A creative storage tank and distribution system pumping strategy helps a Massachusetts utility reduce water age and improve water quality
New Englanders look forward to summer for a variety of reasons, but for water utilities, the rising temperatures can be problematic, leading to issues of water age and water quality.
The Ashland (Massachusetts) Water Department is now an exception to the rule. Until recently, while plant operators spent significant time, effort and money throughout the year treating water to strict compliance parameters, that water was pumped into a distribution system that historically hadn’t received this same level of oversight.
With the distribution system lacking the pump strategies needed to maintain high water quality and suitable water age, operators had to shock one of the water tanks frequently with sodium hypochlorite, a major task that required someone to climb the tank and pour in the chemical with support from the fire department and safety officials.
This protocol was technically working, but the need to do it indicated that the distribution system and storage tanks were not in a condition that would allow the utility to see the desired return on its year-round investment in treating water to high regulatory standards.
To address the problem, the staff at the treatment plant, contract-operated by Woodard & Curran, implemented strategic pump operating procedures to restore the system’s hydraulic integrity and effectively manage water age and monochloramine disinfectant residuals in the tanks and all distribution piping. The result has been improved water quality at minimal capital investment and operating expense.
The central components of this approach include using real-time water modeling and treating the whole distribution system as a separate water tank. Years before developing the real-time modeling strategy, the town of Ashland had installed a passive hydraulic mixing system in one tank, intending to stabilize the degradation of chlorine.
That set the stage for more effective management. However, the mixing system on its own would not have had a significant effect without the operational changes. The mixing helped by dispersing freshwater uniformly throughout the tank, eliminating stratification. However, this only enabled reactive management of the distribution system, responding to issues after they occurred with relatively limited data.
Real-time modeling now enables operators to assess changes in the system, such as those caused by hydrant flushing, main breaks or pipe maintenance as they occur. This allows the staff to quickly assess potential negative effects from system changes, analyze the most reasonable approach for addressing those effects, and implement changes before larger problems develop. This proactive approach keeps small issues from disrupting service and maintains a consistently higher level of overall system health.
The staff first implemented real-time modeling by evaluating chlorine and monochloramine residuals, the heterotrophic plate count (HPC) in the plant’s two storage tanks, and the water pressure across the system.
This evaluation revealed that the height of the two standpipes and their location played a significant role in ever-changing water quality. Simply put, the tank closest to the plant had no challenges maintaining a disinfection residual, while the tank farthest from it did, largely because of distance and the simple fact that water flowed to the place of least resistance due to the system’s hydraulic design.
Operators then developed a way to strategically manipulate the altitude valves and use variable-frequency drives (VFDs) on the raw-water wells and finished-water pumps to force water in certain directions, changing the directional flow from the plant based on water demand. This helped manage water age by recharging the distribution piping with freshwater, and by not pushing old water back into the farthest tank, which has common inlet/outlet piping, when the plant’s finished water pumps are in production mode.
The critical piece of this strategy is to treat the distribution system’s combined water volume as another aspect of the storage system, specifically as a third storage “tank.” In addition, operators carefully monitor levels in the water towers so that water isn’t forced into the towers until the distribution system has been recharged with freshwater.
Therefore, although the plant is in production mode and water is flowing into the distribution system, the towers are flatlining, allowing the water to be completely turned over. This requires operators at the treatment plant to be aware of the water volume in the distribution service areas and to account for it in water turnover calculations and in monochloramine disinfection water quality analysis.
The real-time computerized water modeling system was a big step in the right direction, but other operational changes were also needed to realize the full benefits the water department desired.
For instance, optimizing the flow into the distribution system by using VFDs based on water demand also allows water production to match demand to within 1,000 gallons. With the aid of automated storage tank altitude valves, monitored and controlled at the treatment plant via the SCADA and wireless communications network, operators also routinely take one tank offline during peak demand and force freshwater into the tank and service areas farthest from the plant.
These changes have significantly increased water turnover in the standpipes by ensuring that the distribution system is fully recharged with freshwater. As a result, the staff achieves quality levels that once seemed far-fetched without major capital investments.
For the past four years, the town has consistently maintained its chlorine residuals above the state Department of Environmental Protection’s recommended standards for total chlorine without having to shock the storage tanks. In addition, all sampling data surrounding the monochloramine disinfection program has confirmed improved water quality as measured by monochloramine, ammonia, phosphorus and pH.
The town’s experience is a good reminder that addressing substantial issues does not always require large capital investments. In this case, there was no need to acquire new assets or disrupt service areas with modifications to distribution or valve appurtenances.
By leveraging earlier investments in plant equipment, SCADA and communications, and through aggressive maintenance, Ashland significantly reduced water age and improved water quality in the distribution system, at little added expense. The adjustments are based on creative distribution system management, using modern technologies and industry-approved practices to make the system work to suit utility needs.
About the author
Jeffery Fournier of Woodard & Curran (www.woodardcurran.com) is a plant manager at the Howe Street Water Treatment Facility in Ashland, Massachusetts.