Determining Amount of Ozone Required for Ozone Water Treatment

Ozone (O3) provides multiple benefits for water treatment including remove of organic compounds, certain inorganic compounds (Fe, Mn, H2S), color, odor and taste. It also acts as a micro flocculent which aids in removal of suspended solids. In addition it is an excellent disinfecting agent capable of killing a wide spectrum of micro organisms. As a result it is being increasing considered for a wide variety of water treatment applications.

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

In removing contaminants from water using ozone, it is important to understand that ozone acts by the chemical process of oxidation. A chemical substance is oxidized when it loses electrons. These reactions can occur with and without the presence of oxygen, but in the present case we are referring to reactions where oxygen in the form of ozone is involved.

The amount of oxidizable material in the water is referred to as the ozone demand.

Inorganic Compounds

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

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

Organic Compounds

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

One way to measure the amount of organic in water is to measure the 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 the 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 an objective in water treatment. In laboratory tests the initial amount of COD is noted and ozone is applied to the contaminated solution. A correlation is developed between the ozone applied and the COD level. This is the most direct way to determine the amount of ozone needed. For organic compounds that are treatable with ozone, a rule of thumb can be applied for an initial estimate of ozone demand. It says that you need 2.5 mg ozone/mg of COD where the COD is composed of organic compounds that can be oxidized by ozone.

Another method of measure organic concentration in water is Total Organic Carbon (TOC). This test measures the total carbon (TC) in water by first removing the inorganic carbon (IC), e.g. carbonates, from the water. By measuring the TC and subtracting the IC remainder is TOC. While 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 great. Generally, to remove TOC requires the use of advanced oxidation processes which can involve the use of ozone as a component.

Disinfection

In order to inactivate micro organisms, it is necessary to expose them to ozone for a certain period of time. A measure of this is referred to as Ct, which is the average concentration of ozone multiplied by the average time of exposure. If one plotted ozone concentration versus time, the area under the curve would be Ct. Different organisms require different Ct at a given temperature for inactivation. Ct values for a variety of organism have been developed.

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

For disinfection the amount of ozone required would equal:

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

Ozone Decomposition

Ozone in aqueous solution has a self decomposition reaction. In pure water ozone, without any oxidizable species, will decompose back to oxygen. The decomposition reaction is a function of temperature. At pH 7 the values are:

Temperature (C) Half Life (Minutes)
15 20
20 20
25 15
30 12
35 8


So in addition to the ozone demand from oxidizable inorganic or organic compounds, one has to account for self decomposition. In developing the Ct value, the change in ozone concentration as a function of the 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

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

The transfer efficiency is mainly affected by the following factors:

  1. The ratio of gas volume to liquid volume (G/L ratio), lower ratio increases efficiency
  2. Bubble size, smaller bubbles increase efficiency
  3. Ozone demand of the water, higher demand increases efficiency
  4. Ozone concentration, higher concentration increases efficiency
  5. Pressure, higher pressure increases efficiency
  6. Detention time, longer detention time increases efficiency
  7. Temperature, lower temperature increases efficiency


Required Ozone Production

Ozone generators are normally rated in pounds per day (lbs/day) or grams per hour (g/h). The required ozone production rate is sometimes referred to the Applied Ozone Dose (AOD). We would also need to know the flow rate since most ozone demand requirements are computed 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) = (Ozone Demand (g/l) ÷Ozone Transfer Efficiency (%)) X Flow Rate (l/h)

In the case of disinfection

AOD (g/h) = (Ozone Demand + (Ct ÷contact time) (g/l)) X Flow Rate (l/h) ÷Ozone Transfer Efficiency (%)

The only way to accurately know the proper amount of ozone required is to conduct pilot trials with ozone transfer equipment similar to that which will be used in full scale. Nonetheless the methodology discussed in this article along with the rules of thumb mentioned can be useful in generating rough estimates to see if ozone might be a candidate for further consideration in a water treatment application.