Challenge

In 1998, the city of Vallejo, CA built a 1 million-gallon-per-day (mgd) Green Valley Water Treatment Plant in order to comply with the Safe Drinking Water Act. The original design for the upgrade included a liquid sodium hypochlorite (NaOCl) system, including metering pumps and a 6,000 gallon storage tank.

“Our maintenance staff didn’t like [the idea] of working with 12.5% NaOCl, due to safety and handling concerns,” recalls Franz Nestlerode, deputy water superintendent. “So we looked at, and ultimately installed, the OSEC® system from Siemens Water Technologies.

When the state and federal codes forced the city to replace the gaseous chlorine system at it Swanzy Post-Chlorination Facility two years later, Vallejo decided to install an OSEC® system. The Swanzy facility supplies up to 7 mgd of water to the city and to the nearby Mare Island Navel Base.

Solution

The Green Valley installation has been operating flawlessly since going online five years ago, so we didn’t have to deliberate too long about which type of [chlorination] system to install at Swanze,” says Nestlerode. “We have been very pleased with the OSEC® system’s efficiency and reliability.

The OSEC® systems serving the Vallejo facilities both operate automaticvally using a compound loop control arrangement incorporating Micro/2000® chlorine residual analyzers, also from Siemens Water Technology. These highly accurate and reliable monitoring instruments provide the systems with continuous online chlorine residual analyses down to 1 ppb. The online analyzers, in turn, provide current chlorine residual information to the facilitiy’s SCADA system, which automatically modulates the chemical feed pumps to feed the precise rate.

A metering pump feeds the hypochlorite solution to the point of application. Level probes in the storage tank start and stop the electrolyzer to maintain a hypochlorite supply, and a float switch and a temperture switch monitor the conditions within the electrolyzer to ensure the proper, efficient operation of the system.

Results

“Both systems” are working great and have required zero maintenance from us,” reports Netlerode. “Plus, our operators like the fact they don’t have to deal with 12.5% liquid NaOCl, which is why we switched to the OSEC® system in the first place.”