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 O3 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 O3 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 O3 is involved.
The amount of oxidizable material in the water is referred to as the ozone demand.
The simplest reactions are where 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 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 O3 is added to make the metal insoluble and they are subsequently 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 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 O3/mg H2S, but in practice and excess of ozone is used (4 mg O3/mg H2S). The H2S is oxidized to sulfate, a soluble salt.
It is more difficult to predict the amount of O3 required to remove organic matter from 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 do react with O3, some of which will oxidize to smaller compounds that don't react. As a result it is difficult to predict the amount of O3 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 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 are treatable with O3, a rule of thumb can be applied for an initial estimate of ozone demand. It says that you need 2.5 mg O3/mg of COD where the COD is composed of organic compounds that can be oxidized by O3.
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 O3 as a component.
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 O3 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 O3 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 O3 must be first removed before the concentration can build up to establish a Ct value.
For disinfection the amount of O3 required would equal:
Ozone Demand from Oxidizable Species (mg/l) + (Ct ÷ contact time)
O# in aqueous solution has 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 25 degrees C (77 degrees F) and a pH of 7 the half life of ozone is 15 minutes.
So in addition to the O3 demand from oxidizable inorganic or organic compounds, one has to account for self decomposition.
In developing the Ct value, the change in O3 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, O3 must be transferred from the gas to 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 referred to as the O3 transfer efficiency.
The transfer efficiency is mainly affected by the following factors:
- The ratio of gas volume to liquid volume (G/L ratio), lower ratio increases efficiency
- Bubble size, smaller bubbles increase efficiency
- Ozone demand of the water, higher demand increases efficiency
- Ozone concentration, higher concentration increases efficiency
- Pressure, higher pressure increases efficiency
- Detention time, longer detention time increases efficiency
- Temperature, lower temperature increases efficiency
Required Ozone Production
O3 generators are normally rated in pounds per day (lbs/day) or grams per hour (g/h). The required O3 production rate is sometimes referred to the Applied Ozone Dose (AOD). We would also need to know the flow rate since most O3 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) = (O3 Demand (g/l) ÷ O3 Transfer Efficiency (%)) X Flow Rate (l/h)
n the case of disinfection
OD (g/h) = (O3 Demand + (Ct ÷contact time) (g/l)) X Flow Rate (l/h) ÷ O3 Transfer Efficiency (%)
The only way to accurately know the proper amount of O3 required is to conduct pilot trials with O3 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 O3 might be a candidate for further consideration in a water treatment application.