Ozone has been used for many years for the purification of drinking water, among other things. In recent years, ozone has also increasingly been used in ponds. Nevertheless, ozone is still an unfamiliar technology to many hobbyists. With this article, I aim to provide more insight into the applicability of ozone in ponds. It describes the basics of using ozone, allowing you to decide for yourself whether ozone may be of interest to you.
What is ozone?
Ozone is a gas that is formed, among other things, during thunderstorms; this is also how most people know ozone. Ozone has the characteristic of being extremely aggressive and oxidises a great many materials. This is the property that is used to purify water. Bacteria, fungi, parasites and various other undesirable substances in ponds are oxidised by ozone. After ozone has done its job, it breaks down back into oxygen. The oxidised contaminants generally break down into water and CO2. This therefore makes it a very environmentally friendly and safe way of treating water.
What does ozone do to pond water?
As described above, ozone oxidises bacteria and fungi, among other things. However, ozone does more than this. The main reasons for using ozone when filtering a pond are the removal of colour and pheromones and, to a lesser extent, the reduction of disease pressure. Even with low ozone dosages, it is possible to remove the yellowish tint from the water. Especially in heavily stocked ponds where large amounts of food are given, a yellow tint is usually visible in the water. White fish therefore appear beige rather than white. Ozone makes the water crystal clear. The term “pheromones” will probably mean little to most readers. In heavily stocked ponds, carp secrete a pheromone that inhibits growth. This pheromone cannot be removed using normal filtration techniques. The only thing you can do about this is replace large quantities of water. Ozone oxidises this pheromone, which means you need to replace less water and the fish will grow faster. Ozone also supports the biological filter of your pond. This aspect is also where most inaccuracies are written about ozone. Let us take a look at the various water parameters and the influence of ozone.
Ammonium (NH4) / Ammonia (NH3)
In practice, ozone has little effect on either substance. Only at very high pH values (pH>9) will ozone begin to break down NH3.
Nitrite (NO2)
Nitrite is converted by ozone into nitrate without leaving any further residual substances. This can be a reason to use ozone in heavily stocked ponds. Personally, I think this is unwise: consider what would happen if the ozone system failed. Your filter would then not be capable of converting all the nitrite into nitrate. Therefore, never rely on ozone to remove nitrite; use a sufficiently large biological filter for this purpose!
Nitrate (NO3)
Ozone has no direct effect on nitrate. Although a reduction in nitrate levels sometimes occurs when ozone is used, this is usually temporary. It is not yet entirely clear why this phenomenon occurs.
Practical test
To gain a better understanding of the effects of ozone on pond water quality, I carried out a test. The pond in question is equipped with an ozone system and an electronic measuring system that continuously measures the most important water parameters. The measuring system was calibrated before the test.
The test pond
Below is a summary of the conditions under which the test was carried out. This is a relatively heavily stocked pond containing 71 koi. At the time of testing, the pond is not clear for 2 reasons. Three days ago, a bottom drain was blocked, resulting in poor circulation, and the installed UV capacity is insufficient.
- Volume: 50m³
- Filter system
- Sieve bend
- 3-chamber filter with 200 litres of moving bed and 800 litres of Matala mat
- 75W UV
- 200 litres of air per minute
- Ozone system
- 250mm high-pressure ozone reactor
- 1.5 gram ozone generator (prototype)
- +/- 15 litres of air per minute
- Fish stock: 71 fish averaging 50~55cm (+/- 175 kg in total)
- Daily amount of food: +/- 1000 grams
- Flow through the filter: +/- 19m³/hr
- Flow through the ozone system: +/- 6m³/hr
Measurement results before using ozone
The ozone system had been switched off for two weeks before the test began. The water parameters on 7 September at 20:00 were as follows: Temperature: 20.4°C pH: 7.08 Redox: 249mV Conductivity: 120µS NH3: 0.6 mg/l NH4: 0.00 mg/l NO2: 0.00 mg/l (Measured with a drop test) NO3: 71.71 mg/l O2: 5.69 mg/l (63.1% saturation)
Measurement results 24 hours after using ozone
The ozone system had been running for 24 hours with the following results: The water parameters on 8 September at 20:00 were as follows: Temperature: 19.8°C pH: 7.01 Redox: 278mV Conductivity: 116µS NH3: 0.7 mg/l NH4: 0.00 mg/l NO2: 0.00 mg/l (Measured with a drop test) NO3: 96.8 mg/l O2: 6.89mg/l (75.7% saturation) The data above have no scientific value, as there are too many external influences. The weather (temperature, air pressure, etc.) and the amount of food given each day have a major influence on water quality. Nevertheless, the results do provide an indication of the effect of ozone. The increase in the redox value can clearly be seen, as can the increase in the amount of oxygen. It is also noticeable that the nitrate level increased during this test.
