The largest clean-water plant in Anchorage makes the switch from chlorine gas disinfection to sodium hypochlorite generated on site.
For years, the Anchorage Water & Wastewater Utility had used chlorine gas as the disinfectant at its water and wastewater treatment plants.
In the late 1990s, the utility changed direction. “Our main concern was risk to the community,” says David Persinger, P.E., director of the AWWU treatment division. “In Alaska, everything comes by truck or barge. The chlorine cylinders were barged in.
“Our port is just north of downtown Anchorage. So the 2,000-pound cylinders, usually 12 at a time, had to be transported on a truck through the downtown and some residential areas to get out to our main treatment plant. Then there were heightened security issues that came about after 9/11. There are also process safety management and EPA risk management issues with chlorine gas.”
In view of all that, the utility looked at alternative disinfectants. For the Asplund Wastewater Treatment Facility, the largest wastewater plant in Alaska (58 mgd design, 26 mgd average), the utility chose an on-site sodium hypochlorite generation system supplied by Electrolytic Technologies. The disinfection upgrade earned the utility a Best Projects Award and a Safety Award from Engineering News-Record, North West Region.
Part of a program
The Asplund plant upgrade, completed in December 2015, finished the utility’s transition away from chlorine gas at all its treatment facilities. First to change over, starting in 2000, were the two water treatment plants with design capacities of 35 and 4 mgd. They received on-site systems (Severn Trent ClorTec) generating 0.8 percent sodium hypochlorite.
Next came the utility’s two smaller wastewater treatment plants. The 2.5 mgd plant with secondary and tertiary treatment serving the Eagle River community and outlying areas north of Anchorage was converted to UV disinfection. The 0.6 mgd Girdwood treatment plant with secondary and tertiary treatment, serving a ski resort and vacation home community about 30 miles south of Anchorage, switched to a 12.5 percent solution of purchased hypochlorite.
For the Asplund primary treatment plant serving Anchorage proper and discharging to Cook Inlet, the utility evaluated UV, ozone and hypochlorite. “We narrowed it down to on-site-generated hypochlorite, either 0.8 percent or 12.5 percent,” Persinger says. “We noted that 0.8 percent requires a much bigger building footprint and is a bit less efficient in energy, water and salt consumption. In addition, with the volumes we would be using, 0.8 percent would require quite a bit of storage and a pretty high feed rate. That narrowed it down to the 12.5 percent solution.”
The utility ultimately installed the Klorigen hypochlorite generation system from Electrolytic Technologies, a system designated by the U.S. Department of Homeland Security as a Qualified Anti-Terrorism Technology. The company custom designed the technology to accommodate the utility’s needs, including compliance to local seismic design standards. It can generate 5,000 gpd.
The system was installed in two phases. “We completely rebuilt our disinfection building,” says Persinger. “About half of that building had been dedicated to cylinder storage and the gas feed system, and the other half was a workshop area.
“The initial phase was to build out the storage and feed system for the hypochlorite process. We installed the storage tanks, the pumps and all the instrumentation required to feed the disinfection process. Once that was completed, we functioned with purchased hypochlorite while the contractor installed the generation equipment in the other half of the building.”
The new system is designed to be compatible with the utility’s current operations and for ease of expansion to meet future demand. It is unique in including a gas split mechanism that sends part of the generated chlorine gas directly to the point of application and the rest to the hypochlorite conversion skid.
“It’s an ion-selective membrane process that basically splits salt so that you end up with chlorine gas on one side of the membrane and caustic on the other,” says Persinger. “In conventional installations, generally more gas is produced than caustic to recombine, so the result is an imbalance. Most of those systems require transport of supplemental caustic so that 100 percent of the chlorine gas can be converted to sodium hypochlorite.
“Because we didn’t want to import caustic, due to logistical and safety issues, to supplement our hypochlorite production, we went with the hybrid system. The excess chlorine gas produced is fed directly into the effluent stream for disinfection, and then we vary the flow of hypochlorite to meet our disinfection residual requirement.”
The system uses extremely pure salt and softened water. The process uses sodium bisulfite to neutralize the depleted brine solution produced before that solution is recirculated to the front end of the process. Hydrochloric acid is used in the automated cleaning process that periodically removes chemical deposits.
AWWU personnel worked closely with the installation contractor and the on-site engineer in charge of construction management to become familiar with the technology and the safety precautions required during operation.
“We also had quite a bit of training from the vendor,” says Persinger. “After completion of the project, we did in-house startup and commissioning training. Anytime we have an issue with the equipment, we involve as many people from our maintenance shop as possible, so they can understand the issue and get to know the system better.”