Ozone is a widely used agent for water treatment including public drinking water, industrial wastewater, purification of process water, aquariums, bottled water, ground water remediation and aquaculture, among many others. Ozone finds value in these applications because it can provide multiple benefits in water treatment while decomposing back to oxygen after use. The benefits of ozone include:
•Disinfection – ozone is a biocide that can inactivate a wide range of microorganisms including virus and bacteria.
•Oxidant – ozone is capable of directly oxidizing various organic compounds, including those of emerging concern such as pharmaceuticals and personal care products found in our drinking water sources, as well as organic compounds found in industrial wastewater.
•Advanced oxidation – In combination with peroxide or UV light, ozone can be the base of an advanced oxidation process that can oxidize virtually any organic compound including difficult to treatment materials such as 1,4 dioxane.
This article will touch on some of the major existing applications and some emerging applications for ozone.
Public drinking water treatment
Chart 1 shows the distribution of ozone use in US public drinking water. Ozone provides several benefits to drinking water facilities. These include primary disinfection, taste and odor removal, color removal, removal of inorganic materials such as Fe, Mn and H2S, enhanced filtration via a micro-flocculation effect and removal of certain organic compounds. In a well-engineered process, several of these benefits can be achieved in a single injection of ozone. Being able to accomplish several treatment objectives at the same time make ozone an excellent choice for certain drinking water plants. This explains the growth of ozone for this application.
Municipal wastewater reuse
Because of water shortages caused by growth in water demand as well as loss of water supply caused by drought, more cities are thinking about reusing their wastewater for drinking water, either directly or indirectly. Several facilities have already been built using membrane-based technologies. This is a proven approach but comes with several notable drawbacks. First, you need a find a place to dispose of the waste brine rejected from the RO membranes. In coastal locations, the brine can be rejected to the ocean. In other locations it is not as simple. The second factor is cost, because of the high pressure needed in RO membranes, the electrical demand to treat the water is high.
Ozone in combination with biologically active filters (ozone-BAF) can be used to treat municipal wastewater to achieve high degrees of water purity at lower cost than membrane processes and without the need to dispose of a brine waste. Currently, large scale trials are being run by the Hampton Roads Sanitation District, in Eastern Virginia to demonstrate this technology through their SWIFT project. Other water authorities are also running ozone-BAF pilot trials. If successful, ozone-BAF could provide inland wastewater facilities with a technology to reuse their wastewater for drinking water.
Advanced oxidation processes
Another emerging application for ozone is in advanced oxidation. Advanced oxidation refers to processes that produce hydroxyl radicals (•OH). The hydroxyl radical is one of the strongest and fastest acting oxidants that can oxidize virtually any organic compound. They can be produced in a variety of ways, but ozone with peroxide and ozone with UV are two such methods. The advantage of the ozone routes is that little if any by product is left after complete and pH adjustments are not required. An important application of ozone based advanced oxidation is the removal of 1,4 dioxane from ground water contaminated with solvents. Dioxane is not readily air stripped from water, absorbed on activated carbon, or oxidized by most chemicals. Advanced oxidation by ozone and peroxide has been proven to be effective in removing the dioxane from the water via oxidation. Ozone with peroxide has also found use in industrial wastewater applications where other methods were not able to remove difficult to treat organic compounds. The use of advanced oxidation appears to be a growing technology for water treatment and ozone-based processes are a growing part of this application.
Wastewater treatment plants utilizing the activated sludge process typically produce excess sludge that needs to be removed and disposed of from the process. Ozone has been shown in the literature and in commercial application to offer an alternative method for reducing excess sludge. This process involves exposing a portion of the activated sludge to ozone. Some of the bacteria that make up the sludge are lysed via exposure to ozone releasing COD into the wastewater. Even though some of the cells are damaged or killed, the overall process generally does not show an increase in BOD/COD leaving the process.
Depending on the amount of ozone required per unit of sludge removed and the costs associated with the current handling of the excess sludge, ozone can represent an economical alternative with conventional sludge handling processes. Ozone also offers other potential advantages to the activated sludge process including a reduction in bulking/foaming, scum control and improved denitrification. A number of wastewater treatment plants in Europe have adopted this technology and have realized a 30-40 percent reduction in excess sludge with no loss of wastewater treatment performance. A new installation is planned for the US later this year at a chemical manufacturing plant.
PFAS are a group of various fluorochemicals including PFOA, PFOS, among others. They are of emerging concern since they are distributed widely in the environment, do not readily breakdown in the environment/human body and can accumulate over time. There is some evidence that they may have adverse impacts on human health. US EPA points to studies of PFOA and PFOS that show: reproductive and developmental, liver and kidney, and immunological effects and tumors in laboratory animals. As a result, the agency is studying regulation of these compounds.
A novel approach to PFAS removal is ozofractionation. Ozofractionation is a form of foam fractionation, a method to remove hydrophobic materials from water. In ozofractionation, ozone is used an oxidant, as well as the foam-forming gas. Proposed advantages for using ozone as the bubble forming gas include co-contaminant reduction and smaller bubble sizes. Smaller bubbles result in higher surface area to volume ratios and thus more material can be captured for a given volume of gas. Formation of hydroxyl radicals around the bubbles may result in negative charges that better hold the PFAS molecule.
Ozone continues to find new applications in water treatment while expanding its existing applications. The future looks bright for the use of ozone in various parts of the water treatment industry.
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Source – Anthony Sacco from Water Conditioning & Purification International Magazine (https://wcponline.com/2021/08/15/state-of-the-ozone-water-treatment-industry/)