What does an ozone system for a pond consist of?
We have now looked broadly at the influence of ozone on pond water, but we do not yet know what an ozone system consists of. A good ozone system consists of a number of components that must work well together. If one of the components is missing, the ozone system will not work, or will not work optimally, and dangerous situations may arise. The various components are described below:
Ozone generator
The ozone generator converts oxygen into ozone and is therefore the heart of an ozone system. Ozone can be generated in two ways: with a special UV lamp or using high voltage. The latter method is by far the most widely used because it is much more economical and compact. The capacity of an ozone generator is expressed in grams of ozone per hour, for example 1 or 2 grams per hour. However, this figure means nothing unless the conditions under which the ozone output was measured are known. Unfortunately, few manufacturers state this, making it very difficult to compare ozone generators. An ozone generator has one major enemy, and that is moisture. Moist supply air will result in the formation of nitric acid. In addition, moisture causes dust particles and contaminants in the air to stick to the electrodes of the ozone generator. Both effects are harmful to the ozone generator; therefore, always make sure that dry air is supplied to the ozone generator.
Ozone reactor
This is a collective term for one of the most important components in an ozone system. An ozone reactor enables the ozone to react with contaminants present in the water. The better the ozone reactor functions, the less ozone will be required to achieve the intended result. All ozone reactors work in roughly the same way. A gas flow containing ozone is introduced into the ozone reactor. This gas flow is mixed intensively with the water to give the ozone the opportunity to react with contaminants. The introduced gas flow must also leave the ozone reactor somewhere. Some ozone reactors have an integrated off-gas facility, while with others you must provide this yourself. No matter how well the ozone reactor works, the outgoing gas flow will always contain a small amount of ozone, known as residual ozone. The most common ozone reactors are explained in more detail below, together with the advantages and disadvantages of each type.
Protein skimmer
This is one of the best-known ozone reactors. In fact, these are saltwater skimmers made from ozone-resistant materials. Although they were originally intended for use with salt water, they are also used with fresh water (ponds). Advantages: • Low back pressure for the water pump • Also immediately removes proteins and contaminants by skimming • Can also be used directly to aerate the pond water • Vents residual ozone Disadvantages: • Large • Expensive • Relatively low efficiency • Requires a lot of air for effective skimming • Must be positioned partly above water level
High-pressure ozone reactor
This type of ozone reactor is a closed vessel in which the ozone is intensively mixed with the water. This type of ozone reactor has been specially developed for mixing ozone into water. Advantages: • Fairly high efficiency • Some models can be installed below water level • Requires little air for proper operation • Vents residual ozone Disadvantages: • Relatively large
Static mixer
Static mixers have been used for mixing ozone for several years. This method has been used in industry for much longer. Many products are sold as static mixers, but there are major differences in quality between the various mixers. A good static mixer reduces the size of the introduced air bubbles. Unfortunately, there are many mixers on the market that merely cause the bubbles to rotate instead of making them smaller. Therefore, make sure you are well informed before purchasing a mixer. In addition, when using a static mixer, you will need to take measures to allow the residual ozone to degas. Advantages: • Compact • Requires little air for proper operation Disadvantages: • High back pressure if good efficiency is to be achieved • An additional provision must be made to vent residual ozone • A good mixer is expensive
Venturi
Using a venturi provides the highest efficiency, but there is a disadvantage. For proper operation, there must be a fairly large pressure drop across the venturi, which requires a powerful pump. As a result, power consumption is very high, which is why venturis have been used less and less in ponds in recent years. In industry, venturis are used almost exclusively, often in combination with a static mixer and a reaction tank. Advantages: • Very high efficiency • Very compact • Requires little air for proper operation • Inexpensive Disadvantages: • Very high back pressure if good efficiency is to be achieved • An additional provision must be made to vent residual ozone.
