Automated residual control improves residual levels, lowers nitrite & keeps tanks full
The official motto of Rowland Water District is “effective action for sustainable progress.” Situated in eastern Los Angeles County, Calif., Rowland Water has a history of adopting innovative technologies to improve the reliability and quality of water it delivers to its 58,000 customers. But because Rowland Water purchases nearly all of its chloraminated drinking water from the Metropolitan Water District of Southern California, it lacks the ability to directly optimize water treatment. The utility’s only option is to improve the efficiency and effectiveness of its water distribution system.
Like many water utilities in Southern California, Rowland Water has been challenged by steadily increasing temperatures and a decline in water quality in its distribution system.
“For years, we have battled nitrification and low disinfectant residuals in our water system, especially in our upper zones,” said Water Systems Supervisor Dusty Moisio. “Each year it appeared to be getting worse.”
Nitrification is the most common water quality challenge for chloraminated drinking water systems. Nitrification is a biological “infection” of a drinking water system in which nitrifying bacteria consume free ammonia in the water and produce nitrite. During warm summer months, nitrification quickly can spread across a distribution system, causing rapid drops in the level of residual disinfectant.
“It used to be that we would only find significant amounts of nitrite during the summer, but now we measure some nitrite in our water all year long,” Moisio said.
Like most chloraminated water systems, Rowland Water has a nitrification control plan that provides operators with steps to address the problem. If nitrite is detected above the “alert level” (0.1 mg/L), operators increase the frequency of monitoring. If nitrite is detected above the “action level” (0.2 mg/L), operators employ several strategies to lower water age and restore higher residual levels, including flushing, lowering tank levels and deep-cycling tanks. Unfortunately, these procedures have only a limited effect.
“Typically our highest zone—zone 6—was our biggest concern, and we would flush that zone along with the supplying zone,” Moisio said. “We would see residual levels recover for about 24 hours before crashing again, and it was a bad public perception to flush water during a drought.”
In 2012, Rowland conducted a study of the water in one of its tanks to determine if thermal stratification was present. Thermal stratification occurs when warm water rises to the top of a tank and doesn’t mix with the cooler water below. It can be particularly problematic for chloraminated water systems because the old, warm water can lose its chlorine disinfectant, accumulate higher levels of free ammonia and create all the conditions favorable for nitrification.
Rowland installed several PAX Water Mixers in 2012; they solved the thermal stratification problem, but the nitrification issue remained.
In 2013, Rowland Water was introduced to the PAX Residual Control System (RCS), a new technology for actively monitoring, boosting and controlling chloramine levels in water storage tanks. The RCS uses advanced water quality sensors to continuously monitor disinfectant levels. If disinfectant levels drop below a set point, the RCS automatically feeds in chlorine and ammonia at the correct ratio to restore the tank to its optimal monochloramine set point. The PAX Water Mixer thoroughly blends the tank to ensure that the entire water volume has a homogenous water temperature and chemistry.
Rowland Water initially piloted the RCS using a portable trailer. The trailer can be brought to the tank site, and in only a few hours can be connected to the tank and operated. Significant improvements in water quality usually are seen in the first 24 hours.
Operators at Rowland Water initially directed the trailer to a tank in the upper zone, where water quality usually was the worst. The RCS was able to significantly raise residual disinfectant levels, but operators saw that each fill cycle brought a new batch of low-residual water into the tank.
“We felt like we were chasing our tails,” Moisio said. “We realized that it might be better to address the root of the water quality problem before it was out of control.”
Operators then moved the trailer to the next pressure zone. They were able to establish better control at this site, and the improved water stabilized water quality in the higher-pressure zone.
Convinced that the RCS could help improve water quality, Rowland had a permanent RCS installed at the next lower-pressure zone, and returned the trailer to the highest-pressure zone to apply a one-two punch to improve water quality. This strategy proved to be effective. Rowland Water then went on to install permanent RCS systems at three other tanks and used the trailer to address spot issues in the highest-pressure zones when they occurred.
“We saw a significant improvement in water quality,” Moisio said. “Residual levels are much higher throughout our system, and nitrite has been reduced by 74%. Knowing my residual levels are being maintained gives me peace of mind.”
After installing RCS at multiple sites in its water distribution system, Rowland was able to maintain much higher residual and nitrite was reduced by 74%.
After implementing the RCS, Rowland Water noticed several significant operational and cost benefits.
“We used to try and deep cycle our reservoirs to very low levels,” Moisio said. “We were often storing a lot more air than water in our tanks. Now we can keep our water levels much higher without any loss in water quality, which provides our community with better emergency response and safety during fire season.”
Rowland Water found an even bigger cost savings with pumping energy.
“When we kept our tanks low, we would have to use our booster pumps at all hours of the day, including during periods of peak electricity cost,” Moisio said. “After installing RCS, we have been able to switch to off-peak pumping.”
This saved thousands of dollars and enabled Rowland to participate in demand-response programs with its power utility.
In February 2016, the District again saw the operational and cost savings associated with RCS when Metropolitan shut off supplies to Rowland and other retailers for maintenance on its system.
“Before, we would try to get by with a small supply of groundwater and water that was aging in our tanks,” Moisio said. “It was very risky to store so much water for such a long time. But with the RCS systems running, we could store water in our tanks for the entire 10-day maintenance period without any loss of water quality or interruption to our customers.”
Rowland Water plans to outfit all its tanks with PAX Water Mixers and add additional RCS for a total of six systems. With this investment, Rowland is living up to its motto of “effective action for sustainable progress.”