Ozone (O3) provides multiple benefits for water treatment, including the removal of organic compounds, certain inorganic compounds (Fe, Mn, H2S), color, odor, and taste. It also acts as a microflocculant that helps remove suspended solids. It is also an excellent disinfectant agent capable of killing a wide spectrum of microorganisms. As a result, it is increasingly being considered for a wide variety of water treatment applications.

A key question in designing an O3 water treatment system is how much ozone is required to achieve the treatment goal. The removal of organic/inorganic compounds and disinfection are the two most common applications for ozone treatment, so they will be the focus of the article.

When removing contaminants from water using ozone, it is important to understand that O3 works through the chemical process of oxidation. A chemical substance oxidizes when it loses electrons. These reactions can occur with and without the presence of oxygen, but in the present case we refer to reactions in which oxygen intervenes in the form of O3.

The amount of oxidizable material in the water is called the ozone demand.

inorganic compounds

The simplest reactions are those in which O3 reacts with inorganic compounds such as Fe, Mn and H2S. In the case of Fe and Mn, the metals are oxidized to insoluble compounds that precipitate from solution. In water treatment, removal of these compounds is important as Fe and Mn can discolor water and deposit on piping systems and materials submerged in water. So, O3 is added to make the metal insoluble and then they are filtered out of the water as a solid. The amount of O3 required is 0.44 mg ozone/mg Fe and 0.88 mg O3/mg Mn.

Hydrogen sulfide (H2S) creates a foul odor in the water (rotten eggs). In potable water applications, H2S is often removed to make the water more palatable. The theoretical amount of ozone needed to remove H2S is 3 mg O3/mg H2S, but in practice an excess of ozone is used (4 mg O3/mg H2S). H2S is oxidized to sulfate, a soluble salt.

organic compounds

It is more difficult to predict the amount of O3 needed to remove organic matter from the water. First, some organic compounds do not react with O3, even though it is a powerful oxidant. These compounds are typically carboxylic acids, ketones, and aldehydes. Even with compounds that react with O3, some of which will oxidize to smaller, non-reacting compounds. As a result, it is difficult to predict the amount of O3 needed without detailed knowledge of the chemicals involved or without conducting laboratory or pilot studies.

One way to measure the amount of organic matter in water is to measure Chemical Oxygen Demand (COD). This test essentially determines the amount of oxygen to convert all of the organic carbon in the sample to CO2. The test uses a powerful oxidant at elevated temperature to oxidize organic compounds. A color change, which measures the amount of oxidant used, indicates the amount of COD.

A change in COD is often used as a target in water treatment. In laboratory tests, the initial amount of COD is noted and O3 is applied to the contaminated solution. A correlation is developed between the O3 applied and the COD level. This is the most direct way to determine the amount of ozone needed. For organic compounds that can be treated with O3, a general rule of thumb can be applied for an initial estimate of ozone demand. It says you need 2.5mg O3/mg COD where COD is made up of organic compounds that can be oxidized by O3.

Another method of measuring organic concentration in water is Total Organic Carbon (TOC). This test measures total carbon (TC) in water by first removing inorganic carbon (IC), eg carbonates, from the water. By measuring the TC and subtracting the CI, the remainder is TOC. Although ozone can oxidize organic compounds, including some to CO2, many of the compounds will remain in the water in an oxidized state, so the change in TOC might not be large. Generally, to remove TOC requires the use of advanced oxidation processes that may involve the use of O3 as a component.

disinfection 

To inactivate microorganisms, it is necessary to expose them to ozone for a certain period of time. A measure of this is known as Ct, which is the average ozone concentration multiplied by the average exposure time. If the O3 concentration were plotted against time, the area under the curve would be Ct. Different organisms require different Ct at a given temperature for inactivation. CT values ​​have been developed for a variety of organisms.

To build an O3 concentration in the water, the demand for ozone in solution must first be met. This means that organic and inorganic compounds that can be oxidized by O3 must first be removed before the concentration can build up to establish a Ct value.

For disinfection the amount of O3 required would be equal to:

Ozone Demand of Oxidizable Species (mg/l) + (Ct ÷ contact time)

ozone decomposition 

O# in aqueous solution undergoes a self-decomposition reaction. In pure water, O3, without any oxidizable species, will decompose back to oxygen. The decomposition reaction is a function of temperature. For example, at 77 degrees F (25 degrees C) and a pH of 7, the half-life of ozone is 15 minutes.

So, in addition to the O3 demand of oxidizable inorganic or organic compounds, self-decomposition must be taken into account.

In developing the Ct value, the change in O3 concentration as a function of contact time would be measured to determine the C vs t curve so that the area under the curve can be defined.

ozone transfer efficiency 

To act as an oxidant in aqueous systems, O3 must be transferred from the gas phase to the liquid phase where it acts in solution as a dissolved species. The percentage of the O3 produced in the gas phase (the applied O3 dose) that ends up in solution (the transferred O3 dose) is called the O3 transfer efficiency.

Transfer efficiency is mainly affected by the following factors:

1. The ratio between the volume of gas and the volume of liquid (G/L ratio), a lower ratio increases efficiency
2. Bubble size, smaller bubbles increase efficiency
3. Demand for ozone from water, higher demand increases efficiency
4. Ozone concentration, higher concentration increases efficiency
5. Pressure, higher pressure increases efficiency
6. Dwell time, longer dwell time increases efficiency
7. Temperature, lower temperature increases efficiency

 Required ozone production

O3 generators are typically rated in pounds per day (lbs/day) or grams per hour (g/h). The required O3 production rate is sometimes referred to as the applied ozone dose (AOD). We would also need to know the flow rate, since most O3 demand requirements are calculated in grams or milligrams per liter. So the amount of water treated over a period of time is necessary.

In the case of organic/inorganic removal

AOD (g/h) = (O3 Demand (g/l) ÷ O3 Transfer Efficiency (%)) X Flow (l/h)

in the case of disinfection

DO (g/h) = (O3 demand + (Ct ÷ contact time) (g/l)) X Flow rate (l/h) ÷ O3 transfer efficiency (%)

The only way to know precisely the proper amount of O3 needed is to pilot test O3 transfer equipment similar to what will be used on a large scale. However, the methodology discussed in this article along with the aforementioned rules of thumb can be useful in generating rough estimates to see if O3 might be a candidate for further consideration in a water treatment application.