What Causes Filter Channeling?
First of all, channeling is a problem specific to pool sand filters. When the sand (or crushed glass) media captures enough material that changes its physical properties, its ability to filter changes. Normally, this material is non-living organic waste, calcification caused by a high-LSI violation of some kind, or maybe a synthetic material that accidentally got introduced to the pool. Regardless of what compromises the sand, the sand's ability to screen particles out of water is reduced. It creates uneven pressure spots where water is trying to force its way through the media.
Related: Organic Waste and Carbon Management (Pillar 2)
Eventually, these uneven pressure spots in the filter can form an opening, called a channel. When the channel opens up enough, unfiltered water can flow through it and bypass the majority of the sand. The water rushes straight to the filter laterals at the bottom and continues on through circulation. Cloudy pool water is almost inevitable at that point.
A common form of filter channeling is when water seeps around the edges of a sand filter tank, avoiding the sand bed almost entirely. This is especially evident in pressure sand filters after backwashing. You can see pictures and video of two examples in this article.
Sand Filtration and Particle Screening
Of the three most common filter types in the pool business, sand filters have the lowest screening capacity. Sand filters can capture particles as small as 20-30 Microns (µ). Compared to cartridge filters (8-12 µ) or Diatomaceous Earth (D.E.) filters (down to 1 µ). This is not necessarily a bad thing though, because sand filters still pull out particles smaller than what the naked eye can see. Indeed it is possible to have world class water clarity with a sand filter.
Sand filters are comparatively easy to manage. They do not require too much cleaning, because sand filters can be backwashed. Backwashing is when the plumbing is reversed so water is pumped in the reverse direction, from the laterals up through the sand, and out to waste.
Symptoms of Filter Channeling
Abnormal backwashing cycles
If you're backwashing often without a noticeable change in pressure...or the water being wasted looks 'okay', it may be a symptom of filter channeling. These problems are often caused by heavy organic loads building up in the filter sand and clumping it together. If not just organic loads, calcium can harden in the sand too, as well as polymer-based pool products, aluminum and phosphate removers. Backwashing with minimal results should raise a red flag. We opened up a filter and saw a bad case of channeling up close.
Ridges inside the filter, around the edges, or holes
If you suspect you have channeling, shut the pump off and look in the filter. There could be pockets through the sand (a channel), but more likely, ridges of raised sand form around the inside edges. These ridges indicate water is going around the sand bed along the sides of the filter tank itself, and form during backwashing. The middle of the sand bed, in this situation, should look fairly settled with lots of debris on top.
These ridges are found in pressure sand filters, not vacuum sand filters. They are a dead giveaway of channeling, but are normally only seen in large commercial sand filters. Residential filters tend to not show such obvious ridges or holes.
Cloudy Pool Water
Obviously, cloudy water can be caused by a myriad of factors...channeling is just one of them. It can't hurt to take a look at the filter and see what's going on.
How to fix a filter channeling problem
Understanding what causes channels to form is the first step: the sand is getting clumped together somehow. Pool operators can "un-clump" the filter sand, and that may fix the problem right there. Operators, service techs and staff can look at a channeled filter, but it still may not be evident exactly what is clumping the sand. Non-living organics involve themselves in filter problems in almost every case. They may not be alone in the clumping effort, but they're almost certainly part of it.
If your pool has high calcium or evidence of carbonate scale, calcium could be part of the problem. If you use polymer-based products like clarifiers, those polymers can contribute to the clumping too. So can aluminum-based products like flocculants and some phosphate removers (don't worry, not ours).
We recommend doing a deep clean/purge of your filters with our enzyme and phosphate remover and see what happens. It's an affordable way to remove organic contamination and refurbish your sand. If a channeling problem persists, it will be easier to problem solve once you get those non-living organics and gunk out of the equation.