Ask Al

BY ALVARO G. MENDOZA

COMMERCIAL ENERGY SPECIALISTS, INC.

WE HAVE SAND FILTERS, BUT ARE A BIT UNHAPPY WITH THE POOL WATER QUALITY, ESPECIALLY AFTER HEAVY BATHER LOADS. THE FILTERS ARE IN GOOD SHAPE AND MONEY IS A BIT TIGHT RIGHT NOW TO MAKE ANY MAJOR CHANGES. IS THERE ANYTHING THAT CAN BE DONE TO IMPROVE OUR WATER QUALITY?

Welcome back to our “Ask the Expert” feature designed to assist you with issues related to swimming pool water, mechanical equipment, space conditioning, and code compliance. Ask a question, and we will try to answer to the best of our ability.

Sure thing. There are lots of swim schools that are in a similar situation, and don’t know what to do. Fortunately, there are a couple of relatively simple options to improve water quality.

First, let’s look at some basics. Sand filters work great if they are properly designed, outfitted, and operated. Filter design sizing is rated by flow (in gallons per minute or GPM) relative to the square foot (sq ft) size of the filter (the size is normally on the filter data plate). While a typical 36” diameter model that contains 7 sq ft of filter area is rated up to 20 GPM per sq ft or 140 GPM, that is far too much flow through the filter media. Best practice has dictated that the REAL maximum flow through a sand filter should be 15 GPM / sq ft or 105 GPM, and many researchers and engineers are now recommending 12 GPM / sq ft or less.

Why? Running the water too fast though the filter means it captures fewer solids (contributing to the cloudiness), and the solids that it DOES capture get driven way too deep into the filter bed, making it nearly impossible to remove through a typical backwash process. Deeply trapped solids cause a strain on the chlorination system, and eventually form mudballs and ruin media, necessitating replacement.

Also, proper design incorporates proper media. The most common media for decades has been silica sand. The best is highly angular, specially-graded sand, with proper and consistent grain size. The tech terms for this consistency is uniformity coefficient (UC). When these specs are adhered to, the media forms a tighter weave capturing more solids and filtering better. Inconsistently graded media of varied sizes or more rounded versions will filter poorly and may never “polish” the water. Unfortunately, an alarming percentage of store-bought silica sand fails to meet the optimum angularity and UC targets. Now, that we have identified some issues, here are some suggestions for supercharging your existing filtration system:

SLOW DOWN THE FLOW:

If your filters are operating at above 12 to 15 GPM / sq ft of filter area, there are only a few options to slow down the flow: upgrade to larger filters or install additional filters. This are both space and capital intensive endeavors, typically requiring filter room engineering services and Department of Health (DOH) approvals before commencing. If you’re going through this process, make sure that the filter sizing will drop below 12 GPM / sq ft to get maximum results.

A quicker and less costly alternative is to slow down the flow below 12 GPM / sq ft during off-peak hours. This is allowed in most codes, and won’t provide a complete solution, but normally yields great results. So, for example, if you have a 36” filter operating at 17 GPM / sq ft or 120 GPM, you could turn down to flow to 70 GPM, or 10 GPM / sq ft from 3-hours after closing to 3-hours before opening. This helps turn mediocre filters into much better filters, even if it is only for part of the day.

The best way to accomplish this is by integrating a VFD into your pool pumping system – a popular self-funding conversion. Since the Department of Energy (DOE) July 2021 pump efficiency regulation requires this conversion for most 1-5 HP pumps on a long-term basis, it will not be a wasted effort. If you have a certain model of chemistry controller, it will automate the entire off-peak flow regulation process, and if you don’t… a certain model of the VFD – in conjunction with a digital flow meter –can automate the process as well. You’ll notice a water quality improvement and save a ton of energy as well.

CONVERT YOUR EXISTING SAND FILTERS TO SPECIAL MEDIA:

Research shows that the type of the filter media can make a huge difference in your water quality. It turns out that FULLY Activated Glass Media (AFM® or equivalent) – made from 98% recycled brown and green bottles and treated with an electronic activation process – has been proven to provide sub-micron filtration. This is relatively unheard of, as lesser filter medias only filter down to 20-microns, allowing all 1- 20 micronsized particle to pass through over and over again. This type of glass media also typically uses half (or even less) of the typical backwash water as it fully cleans in 1.5-2 minutes instead of the typical 3 to 5- minute backwash. The activation process also helps prevent accumulation of organics and biofilm in the filter, thus helping avoid media change outs… and normally allowing the media to last for the life of the filter.

As a part of this glass media upgrade, research shows great results by injecting a suitable poly floc on the suction side of the pool pump and filter. This is best accomplished using an inexpensive programmable peristaltic feeder set to pump 1 oz for every 10,000 gallons of water pumped - not a lot. The poly floc will coat the filter bed, clumping smaller particles together so they can be better caught in the glass media bed. These particles are then removed from the filter during a future backwash. Certain brands of poly floc also remove phosphates without using any additional chemicals – thus saving money.

CONCLUSION:

Filtration is one of the keys to crisp, clear, and irritant-free pool water, but many swim schools were not equipped with the optimum systems to accomplish this on an-going basis. If you are interested in improving your water quality, there are several proven solutions, that are quite affordable. These upgrades may not have existed at the time that your school was built, but represent current best practice for highly volume swim school pools.

Anyone considering these upgrades should consult with a highly experienced source, in order to avoid the risk of doing it wrong and not showing improvement, or even regressing.

Properly done, these upgrades will provide years of great water quality with a minimum of labor and maintenance costs.

Best Regards, Alvaro G. Mendoza