Ozone in water

Ozone is widely used in large potable water plants instead of chlorine-based products to avoid the formation of harmful by-products, but its use extends to small plants with difficulties in supplying chemicals

Ozone Application – Potabilization and Primary Water Treatment

The use of ozone in primary water treatment can address multiple issues but, to simplify, it typically concerns either the initial pretreatment phase or the final phase. Depending on where it is used, it leads to two different objectives.

• Initial pretreatment: pre-disinfection, oxidation of organic compounds;
• Final treatment: sanitization and disinfection.

Some of the advantages of using ozone are listed below:

• It degrades complex organic compounds;
• It has a high oxidizing power;
• It performs a strong disinfectant action;
• It does not add odor or taste to the water;
• It does not form halogenated derivatives;
• It inhibits algal growth, degrading organic micropollutants such as humic acids and algal metabolites;
• It raises the redox potential;
• It improves flocculation.

The efficiency of a disinfectant product depends on its oxidizing power and its diffusivity through the cell wall. Among the products that can be used as disinfectants, ozone has the highest oxidizing power:

Product	Potential
Ozone	+ 2,07 V
Chlorine dioxide	+ 1,50 V
Hypochlorite	+ 1,49 V
Chlorine	+ 1,36 V

Ozone as a Bactericide

A necessary condition for the centralized distribution of drinking water is safe disinfection. The disinfectant action of ozone takes place through the oxidation of the cell wall of microorganisms:

• Bacteria: destruction of the cell membrane;
• Viruses: inactivation of the specific viral receptors used to create the bond with the wall of the cell to be invaded.

Ozone performs a disinfectant action superior to any other oxidizing product, together with the property of not releasing any odor or taste into the water.

Ozone for Pre-Oxidation

Ozone oxidizes organic and inorganic compounds present in water through redox reactions.

In the case of metals, ozone also tends to oxidize those present in the form of organic complexes, which are decomposed and precipitate as insoluble hydroxides.

The figure shows a plant that combines the preliminary oxidation phase with tangential reverse osmosis filtration.

For example, iron and manganese, often naturally present in groundwater, compromise its organoleptic characteristics, such as color, odor and taste.

Below are the chemical redox reactions of iron and manganese with ozone.

2Fe²⁺ + O₃ + 2H⁺ ⇒ 2Fe³⁺ + O₂ + H₂O    E⁰ = + 1,30 V

Mn²⁺ + O₃ + H₂O ⇒ MnO₂ + 2H⁺ + O₂    E⁰ = + 0,84 V

Ozone is a very unstable gas under normal environmental conditions, so it cannot be stored but must be “created” in the place and at the moment it is needed. This result is achieved with equipment that transforms part of the oxygen in the air into ozone and is installed together with the other filtration equipment.

GEOmix Plant

The operating principle is very simple:

• The machine is supplied with compressed air, since air is made up of 20% O₂, or with oxygen;
• The generator transforms oxygen O₂ into ozone O₃ through an electrical discharge;
• The ozone produced is injected into the water through dynamic mixers;
• In the contact tank, ozone oxidizes pathogens or inorganic compounds; the reacted ozone turns into oxygen;
• A degassing system allows the total removal of the bubbles present, oxygen and ozone;
• The residual ozone present in the off-gases can be eliminated by a catalytic destructor.

Ozone for Final Disinfection

Information on the Bactericidal Action of Ozone

The bactericidal, fungicidal and virus-inactivating effect of ozone is known and has long been studied (Sonntag, 1890): these properties have been extensively quantified, providing what is now considered irrefutable evidence (Helse, 1915 and 1918; Payr, 1935; Sykes, 1968; Gould, 1981).

As early as 1921, Kleinmann demonstrated that, in the interaction between ozone and water, the degree of purity of the water is a very important factor: greater purity ensures a better antiseptic effect of ozone. This is explained by the loss of part of the ozone, which reacts with the components of impure water instead of with germs.

Ozone is considered an excellent water disinfectant and sterilizing agent, and this effect has been widely used in the potabilization of water networks (Bingmann, 1954).

The advantage of ozone over chlorine, which is often used for drinking water treatment, is that ozone sterilizes much more effectively against both bacteria and viruses; furthermore, ozone does not alter the characteristics of the water, especially its taste (Viebahn, 1977).

Disinfection Strategies

For both viruses and bacteria, it is important to determine the correct ozone dosage required to achieve sterilization. The inactivation of viruses and bacteria occurs through an “all-or-nothing” type reaction, in the sense that below a “threshold dosage” no effect is observed.

In final water disinfection systems, it is therefore necessary to ensure that the amount of ozone dissolved in water is consistent with exceeding this threshold value.

Normally, a small residual amount is guaranteed immediately downstream of the treatment, which, given the instability of the O₃ molecule, tends to disappear before reaching the water distribution points.

For this reason, in drinking water treatment systems, it is necessary to provide coverage of the distribution network with a dosage of persistent product.

Complementary Technologies

To produce water with high-quality characteristics, even starting from poor-quality sources, ozone can be combined with many other technologies, using the multiple-barrier systems approach.

Some examples are provided below.

To Reduce Suspended Solids and Some Pollutant Loads

• Clariflocculators;
• Pressure or gravity sand filtration;
• Pressure or gravity self-cleaning filters;
• Hollow-fiber ultrafiltration membrane plants.

To Reduce Salinity

• Spiral-wound reverse osmosis membrane plants.

To Produce Demineralized Water Where High Specific Standards Are Required, such as Boiler Feed Water, PW, WFI, …

• Resin plants;
• Electrodeionization.

Standard on Mineral Waters

For illustrative purposes only, we report below some parts of the current legislative directives on ozone and mineral waters.

Directive 80/777/EEC: on the approximation of the laws of the Member States relating to the exploitation and marketing of natural mineral waters.

Article 4 – Paragraph 1: “Natural mineral waters, as they appear at source, may undergo only the following treatments or additions: a) separation of unstable elements, such as iron and sulphur compounds, by filtration or decanting, possibly preceded by oxygenation, …”

The following directive was subsequently published in the European Gazette:

Directive 2003/40/EC: use of ozone for the treatment of mineral waters.

Article 5 – Paragraph 2: “The treatment of natural mineral waters with ozone-enriched air … c) the treatment does not cause the formation of residues at a concentration exceeding the maximum limits laid down in Annex III, or residues that may pose a risk to public health;”

ANNEX III – Maximum limits for treatment residues of natural mineral waters and spring waters with ozone-enriched air.

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