While a relatievly new technology in India, the use of closed pipe, medium pressure UV disinfection systems for wastewater treatment has seen tremendous growth in Europe, the USA and Australia over the last 10 years. More and more operators of wastewater treatment facilities in these regions are now opting for closed pipe UV disinfection systems instead of older, open channel low pressure UV systems. This change is being driven by the very real advantages of closed pipe systems.
The disadvantages of open channel disinfection include the large footprint required for the disinfection channels; the large number of low pressure lamps required; the difficulty in cleaning the lamps, which sometimes have to be cleaned manually - a laborious procedure – or physically lifted and moved to an acid bath; significant pressure drop; the danger of personnel being exposed to UV light; and the growth of algae in the open channels. Also, the hydraulic movement of liquids through open channels is not particularly turbulent, so some sections of the wastewater may not pass close enough to the UV lamps to receive the minimum required UV dose.
Closed pipe medium pressure UV systems, on the other hand, have a much smaller footprint, as the UV chambers are inserted into existing pipework; the number of lamps is much less as medium pressure lamps have a significantly higher UV output than low pressure lamps; the lamps are fitted with a mechanical wiper on their protective quartz sleeve which keeps them clean – something that is not possible with low pressure systems. Periodic chemical cleaning, if required, is simple and can be done ‘in-line’ without removing the lamps. Pressure drop is much less as the wastewater passes directly through the treatment chambers. In addition, as the chambers are completely enclosed there is no danger to staff; this also eliminates the problem of algal growth. In addition, lamp change-over is easy and can be done in minutes. The hydraulic design of closed-pipe systems also means the movement of wastewater through the treatment chamber is more turbulent than in open-channels, ensuring all the wastewater receives the minimum required UV dose by passing close to the lamps. This has been confirmed by CFD modelling.
Medium pressure lamp technology
Medium pressure UV lamps emit UV over a broad wavelength and have 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 a single peak of UV output. It has been shown that many microorganisms are able to repair themselves after exposure to UV from these low pressure lamps, especially if they are subsequently exposed to sunlight – as is often the case in wastewater treatment facilities (see references above).
In addition, only a few medium pressure lamps do the same job as many low pressure lamps – this makes medium pressure systems much easier to operate, monitor and maintain. Because of these factors, low pressure UV lamp technology should be avoided in wastewater applications.
Cleaning of quartz sleeves
A major factor to consider with UV wastewater treatment plant is fouling of the protective quartz sleeves surrounding the UV lamps. Suspended solids and minerals in the wastewater attach themselves to the sleeves and must be removed at regular intervals to ensure maximum UV output. This is something that happens to both low and medium pressure UV lamps, and in both open-channel and closed-pipe systems.
There are two main ways to control fouling: mechanical cleaning of the sleeves (with O-rings or brushes) or chemical cleaning with acids. Even when mechanical cleaning is used, the sleeves will still need to be chemically cleaned from time to time. As explained above, with open-channel UV systems the UV lamps must be physically lifted from the channel and transferred to an external chemical bath.
With closed-pipe UV systems, cleaning agents are simply added to the UV chamber and cleaning takes place internally. When the UV system is on, automatic wipers move up and down the quartz sleeves, removing any deposits. At the same time, a small volume of low concentration acid is applied directly to the sleeves. This ‘direct dosing’ means significantly less chemicals are required to keep the sleeves clean than with conventional chemical dosing. The chemicals used are not harmful in any way either to the environment or to the wastewater plant’s pipeline infrastructure.
Berson’s closed vessel UV disinfection equipment is helping the award-winning Flat Creek Water Reclamation Facility (WRF) in Georgia, USA, exceed permit limits for faecal output by a significant margin.
“We installed three Berson medium pressure InLine UV systems over six years ago, adding three more in 2004, and they have performed exactly as expected, with all our faecal samples well below the permit limit,” commented Flat Creek’s Plant Manager Michael West.
The Flat Creek WRF recently gained a second place National Clean Water Act Recognition Award for operations and maintenance from the US Environment Protection Agency (USEPA). According to the USEPA, the Flat Creek Water Reclamation Facility is one of the two treatment plants in Georgia recognized for “their innovative approaches and achievements,” which “improve water quality and protect public health and the environment in the communities they serve.”
The Berson units are arranged in three trains of two chambers each, providing a ‘series’ approach to ensure adequate disinfection to the 23 faecal colonies per 100ml sample required by the National Pollution Discharge Elimination System (NPDES) permit. The systems are designed to treat a combined total of over 45 million litres per day of wastewater (up to 80% industrial and 20% low level commercial and residential) for discharge into nearby Lake Lanier. “With the impact of the severe drought in our area causing the level of the lake to fall to historically low levels, every drop of reclaimed water that can possibly be returned to the lake is essential,” added West.
This closed vessel design was an important feature for the City of Gainesville, which Flat Creek serves. When first selecting UV equipment for the Flat Creek WRF, representatives from the City’s Public Utilities Department visited several neighbouring metro Atlanta facilities and examined a range of open and closed pipe systems from various manufacturers. “The excessive man-hours required to keep an open channel unit clean was a large factor in choosing the Berson closed channel system initially,” said West.
Ease of handling was another positive feature, according to West, as rather than handling an entire bank of UV lamps at a time, the InLine’s single lamps may be changed quickly and easily by plant personnel. To further reduce maintenance, the chambers are equipped with an automatic cleaning mechanism to keep lamp sleeves free of organic deposits.
Two golf courses in Anthem, Arizona, are using UV-treated wastewater for irrigation. Founded less than 10 years ago Anthem, a town just north of Phoenix, now has a population of over 40,000. As part of its rapid expansion the town recently installed three closed chamber, medium pressure Berson UV systems to disinfect its wastewater. This allows the town to not only meet increased demands in its water and wastewater treatment capacity, but also to exceed the output quality standards.
“The wastewater is treated by three Berson InLine systems handling a combined flow of three million gallons per day,” explained Anthem’s wastewater Foreman Jeff Marlow. “They work in conjunction with microfiltration and nitrification/denitrification. We chose the Berson UV systems because they are optimized to meet the upcoming Arizona Pollutant Discharge Elimination System (AZPDES) Permit Program,” he added.
The two local golf courses currently use a combination of UV treated wastewater and fresh river water for irrigation, but with increase in population, it is expected that the courses will soon be using wastewater exclusively.
An automatic cleaning mechanism keeps the lamp sleeves free of organic deposits for consistent UV dosing. Each chamber is also fitted with UV monitors to measure actual UV dose for record keeping. With the addition of an optional online transmittance monitor, real time transmittance values are used to automatically adjust the dose pacing of the UV system.
Closed-pipe UV wastewater treatment systems are increasing in popularity with operators of wastewater treatment plant. There are many reasons why these systems are now taking over from older, open-channel systems. Firstly, closed-pipe systems are safer. Secondly, cleaning the UV lamps’ protective quartz sleeves is straightforward – either mechanically or chemically – without having to remove the lamps. Thirdly, the hydraulic design of closed-pipe systems ensures most of the wastewater receives the minimum required UV dose. Finally, closed-pipe systems, in conjunction with medium pressure UV lamps, ensure that microorganisms are permanently deactivated and cannot repair themselves.
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 Sensitivite 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.