Tech Talk: CO2 – Carbon Dioxide, All it's Gassed Up To Be

In this age of concern over the care and feeding of hazardous pool materials, a needed replacement for muriatic acid seems to have been found. If you're using a high-pH sanitizer in your pool (bleach, cal hypo or lithium hypo being the most common) chances are you've been using an offsetting acid – hoping each day you don't splash it on you clothes, squirt it in your eye, siphon it into the pool or spill enough to wreck the pump room. Maybe you should be using carbon dioxide... At first glance, its the answer to all those concerns and more. But is it, really?

This harmless, abundant gas is now being used very effectively as an acid substitute by a small-but-growing group of public-pool owners. Carbon dioxide (CO2), bubbled into the return line, produces carbonic acid which can offset those high-pH sanitizers, forever to take the place of troublesome, dangerous acids. Here's the list of the bubbly's advertised advantages: 1. CO2's completely safe; you can breath the stuff. 2. It's available everywhere and no more expensive than other methods of downside pH control. 3. It's easy to feed and to automate. 4. CO2 results in a pH no lower than 6.8, so even major feed errors can't drive the pH down to damaging levels. 5. It can replace acid in almost any pool.

Trouble is, none of these five advantages is really, completely true!

Let's look at the claims one at a time, discussing the true advantages and the honest weaknesses. Then we'll highlight some solid advances in this technology, finally allowing pool owners to consider anew the real possibilities for using CO2!

1) As you take this gas in with every breath (CO2 is about 3% of our atmosphere) and exhale even more of it, it's obviously pretty safe. The main concern of designers and users is ventilation of equipment compartments and chemical rooms; a massive leak of this heavier-than-air, odorless gas might displace all the air in an enclosed area. An unsuspecting operator could faint – even die – from lack of oxygen, without so much as a cough.

2) Yep, it's available everywhere, as CO2 is the carbonation in every fizzy drink or soda pop you've ever gulped down. And the route truck that just serviced the local fast-food restaurant can stop at your pool site next. But it won't match inexpensive, reliable acid. Carbon dioxide hasn't proven to be nearly as cheap as the eager salesman claimed or the hopeful purchasing agent dreamed, since so many variables can effect the consumption rate – and they usually do. This bleak picture is changing now, so read on.

3) CO2 is fed through a simple regulator/rotometer, a little like gas chlorine. It's not corrosive, carries no solids to clog things up, and automated controllers can switch the feed on with the simple click of a solenoid. Lots of gas can be stored at your site. bulk delivery into 500-pound low pressure systems is common, where CO2 fills a big, stainless-steel "Thermos jug" called a cryogenic tank. (Small, 50-pound high-pressure steel bottles can be delivered and used for smaller pools, too.) The real feed difficulty lies in the "injection" – where all the problems arise and the costs go up. The commonly used fish-tank-like diffuser stone mounted in the flowing stream just can't get the right amount of gas dissolved, and the mixed results are virtually always counterproductive.

4) Almost all of us believed that carbonic acid had a pH around 6.8, as we spread this false assumption through text and classroom. Credentialed chemists and a few talented pool researchers knew better, and are recently taking full advantage of the fact that injected circulation water can be much lower in pH. The reason CO2 has such a difficult time getting water to lower pH values is the same reason it costs so much to use, and the "no damaging pH" conversation is simply a smoke screen for poor feeder technology! We'd better look at some simplified chemistry and a little history, then see how it's been resolved...

First, we suppose you know about CO2 and water making carbonic acid and, ironically, bicarbonates too. The first resultant has an down-side pH influence, while the other can have an influence up while increasing TA to boot. Maximizing the "down" and minimizing the "up" results in the desired offset for high-pH sanitizers with little bicarbonate effect; but feed the gas in a sloppy, gross fashion (large bubbles, less water) and get the costly opposite – lots of home-made bicarbonate of soda! What needs to occur here is excellent dissolving of the CO2 into the pool water. That's just not been happening. Folks were trying to do it right, but the fish-stone bubblers were simply not effective enough.

For years, responsible designers and equipment suppliers performed a total alkalinity test on local water before choosing CO2 as the choice for pH management. If the TA was high, say, above 120 to 130 ppm, the owner'd better go with acid. Low TA make-up water, on the other hand, made him or her a solid candidate for this new miracle application so in went the CO2 container, regulator and injector. Many were happy, but about as many were not. The TA tended to rise steadily in about half of these pre-qualified pools. Then consumption began to rise, and rise some more. Many pools experienced TAs of 200 ppm or higher, and a few actually reached the lofty levels around 800 ppm!! "Hold a lower pH in the pool", the owner was told, then "get your calcium hardness up above three times your TA value". "Feed at slower rates, not faster, so the pH effect is greater." (But slow feed doesn't keep up with demand, much less catch up, on busy days – so pH automation becomes valueless.) "Split your CO2 injections to two locations, then the each will feed slower and give you more pH effect, less bicarb effect." All this advice was appropriate, and it often worked. But there had to be a better way...

About this time we're seeing a surge in ozone-system development and sales. Ozone's tougher to dissolve than CO2, so boost pumps and venturi injectors have become the norm. Why not try these highly refined injectors with CO2? A major Florida equipment distributor, frustrated with the old way, did just that. Pressure gauges, flow meters, even a pH probe and meter were installed, and what do you know... pH values in the low 5s were developed in the side stream water! The instant result? Better and quicker pH management, less CO2 consumption, and the total alkalinity actually went down. (Remember, when water's pH gets below about 5.5, all alkaline components turn into more CO2 and water, and the TA of the effected water becomes essentially zero!) With a little balancing of gas feed and side-stream flows, with the sanitizer's and make-up water's collective influence, we finally can get what we're after. Safe pH control we can afford.

So what about promise 5. ? Maybe the salesman was right after all. Carbon dioxide can replace acid in most any pool after all!

© 2002 Professional Pool Operators of America