Ask the Expert
Question: How much ozone is necessary to kill bacteria in a 500 gallon hot tub? How much is needed for a 20,000 gallon pool? We currently sell and install corona discharge ozone units on Hot Springs portable spas. I also need to know, can there be too much ozone generated? What is a safe level in a small environment such as a hot tub?
Jobi D. O’Neill
Buyer, Baker Pool & Spa, Inc.
314/965-8280 ext. 115; email firstname.lastname@example.org
Answer: Ken Mouw, fisherman extraordinaire and president of Ozotech, Inc. of Yreka, Calif., kindly offered his expertise on this question. With regard to determining how much ozone is necessary to kill bacteria in a 500-gallon hot tub, he began by noting that his answer is based on a Hot Spring (inquirer’s reference) spa line that utilizes a dedicated 24/7 recirculation pump for the injection of ozone. Numerous other spa manufacturers and older type spas do not, or have not, incorporated a dedicated pump in their configuration, thus the rules of ozone injection and sizing may not directly apply in the below response.
“In the interest of bather safety and keeping cost to a minimum, spas are most often dosed very lightly with ozone. Effective ozone treatment is realized by the cumulative effects of continuous, low-level application,” Mouw explains. In most modern spas, average ozone generator output is 0.05—0.15 grams of ozone per hour with gas concentration by weight in the 0.1 g/m3 region.
Mouw cautioned that ozone activity in a spa should be confined to the piping of the circulating filtration line. It is a mistake to attempt to carry ozone into the main body of water, both in terms of bather safety and ozone efficacy. Likewise, it is a mistake to attempt to inject ozone into the main body of water, as would be the case if using one of the air or water jets or a bubble diffuser in the lower cavity of the spa.
While ozone certainly destroys bacteria in a spa, its time proven method of application does not follow the established rules for ozone disinfection, which require that ozone residual levels be measured after contact time.
Ozone in a swimming pool has the ability to limit the formation of THM’s and HAA’s that are created when chlorine oxidizes organic molecules. This serves to improve the overall bathing experience through reduction of the chlorine odor, as well as dry skin and red eyes. Additional benefits include sparkling clear pool water made possible by the ability of ozone to act as a micro-flocculent, coagulating small particles into larger particles that can be removed by the filter. Ozone cannot be the sole sanitizer, however. Chlorine or bromine must be added to maintain a residual disinfectant in the pool body, because ozone is applied in such a manner that it is totally consumed prior to the water re-entering the pool.
Determining the required output of the ozone generator is usually tricky, partly because of the disparity in the approach of different ozone suppliers, as well as the sometimes difficult task of quantifying to the consumer the advantages/disadvantages of each method.
The designer is faced with the decision to treat the entire volume of the filter line flow, or to split the flow and inject ozone into a reduced volume of the filter flow, usually 25-50 percent, called a slipstream or sidestream. It is a given that the more water being ozonated, the more expensive the system. So, what is the correct approach?
The only way to come up with an answer is to educate the customer regarding the benefits that ozone can provide, Mouw instructed, and how those benefits are realized. For example, one might try reducing the formation of THM’s and HAA’s, and thereby limiting the odor, dry skin and red eyes experienced by bathers. Will these benefits still be possible in the reduced flow slipstream method? Certainly, but to a reduced degree, he stated. Assuming a six hour pool turnover time, each “packet” of water (possibly containing an organic molecule) is subject to contact by ozone once every six hours in a full flow system, and only once every 24 hours in a 25-percent slipstream. It is therefore apparent that treating the full flow will provide the maximum benefit.
“Given a full flow Din Standard approach, you would need approximately 15.1 g/hr of ozone for a 20,000 gallon pool. With reduced benefits fully discussed and understood by the consumer and a 25 percent slip-stream approach as the agreed upon methodology, approximately 3.8 g/hr of ozone would be utilized,” Mouw surmised.
Can there be too much ozone generated? And what is a safe level in a small environment such as a hot tub?
As to the reader’s query about whether there can be too much ozone generated, Mouw said the short answer is yes. “It is possible to generate too much ozone for a given application, although due to economic reasons, it is more likely that ozone is misapplied. Generally speaking, if the ozone system allows residual ozone to enter the main body of water, be it a pool or a spa, exposure of the bathers to ozone gas is possible, “ he explained.
Following this line of reasoning, if ozone gas exposure is possible, it is also possible for a person to be exposed to a level that exceeds regulatory limits (OSHA 0.1ppmv TWA 8-hours; 0.3ppmv STEL).
Therefore, engineering controls are used to prevent ozone from entering the occupied space. First, ozone is applied in a manner which will cause it to be consumed prior to its being able to enter the main water body. In the case of a spa, this means injecting the ozone in the circulating filter line and allowing it to do its job before the water re-enters the spa. “The relatively low ozone levels applied, the high water temperatures, and the ozone demand all contribute to giving ozone a very short life,” Mouw noted.
Secondly, devices are available to destroy excess ozone, in either the dissolved phase or gas phase. In the case of swimming pools, in which relatively high ozone levels are applied to the filter flow and water temperatures are much lower, ozone is consumed during the time it is detained in a contact tank following the ozone injection point, with any residual ozone destroyed by passing through a GAC filter or UV unit prior to reentering the pool.
Those with additional ozone questions (or who love bass fishing) are encouraged to contact Kenneth W. Mouw: Ozotech, Inc, 2401 E. Oberlin Road, Yreka, Calif. 96097, telephone (530) 842 4189, fax (530) 842 3238 or email at email@example.com