have 500 or more square feet of area on which to collect dirt (a typical size for large cartridge filters), there is a large area for water to move through, which translates to needing less energy to move that water.

This also means that resistance to flow increases at a slower rate as dirt collects inside a large cartridge filter because it takes longer for dirt to accumulate on a measurable percentage of the media.

The greater efficiency of cartridge filtration was central to another Chafey recommendation. Red Rock Pools was asked to bid on an overall efficiency upgrade, a bid which included two new cartridge filters in place of two aging DE filters. "They didn't realize there was an option other than DE," Chafey recalls, adding that this pool's features did not require the extraordinary water clarity associated with DE.

"If you have a customer who wants to view the pool at night through an acrylic window, then the water clarity you get with DE will be noticeable, but you won't see the difference during the day in a normal pool," according to Chafey.

Another reason for swapping in cartridge for DE on this job is that the filter elements had reached the end of their useful life. The DE filters could have been repaired and serviced, but Chafey saw that the cost of getting the DE filters back to working condition (taking them apart, replacing the elements, etc.) was closing in on the cost of new cartridge filters. The homeowner agreed to include new filters in what turned into a $20,000 equipment upgrade. (The job also included a heat pump in for a gas heater, variable-speed pumps in for older technology and a new automation system.) A photo of the equipment pad prior to the upgrade is on page 30. This project also presented a challenge in that there was no good place on the property for disposal of the DE while backwashing. The property is on the side of a mountain in Phoenix. It has no lawn to speak of (which is where used DE is sometimes distributed), nor was backwashing into the sewer practical because of the distance and elevation between the filter and the nearest sewer. (Many municipalities have rules against this practice.)

In both cases, Red Rock's customers ended up with more-efficient, easier-to-care-for equipment pads and the right filters for the jobs.

Comments or thoughts on this article? Please e-mail editors@aquamagazine.com.

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Scott Hertzog wrote:
I feel it's of utmost importance to consider the end user's ability to maintain the pool/filter. Those with physical limitations might enjoy the simplicity of sand filtration. The hands-on tinkering personalities might like DE since there are more parts and perhaps a greater need to more frequently chemically clean the filter to keep it from bogging down and short cycling. They all remove microscopic particles and yield crystal clear water, so bring your customer's needs into consideration.

You hit the nail on the head...a hundred years ago in Indiana, when I visited a homeowner to sell a new pool, I would always try to peek into the garage. If I saw a nice, organized workbench with tools hung on pegboard and everything in its place, I would pitch the DE filters. If it was cluttered with half-finished projects (like mine LOL) I pitched sand.
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I have found that cartridge filters will require the same amount of water for cleaning once you drain the tank, which can hold 30-50 gallons of water, then the garden hose to clean the cartridge can require 50 or more gallons, while back washing a sand filter uses about the same. The other concern is environmental aspect of receiving the old cartridges every two years to slowly degrade. Sand can last as much as 10 years or so and be spread over the yard when removed! when it comes to pump efficiency, the NSF determines the turnover, so properly sizing the pump with a single speed motor to properly meet this demand has no parallel, and the initial as well as replacement cost WILL SAVE the homeowner a lot of money!!!
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Thank you for explaining the differences. I have my original 4 cartridges from Rainbow Plastics and want to upgrade while I upgrade my pump. I am going with a variable Pentair pump and now feel more comfortable with my decision to stay with cartridges despite more pool companies steering me towards DE. There is an additional reason to not use DE. It can cause cancer!
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I want to thank you very much for the report on the problem with frequent dust storms in AZ. I live in N Texas where there are massive rebuilding of the infra structure. Specifically the interstate and toll roads. The resulting dust (concrete, road dust and general building dust) which would explain why we have had to increase the cleaning cycle on all filter types.

Now to my question what's the drill on DE grid filter vs DE cartridge filters?
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I really like most of this breakdown describing filter differences! In the comments describing a backwash valve, to vac to waste, is a must, with even the best intentions, a pool will at some point go south, and the ability to remove debris and algae to waste is the best answer. Another observation is regional operation also dictates filter types, the sand filter is by far the most forgiving when in areas of high dust and in the central mideast, where the diversity of flora and fauna cause high filter loads, a sand filter is a must!
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Scott Hertzog Monday, 02 May 2016
I feel it's of utmost importance to consider the end user's ability to maintain the pool/filter. Those with physical limitations might enjoy the simplicity of sand filtration. The hands-on tinkering personalities might like DE since there are more parts and perhaps a greater need to more frequently chemically clean the filter to keep it from bogging down and short cycling. They all remove microscopic particles and yield crystal clear water, so bring your customer's needs into consideration.
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Having read many of Rick's comments on other forums, I can say technically, as a peer, he is exactly on the mark. I would like to add that while most DE systems and side mount sand filters are installed with MPVs, a push/pull valve will also help reduce the resistance. As for cartridge systems, I usually see them directly plumbed or with a T, one side to the pump discharge, one to the filter and a 3rd with a bushing and a garden hose spigot to lower a pool that is too full. This is annoying. I think a better way is to use a 3 way valve instead of the T. The drain port can then be plumbed to a backwash line and the homeowner can vacuum to waste without gunking up the cartridges, say, after an algae bloom. It also makes lower the pool level much faster. The $45 delta in costs vs convenience is huge. There is a push to make pools that cost less to operate. Energy costs are going up and will continue to do so. VSP, big pipes, and big filters will all play a role in system design. With automation making significant leaps in capabilities, the opportunities to make it easier than ever to own and operate a pool successfully will temp new and existing customers like never before. I expect that in the next few years, it will be common for techs to check the customer's pools from a PC or phone. It can be done today but will become common before long. It's going to be a fun ride!
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Hot Tub Filters Tuesday, 26 July 2011
I agree...right on the mark. I learned some new info. Great info!