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

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 organic contamination.

The Hypothesis

We know that in an aqueous environment that contains bacteria, organic contamination 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 organic contamination over time. Organic contamination is very sticky so the particles stick together. As time and pressure continue to pack the organic contamination-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 organic contamination. In fact there are very few things that will remove organic contamination. 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 organic contamination in one hour, and that many other solutions that claim to remove organic contamination 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 organic contamination, filters and water we are aiming to create organic contamination free aquatic systems that require less chemicals, maintenance, and unwanted side effects.