Jay Hemdal
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THE USE OF OZONE IN MARINE AQUARIUM SYSTEMS
By Jay Hemdal
By Jay Hemdal
Ozone, (O3) has the same elemental composition as oxygen, (O2) but very different chemical properties. Ozone, (tri-atomic oxygen) is a very unstable molecule. It is a powerful oxidizer. When ozone comes in contact with a compound exhibiting an “oxygen demand”, it releases one of its oxygen atoms resulting in the formation of an oxygen molecule, with the third oxygen atom becoming attached to the now oxidized compound. A theoretical example of this reaction would be the addition of ozone to an aquarium containing a certain amount of nitrite (NO>-). The third oxygen atom in the ozone molecule would contact the nitrite molecule oxidizing it to nitrate (NO3-) and leaving an oxygen molecule (O2).
For use in home aquariums, there are two methods that this gas is commonly produced: either by a photochemical reaction using ultraviolet light or through a hot or cold corona discharge device. In almost every case, ozone is dissolved in the aquarium water by means of a contact chamber, usually a protein skimmer. Ozone gas dissolved in marine aquarium water has been used both as a disinfectant, as well as a means to oxidize certain toxic metabolites produced by the aquarium inhabitants. The various chemical reactions ozone exhibits when exposed to seawater are extremely complicated. Some aquarium “chemists” may be a bit too quick to describe these theoretical reactions, and do so at the risk of over-extrapolation, and the real-world results may be very different. Industry manufacturers also attempt to define the activity of ozone in water, but since there is the underlying message to buy their products, one cannot always accept their information at face value. What I propose to do in this article is describe my findings regarding the use of ozone in synthetic seawater, using primarily the easily discernible results of first applying ozone to synthetic seawater and then measuring the results. I’ll not conjecture on the results of the theoretical chemical reactions involved, nor repeat sales pitches from manufacturers. Please understand that this is a hugely complicated topic. While I believe the information presented here is accurate, it is not all encompassing; there is still a lot more research to be done in this field.
First, a few definitions might be in order. In this article, the term DOC refers to organic carbon compounds dissolved in the water such as oils, humic acid, phenols, etc. Particulate organic carbons, POC are the organic particles suspended in the water including uneaten food, detritus, algae, bacteria and protozoans. These two categories in combination are referred to as TOC — Total Organic Carbon. Ozone can convert some of an aquarium’s POC into DOC, as well as change the chemical form of some of the DOC, but there is very little direct reduction of TOC by ozone. In other words: ozone may break down algae, protozoans and bacteria to their non-living, chemical components. It may also oxidize some organic compounds into new forms. But without some additional mechanism, such as foam fractionation, ozone by itself completely oxidizes very few of these materials. In addition, ozone can react with some inorganic compounds and oxidize them as well.
It is a commonly held belief that ozone enhances bubble formation in protein skimmers. In my experience, injecting small amounts of ozone into a properly adjusted skimmer has little real affect on the amount or character of the foam produced. However, suddenly injecting very high doses of ozone seems to have an effect of producing clearer, more watery foam. Injecting moderate amounts of ozone for long periods of time tends to produce clearer, but relatively dry foam. On a skimmer that has had its foam production temporarily reduced (by cleaning out the unit or feeding the aquarium) abruptly adding ozone seems to hasten the rebound of the unit back to its normal level of foam production.
Safety is an important issue when working with ozone. Humans can sense ozone in air at a level of around 0.02 to 0.05 mg/l, very near the critical exposure level cited in many industrial applications. The real danger in working with ozone is in the case of large commercial units. In most instances, home aquarium ozone generators with outputs less than 250 mg per hour will not cause acute toxicity in humans. They can however, cause chronic health problems such as coughing and headaches if not used properly. Remember, if you can smell ozone, the concentration in the air of the room is too high!
