Wastewater re-use using UV disinfection

The UV disinfection industry has experienced tremendous growth over the last 25 years - particularly in Europe, the USA and South-East Asia. The development of new UV technologies over this period is a perfect example of an industry investing to meet market demand - in this case, demand for an effective, low-cost and environmentally friendly way to disinfect wastewater for re-use.

The acceptance of UV disinfection at wastewater plants treating almost four billion litres daily is proof that UV is no longer an ‘emerging’ technology, but rather an accepted technology to be used routinely by engineers to safeguard human health and alleviate environmental pressures.

Wastewater re-use has been practised in various forms for decades, with the USA leading the way in re-use research. It is now a major issue in southern USA, southern Europe, the Middle East, Australia and many parts of Asia, where chronic water shortages are driving investment in re-use technology.

New technology

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 will soon use this tool to optimise the dose delivery of their reactors and minimise energy costs. Also, as manufacturers develop and improve optimised UV reactors, they will be able to validate the designs using recognised validation protocols.

Conventional UV lamp technology has also improved 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 h.

New mercury-free UV-LED systems are now coming online. These small systems are ideal 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) multilamp UV systems is being developed. Two versions are available: a ‘U’ configuration with a smaller footprint for lower UV transmittance (UVT) applications where higher doses are required (eg, wastewater re-use or virus removal) and an ‘L’ configuration for higher UVT and lower dose requirements (eg, drinking water applications). Both configurations create a rotational liquid flow, ensuring extensive mixing of the fluid and optimal disinfection performance while minimising head-loss.

Concerns

A major concern to the UV industry is the issue of reactivation - the apparent ability of some microorganisms to repair the damage done to their DNA by UV, reactivating their ability to infect. DNA repair can occur in a closed (dark) system, but is more likely in open systems under direct sunlight (photoreactivation). The dose level and lamp type seem to affect the degree of reactivation, with low-pressure (single-wavelength) UV lamps appearing to be more susceptible to photoreactivation than medium-pressure (multiwavelength) lamps. A much larger research effort into the area of photoreactivation is required and will most likely be forthcoming over the next five years.

A significant amount of research has also targeted the question of UV disinfection by-products, specifically the most common water constituents such as chlorine, bromide, nitrate, ozone, natural organic matter and iron. At normal UV disinfection doses, no significant disinfection by-products have been shown to form.

Benefits of UV for the re-use market

The most common method of wastewater disinfection for re-use 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 re-use 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, with the accompanying rigour, 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 re-use

Potential applications for wastewater re-use 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 stabilisation in the construction industry and artificial snow generation. In all these applications, re-used wastewater relieves the burden on existing municipal potable supplies.

Case study

Arizona, USA

Two golf courses in Anthem, Arizona, are using UV-treated wastewater for irrigation. Founded less than 10 years ago, Anthem now has a population of over 40,000. 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 optimised to meet the Arizona Pollutant Discharge Elimination System (AZPDES) Permit Program.”

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.

Conclusion

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 re-use 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.