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Water Utility selects advanced WEDECO treatment technologies to purify urban runoff water.
In February 2010, the Dempsey E. Benton Water Treatment Plant (DEBWTP) added 16 million gallons per day (MGD) of capacity to the water utility operated by the City of Raleigh, North Carolina. The project, located on 55 acres in the nearby town of Garner, North Carolina, represents a milestone in the history of public service.
Scope
Lack of rain combined with an increase in population in Raleigh and the area surrounding Research Triangle Park pushed the existing processing plant to its limits. Drought conditions that lasted from 2005 to 2008 altered the daily peak demand of the plant, especially during the summer months. In 2009, members of the DEBWTP facility began working with an engineering consultant to design a second plant. “It seemed like Raleigh had started to change overnight in the years leading up to the new plant,†recalls Kenneth Best, manager of Raleigh’s two water treatment plants.
Solution
The drought conditions that lasted from 2005 to 2008 reduced the porosity of the soil, resulting in excess runoff every time it rained. The DEBWTP was to manage and address the increased urban runoff, which contributed to contaminants, organics and high turbidity, and was discharged into the relatively shallow area
Benson Lake. Turbidity alone can reach 120 NTU, which played a major role in the selection and exclusion of processing technologies from the plant, according to Shannon Dorsey, PE, associate project manager at Arcadis.
The ozone system is used as an additional multi-purpose treatment barrier to reduce the amount of chemicals in the process and improve the performance of the two-stage filtration system by removing total organic carbon (TOC) and oxidizing the iron and manganese in raw water.
The new plant’s WEDECO high-performance ozone system consists of two ozone generators rated at 700 lbs / day, each operating at a concentration of 10% by weight (% by weight), fitted with a supercharging system with nitrogen. Only one generator should be running to produce the required ozone dose of 4 mg / L at 20 MGD. There are two 62,380 gallon pre-ozone switches and two 9,000 gallon LOX tanks. An in-line flash reactor transfers gaseous ozone to the plant’s influent via side injection. In the last step of the ozone oxidation process, a flue gas ozone destruction unit, one per reactor, converts any residual ozone back to oxygen.
WEDECO UV disinfection has been added at the end of the processing chain to enable the plant to no longer rely exclusively on chemical disinfection and to enable the plant to comply with more stringent federal requirements for sub- disinfection products.
The factory has three trains (one on hold) rated at 10 MGD each, configured in five rows with 12 lamps in each row.
Results
The DEBWTP is an award-winning model of several advanced processing technologies. Upon completion of the second plant, the City of Raleigh has achieved an increased capacity of 40 MGJ and can handle fluctuating flow and turbidity levels. The new treatment process reduced turbidity levels to 0.07 NTU, significantly less than the state maximum of 0.30 NTU.
Editor’s Note: Scranton Gillette Communications and the SGC Water Group are not responsible for the accuracy, effectiveness or validity of any claims made in this article. The opinions expressed in this content do not reflect the position of the editorial teams of Water & Wastes Digest, Water Quality Products and Storm Water Solutions.
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