The Benefits of UV for Wastewater Reuse| Damian Corbet
The widespread use of UV disinfection technology for wastewater treatment globally is evidence, if it were needed, that it is no longer an ’emerging’ technology. Indeed, it is now an accepted, mainstream disinfection method used routinely by water and wastewater engineers across the world to safeguard human health and alleviate environmental pressures.
UV is particularly useful when it comes to reuse. Wastewater reuse has been practiced in various forms for decades, with the USA leading the way. It is now a major issue in the southern USA, southern Europe, the Middle East, Australia and many parts of Asia where chronic water shortages are driving investment in reuse technology. UV is now playing its part.
The use of computational fluid dynamics (CFD) modelling has vastly improved UV equipment manufacturers’ ability to predict with confidence the level of treatment required for wastewater using their proprietary equipment. All manufacturers now use this tool to optimize the dose delivery of their reactors and minimize energy costs. Also, as manufacturers develop and improve optimized UV reactors, they will soon be able to validate the designs using recognised validation protocols.
Conventional UV lamp technology has improved significantly over recent years, with medium pressure lamps continuing to see gains in energy efficiency, lamp life and power density, and quartz coating techniques extending lamp life to well over 12,000 hours.
New mercury-free UV-LED systems are now also appearing. These small systems are idea for low-flow applications and easily integrated into existing treatment applications. In addition, a brand new LED-based UV transmittance monitor, the BersonSense – the first in the world to use UV-LED technology – provides highly accurate UV transmittance readings in all conditions over an extended lifetime, ensuring highly accurate UV dosing.
Finally, a new range of low pressure, high output (LPHO) multi-lamp UV systems are being developed. Two versions are available: a ‘U’ configuration with a smaller footprint for lower UV Transmittance (UVT) applications where higher doses are required (such as wastewater reuse or virus removal); and an ‘L’ configuration for higher UVT and lower dose requirements (for example drinking water applications). Both configurations create a rotational liquid flow, ensuring extensive mixing of the fluid and optimal disinfection performance while minimising head-loss.
Benefits of UV for the reuse market
The most common method of wastewater disinfection for reuse has long been chlorination. Despite chlorine’s impressive track record, concerns regarding disinfection by-products (DBPs) and, more recently, disinfection performance with respect to pathogen inactivation, are driving the conversion from chlorine disinfection to other disinfection methods such as UV, which does not produce any DBPs.
Closed vessel UV systems are easy to install within existing pipework, so there is minimal disruption to plant operation. Day to day operation is simple and only minor maintenance is needed. The only regular requirement is changing the UV lamps and wiper rings once a year, a straightforward operation that can be carried out by on-site personnel.
UV systems for wastewater reuse are also validated to much higher doses than drinking water systems, according to protocols established by the National Water Research Institute (NWRI) in the USA. Drinking water-type product validation will emerge as the dominant method of assessing suitability for these critical applications. The ability to prevent photo repair will also emerge as key.
Applications for wastewater reuse
Potential applications for wastewater reuse are extremely wide-ranging and include any instance where water is needed for non-potable use. The most popular and widespread use is for agricultural irrigation, with California and Florida leading the way in the USA and a number of Australian states also making significant progress. Other irrigation uses include landscape and recreational applications such as golf courses, parks, and lawns.
Reclaimed wastewater is also used for groundwater recharge applications such as aquifer storage and recovery or preventing saltwater intrusion in coastal aquifers. Other uses include toilet and urinal flushing, firefighting, foundation stabilization in the construction industry and artificial snow generation. In all these applications, reuse wastewater relieves the burden on existing municipal potable supplies.
Two golf courses in Anthem, Arizona, are using UV-treated wastewater for irrigation. Founded 15 years ago Anthem, a town just north of Phoenix, now has a population of over 20,000. As part of its rapid expansion the town recently installed three closed chamber, medium pressure UV systems from Berson UV-techniek 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 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 an 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.
The UV industry has matured considerably over the last decade and is now highly regulated and dominated by the world’s major water technology companies. Conventional UV technologies have been field tested and now have considerable track records in a wide range of applications. Uncertainties surrounding regulations, royalties, technology and engineering have decreased and acceptance of UV is expected to grow rapidly over the next 20 years. Conventional UV designs have been greatly aided by CFD, which will be used as a routine sizing tool for future designs. The addition of UV-LED technology is also opening up many new markets for UV disinfection.
The stage is now set for dramatic growth in the wastewater reuse market, especially with increasing populations putting even more pressure on already overstretched water resources in many regions of the world. Tighter limitations on pollution discharge will also play an important role in the development of this technology.