The action of ozone on seawater produces various forms of organic and inorganic oxidants (Crecelius 1979). Administering a proper dosage of ozone in aquarium applications is frequently a problem. The result of improper dosage is either a low concentration that does not produce the expected benefit, or an excessive amount that can prove toxic to the aquarium inhabitants. The oxidants produced by the reaction of ozone in seawater can most easily be measured indirectly by means of a DPD (N,N-diethyl-p-phenylenediamine) total chlorine colorimetric method (APHA et-al 1980). As the color change of the DPD method is very small at low doses, a spectrophotometer is usually required to interpret the results at low concentrations of ozone. The results of the DPD test are affected if the alkalinity of the sample is greater than 300mg/1 or if there is chlorine, bromine or iodine present at high enough levels. A much more accurate method, (but vastly more expensive procedure) is the Hach Company’s Indigo trisulfonate Accu Vac method. While this test is free from most interferences, use of a spectrophotometer is mandatory, and it seems that this test only works for freshwater systems. Many home aquarists use an Oxidation / Reduction Potential (ORP) probe to indirectly measure ozone levels. In some cases, the ORP probe is used to automatically control the output of the ozone generator. Care should be taken using this method on small aquariums. ORP probes need frequent calibration, and are prone to sudden failure — sometimes in the “full on” setting. Furthermore, ORP only indirectly measures ozone, and does not differentiate between free ozone, ozone-oxidants or any other type of oxidant or reducing agents that might be present in the aquarium system.
Ozone-oxidants such as bromite, (Obr’) bromate, (BrO3°) hypobromous acid, (HOBr) and hypochlorites (OCI) are much more stable than free ozone. I had originally surmised that more than one type of oxidant might be formed at the same time. What that meant was that ozone reacted with marine aquarium water would create a mixture of oxidant compounds that would vary in their stability. The instable products would more quickly decompose, while the more stable oxidants would persist for a longer time. My early experiments seemed to bear this out. During one 96 hour trial, the decomposition rate was found to average 0.04 mg/l per hour, but was much faster early on in the trial as compared to the end (Hemdal 1992). It wasn’t until I examined the decomposition rate of ozone again many years later, that I discovered that the rate is logarithmic, but fairly consistent in that it doesn’t show the expected plateaus and sharp declines (Graph #1). The equation I used for the calculated decay rate was:
A =(LnY)/((LnX)/T). Where A is the time it takes the ozone to decay to the level Y (expressed as a decimal of the percentage; so if the amount of decay to be determined is 50% then Y would be 0.50). X is the previously measured percent of the material remaining after T time. Ln is the natural log of each of these numbers, and is most easily determined using a scientific calculator.
I don’t have any explanation as to why the measured decay rate was faster than the calculated rate during the middle of the time frame, when both rates were equivalent at the beginning and the end of the graphed curves. When using this data to predict the amount of potentially toxic ozone-oxidants, it would be prudent to follow the slower, calculated decay rate.
During aeration of samples containing three oxidants, the decomposition rate of chlorine in tap water was found to be the fastest, followed by the decay rate of ozone oxidants in non-brominated synthetic seawater. Ozone-oxidants in seawater containing bromine had the longest half-life. In using logarithmic models for each of these cases, the decomposition rate was a consistent, logarithmic decline (Table 1).
The time needed for enough of the oxidant to decompose through simple aeration to make the water safe for aquatic life varies on the starting concentration of the oxidants to begin with, but table 1 can be used to get a rough idea of this decay rate. For example: the old adage, “Aerate tap water for 24 hours prior to using it in an aquarium in order to dechlorinate It.” seems to hold true. Most city water systems deliver tap water containing a maximum of 1.0 mg/I total chlorine. About 85% of this chlorine will be neutralized during 24 hours of aeration — and the residual amount of .15 mg/I will have no detrimental effect on the aquarium inhabitants.
