If implemented, distribution system optimization will lead to increased public health protection through increased monitoring and operational oversight, resulting in improved physical protection and improved water quality for all customers. In addition to a more proactive sampling program, goals must be established in order to successfully determine if drinking water distribution optimization is being achieved. These goals can be water quality based or operations based and should be established to provide an extra level of protection when compared to the current regulatory requirements. The Department of Environmental Protection and Environmental Protection Agency have established some distribution system optimization goals which can be used by systems trying to optimize their distribution systems. The current goals are as follows:
| Secondary Disinfectant | Parameter | Goal |
|---|---|---|
| Free Chlorine | Free Chlorine | ≥ 0.20 mg/L at all locations at all times |
| Chloramines | Monochloramine | ≥ 1.50 mg/L at all locations at all times |
| Free Ammonia | Detectable, but ≤ 0.10 mg/L at Entry Point | |
| Free Ammonia Dosing | 4.5 - 5 parts Cl to 1 part NH3-N | |
| All | Storage Tank Turnover | ≤ 5 days |
| Storage Tank Mixing | Performance Ratio ≥ 1 | |
| Disinfectant Byproducts | 20/15 µg/L at Entry Point | |
| 70/50 µg/L at individual site LRAAs |
It is important to note that the goals presented are general goals and are a good starting point for systems that may be considering distribution optimization. A system may wish to establish additional or more stringent goals to ensure enhanced public health protection.
In addition, a water system may choose to participate in the Partnership for Safe Water Distribution Optimization Program. This Partnership for Safe Water has goals similar to those presented, as well as goals for areas beyond those listed. More information for the Partnership for Safe Water Distribution Optimization Program can be found on the American Water Works Association website:
Partnership for Safe Water Distribution System Optimization Program
Additional Resources
Plant Effluent / Distribution Influent Hold Study
If having DBP or chlorine degradation issues, a system may wish to conduct a plant effluent /distribution influent hold study in order to determine the bulk water DBP formation potential or chlorine decay rate. Instructions for conducting a hold study can be found at the following link:
Plant Effluent / Distribution Influent Hold Study (Excel)
Plant Effluent / Distribution Influent Hold Study Instructions (Word)
A hold study can be an extremely useful tool when trying to determine whether to focus DBP control strategies at the plant or within the distribution system. It can also help identify whether chlorine degradation issues are a result of distribution system deficiencies or as a result of demands in the bulk water (e.g. dissolved organics).
Water Loss Control Controlling the amount of water that leaves a distribution system that is not billed (non-revenue water) is not only financially beneficial, but can also lead to the identification of leaks that may have the potential to cause adverse effects on water quality and public health. Through a reliable and consistent auditing procedure, a water system can identify system needs and implement corrective measures to help control losses. By continuously auditing, system performance can be tracked to document improvements as the control measures are implemented. Additional information and free auditing software can be found on the American Water Works Association website:
Maintain ≥ 0.20 mg/L free chlorine at all monitoring sites in the distribution system, at all times.
This goal only pertains to drinking water systems utilizing free chlorine as the primary disinfectant within the distribution system.
In a free chlorine system, the chlorine acts as the final chemical barrier to provide protection prior to consumer use and consumption. Waterborne disease outbreaks have been attributed to inadequate (or no) disinfection in distribution systems when contamination was present. It is therefore important to maintain a suitable amount of free chlorine throughout the entire distribution system to react with any contaminants that may be introduced into the distribution system through breaks, backflow, treatment breakdown or other avenues.
In addition, changes in free chlorine residual can be a primary indicator of contamination or operational issues. By maintaining a sufficient residual at all times, changes will be more readily detected.
This goal can be tracked using assessment software developed by the Environmental Protection Agency (EPA). This software and instructions can be found at the following links:
Distribution Water Quality Assessment Software (Excel)
Distribution Water Quality Assessment Software Instructions (Word)
It is important to note, when making system decisions to meet distribution free chlorine goals, distribution system operational controls should be investigated prior to making chlorine dosing adjustments at the plant entry point. A few options include improving tank turnover and mixing; improving routine flushing programs; and controlling water age through targeted flushing, line looping and valving to better regulate flow.
