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Water Purification – Mechanical vs. Chemical Methods

There are two basic ways to remove contamination from water: remove it physically or zap it with chemicals. I would argue that based on the way we do things in the aquatics industries in the U.S., our focus is largely on the chemical treatment side, and we pay less attention to the physical removal part of the process.  

I think that’s a mistake because both are equally important in the quest for ultra-pure water. On that charge, I have to plead guilty to having paid a tremendous amount of attention to the use of chemicals to purify water, and rightfully so; it’s an indispensable part of the overall treatment equation. As a result, I’m among a majority of people who probably have not talked enough about physical removal of pollutants (meaning filtration, along with skimming, draining/replacement, and possibly flocculation).   

The virtue of those treatment elements is that they remove material that would otherwise have to be treated chemically, processes that create nasty things such as disinfection by-products. I contend that it makes more sense to physically remove unwanted dissolved and free-floating constituents from the water before you undertake the chemical treatment.  

In other words, maybe we should be thinking about filtration on equal footing with chemical treatment.    

Contrary View 

Here is where I part company with many of my peers who rely on either cartridge filters or diatomaceous earth (DE) filter media.  

Based on my many years of experience running a service business, I believe both of those technologies are inferior to sand filtration, especially when compared to slower and larger sand filtration. I know, I know, cartridges save space and water, and they filter to a small micron size. And I also know that DE filters can do a great job of polishing dirty water very quickly and, when applied and maintained correctly, result in good water quality.  

But I’ve always experienced problems with both of those filter types precisely because of the maintenance requirements. If you don’t regularly clean cartridge filters, they collect the muck and basically constantly reintroduce the contamination into the water — the way a dirty oil filter in a car contributes to the buildup of lubrication and combustion by-products. Likewise, cleaning a DE filter is a horrid mess, and if that’s not done regularly, it also re-contaminates the water.   

Sand doesn’t have those problems. Particulates don’t absorb in sand the way that they do in DE filters and cartridges, and they’re easily released during backwashing, which can be done automatically, essentially requiring no maintenance. Although backwashing does use water, from a treatment standpoint, draining and replacing water dramatically contributes to water quality. With sand filtration, along with robust skimming and regular maintenance, we effectively remove a tremendous amount of the stuff that would otherwise have to be handled by the chemical treatment.  

Less Really IS More 

This is all a big part of our SRK HydroZone 3® concept. We don’t put as much pressure on the chemical treatment side of the process because we remove much more of the organic and biological load by slowing down the filtration and using upsized sand filters and by constantly and rapidly removing the top layer of water via skimming action.  

When you combine that approach to filtration with our multi-layered chemical treatment — ozone, UV, and low chlorine residuals — you have superior water quality that is resilient to things such as a sudden surge in bather load. With the power of sand, it’s easier to remove contaminants ahead of chemical treatment and everything just becomes easier and more reliable.   

It’s interesting to note that we’re not the only ones on the planet that take this approach. In Germany, for example, they base the treatment of public pools on a document known as the DIN Standard. Many consider it to be the most advanced set of water treatment standards in the world, a point I’ll leave for others to debate.  

Interestingly, the DIN Standard mandates massive low-rate, multi-layered sand filters with large freeboard. It also requires drain/replace regimens based on bather load, and all pools built to this standard have 360-degree perimeter overflow systems in order to absolutely maximize skimming action. And, the DIN Standard specifies chlorine residuals of 0.2 to 0.6 parts per million (ppm), which is only possible because they are physically treating the water ahead of treating it chemically.  

I cannot say that our HydroZone 3 pools would be perfectly compliant with the German standard, but there’s no question they benefit from many of the same important ideas. Yes, upsized and slowed-down sand filtration is a key element, but it is just one of the many important factors that creates a synergistic and efficient treatment scheme. It all works hand in hand.   

the Water Geek


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