TASK 2

OZONATION

An alternative to the chlorination in the swimming pools can be the ozonation. Ozone was first used in a water treatment in the late 1800s and now is more widely used in Europe and Asia than in the United States. In Europe, ozonation for water swimming pool treatment has been in regular use for over 50 years in Germany, France…

Currently, more than 300 000 swimming pools in Europe are treated with ozone.

What is ozone?

Ozone is a colourless gas at all concentrations experienced in industry. It has a pungent characteristic odour (similar to the smell after a major thunderstorm) which is generally detectable by the human nose at concentrations between 0,02 and 0,05 ppm (≈1/100^th of the recommend 15 minute exposure level).

Ozone is an unstable gas composed of three oxygen atoms, which readily degrade back to diatomic oxygen at normal temperature. Its decomposition is accelerated by contact with solid surfaces, by contact with chemical substances and by the effect of heat. During this transition, a free oxygen or free radical is form. The free oxygen radical is highly reactive and short lived.

It is the second most powerful oxidant known after fluorine. But fluorine is not used since it is dangerous and like the chlorine, it decreases the pH of the water and increases the mineral content.

Ozone purifies water by chemical oxidation. All forms of microbe are destroyed by ozone, colloids are destabilized by neutralization and dissolved organic materials are partially oxidized. It has a greater disinfection effectiveness against bacteria and viruses compared to chlorination. Its oxidizing properties can also reduce the concentration of iron, manganese, sulphur, and reduce or eliminate taste and odour problems.

Health effects of ozone

The MAC-value is the Maximum Acceptable Concentration a human is allowed to be exposed to for a certain time and certain agent. 

For ozone the MAC-values is 0.06 ppm for 8 hours a day, 5 days a week (ppm = parts per million). For a maximum of 15 minutes a MAC-value of 0.3 ppm is applied. 
The concentrations mentioned above are much higher than the odour threshold (0,02-0,05 ppm) at which ozone can be smelled, so critical concentrations will be noticed quickly.

When people are exposed to high ozone concentrations the symptoms can vary from dryness in the mouth and throat, coughing, headache and chest restriction. Nearby the lethal limits, more acute problems will follow.

Ozonation process

Since ozone is an unstable molecule, it is produced on site. The formation of oxygen into ozone occurs with the use of energy. Commercially, ozone is generated by accelerating electrons between two electrically charged plates in the presence of air. A silent corona discharge is produced between two charged electrodes. The corona discharge propels electrons which in turn dissociates oxygen found in air molecules into oxygen atoms. Thus, a part of oxygen is transformed into ozone.

3 O₂ à 2 O₃

Ozone generators must be installed in a well-ventilated and dust-free room. In most cases, only a few square feet of floor space is needed.

Then, ozone is injected by diffusor or venturi. With a venturi, the ozone gas is injected in the water via a vacuum. The advantages of this system are a compact installation and a high yield (up to 90%). A diffusor works under pressure and creates a bubble column. Its advantages are a high yield, a simple construction and it is advantageous for high flow rates. But it required surface area and the need of tall buildings to increase efficiency.

Advantages of ozonation

ü Ozonation needs no change of the water temperature.

ü The pH of the water is not changed.

ü Ozone is effect over a wide pH range.

ü It can eliminate a wide variety of inorganic, organic, microbiological problems but also taste and odour problems.(Properly dissolved ozone residual of 0.05 mg/L or higher assures a 100% kill of all bacteria, viruses and fungus)

ü It reacts rapidly with all forms of microbe including bacteria, viruses and protozons. It has stronger germicidal properties than chlorination. (the rate of speed of bacteria killed by O3 is over 3000 times faster than chlorine)

ü Ozone has a very strong oxidizing power with a short reaction time.

ü The treatment process does not add chemicals to the water.

ü Chloramines are oxidized by ozone into chloride and nitrate. Precursors of disinfectant by-products are also destroyed, resulting in very low levels of THMs (<0.02 mg/litre) and other chlorinated organics.

ü Ozonation also helps to add great clarity to the water.

ü Ozone assists flocculation of organic waste materials, thus enhancing the effectiveness of sand filters.

ü It leaves no toxic residues in the treated water. (Researches have begun)

ü Dissolved ozone at levels below 0.15 mg/L do not cause adverse health effects even during long term exposure.

ü Ozone is produced on site and does not require storage or transportation.

ü Undissolved ozone off-gas is easy to separate from treated water in the off-gas tank, long before entering the pool, while chlorine is being constantly released in its highly toxic gas form right above the pool water.

Disadvantages of ozonation

ü There is a higher equipment and operational cost than chlorination. Besides it may be more difficult to find professional proficient in ozone treatment and system maintenance.

ü Ozonation provides no germicidal or disinfection residual to inhibit or prevent regrowth. So a final disinfection with chlorine is needed. (but ozone in conjunction with chlorine is considerably more expensive than that of chlorine alone) *

ü Ozonation by-products are still being evaluated and it is possible that some of them (such as brominated by-products, aldehydes, ketones, carboxylic acid) may be carcinogenic. That is why the post filtration system may include an activated carbon filter. But it is also because of the excess ozone remaining in water (ozone level in pool water should not exceed 0,05 mg/L).

ü Ozone is less soluble in water, compared to chlorine. Therefore, special mixing techniques are needed.

* Some people say that ozone systems may still require that “all ozone is destroyed prior to entering the pool by passing through a carbon bed, then chlorine is introduced instead to maintain a disinfecting residual”. Such requirements are partially based on a 30 years old German DIN procedure, an old design which has been changed. Now, reliable off-gas systems are able to strip all undissolved ozone gas from the water flow safely. Thus no free ozone gas can escape to the pool. Today's technology does not always require chlorine assistance, except as a backup in case of maintenance or power blackout.

If there is need to complement an ozone system, bromine is much more suitable than chlorine. While bromine is more expensive to buy, much less bromine is needed since ozone recycles bromine up to 14 times in the pool water.

Ozone system design

For optimum design, the most important variables to take into account are the quantity of ozone transferred into the water and the ozone demand. Experience has shown that a minimum dosage level of 0,5 grams/m³ must be used in swimming-pool water treatment. However, the recommendations for calculations show 0,8 to 1,2 grams/m³. But these recommendations give expensive ozone plants if all the circulated water gets ozonated. That is why most of the ozone treated swimming-pools do not operate on a 100% ozone generating level. Indeed, if more ozone gets generated than necessary more ozone needs to be destroyed and the energy costs are going to be excessive.

The “Slipstream” ozone design can be an appropriate solution to this problem. It is a new form of ozone water treatment where only part of circulated water gets ozonated. It is based on a ozone dosage level minimum 0,8 grams/m3 (depending on water temperature) for 15 % to 30 % of the total amount of circulated water. It has the all benefits of ozone and the lowest energy costs and investment costs.

A combination of a Bi-Flow Filters (combi-flow sand filter) with a Slipstream ozone system also exists. It is found that the ozone residue after the ozone reaction tank, which enters the Bi-Flow Filter, oxidates the pollution accumulated in the Bi-Flow Filter very efficiently. This oxidation of the accumulated pollution makes the ozone residual disappear completely so that no carbon filter is necessary.