Ozone is produced when ozone (O2) molecules are dissociated by an energy source into oxygen atoms and subsequently collide with an oxygen molecule to form an unstable gas, ozone (O3), which is used to disinfect wastewater. Most wastewater treatment plants generate ozone by imposing a high voltage alternating current (6 to 20 kilovolts) across a dielectric discharge gap that contains an oxygen-bearing gas. Ozone is generated onsite because it is unstable and decomposes to elemental oxygen in a short amount of time after generation.
Disinfection is considered to be the primary mechanism for the inactivation/destruction of pathogenic organisms to prevent the spread of waterborne diseases to downstream users and the environment. It is important that wastewater be adequately treated prior to disinfection in order for any disinfectant to be effective. Table 1 lists some common microorganisms found in domestic wastewater and the diseases associated with them.
The effectiveness of disinfection depends on the susceptibility of the target organisms, the contact time, and the concentration of the ozone. The components of an ozone disinfection system include feed-gas preparation, ozone generation, ozone contacting, and ozone destruction. Air or pure oxygen is used as the feed-gas source and is passed to the ozone generator at a set flow rate. The energy source for production is generated by electrical discharge in a gas that contains oxygen.
Ozone generators are typically classified by:
• The control mechanism (either a voltage or frequency unit).
• The cooling mechanism (either water, air, or water plus oil).
• The physical arrangement of the dielectrics (either vertical or horizontal).
Ozone Generators manufactured by different companies have unique characteristics but also have some common configurations. The electrical discharge method is the most common energy source used to produce ozone. Extremely dry air or pure oxygen is exposed to a controlled, uniform high-voltage discharge at a high or low frequency. The dew point of the feed gas must be -60?C (-76?F) or lower. The gas stream generated from air will contain about 0.5 to 3.0% ozone by weight, whereas pure oxygen will form approximately two to four times that concentration.
After generation, ozone is fed into a down-flow contact chamber containing the wastewater to be disinfected. The main purpose of the contactor is to transfer ozone from the gas bubble into the bulk liquid while providing sufficient contact time for disinfection. The commonly used contactor types diffused bubble (co-current and counter-current) are positive pressure injection, negative pressure (Venturi), mechanically agitated, and packed tower. Because ozone is consumed quickly, it must be contacted uniformly in a near plug flow contactor.
Ozone disinfection is generally used at medium to large sized plants after at least secondary treatment. In addition to disinfection, another common use for ozone in wastewater treatment is odor control. Ozone disinfection is the least used method in the U.S. although this technology has been widely accepted in Europe for decades. Ozone treatment has the ability to achieve higher levels of disinfection than either chlorine or UV.
Industrial Process Water
Ozone treatment systems oxidize organic solvents, phenolic compounds, sulfides, mercaptan, odors, and heavy metals. In addition ozone can increase the efficiency of sand filters, carbon filters, stripping columns, and clarifiers.
There are two very basic objectives for any cooling water treatment program: maintaining system efficiency and prolonging system life. Toward these ends, Ozone systems coupled with cooling water treatment programs control biological activity, prevent scale formation, and inhibit corrosion.
Wastewater treatment plants use ozone treatment systems and ozone treatment programs to oxidize organic solvents, phenolic compounds, sulfides, mercaptan, odors, and heavy metals. Wastewater treatment plant managers also find the ozone systems coupled with a good maintenance program are effective at increasing the efficiency of sand filters, carbon filters, stripping columns, and clarifiers.
Ozone treatment process for food plants using brine for product processing. The treatment systems reduce bacteria counts in the brine to extremely low levels. A specially designed mixing system improves clarity by separating fly ash, fats, and other impurities from the brine. The system provides an effective means of controlling bacteria, yeast, and mold – and without the taste, quality, and health problemsassociated with chlorine.
Textile plants using Ozone generators benefit from the technology with cleaner spray nozzles, low bacteria counts, increased run-times between air washer cleanings, and cleaner running air washings without the use of poisonous biocides.