What are the best ways to control ozone/oxidant levels in aquariums? If an ozone test indicates low or no concentration of ozone, it is a simple matter for the aquarist to increase the output of the ozone generator by either turning up the rheostat running the unit for longer periods of time, or by using an air-dryer to increase efficiency of the generator unit. However, if the test shows excessive amounts of ozone-oxidants in the water, there is little published data as to how the concentrations of the ozone-oxidants may be quickly reduced. Spotte (1979) indicates that sodium thiosulfate may be used to reduce the level of ozone vented to the atmosphere during the reaction process as a means of alleviating the potential health hazard to humans. Sodium thiosulfate is routinely used in aquariums to neutralize free chlorine (Amlacher 1970). In a previous study, (Hemdal 1992) ozone gas was dissolved at various concentrations in synthetic seawater containing bromine. The toxicity of the resultant ozone-oxidants on Brachionus rotifers was noted. Fifty percent mortality occurred in as little as ten minutes at 9.0 mg/l ozone-oxidants. This ranged to greater than 23 hours for 0.43 mg/l ozone-oxidants. Sodium thiosulfate was added to water samples containing known toxic concentrations of ozone-oxidants. One milligram of sodium thiosulfate neutralized an average of 1 mg of ozone-oxidants (The actual amount varied and was dependent on the length of time the sample was aerated prior to the addition of the sodium thiosulfate. The longer the sample was aerated, the less effective sodium thiosulfate was in neutralization of oxidants). So: an aquarist faced with the challenge of detoxifying excess ozone-oxidants in an aquarium can simply add the same amount of sodium thiosulfate as there is ozone present. For aquarists without access to sodium thiosulfate, remember that this is the main component of aquarium water dechlorinators. Most of these products are designed to dechlorinate water on the order of 1 mg/I chlorine, so if your ozone level is at 2 mg/l, simply add a double dose of dechlorinator.
Rather than relying on potentially inaccurate ORP controllers, home aquarists might want to try the following method to achieve a steady state ozone dose for their aquarium — one that remains at a low, but constant level. This small dose of ozone will not have any real disinfectant ability, and it probably will not break down too many of the POC’s in the water. On the other hand, there is a high margin of safety in this method if applied properly.
- Purchase an ozone generator with a rheostat control that can supply between 0.10 and 0.15 mg of ozone per liter of tank capacity per hour. Two small units operating in parallel may be the most cost effective way to boost ozone production.
- Using a supply side air-dryer if the room is humid, add ozone at a relatively high rate into the tank through a protein skimmer, (0.10 mg ozone per liter of tank capacity per hour)
- Monitor the level with a DPD ozone test. A spectrophotometer is best used for this, but this alternative method will work:
- Create a “reagent blank” using deionized water and the DPD reagent
- Add 1 drop white vinegar to 50 ml of the sample as a pre-treatment to avoid any interference from high alkalinity levels.
- Add the DPD reagent to the tube containing the aquarium sample.
- After 3 minutes, but before 5 minutes, hold both tubes over a piece of white paper.
- The slightest pink color at all in the test sample indicates some ozone is present.
- Higher amounts of ozone (> 0.01mg/l) should be discernable on the test’s color comparator chart.
- Iodine and chlorine will react with the DPD test just like ozone will. Adding even a small amount of chlorinated tap water to an aquarium, or dosing with high amounts of iodine supplements will skew the results.
- Turn off the ozone when the level in the tank reaches 0.02 mg/l or if ozone can be smelled in the room. Ozone in a room quickly desensitizes the human olfactory system, so be aware that you may not be able to smell it after a while. Continued exposure will cause coughing and headaches. Immediately turn the ozone generator off and move to fresh air if any symptoms occur.
- Repeat this process for 4 to 6 hours each day for three days. Check the levels hourly.
- Once the more easily oxidized compounds have reacted with this relatively high level of ozone, begin adjusting for a steady state ozone concentration.