Redox controller
The operation of a redox controller is best explained by comparing it with a room thermostat. As you know, a thermostat switches the heating on or off depending on the desired and current temperature. A redox controller essentially does the same thing, except that it does not look at the temperature but at the redox value. The redox value provides an indication of the oxidising capacity of the water. A redox controller therefore limits the oxidising capacity of the water. For the safe operation of an ozone system, a redox controller is therefore indispensable! In ponds, a redox value of 275mV is a guideline. In fact, the optimum redox value depends on the pH, but it would go too far to describe this here.
Air dryer
As indicated in the section about the ozone generator, moisture is enemy number 1. An air dryer reduces the amount of moisture in the supply air to the ozone generator. An air dryer usually consists of a tube filled with silica gel that absorbs the moisture. These dryers work well, but the silica gel must be replaced or regenerated regularly. Automatic air dryers that regenerate automatically are also available. These dryers are quite expensive and consume a fair amount of electricity. For this reason, they are rarely used in pond applications.
Residual ozone destructor
The gas flow from an ozone reactor contains residual ozone. We cannot simply allow this to discharge into the filter pit or the garden. Ozone is harmful to health, even at the low concentrations coming from the residual ozone outlet of an ozone reactor. Fortunately, ozone can be neutralised quite easily. In its simplest form, a residual ozone destructor consists of a tube filled with activated carbon. As soon as ozone comes into contact with activated carbon, it breaks down back into oxygen. A residual ozone destructor is often omitted from an ozone system, but remember that it is there for your own safety! PLEASE NOTE: activated carbon must never be used in an ozone system in which ozone is generated from almost pure oxygen!
Installing an ozone system
When installing an ozone system, there are several important aspects concerning its placement. These mainly relate to the positioning of the ozone reactor and ozone generator. The operation of an ozone system depends entirely on the way in which the system is applied.
Ozone generator
Installing an ozone generator is not complicated, provided that a number of simple rules are followed. Always install an ozone generator in a dry, cool and well-ventilated area. Moisture is enemy number 1 of an ozone generator, so always ensure a dry environment.
Ozone reactor
The ozone reactor can be installed in several ways, by which I actually mean that the supply water can come from different locations. The water can also be discharged to various locations. Preferably, relatively dirty water should pass through an ozone reactor, but the water must already have had most organic contamination (leaves, algae, coarse particles) removed. The most logical place to take the water from is after the pre-filter. If it is possible to take the water from a skimmer, this is even better, because the greasy layer floating on the water is then treated with ozone at the same time. The best place to discharge the water partly depends on the type of ozone reactor you have.
A high-pressure reactor or skimmer
The water from the ozone reactor may always contain some dissolved ozone, so it is not advisable to send this water directly back into the pond. With a multi-chamber filter, a good option is to allow the water from the ozone reactor to flow into the final filter chamber. With closed filters (bead filters), another option will have to be chosen. The water can then be returned to the pre-filter. You can also allow the water from the ozone reactor to flow into a plant filter. In winter this is not really a good option, but ozone systems are generally switched off during winter. Another alternative is to return the water from the ozone reactor to the pond through a vessel filled with activated carbon.
A venturi or static mixer
With a venturi or static mixer, there will be (large) bubbles in the water flow. These bubbles always still contain ozone, so do not allow the water to discharge directly into the pond. After a venturi or mixer there must always be something that allows the water to degas. In practice, this often means that the water from a venturi or mixer flows into a vessel where the flow velocity is low. The air bubbles will then rise out of the water. The water from the vessel can now be discharged in the same way as described under the heading “A high-pressure reactor or skimmer”. The outgoing air, just as with an ozone reactor, must be neutralised via a residual ozone destructor.
Redox controller
I will not describe the installation of the redox controller itself here; for this, it is best to follow the manufacturer’s manual. However, I would like to say something here about the positioning of the redox electrode. One manufacturer recommends measuring in the incoming water, while another recommends measuring in the outgoing water.
Measuring the redox value in the outgoing water:
Measuring the redox after the reactor has the advantage that very clean water flows past the electrode, which means that it becomes dirty less quickly. The disadvantage of this method is that it results in very unstable control behaviour. As soon as the ozone generator switches on, the redox value in the outgoing water will rise relatively quickly. After some time, the ozone generator will be switched off because the redox is high enough. This process will repeat continuously, causing the ozone generator to switch on and off frequently. This results in a shorter service life for the ozone generator.
