38. Reclaimed Water Pasteurization and Energy Recovery
The process of heating water for disinfection has been proven effective for greater than 150 years. However, only within the last 8 years have engineers and scientists discovered how to capture waste heat and continuously utilize that waste heat to cost effectively disinfect water. In late 2011, the City of Ventura, working with the Pasteurization Technology Group and Carollo Engineers, constructed the world’s first demonstration scale reclaimed water pasteurization system. Sized at 350 gpm, the system uses full-scale components to demonstrate low energy sustainable disinfection meeting California’s Title 22 reclaimed water criteria. For Ventura, the pasteurization system is planned to:
Burn a combination of natural gas and digester gas,
Use the power to run the entire treatment plant, and
Use the waste heat generated from the turbines to disinfect up to 21 mgd of effluent meeting Title 22 standards for unrestricted water reuse applications.
The economy of pasteurization is based upon the capture of a waste-heat source and the transfer of that heat to the water for disinfection. Economic analyses for utilities nationwide have shown pasteurization to be the lowest-cost system in all but a few locations, in terms of both energy use and total cost. For Ventura, pasteurization would fulfill two immediate needs, reduced energy (power) costs, and low cost disinfection. The engineering approach for Ventura included the installation of 1.2 MW of gas turbines, purchase of natural gas to use in combination with digester gas to feed the turbines, and the capture and use of turbine waste-heat to disinfect Ventura’s entire flow stream.
The true value of pasteurization is the continual reuse of heat. The Pasteurization Technology Group’s patented pasteurization system consists primarily of plate-type (water‑to‑water) and stack-type (air-to-water) heat exchangers. The plate-type heat exchanger, or preheater module, transfers the heat from the already disinfected wastewater to the influent wastewater. The stack-type heat exchanger, or waste-heat recovery module, transfers the waste heat from the external heat source to the water. The external heat source can be turbine-exhaust heat, burner- (flare-) exhaust heat, hot water, or other forms of heat (exhaust heat for Ventura). Following the waste-heat recovery module, in-line pumps are included to maintain a higher pressure on the effluent side of the system, compared to the influent side. Many assume that pasteurization cannot possibly be cost-effective to heat wastewater to greater than 71°C (160°F). While each drop of water attains a the peak temperature, the reuse of heat within the reactor means that only 2 to 5 degrees F must be continuously input into the reactor. In fact, the use of the various heat exchangers keeps more than 96% of the reactor heat within the reactor. The secondary benefit of the capture and maintenance of the heat within the reactor is that effluent temperatures are typically within 1.7°C (3°F) of influent temperatures.
The pasteurization concept for Ventura thus requires gas turbines, heat exchangers, and a pump to maintain pressure through the system. The large economic value to Ventura (and others) is the low cost to make power with natural gas and digester gas, compared to the price of power in Ventura ($0.11/kW-hr). Over 20 years, the net present value of pasteurization is $3M to $6M less than UV disinfection, and less than the cost of their current chlorine gas system.
The reactor was installed in late 2011 for long term demonstration testing, and has been running consistently at between 350 and 400 gpm ever since. The reactor has run through various flow and temperature scenarios to fine tune the operational temperature for a future full-scale system. Total coliform results have shown that much lower temperatures are required (155 degrees F) compared to the values used in the prior economic analysis (180 degrees F). The treatment plant has seen heat exchanger fouling and has worked with the Pasteurization Technology Group to devise an effective clean-in place procedure. Carollo Engineers has just completed detailed virus testing to better assist the California Department of Public Health’s understanding of pasteurization to reclaimed water standards. In total, the demonstration testing has shown cost effective treatment, effective capture and reuse of waste energy, and more sustainable wastewater treatment and energy use. Once the demonstration testing is complete (late 2012), Ventura has plans to move ahead with a full-scale (21 mgd) pasteurization project.
Why it should be recognized:
Energy and Water are inextricably linked. The Sandia National Laboratories states, “The continued security and economic health of the United States depends on a sustainable supply of both energy and water.” Because energy requires water and water requires energy, there is substantial economic value in fully utilizing both commodities. Capturing waste heat for disinfection of reclaimed water accomplishes this goal.