- Start with injecting 0.005 mg of ozone per liter per hour through the protein skimmer. Turn unit off at night, or whenever you are unable to monitor it closely. It is a little ironic that after all this research, this value is very close to the 0.003 mg ozone per liter per hour given by Robert Valenti over 30 years ago (Valenti 1968).
- Raise the ozone level a little each day until one of two things occurs; either free ozone can be smelled in the room, or the DPD ozone test of the main portion of the aquarium reacts with any pink color at all (generally this happens at 0.005mg/1 ozone in water).
- Lower the ozone production rate 25% from the previous setting. The aquarium should be able to operate continuously at this dosage over long periods of time, except in the case where activated carbon usage changes (The carbon “competes” with the ozone for many of the same compounds, changing in turn, the demand for ozone).
- Never use this method with natural seawater, or any synthetic seawater containing natural levels of bromine (0.0673 mg/l). The ozone-oxidants formed when the ozone reacts with the bromine may be harmful to the animals.
- For added safety the water returning to the tank from the protein skimmer / ozone reactor can be passed through activated carbon, which will remove any residual ozone. Be aware that the carbon once exhausted, will lose its capacity to remove ozone from the water. The difficulty of course, lies in knowing when the carbon needs to be changed.
Except in public aquariums needing to process huge amounts of water, ozone systems seem to have fallen out of favor in recent years. For these public aquariums needing “gin-clear” water for their exhibits, ozone remains an almost obligate choice. Many home marine aquarists seem to have gravitated towards different methods of water quality management. It is a little ironic that almost all of the home aquarium interest in ozone has been focused on the marine side of the hobby. Large freshwater aquariums can potentially benefit even more from the use of ozone. In these systems, there is no fear of producing residual ozone-oxidants, so ozone levels can be raised to levels that will sterilize the water, as well as oxidize most organic waste molecules, while still easily allowing the ozone to quickly decompose back to oxygen, before causing harm to the animals.
Do you need ozone for a marine aquarium of the size of most home systems? Probably not, there may be safer and less expensive methods, (like foam fractionators alone, or performing more frequent water changes) to achieve the same level of water quality. However, if you like to experiment with your aquariums, you might find one these units interesting to work with. If you already own an ozone generator, hopefully the information in this article will allow you to operate the unit in a safer, more effective manner.
Bibliography:
Amlacher, E. 1961. Taschenbuch der Fischkrankheiten. Translated by D.A. Conroy and R.L. Herman. Textbook of Fish Diseases. Neptune, New Jersey, T.F.H. Publications, Inc., 1970
APHA (American Public Health Association), American Water Works Association, and Water Pollution Control Federation. 1980. Standard methods for the examination of water and wastewater, 15th edition. American Public Health Association, Washington, D.C.
Crecelius, E. A. 1979. Measurements of oxidants in ozonized seawater and some biological reactions. Journal of the Fisheries Research Board of Canada 36:1006-1008
Ge-Ling Lu, Biao Jiang, Zhi-Cheng Li, An-Xing Li. 2022. Ultraviolet light and ozone controls Cryptocaryon irritans infection in factory aquaculture. Aquaculture, Published by Elsevier B.V.
Hemdal, J.F. 1992. Reduction of ozone oxidants in synthetic seawater by use of sodium thiosulfate. Progressive Fish-Culturist 54(1):54-56
Hoff, F. & Snell, T. W. 1987. Plankton Culture Manual. Florida Aqua Farms, Dade City Florida
Segedi, R. and W. E. Kelley. 1964. A new formula for artificial sea water. Sea-water systems for experimental aquariums: a collection of papers, J. R. Clark and R. L. Clark (editors) Research Report 63, Bureau of Sport Fisheries and Wildlife, Washington D.C.
Snell, T. W. & Persoone, G. 1989. Acute toxicity bioassays using rotifers. Aquatic Toxicology 14:65-80
Spotte, S. 1979. Seawater aquariums. John Wiley and Sons, New York.
Valenti, R.J. 1968. The saltwater aquarium manual. Aquarium Stock Company, New York
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