In this posting we will discuss the use of ozone for residential wastewater disinfection. While not practiced extensively in the US versus chlorination and UV, ozone is used more often for wastewater treatment in Europe. The advantages of ozone include: it is more effective than chlorine in destroying viruses and bacteria. The ozonation process utilizes a short contact time (approximately 10 to 30 minutes). There are no harmful residuals that need to be removed after ozonation because ozone decomposes rapidly and, after ozonation, there is no regrowth of microorganisms, except for those protected by the particulates in the wastewater stream. Ozone is generated onsite, and thus, there are fewer safety problems associated with shipping and handling. Ozonation elevates the dissolved oxygen (DO) concentration of the effluent (the increase in DO can eliminate the need for reaeration and also raise the level of DO in the receiving stream).
The key process control parameters are dose, mixing, and contact time. An ozone disinfection system strives for the maximum solubility of ozone in wastewater, as disinfection depends on the transfer of ozone to the wastewater. The amount of ozone that will dissolve in wastewater at a constant temperature is a function of the partial pressure of the gaseous ozone above the water or in the gas feed stream. It is critical that all ozone disinfection systems be pilot tested and calibrated prior to installation to ensure that they will meet discharge permit requirements for their particular sites.
Ozone generation uses electrical power. Attention should be given to the system to ensure that power is optimized for controlled disinfection performance. The operator must on a regular basis monitor the appropriate subunits to ensure that they are not overheated. Like oxygen, ozone has limited solubility and decomposes more rapidly in water than in air. This factor, along with ozone’s reactivity, requires that the ozone contactor be well covered and that the ozone diffuses into the wastewater as effectively as possible.
Ozone in gaseous form is explosive once it reaches a concentration of 240 g/m3. Since most ozonation systems never exceed a gaseous ozone concentration of 50 to 200 g/m3, this is not a problem. However, ozone in gaseous form will remain hazardous for a significant amount of time; thus, caution is needed when operating the ozone gas systems.
It is important that the ozone generator, distribution, contacting, off-gas, and ozone destructor inlet piping be purged before opening the various systems or subsystems.
Key O&M parameters include: Clean feed gas with a dew point of -60° C (-76° F), or lower, must be delivered to the ozone generator. If the supply gas is moist, the reaction of the ozone and the moisture will yield a very corrosive condensate on the inside of the ozonator. The output of the generator could be lowered by the formation of nitrogen oxides (such as nitric acid). Maintain the required flow of generator coolant (air, water, or other liquid). Lubricate the compressor or blower in accordance with the manufacturer’s specifications. Ensure that all compressor sealing gaskets are in good condition. Operate the ozone generator within its design parameters. Regularly inspect and clean the ozonator, air supply, and dielectric assemblies, and monitor the temperature of the ozone generator. Monitor the ozone gas feed and distribution system to ensure that the necessary volume comes into sufficient contact with the wastewater. Maintain ambient levels of ozone below the limits of applicable safety regulations.
The cost of ozone disinfection systems is dependent on the manufacturer, the site, the capacity of the plant, and the characteristics of the wastewater to be disinfected. Normally, costs for wastewater ozonation are based on at least a secondary level of treatment (i.e., average BOD and SS of 30 mg/L or less). Costs are influenced by many factors that are process- and site specific. Because the concentration of ozone generated from either air or oxygen is so low, the transfer efficiency to the liquid phase is a critical economic consideration. For this reason, the contact chambers used are usually very deep and covered. The annual operating costs for ozone disinfection include power consumption, chemicals and supplies, miscellaneous equipment repairs, and staffing requirements.