Automated residual boosting system improves chloramine levels & eliminates nitrification in tank
Like many municipalities in urban and suburban areas, San Bruno, Calif.’s source water comes both from its own groundwater supply and through a purchase agreement with a major water utility—in this case, San Francisco Public Utilities Commission (SFPUC). And, like many municipalities in California, SFPUC switched from free chlorine to chloramines in 2003 largely to reduce disinfection byproducts.
At first, the conversion to chloramines did not present a major challenge for San Bruno, other than having to fluctuate tank water levels to maintain proper chlorine residual. However, in 2010, San Bruno and the 26 other agencies receiving SFPUC water started experiencing issues with nitrification. San Bruno first detected low chlorine residual and high free ammonia with elevated nitrite levels in two of its remote tanks. Distribution operators at San Bruno worked quickly to isolate the affected pressure zones and water tanks, flushing and draining these parts of the system in an attempt to restore acceptable water quality.
“September and October were especially bad,” said Water System and Conservation Manager Mark Reinhardt. “That’s our Indian summer—the hottest time of year in the Bay Area. At one point, we were detecting chlorine residuals as low as 0.5 ppm, far below our action level of 1.4 ppm. The storage tanks were drained in the excess of 60,000 gal every two weeks and we’d have to send out a crew of four to five to flush water in this entire pressure zone. It was a big maintenance burden.”
Seeking a way to reduce the cost and manpower associated with nitrification, San Bruno’s utility managers researched technologies to lower the risk of nitrification in their water storage tanks. San Bruno managers implemented PAX Water mixers in all their water storage tanks in 2012. This helped maintain residual in tanks, but operators knew that mixing alone would not solve all the nitrification problems the utility was facing.
“Part of the problem was that we were receiving water of variable quality,” Reinhardt said. “Our goal is to maintain 2.6 ppm chlorine residual in our tanks, and we were occasionally receiving purchase water below 1.9 ppm.” The utility needed a combination of active mixing and automated disinfectant boosting. So when PAX engineers were seeking trial sites to demonstrate the effectiveness of their newly developed Residual Control System (RCS), San Bruno was first in line.
Sensing a Solution
RCS combines the mixer with a set of water quality sensors, a pair of chemical feed systems for adding chlorine and/or ammonia, and a closed-loop control system to automatically and continuously monitor and adjust chloramine disinfectant levels in a distribution system storage tank.
“The RCS acts like a thermostat on a building—setting and maintaining a constant level of residual disinfectant with the tank in continuous operation,” said Robin Giguere, Ph.D., head of the PAX Water research and development team.
The 400,000-gal Sweeney Ridge tank was selected as the site for the trial. This tank is isolated by one mile of 14-in. pipeline and often experienced high water age, thermal stratification and elevated free ammonia levels that left it prone to nitrification. Dumping water from the tank was particularly problematic because Sweeny Ridge is located in the Golden Gate National Recreation Area, a protected environmental site. While mixing solved the thermal stratification problem, RCS was needed to correct the ammonia-to-chlorine ratio. The system was installed in the summer of 2013.
“The site is wonderful in some ways,” Giguere said. “With chlorine sensors already installed on site and a sweeping panoramic view of the San Francisco Bay and Pacific Ocean. Unfortunately, the weather conditions can go from beautiful and sunny to 50-mph winds, fog and large temperature drops in a matter of minutes. So it ended up being an extreme test for the environmental performance of our system.”
Installation of RCS was completed in late May 2013 and testing began in June. In the first phase of tests, the goal was to use the RCS to continuously monitor the water chemistry in the tank, detect levels of free ammonia, and add just enough chlorine to fully combine with the ammonia to produce monochloramine. Rather than try to control a specific chloramine residual set-point ,which would require the simultaneous addition of chlorine and ammonia, the system added enough chlorine to eliminate the free ammonia present in the water and then stopped.
The average free ammonia level when the RCS was off was 0.14 ppm whereas when the it was in operation, free ammonia levels averaged less than 0.02 ppm (the detection limit for the method used). Lab samples confirmed that the RCS maintained ideal monochloramine chemistry during this phase of the trial. The results showed that the system could reliably eliminate excess ammonia while maintaining consistent monochloramine chemistry.
After a year of site and environmental tests, Phase 2 of the trial began: direct monochloramine set-point control. For this phase, an ammonia chemical feed skid was installed and integrated with the RCS system.
Prior to turning on the system, total chlorine levels in the tank averaged around 1.5 ppm. Operators chose an initial set point of 2 ppm total chlorine and turned on the RCS. Within the first 24 hours, the system added chlorine and ammonia and continuously monitored water chemistry until the 2-ppm set point was reached. For the next three weeks, the system held the 2-ppm set point as the tank was filled and drained. In late July, operators bumped the set point up to 2.2 ppm and the RCS quickly established this new level and held it through the summer.
PAX Water also activated the remote monitoring system, which wirelessly transmits water quality data and operational parameters from the controller to the operators. This allows operators to monitor the performance of the various RCS subsystems (such as to schedule refills of disinfectant chemicals), and also provides them with continuous, real-time monitoring of the water quality inside the Sweeney Ridge tank.
Following completion of Phase 2 of the trial, San Bruno moved forward with the permanent installation of RCS at Sweeney Ridge and implementation of RCS at two additional tanks in their system: the reconstruction of their 2-million-gal Glenview tank and tank No. 4. “With the combination of active mixing in all our tanks and RCS, we’re one of the only municipalities in the area to keep our tanks 3⁄4 full and maintain good residual levels,” Reinhardt said. “With the addition of another two RCS systems, I expect to keep my tanks completely full, with steady residual, no nitrification and year-round fire protection.”