Homemade Disinfection

A Scottsdale water plant team converts from chlorine to on-site sodium hypochlorite generation in a smooth process and in an existing footprint.
Homemade Disinfection
The OSEC B-Pak on-site hypochlorite generation system at Scottsdale’s Central Arizona Project (CAP) surface water treatment plant.

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Water treatment plants near and far are moving away from conventional chlorine gas for disinfection. The decision to convert is driven by concerns over the safety of transporting and handling chlorine gas, which is toxic; costs; and the efforts of many communities and utilities to make their operations as green as possible.

One alternative is to generate sodium hypochlorite on site. On-site generation systems (OSG) use water, salt and electricity to produce a stable sodium hypochlorite solution with a free available chlorine content of about 0.8 percent. In addition, OSG systems typically store only a two- to- three-day supply of sodium hypochlorite at a time, eliminating the risk of the chemical degrading, as can happen when sodium hypochlorite is brought in and stored for long periods.

While the capital costs for on-site generation systems may be higher than for conventional chlorination, plants using OSG hypochlorite usually see a return on their investment of two to three years, according to online information from the National Environmental Services Center.

A case in point

The 13.5 mgd Central Groundwater Treatment Facility (CGTF) in Scottsdale, Ariz., changed out its disinfection from chlorine cylinders to sodium hypochlorite within a four-month installation window. The new equipment required no increase in footprint, fitting inside the existing chlorine storage and feed facilities.

Scottsdale is home to more than 200,000 residents in Maricopa County near Phoenix. Historically, the city was totally dependent on groundwater, but in the mid-1980s it began putting together a multifaceted water resource portfolio, intended to provide a long-term sustainable water supply.

This portfolio includes surface water supplies, groundwater resources and reclaimed water. In the process, the city has become a national leader in innovative water reuse and groundwater recharge projects. The CGTF is one of several treatment and reuse facilities that ensure an adequate supply of high-quality water to residential, industrial and commercial customers.

The plant draws groundwater from wells previously contaminated with trichloroethylene (TCE), and passes it through packed towers where the TCE is removed. The flow is disinfected, then pumped to storage reservoirs. From there, it is directed to the city’s distribution system.

Making the switch

Since startup in the early 1990s, the CGTF had used a conventional Wallace & Tiernan chlorination system supplied by Siemens Water Technologies. Chlorine was supplied by two 1-ton cylinders — one in service and one in reserve. However, in 2007, as the treatment plant was being upgraded, Scottsdale water officials determined that on-site chlorine generation would be a better and safer choice, especially since the facility lies in a residential area and next to a community park.

The CGTF rehabilitation project was on a tight timeline and had to be completed without interrupting water production at the plant. The city installed a pair of OSEC B-Pak 500 on-site hypochlorite generation systems and four Chemtube PPS peristaltic hose pumps from Siemens to replace the chlorine gas system.

According to Daryl Weatherup, product manager for Siemens, the OSEC B-Pak is a fully automated, prepackaged system designed for fast and economical installation, safe operation and easy maintenance. Skids are shipped completely piped, wired and tested.

The system generates a sodium hypochlorite solution through the electrolysis of brine, using only water, salt and electricity. The converted plant has been operating successfully since late 2009.

Brian Paulson, Scottsdale water production manager, cites the key reasons for the transition to OSG: more control over the disinfection process, and greater operational safety and security.

Transportation and storage of chemicals is an important area of concern these days, he notes.

Overall, the move to OSG has gone as planned, with no major obstacles. “We were able to fit the new equipment into the existing chlorine storage and feed room,” Paulson says. “The existing electrical service was adequate. It worked out very well from a footprint standpoint.”

Temporary supply

During the four months while the old chlorination system was taken offline and the new equipment installed and started, the plant purchased sodium hypochlorite from a local vendor and stored it in tanks that fit into the space previously occupied by the chlorine scrubber.

In Paulson’s mind, one of the most important lessons learned in the change to on-site generation is to pay close attention to the transition process. “It’s hard to design and construct a temporary system that will operate flawlessly,” he says. “From an engineering and setup standpoint, temporary systems are different than full-scale systems that are designed to function for years.”

Every situation is unique, he says, noting that differences in water quality, flow rates and potential scaling can affect the operation of temporary systems. “Look closely at your temporary system,” he says. “Examine all the variables before designing or using it.”

Weatherup reports that since startup of the on-site system at the CGTF, Scottsdale has partnered with Siemens on additional conversion projects at the 70 mgd CAP plant which treats Central Arizona Project water from the Colorado River, and at the city’s AWT plants, which supply 20 mgd for turf irrigation and groundwater recharge. In both projects, teams completed the conversion from gas chlorination to on-site generation without interrupting water production by retrofitting existing chlorination structures.



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