Reclaiming Wetlands Using UV Light

Carnation, a community of about 1,900 residents, has never used a central sewage system, relying instead upon individual septic tanks and drainage fields to handle its wastewater disposal needs. However, soil surveys conducted in 1987 revealed that the city's current wastewater disposal method had become insufficient for the growing population's needs and continued usage of the current system posed a contamination threat to the local unprotected aquifer. Carnation anticipated its future growth and recognised the associated health and environment problems that could come without a more developed wastewater system.

To deal with this issue, Carnation worked with officials from King County to develop plans for a new sewage system and wastewater treatment facility. Officials chose to integrate a Membrane Bioreactor (MBR) system with UV disinfection technology to ensure that the treated water would be clean and environmentally safe. Reclaimed water would then be discharged to the nearby 24 hectare Chinook Bend Natural Area to foster growth of wildlife and restore the wetlands. The resulting facility was awarded the "Small Project of the Year Award" at the 2008 WateReuse Symposium in Dallas, Texas.

To find UV disinfection equipment suitable for this application, King County turned to Berson (through its US sister company), who provided two parallel InLine 7500+ UV units installed in-series after the MBR system. The units are closed vessel, which allows them to flange directly to the piping from the MBR. Each UV unit is capable of treating large volumes of water, with one unit treating up to 5.3 million litres per day, with the second unit providing back-up treatment. The system utilises Berson's medium pressure, high intensity lamps to provide a compact footprint for disinfection. The InLine systems are low maintenance, with automatic mechanical cleaning to keep quartz sleeves surrounding each UV lamp deposit free.

The Carnation wastewater plant's discharge is Class A, the highest quality level of reclaimed water recognised by the state of Washington. Water is released into Chinook Bend Natural Area throughout the year, except during maintenance periods when output can be switched to the nearby Snoqualmie River.

According to Carnation wastewater plant supervisor, Dan Zimmer, the UV equipment's performance has met expectations producing reclaimed water for the facility without any permit violations. In May 2010, after two successful years of operation, the automatic cleaning system had worked flawlessly and the UV lamps only required a single change.