Ultraviolet Disinfection of Wastewater

The Old Way

In 1991, AWA spent about $8,000 for chlorine. Stored on site in 2,000-pound canisters, 80 to 150 pounds of chlorine were added to the chlorine contact tank daily. After disinfection, plant effluent contained low levels of chlorine which threatened local aquatic life. Also, while connecting, emptying, and changing canisters, plant operators risked exposure to chlorine gas. Several times toxic leaks forced workers to use protective breathing equipment.

The New Way

UV disinfection helps AWA treat wastewater economically while minimizing environmental health and safety impacts. All AquarayTM closed chamber horizontal system, supplied by Ultraviolet Purification Systems, Inc. of Bedford Hills, NY, was installed at AWA in December 1991. The system operates continuously treating up to 20 million gallons per day. Composed of 2 horizontal Type 304 stainless steel cylindrical reactors, it operates under pressure up to 30 pounds per square inch.

Both reactors contain 348 UV quartz enclosed lamps divided equally into 2 banks. Controlled by electromagnetic ballasts, lamps activate sequentially as wastewater flow increases. The banks in the first reactor handle flows up to 10 MGD while heavier volumes activate the banks contained in the second reactor. As wastewater passes through the unit, UV light disrupts DNA in bacteria, viruses, molds, algae, and other microorganisms causing cell death or inactivity. Treated wastewater is then discharged.

Performance data are displayed on a remote UV intensity meter and chart recorder. If a lamp burns out or light intensity drops, alarms alert personnel. The UV unit can be operated by remote control or manually and requires no special training.

The Results

AWA has found an environmentally responsible, convenient, and cost-effective way to disinfect municipal wastewater discharge. Performance is equal to chlorine. Effluent easily meets NPDES permit requirements. As a result, adequate disinfection is provided, permit requirements are met, and local aquatic life is protected. By lessening reliance on chlorine, AWA also reduced the potential for worker exposure to chlorine gas.

The Advantages

UV disinfection is safer than wastewater treatment systems that rely on chlorine gas. By eliminating transport and handling of large quantities of a hazardous chemical, the UV system reduces potential liability from worker/community exposure. Studies also suggest that UV disinfection controls viruses and many disease-causing bacteria better than chlorination/dechlorination.

Simplified Compliance:
UV disinfection has helped AWA comply with NPDES permit requirements and Fire Code regulations. Continued use of chlorine would have required AWA to build a new disinfection area incorporating secondary containment and a scrubbing/neutralization system. Instead, in less space, AWA installed the UV system. By avoiding major construction, AWA met NPDES requirements cost effectively and on time.

Reduced Effluent Toxicity:
Even at low concentrations, chlorine is toxic to aquatic organisms. Aqueous chlorination practices also generate halogenated organic compounds, which may also be toxic. By using UV, AWA protects the aquatic habitat of the receiving waters.

Maintenance and Cleaning:
Maintenance of UV disinfection systems is becoming easier. For AWA's UV system, two workers scrub lamp jackets with a nontoxic 5 to l0 percent citric acid solution twice a month. About 3 times per year, the jackets must also be cleaned to remove algae, a process that takes around 40 manhours. Lamps are replaced due to electrical failure or after 7,500 hours of use when output decreases to about 70 percent. Since the system is accessible from the chamber ends, disassembly is not required.

The AquarayTM closed chamber horizontal system is highly reliable. In-place chemical cleaning systems automatically respond when UV intensity decreases. If this happens, visual and audible alarms activate. Valves shut off wastewater flow and the system circulates a citric acid solution with a pH of approximately 3 throughout the disinfection chamber. Combined with air scrubbing, the process removes coating from lamps in under an hour. Used solution is sent to the plant inlet where it biodegrades in the normal treatment process. Once reactivated, the unit is operational in less than one minute.