Innovative Wastewater Disinfection

The Kent County Regional Wastewater Treatment Plant (WWTP) in Delaware is internationally recognized as a model of excellence. Representatives from utilities in many countries, including Brazil, South Africa, Germany and Russia, have visited the plant to observe its operational methods, so it is no surprise that the Milford, Del., plant recently claimed another distinction when it incorporated the world’s largest microwave UV system into its plant operation.

The Kent County facility is Delaware’s second largest treatment plant, taking in 12 million gal per day (mgd) from a service area with a population of 130,000 in Kent County and parts of New Castle and Sussex counties. The treated wastewater is then discharged to a tributary of the Murderkill River. The original plant was built in 1973 and has since won several high-profile awards for innovation and operational excellence, including the inaugural U.S. Environmental Protection Agency (EPA) Region II Environmental Achievement Award. The plant also was the first wastewater facility in the United States to be certified to the ISO 14001 Environmental Management System (EMS) standard, the OSHA Assessment System 18001 standard and the National Biosolids Program EMS program.

Search for Innovation
According to Jim Newton, environmental program manager, improving plant performance is an ongoing process, and personnel are encouraged to explore innovative alternative technologies to achieve continuous improvement. Newton also noted that safety and the environmental impact of plant operations are ongoing focuses of Kent County management. “Our goal is to reduce our environmental and health and safety footprint, especially as it affects our employees and neighbors,” he said. “That has driven our efforts to move to sustainable energy sources, reduce power consumption and eliminate the handling of chlorine.”

The Kent County WWTP successfully used gaseous chlorine to disinfect the wastewater for almost 40 years. However, the utility’s Environmental, Health, Safety and Sustainability Management System (EHSS-MS) set as an objective the replacement of this process with UV disinfection because of the health, safety and regulatory burdens the chlorine process required.

True to its reputation as an innovator, Kent County decided to explore the use of microwave UV technology based on a study comparing all disinfection processes conducted by master’s degree students in the environmental management department of the University of Maryland. The new technology, Severn Trent Services MicroDynamics microwave UV system , uses microwaves to generate monochromatic UV light from electrode-less lamps. With these lamps, there are no electrical connections to fail, corrode or leak, which increases system efficiency and lamp life when compared to traditional UV lamps. The MicroDynamics system is modular and can operate at ambient water temperatures, making it less susceptible to rising temperatures during periods of low flow. And the lamps can operate in air, so the system does not require lamps to be submerged in the channel, thus reducing concerns regarding precise water level control.

UV disinfection provides a number of advantages for the wastewater treatment plant compared to gaseous chlorine. Its employment reduces chemical usage, simplifies regulatory compliance issues and improves employee and community safety.

Pilot Project Success
Before the Kent County Department of Public Works would commit to the MicroDynamics system, Severn Trent Services was contracted to provide a 1-mgd pilot plant to ensure that the unit could meet National Pollutant Discharge Elimination System disinfection permit conditions. Over a 35-day period in 2008, the system was found to meet the permit limits of 33 colonies per 100 mL of Enterococcus. The unit was operated at 150% of its 1-mgd design flow capacity and was able to produce an effluent with a geometric mean of 3.5 colonies per 100 mL.

“With a three-year lamp life guarantee, the MicroDynamics system offered the lowest overall operational costs and maintenance requirements of any UV technology we evaluated,” Newton said. “The further appeal of this innovative technology was the improved reliability it offered compared to traditional UV systems and lamps. The system’s unlimited on/off capability was perfect for our peak treatment capacity needs. And the safety of the MicroDynamics electrode-less lamps, which have no electrical connections in the water, just couldn’t be matched.”

The full-scale system, which was placed into operation on Oct. 7, 2010, is designed to handle the plant’s normal and peak flows of 12 mgd and 18 mgd, respectively. Twenty-four microwave UV modules are configured in six trains of four each. Until a control panel is installed, which will enable automatic operation based upon flow, the system is being operated manually.

Microwave System Benefits
In the short period that the full-scale microwave UV system has been in operation, Newton said its efficacy has been verified. “With permit limits of 33 colonies per 100 mL of Enterococcus, we’re consistently achieving 2 to 10 [colonies] per 100 mL of Enterococcus.”

In addition, Kent County now saves $100,000 per year that was being spent on chlorine and sulfur dioxide when gaseous chlorine disinfection was used. Risk planning training and paperwork has been eliminated as well. “We donated hazmat suits to the local hazmat team,” Newton said. “And since we no longer had to maintain a 1/2-mile evacuation route in the event of possible chlorine gas releases, we were able to completely decertify the U.S. EPA risk management program.”

One benefit of the microwave UV system that does not show up on the balance sheet is its safety. Because the electrode-less lamps have no wires underwater, safety concerns inherent with the repair of possibly corroded electrodes or wiring are eliminated.

The electrode-less lamps also offer benefits that are reflected in bottom line calculations. “Because electrode-less lamps operate at lower temperatures than traditional UV lamps, you don’t get the build-up of solids as with traditional lamps. And the lamps take less time to generate full power compared to traditional UV lamps,” Newton said.

The county already is planning to propose eight more microwave UV systems, as the current system is operating at 85% of capacity, and a state Department of Natural Resources and Environmental Control rule requires an increase in system capacity when that rate of usage is reached.

To cover the costs of the facility upgrades, the public works department received low-interest State Revolving Fund stimulus funding, which included adding a 1.2-NW solar farm to power the UV system and portions of the biosolids operation. The solar power system is due to go online in February 2011.

“The initial plan for the solar farm was to offset electric costs. When the solar power system comes online in February, it’s anticipated that all the Kent County WWTP’s power will be provided by the 6,000 solar panels,” Newton said.

The new solar farm and microwave UV disinfection system are just the latest examples of how the Kent County WWTP uses innovation to improve health, safety and environmental sustainability, and satisfy customers. It is likely that one of the country’s most highly-praised wastewater plants will need to make room in its trophy case for the additional awards that are surely coming its way.

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