British Columbian WWTP Chooses Medium Pressure UV

Aquionics technology replaces low pressure, multi-lamp system

A wastewater treatment plant (WWTP) in Richmond, BC, Canada, has opted to replace its existing low pressure UV disinfection system with a medium pressure UV system from Aquionics Inc. The WWTP serves the Riverport Sports and Entertainment Complex and surrounding residential areas in the City of Richmond, south of Vancouver.

Riverport, BC

The facility decided to switch to medium pressure UV because cleaning the low pressure system was proving to be very difficult. Each of the two disinfection chambers in the old system contained 24 low pressure lamps – automatic wiping is not an option with that number of lamps, so cleaning had to be done either manually or with chemicals. Both of these methods is extremely time consuming and requires the entire system to be shut down.

The operators therefore decided to look at other options, and medium pressure UV seemed an obvious alternative. While having a similar footprint, medium pressure systems utilise UV lamps with a much higher UV output than low pressure lamps. In fact, only two lamps per disinfection chamber are required for the same level of disinfection. Each of the medium pressure lamps is fitted with a mechanical wiper, which automatically moves up and down the lamp’s protective quartz sleeve, keeping it clean. This is simply not possible with multi-lamp, low pressure systems. Periodic chemical cleaning of the medium pressure lamps, if required, is simple and can be done by injecting the chemical into the treatment chamber where it cleans the lamps ‘in-line’, without the necessity of removing the lamps or shutting down the system.

How UV works

UV is the part of the electromagnetic spectrum between visible light and X-rays. The specific portion of the UV spectrum between 185-400nm has a strong ‘germicidal’ effect, disrupting the DNA of microorganisms, rendering them unable to reproduce. It is generally accepted that microbial DNA absorbs UV most effectively at 265nm, a wavelength that MP lamps produce in abundance. In addition to emitting UV at this optimum ‘peak’ wavelength,  medium pressure lamps also emit UV over a much broader range of wavelengths (between about 185 – 400nm) than low pressure lamps. This broad output, has been shown in independent tests to cause permanent inactivation of both pathogenic and non-pathogenic microorganisms such as E.coli  (references 1, 2, 3).

Low pressure UV lamps, on the other hand, produce just a single peak of UV output at 254nm. While also having a strong germicidal effect, the independent tests mentioned above showed that some microorganisms were able to repair or ‘reactivate’ themselves after exposure to UV from low pressure lamps, especially if they are subsequently exposed to sunlight (known as photoreactivation) – as is often the case in wastewater treatment facilities.

It is thought that the permanent damage caused by medium pressure UV may be a result of the UV at other wavelengths, such as 240nm and 280nm, having an effect on other intracellular molecules such as RNA and enzymes, which are unable to repair themselves.

The implications of the research into photoreactivation are far-reaching. For any industry where UV is used to disinfect water or wastewater, the operator needs to be sure that the treatment is permanent. This is especially the case when the treated liquid will subsequently be exposed to light and make its way back into the environment. Zimmer at al (reference 1) suggest that medium pressure UV could therefore provide better protection against photoreactivation if UV treatment occurs prior to any process units in which water is exposed to light for even a short time. According to their study, “Using low pressure UV in this type of situation should be avoided, since repair occurs rapidly following exposure to light.” A much larger research effort into the area of photoreactivation is still required, however, especially research involving real water and wastewater treatment plants.

The two Aquionics medium pressure units installed at the Riverport facility are situated after a Sequencing Batch Reactor and treat a combined flow of up to 2271 litres per minute (600 gallons per minute) of wastewater from the adjacent sports complex and residential areas. The disinfected wastewater is discharged into the Fraser River and meets all necessary quality standards for emission into watercourses.

Each of the two UV chambers is fitted with a UV monitor which measures actual UV intensity and dose from the two lamps. This provides real-time disinfection information which can be downloaded for record keeping. Operating the UV system is simple and, when lamps need replacing, it can easily be carried out by on-site staff.

The Riverport Sports and Entertainment Complex is a major development covering 14.5 hectares (36 acres) in southern Richmond, BC. It contains shops and restaurants, cinemas, bowling alleys, ice rinks swimming pools and spas. The WWTP is a private facility serving the complex and some surrounding residential areas. The complex is situated along a stretch of the Fraser River known as the Fraser River Port, which begins at the mouth of the river and extends 270 kilometres (168 miles) inland. 38.8 million tonnes of cargo are shipped through the port annually, making it the second largest in Canada.

References:

1. Zimmer, J. L., Slawson, R. M. & Huck, P.M. Potential repair of Escherichia coli DNA following exposure to UV radiation from both medium- and low-pressure UV sources used in drinking water treatment. Applied & Environmental Microbiology, Vol. 68 (2002), No. 7, 3293-3299.
2. Oguma, K., Katayama, H. & Ohgaki, S. Photoreactivation of Escherichia coli after Low- and Medium-Pressure UV Disinfection Determined by an Endonuclease Sensitivity Site Assay. Applied & Environmental Microbiology, Vol. 68 (2002), No. 12, 6029-6035.
3. Hu J. Y.,  Chu, S. N.,  Quek, P. H., Feng, Y. Y.,  and Tan, X. L. (2005). Repair and regrowth of Escherichia coli after low- and medium-pressure ultraviolet disinfection. Water Science and Technology: Water Supply, Vol. 5, No. 5, 101-108.

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