FWEA FOCUS
Don’t Overly Disinfect Your Organization Sondra W. Lee, P.E. President, FWEA
W
hen it comes to disinfection at the Thomas P. Smith Water Reclamation Facility (TPSWRF) in Tallahassee, the first thing that pops into my mind is how easy it is to forget about this part of the treatment process. It nearly fits into an “out of sight, out of mind” category. A system like this, which runs well and doesn’t demand too much attention, doesn’t happen without some planning, great teamwork, and a little bit of flexibility.
Planning Prior to 2011, the TPSWRF was rated for 26.5 million gallons per day (mgd) of disinfected effluent using gaseous chlorine. Due to the dangers of elemental chlorine,
personnel at the facility underwent a fair amount of training to maximize the safety of everyone there, but when planning was underway for a facilitywide upgrades project, the City of Tallahassee wanted to consider alternatives for disinfection. A study was completed in 2008 and evaluated three options: gaseous chlorine, onsite generation, or delivery of 12 percent sodium hypochlorite. Ultraviolet (UV) was not considered due to the desire to deliver public access reclaimed water directly from the facility, which may require adding chlorine into the effluent to meet permit requirements. Equipment Selection Tallahassee decided to place safety concerns at the top of its criteria for disinfection alternatives. The 20-year present worth analysis for capital and operation and maintenance costs resulted in chlorine gas as the most cost-effective alternative. With the safety concerns of this option and the potential for significant future regulatory restrictions regarding the transportation of hazardous materials, Tallahassee discarded
20 August 2022 • Florida Water Resources Journal
this option. It also discarded the second most cost-effective option to generate sodium hypochlorite onsite. At the time of the study, technology to create sodium hypochlorite at treatment facilities was somewhat new and staff had concerns about issues in handling the hydrogen gas byproduct of the onsite process. The city decided it would rather wait a few years to let the technology improve; therefore, bulk delivery of sodium hypochlorite was selected, even though it was not the most cost-effective option when evaluating a 20year life cycle cost. Ten years later, this appears to have been a great choice.
Teamwork In general, the disinfection process at TPSWRF appears to operate trouble free. Admittedly, when compared to many processes at the facility, the disinfection system is rather simple, but when something runs well and doesn’t pose many problems, it’s so easy to forget about it. A large part of the success is that the
