Sand Filtration: the GOOD, the BAD, and the UGLY

April 26th, 2010


Anyone who owns, cares for, or cares about pools, spas or any kind of recreational water, knows that water filtration is an integral part of every water system.  Using sand for filtration is commonly used on recreational and residential pools and large spas.  Here are some of my observations, thoughts and concerns about sand filtration.

The Good

Sand is cheap, plentiful, and when it is a particle, it works well as a filter medium.

The Bad

Sand filters are usually filled, sealed and the sand is forgotten.  The commonly held belief is that back washing the sand periodically, “fluffs it up” and returns the sand to a particle state where it can again work its magic as a particulate filter.  Some sand filters have never been opened for 5-10 years to inspect the sand.

Back-washing the sand filter is costly.  Water lost during back washing needs to be replaced, heated and treated.  Ideally, the pool operators backwash often enough to keep the sand working as a filter, but do not needlessly back wash so water, heat, chemicals and time aren’t wasted.

The Ugly

Inspecting and analyzing the sand from pool sand filters in both residential and commercial pools has been enlightening, to say the least.  At the bottom and sides of many filters we found sandstone.  Actual sand in the process of forming sandstone. It wasn’t the gravel that is often put down underneath the sand, but sandstone.  The sand in those filters was anywhere from 2-10 years old.  The sand that wasn’t rock was sticky and foul.  When we tested it in our laboratory, we found that it was full of biofilm.

The Hypothesis

We know that in an aqueous environment that contains bacteria, biofilm forms on every surface.  To be effective, filters have enormous surface area whether they are made from sand, charcoal, paper, glass or diatomaceous earth.  The particles become covered with biofilm over time.  Biofilm is very sticky so the particles stick together.  As time and pressure continue to pack the biofilm-coated particles together they eventually become rock.  So what happens during backwashing?  The water will take the path of least resistance.  We observed in these filters that there were channels in the sand.  We think that the water follows channels through the sand that have become established over time.

We know that backwashing will not remove biofilm.  In fact there are very few things that will remove biofilm.  Strong acid or base solutions work but they destroy the filter, pumps, valves etc.  We have demonstrated that a flush used in spas removes 90% of laboratory created biofilm in one hour, and that many other solutions that claim to remove biofilm don’t.

Getting Better Results

We also have observed that sand in filters where the water is treated with PoolNaturally® Plus (the commercial version of the residential product PoolNaturally®) appears to remain as particles.  We think this is why we needed to backwash filters with PoolNaturally Plus much less often than those with conventional water treatment.

By understanding the relationship between biofilm, filters and water we are aiming to create biofilm free aquatic systems that require less chemicals, maintenance, and unwanted side effects.

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2 Responses to “Sand Filtration: the GOOD, the BAD, and the UGLY”

  1. Colleen Says:

    I believe I have a biofilm problem. When we changed our sand this year, it looked just as you described in one of your posts. Also, it has thus far been impossible to get a free chlorine reading since opening the pool in May, even though large amounts of shock have been added (three different kinds). If I do have biofilm, would that prevent the shock from working properly?

  2. gina Says:

    If there is still a large quantity of biofilm in your pool plumbing and pool surfaces (which there likely is), it is absorbing much of the chemical you are adding to your pool, leaving little to produce a free available chlorine (FAC) residual. Changing your sand was a great first step toward cleaning up your pool, greatly improving filtration, and using PoolNaturally would further improve the quality of your pool water. Allow the moss up to a week to condition the water and you should see a FAC residual develop without intense shocking.

    Without additional information about your pool and what you are using to treat it, it is difficult to offer other specific advice. Please send more information to or call us at 1-877-212-6493 and ask for Customer Service and we would be happy to assist you.

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