Water Innovations gives Water and Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.
Issue link: http://wateronline.epubxp.com/i/672151
national regulatory trends and guidelines, preference was given to these technologies. Of the alternatives tested, ozone could provide the additional advantage of AOP disinfection per EPA water reuse guidelines. While UV disinfection provided the greatest economic savings to the Authority, the potential long- term benefits of AOP by ozonation led the Authority to select ozone disinfection as the preferred method. RETTEW designed the system to allow for future higher dosages, complying with recommended AOP methodologies and providing biofouling control. The AOP System At UAJA The AOP process consists of an ozone gas generation system combined with support processes to inject the ozone gas into the RO feed and secondary safety structure. System requirements include the following: • Liquid oxygen (LOX) supply • Ozone generator • Gas cooling and chiller • Sidestream gas injection system • Ozone contactor • Ozone quench injection system • Off-gas collection and destruction system A third-party vendor supplies the oxygen, which is stored in an exterior cryotank specially made for that use. When required for the process, a vaporizer converts liquid oxygen to high-purity gaseous oxygen and conveys the substance, under pressure, into the AOP system. To improve operation performance, the Authority chills its oxygen gas to a minimum of minus 60 degrees Fahrenheit dew point. The ozone generator converts chilled gaseous oxygen into ozone gas, with a maximum capacity of 53 pounds per day of ozone at a 5 percent concentration. Additional capacity could be gained in the future by increasing the concentration of ozone in the gas. The ozone gas is first applied to a sidestream water feed through a venturi injector, and then mixed with the RO feed to the ozone contactor. The ozone contactor is a 4,200-gallon stainless steel baffled tank designed to provide the minimum contact time for reaction. Based on the treatability study conducted, the design contact time is five minutes at a dose of 4.0 mg/L of ozone. Off-gassing is controlled through the removal of headspace gases in the ozone contactor, through an ozone destruction skid. The gas is then discharged into the atmosphere outside the building. The Authority monitors the off-gas for ozone concentration, ensuring an ozone-free discharge. As an aggressive oxidant that would damage thin-film composite RO membranes, the Authority must quench residual ozone in the water. RETTEW adapted a portion of the existing chemical dosing system to feed liquid sodium bisulfite into the water before the RO feed, which provides sufficient removal and protection. Construction The Authority initiated construction in the summer of 2014 and began operations in early 2015. Equipment costs were lower than anticipated during the AOP evaluation, resulting in an additional $100,000 in capital cost savings to the Authority. Total project costs related to the AOP portion were approximately $800,000. To date, power and oxygen consumption have been lower than predicted. The energy savings from the project were enough for the Authority to receive a rebate from the local electrical utility, further enhancing project returns and payback. Operational monitoring is underway to characterize the reduction in total organic carbon (TOC) and contaminants of concern to document removal efficiencies and their impact on the RO system operations. System operations are thus far stable, not showing any performance decrease with the use of ozone for AOP, as opposed to the previous chlorination/dechlorination scheme. Conclusions Proper biofouling control is critical to the cost-effective operation of an RO system; however, the economic merits of any biofouling control require periodic evaluation and modifications. While AOP is not necessary for the UAJA to meet its current regulatory limits for water reuse, its application has proved to be a cost-effective means for controlling membrane biofouling. Coupled with the removal of contaminants of concern, AOP proved to be an effective alternative to traditional biofouling control strategies. n 10 wateronline.com n Water Innovations FILTRATION Braas is an environmental engineer and the firm's water reuse expert, engaged in projects across the U.S. She has more than 20 years of experience in designing wastewater systems and drinking water distribution systems, including coordinating design, evaluations, and implementing industrial pretreatment programs. She also manages grant applications, permitting, operation analysis, and construction- related activities. About The Authors Jason Wert is a senior technical engineer responsible for the development and technical evaluation of traditional and renewable energy projects throughout the U.S. With more than two decades of experience, he brings a practical project knowledge in financing, development, and structuring of public and private energy projects. He is a national expert in the planning, development, design, construction, and operation of membrane treatment, anaerobic digestion, and renewable energy projects.