Re: Domestic pool turnover rates
Originally Posted by Teapot
You don't filter out pathogens. Bacteria and viruses are to be killed by chlorine in the bulk pool water to prevent person-to-person transmission. It's only the protozoan oocysts that take a lot longer, especially for Cryptosporidium parvum that for practical purposes isn't killed by chlorine (at normal levels), but that is very uncommon in residential pools. Those Crypto spores are also too small to get caught in sand or even most cartridge filters without coagulants. In the U.S., the CDC Model Aquatic Health Code (MAHC) proposal requires the use of supplemental systems such as ozone or UV in the circulation system to kill Crypto in high-risk venues. There are some other unusual pathogens that aren't killed readily by chlorine such as iron bacteria and of course if one isn't diligent about maintaining the proper FC/CYA level then biofilms can form and be more difficult to remove.
If you are running on low speed, then unless you have an outstanding circulation system with many returns, you won't get great skimmer action. At low flow rates one simply doesn't have enough water flow to move leaves around the pool and into the skimmer quickly enough. This depends, of course, on pool design. Some pools will work OK at low speeds, but many won't since they were designed originally for high speed pumps so the placement and number of returns is pretty paltry when run at low speeds.
I agree with you though that if you don't run during some times throughout the day (not necessarily 24 hours, but on and off so the "off times" aren't too long, then that will help prevent surface debris from getting water logged and sinking. Of course, with a pool sweep such items can get collected from the bottom so I'm not sure why that's a problem. If you don't have a pool sweep, then what you propose keeping the water moving at the surface longer would make some sense, with the caveat of sufficient flow to actually move things towards and into the skimmer.
By the way, we aren't proposing to run at high speeds for long times. If a pool can run at low speeds all the time that the pump is on with reasonable skimmer action, then great. What Mark is saying is that whatever the pump speed is, it doesn't usually need to be run for as long as people think. You can run your low speed for perhaps hours over the 24 hour period and have great water clarity. If one needs some skimmer action, then perhaps one needs an hour every 8 hours at high speed for that. And so on. In other words, use the low speed whenever possible and use the shortest times to achieve the desired result. That results in the lowest energy usage to achieve the desired water clarity and skimming action.
As for low TA levels, that has been suggested for many years to have more stable pH. I don't remember when Ben suggested it but it was a long time ago. Just remember that if you have a plaster pool then you need to have a higher pH target and/or higher CH to compensate for the saturation index to saturate the water with calcium carbonate to protect the plaster.
See the section "Turnover Rate and Law of Dilution" in the first post of the thread Certified Pool Operator (CPO) training -- What is not taught. The Gage and Bidwell model does NOT tell you how much of the pool water has "seen" the filter. It is less than that because they assume a specific amount of contamination introduced once per day and then calculate the clarification percentage. He says that one turnover has only 42% clarification. Fortunately you aren't quoting that number and are instead quoting the correct 63% number, but that has nothing to do with Gage and Bidwell. So your numbers are correct, but you shouldn't refer to Gage and Bidwell since that can be confusing.
Believe me, you won't be filtering out bacteria. Most bacteria are going to attach themselves to surfaces or to particles because they generally only reproduce affixed and not when free-floating. You can't rely on filtration for most pathogens. Oocysts and spores are difference since they generally don't attach and instead float through the water so will generally get circulated. Though the same is also true for viruses, filters won't capture them (without coagulation).
Mark has always said that using low-speed (or the lower speeds of variable speed) pumps saves a lot of energy and you agree with that. So the only question left is how long to run at this lower speed. This is where you two differ, but I think would agree that you need to run at higher speed part of the time if that is necessary to achieve a specific goal such as getting proper skimmer action and that clearly depends a lot on specific pool design.
Then the question becomes how to space out this higher speed over the day and how long to run at low speed and how to space that out. Here is where you differ where Mark is saying you don't need even a full turnover in most residential pools whereas you are going for 24/7 filtration and have more turnovers. That obviously uses more energy than if you cut back your total runtime, even if you spaced it so that you didn't have long "off" periods to still achieve your goal of having mostly skimmer action rather than picking up debris in a pool sweep.
My pool runs for roughly one turnover per day but not because of any filtration need but rather because my solar system runs at 48 GPM and is on longer, usually 4-6 hours per day depending on the time of year. The pump is on low speed with 26 GPM for the other 2-4 hours for a total of 8. With my 16,000 gallon pool this translates into 1.11 to 1.275 turnovers. My pool looks like the following:
and a photo of one of my main drains at night is the following:
Since I only have a cartridge filter, I'm sure I could get even better water clarity but it's plenty fine for me as it is. The only thing that might improve from even longer circulation is reducing the number of floaties one sees in the pool light assuming that such floaties aren't just in a "dead" pattern going around the pool (i.e. a bad circulation pattern that extra circulation time won't fix). I have a pool cover that keeps the pool relatively free of debris though it is used nearly every day so during that time stuff does get blown in. I do clear the skimmer basket, pool sweep bag, and pump basket of debris at least weekly (skimmer basket sometimes more frequently, especially during fall).
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