Ultraviolet (UV) disinfection is now a standard feature in many wastewater treatment plants. It has also been adopted by the drinking water community as a barrier against chlorine-tolerant species such as Cryptosporidium and Giardia.

The technology is widely favored for its non-chemical nature, its elimination of subsequent dechlorination, and its ability to be unselective in disinfection performance. While wastewater treatment plants traditionally have used open-channel systems for UV disinfection, closed-vessel systems have been refined and improved in recent years and have benefits that make them deserving of serious consideration.

 

Proving performance

UV light works by causing permanent damage to the organisms’ DNA. Once the DNA becomes damaged, or dimerized, the organism is unable to carry out the routine cell functions of respiration, the assimilation of food, and replication. Once the cell is rendered non-viable, the organism quickly dies. The difference in UV system efficiency among different manufacturers was made transparent with the advent of UV system validation using bioassay techniques.

A bioassay involves the introduction of a non-pathogenic organism (biodosimeter) into the fluid stream before the UV system. Examples of biodosimeters include MS2, T1, and T7. The entire procedure is performed under controlled conditions, and system variables (flow, transmittance, power loads and lamp intensity) are carefully recorded as samples are taken before and after the UV system.

Once the sample data is returned from the analyzing laboratory, the system’s ability to disinfect can be compared to the manufacturers’ claims. Of course, bioassays should be carried out under the auspices of a credible third party.

Such studies have documented the effectiveness of closed-vessel UV systems. The accompanying table shows results from a system using medium-pressure lamp technology. The performance is based on three separate reactors that underwent validation testing for a range of transmittance levels, flow rates, and power input levels to the lamps.

This example shows that when the required disinfection goals (log reduction of fecal coliforms) are increased while keeping the water transmittance constant; the capacity of the system is reduced. This reduction is expected since UV performance is a function of dose and exposure time. For greater log removal of coliforms, the flow must be exposed to the UV intensity for a longer time.

 

Closed-vessel advances

As bioassay validations became the norm, engineers began to notice that hydraulics play a vital and often overlooked role in system performance. In essence, if a UV system design allows short-circuits, or poor flow paths, then the water will receive differing degrees of UV dose. In extreme cases, the water can short-circuit straight through a UV system, rendering it grossly inefficient. Most UV systems need to cope with a variety of flow rates, and usually an operating flow range is considered when designing the system.

The standard in the municipal wastewater industry is to put UV lamps in an open-channel configuration. The industry went with this original configuration largely because early municipal adopters of UV disinfection retrofitted their chlorine contact basins for the purpose.

Even today when new plants are constructed, the norm is to install open-channel UV systems because they are familiar to engineers and operators. However, there are alternatives, including closed-vessel reactors.

Early closed-vessel reactors received poor grades because they were fitted with large numbers of low-pressure lamps and did not have automatic wiping mechanisms to keep the lamps clean. Today’s closed-vessel UV chambers are much different, as they are fitted with high-powered amalgam or medium-pressure lamps, automatic wiping mechanisms, air release valves, and hatches to provide access to the interior of the chamber.

Closed-vessel UV systems are proven in municipal water systems, industrial process water systems, swimming pools, water parks, ballast water systems, and many other applications. Closed-vessel systems offer many benefits for treatment plant operators and consulting engineers. These include:

Quick, easy, low-cost installation. A closed-vessel system can be thought of as a spool piece in a pipe. The systems are fitted with ANSI flanges and can be installed as a contractor would install plant piping. There is no need for pipe support directly under the chamber, as pipe supports can be added to influent and effluent piping. Closed-vessel chambers eliminate the need of precision alignment of poured concrete walls and floor. Some estimates are that closed-vessel systems can reduce contractor installation costs by up to 80 percent.

Installation flexibility. Closed-vessel systems can be installed in horizontal or vertical pipe runs. This allows consulting engineers greater flexibility when designing a new system or retrofitting an existing treatment plant. Vertical installations lead to a smaller footprint and eliminate air entrapment, but require additional head for gravity-fed systems.

Wiping mechanism is external to water. Systems are typically fitted with a fractional-horsepower motor external to the chamber and water. The motor is coupled to an internal threaded screw that turns and drives the wiping carriage across the quartz sleeves and UV intensity monitor. It is critical to keep all optical paths free from fouling to ensure optimum disinfection.

Simpler service. Wiper rings can be replaced without removing the wiping carriage from chamber. Systems with access hatches allow operators to replace wiper rings without having to remove the complete wiper assembly. The design also allows operators to replace lamps without draining the chamber and to replace individual lamps with relative ease. The lamps are removed without any contact with effluent, reducing the risk of disease or infection. To replace the quartz sleeves, the chamber needs to be drained, but each sleeve can be replaced individually.

Nuisance prevention. A closed system eliminates fly and mosquito nuisances and protects electrical components against corrosion that can be caused by elevated humidity. It also excludes sunlight, which causes algae to grow and stimulates an enzyme that can repair DNA damage to pathogens and thus reduce system effectiveness. In addition, operators are protected against inhalation of aerosols containing pathogens.

Reduced safety concerns. Closed-vessel systems keep UV light inside the chamber, reducing the chance for operator exposure. Open-channel lamps running under effluent pose little risk, but if the level control fails, or if the lamps are turned on when the racks are lifted out, operators are at risk. UV light can burn exposed skin in seconds, causing sunburn. Burns to the inside of the eyeball (sometimes called arc eye or welding flash) are extremely painful and can lead to retinal lesions, cataracts, and yellowing of the lens on prolonged exposure.

 

Conclusion

A persuasive case can be made to put the UV disinfection system for wastewater into a closed pipe, as in nearly every other application that uses UV. Closed systems optimize hydraulics and reduce operators’ exposure to the wastewater and the UV light itself. Manufacturers offer reactors up to 30 inches in diameter designed specifically for wastewater treatment applications.

 

About the author

Jon McClean is president and Patrick Bollman, P.E., is municipal operations manager for Engineered Treatment Systems (ETS), LLC, a manufacturer of UV disinfection equipment based in Beaver Dam, Wis.