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Cartridge Filters – What People Might Not Know

Making great water, the kind you can’t wait to get into, ultimately comes down to making the best choices among the available pool water treatment options. That can be tough because the pool and spa industry is loaded with various chemical products and devices that claim to be the best thing since canned beer.

The selection process becomes a challenge because unless you have experience with a given pool water treatment option, there’s really no way to know how effective it might actually be. That’s certainly true when it comes the type of filtration you choose. We have basically three primary categories of filter types used in pools and spas: sand, diatomaceous earth (DE) and cartridges. All three have their advantages and disadvantages and many people within the aquatics industry have strong opinions as to why they prefer one over the other two.

For the past several years, there is little question that cartridge filtration has become the choice for many designers and builders. Almost every set of plans I see these days from companies other than ours includes cartridge filters. Anecdotally speaking, it’s clear to me that this technology has become the most popular.

I’m writing this blog because I respectfully, but also strongly, disagree. Based on nearly three decades of experience servicing and building swimming pools, I believe cartridge filters are not the best choice. I’ll explain why, but first let’s look at the advantages that have attracted so many water quality professionals and consumers to these products that have, for better or worse, become water treatment mainstays.

Not So Fine Filtration

Perhaps the biggest selling point is that cartridge filters remove particles down to 1-5 microns, compared to 15-20 microns in a sand filter. Naturally, the idea of removing smaller particles to increase pool water clarity is very appealing. And it’s true – if you put a cartridge filter on a pool that has water quality issues, it will polish the water. Cartridge filters are not backwashed, so they also have the appeal of water conservation. And, they are easier to plumb than sand or DE, with no multi-port backwash valve – it’s just one line in and one line out.

For a long time, I didn’t disagree, at least not entirely. But, when I started looking at the issue more closely, rather than simply taking the product claims at their word, things changed. I started noticing more and more water quality problems in pools with cartridges, far more so than those with sand or DE, and the maintenance challenge of the necessary routine cleanings became a logistical nightmare.

Over the years, I’ve been called into troubleshoot many a swimming pool, commercial and residential. When I’d take apart the cartridge filter, very often the inside of the filter was disgusting. We’re talking inches upon inches of biofilm, festering in the filter, contaminating the water rather than cleaning it. Obviously, the immediate answer was to put in clean cartridges, and that the problem was that someone was neglecting the maintenance rather than the filter itself. That’s true, but therein also lies the catch – the need to take these things apart every month or two to replace and clean the cartridges.

It’s a fact that maintaining cartridge filters is an unpleasant mess. It’s a painstaking process that requires  time and energy. The process also adds ongoing expense for the swimming pool customer, unless they simply decide to live with contaminated water.

Alternatively, with sand and DE, you backwash the filters which requires virtually no effort, time or expense. Some people assume that if the pool is not used often they can go longer between cleanings, without taking into account the dust and pollen constantly falling into the water, causing the cartridges to quickly load up with biofilm. As a result, in the course of real-life human events, it is true that it’s easy to go too long between cleanings. Because the filtering is down to such a fine particle size, the pool water becomes fouled really fast. In that sense, their greatest advantage, the fine filtration, becomes the biggest problem. It accelerates the frequency of needed cleaning, and that ultimately means too many filters remain dirty for too long. So, as the dirty filter does its thing, the pool water quality is compromised, chemical consumption goes up, and the situation deteriorates. It’s a dirty water snowball effect.

Why Sand

In my book, “The Water Quality Professional: transforming aquatic management”, I describe my preference for sand filtration. I believe if you’re doing everything correctly in terms of pool water balance, sanitization, oxidation, skimming, water distribution and service routines, the 15-20 microns filtered by a sand filter is more than adequate.

The fact is that sand is much easier to maintain, and as a result, you’re always running the pool on clean filtration media. In fact, with sand, as the filter bed loads, it becomes more efficient. That’s up to a point, of course, but that’s when you backwash, which these days can be totally automated.

the Water Geek


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A blog about all things water, written by SRK's founder Steve Kenny.