Measuring the redox value in the incoming water:
The redox value at the inlet of the ozone reactor is equal to that of your pond water. When you switch on the ozone generator, the redox value of your pond will slowly rise. This is in contrast to the redox value at the outlet of your ozone reactor. The advantage of this is that your ozone generator will be switched on and off far less frequently, which benefits its service life. This measuring method also has disadvantages: the redox electrode will need to be cleaned more frequently. Measuring the redox in relatively dirty water is not a problem in itself, but it does cause the electrode to become dirty more quickly, which leads to unrealistic readings. When your electrode becomes dirty, it will generally indicate a higher redox value and therefore switch off your ozone generator sooner. This makes the system intrinsically safe, because when contamination occurs it will actually switch itself off sooner. This option therefore has my personal preference.
Other peripheral equipment
For the correct installation of, for example, an air dryer or a residual ozone destructor, it is best to consult the manufacturer’s manual. The way in which these products must be installed depends greatly on their design. Using a UV lamp in combination with ozone Sometimes it is recommended to use a UV lamp to neutralise dissolved ozone. Personally, I would place a UV lamp before, or parallel to, the ozone reactor. In most cases, a UV lamp will still be required because ozone does not kill suspended algae in all situations.
Maintenance of an ozone system
Like many devices, an ozone system also requires maintenance. Maintenance can basically be divided into 3 categories. Regularly (once every 2 weeks): This mainly concerns cleaning the redox electrode and checking the air dryer. Over time, a layer forms on the measuring tip of the redox electrode. This generally causes the electrode to indicate a higher redox value. You can remove this layer by gently rubbing the measuring tip with a fingertip. If necessary, this can also be done with a lint-free cloth. Never use a toothbrush for cleaning, as this can cause scratches on the measuring tip. The air dryer should also be checked regularly. Most air dryers contain granules that change colour when the dryer is saturated. You will then need to replace or regenerate the granules. How often this needs to be done in your situation depends entirely on the humidity. In some cases it must be done every week, while in other cases it can take months. Occasionally (once every 2 months) Check every 2 months whether the ventilation grilles of the ozone generator are still clean. Most ozone generators are equipped with fans to cool the device. Make sure that the inlet and outlet grilles are clean so that air circulation is not blocked. At the same time, inspect the hoses between the various components. The hoses can be affected by nitric acid and other contaminants. Check the hoses for cracks; if cracks appear, the hose has most likely been affected by nitric acid. Moisture may sometimes also accumulate in the hoses. This is usually nitric acid; make sure that this liquid does not come into contact with your body or clothing, as nitric acid is highly corrosive. Annually The internal components of an ozone generator become contaminated. This contamination must occasionally be removed, otherwise it can damage the ozone generator. Most ozone generators must be sent back to the manufacturer for annual maintenance.
Safety
The text above describes what ozone is, how it works and how you can use it. However, there is one very important point that certainly must not be forgotten: safety. I wrote earlier that ozone is a very aggressive gas. Unfortunately, this effect is not limited to contaminants in water. Ozone does not distinguish between bacteria and parasites or people and fish. The warning not to discharge ozone-treated water directly into the pond is therefore not without reason. You must give the ozone the opportunity to finish reacting before it enters the pond. If you do not, the ozone will react with the first thing it encounters. This may be algae or organic contamination, but it could also be your valuable koi! The same applies to the warning about residual ozone. Residual ozone must always be neutralised. If you allow residual ozone to discharge directly into your filter pit or garden, the ozone will again react with everything it encounters. Accidentally inhaling a low dose of ozone is not immediately harmful, but try to avoid it. Prolonged exposure to ozone can be harmful to your health. You should not be able to smell ozone when your ozone system is operating. If you can smell it, there is a leak or a fault somewhere in the system. In such a case, switch the system off immediately and correct the problem. This will keep it safe for both you and your fish.
Conclusion
Ozone can remove certain substances from water that can hardly be removed using other techniques. In that respect, ozone is a good addition to any pond. However, ozone can never replace an existing filter because it is not capable of converting ammonium/ammonia into nitrite. A properly functioning filter therefore remains essential, although ozone can certainly help to purify pond water. Ozone is particularly effective in heavily stocked ponds because this is where discolouration and the accumulation of pheromones are most likely to occur. An ozone system must consist of a number of components that are properly matched to one another. If one of these components is missing, there is a risk of a poorly functioning system with potentially harmful consequences. If you use ozone correctly and responsibly, it is safe for both humans and animals and can be a very valuable addition to your filtration system. ©Tol Watertechniek