Tech Talk: Turnover vs. Turnover Rate

Do you know the difference between a "turnover" and a "turnover rate"? They are quite different, often confused, even used erroneously in popular pool-operator texts. A turnover is the time it takes for a pool’s circulation system to handle as many gallons as the pool holds (not all of its water; only about 40% of it!), and the turnover rate is the number of times per day all this happens. A six-hour turnover is equivalent to a turnover rate of four. A two-hour turnover becomes a turnover rate of 12. So a short turnover means a high turnover rate! If you’re a pro, you won’t mix ‘em up!

Let's look some more at this topic:

In most states and in Canada, standard public pools are required to turn over their pools four times a day. This term "turnover", even when used correctly, is a misnomer as a noun or a verb, as the implication is that the entire pool’s content is run through the system in that many hours. "I turn my pool over in five hours" means only that in five hours the number of gallons circulated equals the number of gallons contained in the pool.

According to two scientists – Doctors Gage and Bidwell – and their law of dilution, over half of the pool’s water during the first so-called turnover gets nowhere near the filter. The other "half" of the pool’s volume has seen the filter twice during that time. Indeed, some "parcels" of that water went through the system three times. Do you suppose they tagged little water bundles and watched them flow around, or dyed them different colors? We’re told it was actually done with "calibrated" soil additions and periodic measurements of turbidity, which apparently is more scientific. The concept illustrated is really all we need to know.

Looking at the height of columns in the chart below, representing the water that has yet to see the filter at each successive cycle, one can see the 58% unfiltered water after that first turn. Note that at the second turnover the filtration exposure improved from 42 to 84 percent (16% not yet circulated), then for the third turn, 95 percent made it with 5% still not processed. Here is where earlier pool engineers stopped, as the point of diminishing returns appeared to have occurred. Achieving the three cycles in a day, eight hours each, became the design target.

Columns represent water not yet filtered.

As pool designers and health departments became more conservative, one more step was added - now four cycles per day - improving filtration exposure to 98 percent. Certainly by now "diminishing returns" have been reached. This change was not without considerable cost to new-pool owners. With the fourth turnover (now six hours long), we have increased the size of the pipes, filters, pumps, feeders and some operating costs by a full 25 percent and improved the filtration by only a miniscule three!

Apparently it was an appropriate move for the industry, as virtually all of the health departments in the USA have made the change and now require the six-hour rate – at least while the pool is open for use. Separating theory from practice helps us see the real reason why the change was wise; the scientific tests were done with just one dose of "soil" per day, while a pool experiencing heavy use has introductions of solid contaminants rather constantly.

And in the cases of spas, waterslides, therapy pools and the like, with tremendous organic and particulate loads, the dilution law is completely out the window. Much more frequent circulation "opportunities" for filtration and treatment are achieved by turnover times (dictated variously with different regulatory bodies) from two hours down to as little as 30 minutes. Forty eight times per day at 30 minutes – that’s a lot of circulation turnovers. Maybe so, but many busy public spas are now designed for ten minute loops, and owners of the busiest of these find filtering that steamy soup 142 times a day is barely enough! Thank goodness six hours is enough for most public pools.

"Most", however, doesn’t mean "all". You may have read – in PrP 10, Pumprooms are Getting Bigger – that worldwide trends are moving towards shorter turnovers yet. (If you didn’t read it and are remotely interested, you should...) From four hours to as quick at two hours are turnover values now becoming design targets in Europe and the Orient. If your pool is staggeringly loaded with swimmers and their organic gifts, you have probably discovered that your filtration gets behind, sanitizer/oxidizer feed can’t keep up, clarity suffers and your swimmers are at risk.

Filtration becomes somewhat more important than it was back when we all were injecting gas (especially if you are using bromine). If you are using serious ozone, more turnovers give you more exposure to the enhanced oxidation as well as to the filter. The rule is simple: If your soil removal rate is greater than your introduction rate, then – and only then – you will keep that sparkling pool you desire.

When you are on a planning committee, consider faster flows, higher turnover rates, shorter turnover times. You’ll be considering better water.

~kw

PrP 15, modified

©1997 Professional Pool Operators of America