Clean Treatment

The City of Ripon maintains VOC compliance and keeps chemical and energy use low with tray tower aeration and support from the Wisconsin Department of Natural Resources.
Clean Treatment
Jim Jacobs takes a water sample.

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In February 2007, the City of Ripon, Wis., received bad news that a well would be shut down due to high concentrations of the carcinogen trichloroethylene (TCE).

Quarterly samples tested by the state Department of Natural Resources (DNR) showed that the TCE exceeded the maximum contamination level of 5 µg/L. Today, an innovative technology for TCE removal helps the city limit its chemical usage and maintain energy efficiency for its drinking water operations.

Since TCE is not a naturally occurring compound, the DNR sought to determine its source while supporting Ripon’s evaluation of treatment alternatives and fully funding the treatment process.

“The DNR realized that Ripon’s budget needed help and they were willing to support us,” says Jim Jacobs, water plant lead operator. “If they hadn’t done it, it would have been a hardship. They stepped in and provided the financing.”

After extensive research, the DNR failed to locate the source of TCE. One theory was that a nearby defunct landfill may have leached industrial chemicals, but the link was never proven. The DNR continues searching for the source and would hold that party responsible for the treatment costs.

Combining air and water

Project manager Angel Gebeau of the AECOM engineering firm worked with Ripon and the DNR to evaluate treatment options, which included aeration, granular activated carbon, and construction of a new well.

The city chose aeration based on cost analysis. Aeration adds large volumes of air to the water, encouraging the volatile TCE to evaporate into the air. AECOM evaluated a traditional packed tower aeration system, as well as a newer tray tower aeration method.

Packed tower aeration uses packing material made of perforated plastic balls. Water is pumped to the top and flows by gravity through the tower. Meanwhile, air blows upward, creating counter-flow aeration. The packing material moves around to lengthen the contact time between the air and water, providing more surface area for the TCE to escape.

In tray tower aeration, water flows in a serpentine fashion along a series of perforated trays through which a high volume of air is blown. In this cross-current design, the trays (in place of packing material) provide longer contact time between air and water.

No cleaning chemicals

Ripon chose the tray tower method for multiple reasons. First, the packed tower would have been 30 feet high, causing aesthetic concerns from nearby homeowners. However, the true deciding factor was ease of maintenance. As iron and other sediments coat the small holes in the tower’s packing material over time, chemicals such as citric acid, muriatic acid or hydrochloric acid are needed for cleaning. The tray tower allows simpler and faster cleaning without chemicals.

“The ports in the tray system open, and we can just use a pressure washer to spray it out,” Jacobs says. “It makes it a better solution because we won’t be using and disposing of chemicals for maintenance. The system has been in operation for two years, and looks the same as the day it was put in. So far, we haven’t had to do any cleaning on it.”

Building the plant

The city water department, comprised of three full-time operators and a meter reader, serves 2,500 connections and distributes 925,000 gpd. The city uses three wells with a total capacity of 1,525 gpm, but has an additional 730 gpm well available when needed.

While the TCE-contaminated well was shut down during construction of the treatment facility, the backup well was put online. Water treatment is minimal: chlorine disinfection and fluoride are added, iron filtration is used at one well, and phosphates are added to sequester iron and manganese at two wells.

Construction of the treatment system began in spring 2009 and was completed by the following November. It was the first tray tower aeration system in Wisconsin. In addition to the Carbonair aeration tower, the project included a new 35,000-gallon-capacity reservoir, and a building for the pump room and chemical storage. The facility removes 99 percent of TCE. A provision in the design allows for the addition of trays if needed in the future.

With sustainability in mind, AECOM designed several energy-conserving features, including energy-efficient fluorescent lighting, premium-efficiency motors on the well pump and high-lift pump (U.S. MOTORS/Nidec Motor Corporation), variable-frequency drives (Allen-Bradley/Rockwell Automation) on the pump and blower motors, and extra-insulated exterior walls.

Sampling for safety

Operators’ daily responsibilities have changed little since the treatment plant went online: They make a few extra gauge checks and measurements and occasionally test the effluent and influent to confirm that VOCs remain unchanged. Jacobs, who does the majority of the sampling, finds it rewarding to get affirmation that the system is keeping the water safe.

“I enjoy sampling,” he says. “It’s a good feeling to know what’s in our water. After going through development of the treatment system, there’s a satisfaction in knowing it’s working. From the moment it turned on until now, the water that exits into the distribution system has had no VOCs detected — not even a trace.”

Ripon has found other ways to operate more sustainably. That includes inspecting water towers without wasting water or disinfectant. For a tower inspection in 2008, the city used a traditional method, emptying the entire 350,000 gallons so that inspectors could enter the tank. After inspection, the tank had to be refilled with heavily chlorinated water for disinfection, then emptied and filled again. That took three days and wasted 700,000 gallons.

In September 2012, for inspection of the second water tower, Ripon hired a specialist from Water Tower Clean & Coat to enter the tank while it was full. The inspector, wearing a chlorine-disinfected diving suit and carrying an underwater camera, swam through the tank to inspect and clean. The process took four hours.

“When the inspector was done, the water could still be used and we didn’t have to dump it,” Jacobs says. “We didn’t waste water or need to use a large dose of chlorine.”

 



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