Disinfection By-Products Goal:
Individual Site Goal - Quarterly Max LRAA TTHM/HAA5 values not to exceed 70/50 ppb.
Under the Stage 2 Disinfectants and Disinfection By-Product Rule (Stage 2 D/DBPR) compliance for TTHM and HAA5 is calculated as a Locational Running Annual Average (LRAA). The LRAA MCL values for the Stage 2 D/DBPR are 0.080 mg/L (80 ppb) for TTHM and 0.060 mg/L (60 ppb) for HAA5. The goal of a quarterly maximum LRAA ensures that all sites being used for compliance monitoring are below the MCL and provides for extra public health protection.
Goal: Long-Term System Goal - Average of Max LRAA TTHM/HAA5 values not to exceed 60/40 ppb (the average of the last 8 quarterly Max LRAA values not to exceed 60/40 ppb).
The long-term goal averages the last 8 quarterly maximum LRAA values for any given quarter. By meeting the goal, it not only provides for extra public health protection, but also helps with evaluating increasing trends that may indicate either treatment or distribution system issues that may not have otherwise been detected.
Goal: Treatment Plant Entry Point Goal - LRAA TTHM/ HAA5 values not to exceed 20/15 ppb.
Disinfection by-products start to form as soon as chlorine is added to water containing natural organic matter (NOM). This can be during pre-treatment; if pre-chlorinating for inorganic oxidation, taste and odor control, algae control or to meet CT requirements, and/or during post-treatment. After leaving the treatment facility, DBPs will continue to form as the reaction continues between the chlorine and organic matter. Identifying an entry point goal provides a target for minimizing DBP formation within the treatment plant while allowing for formation to continue within the distribution system without exceeding the MCL for either TTHMs or HAA5. Although the values for this goal is a good starting point, it may need adjusted in order to meet the distribution system goals (i.e. the entry point goal may need to be lower in order to meet the individual site and long-term system goals at a particular system.)
All of these goals can be tracked using assessment software developed by the Environmental Protection Agency (EPA). This software and instructions can be found at the following links:
Distribution Water Quality Assessment Software (Excel)
Distribution Water Quality Assessment Software Instructions (Word)
Guideline: Maintain turnover time less than or equal to 5 days at all times or establish and maintain an optimal water turnover rate at each storage facility.
Tank turnover is essential to ensure that fresh water is entering the tank and replacing a specific volume of water within the tank. This is needed to maintain water quality within a tank and to minimize the impacts of the tank on the quality of water entering the distribution system from the tank.
Turnover time is the amount of time that it takes for the volume of the water in a tank to be completely replaced. For example, if a 1 million-gallon tank draws 20% of its tank volume once per day, the total volume of the tank, or 1 million gallons, will be replaced in 5 days. In this example, the tank turnover time is equal to 5 days, which meets the goal presented.
It is important to keep system pressure in mind when manipulating tank operations to meet tank turnover goals. Lowering a tank to a point where pressure issues arise should be avoided.
Guideline: Maintain good mixing performance ratio (PR ≥1.0) at all times.
In distribution system storage facilities, once tank turnover has been optimized, mixed conditions should be achieved. It has been shown through studies that the disinfectant residual decays more rapidly under a plug-flow versus a completely mixed scenario. Although, without proper turnover and sufficient exchange of the volume of water present within the tank, mixing may not show any benefits and may even be detrimental when very poor tank turnover is occurring.
Tank mixing can be measured by calculating the volume exchange fraction or the fraction of water that has been added to the tank during a fill period. In most tanks, mixing can be improved by increasing the volume added during a given fill period or decreasing the inlet diameter (i.e., increasing inlet velocity). Other mechanical or passive mixers can be utilized to enhance mixing as well. A mixing performance ratio (PR) is used to assess the actual mixing compared to the desired mixing.
The tank turnover and mixing PR can be calculated using software developed by the Environmental Protection Agency (EPA). This software and instructions can be found at the following links:
Drinking Water Storage Tank Assessment Software (Excel)
Drinking Water Storage Tank Assessment Software Instructions (PDF)
It is important to note that these are calculations based on theory. Although they provide a starting point for enhancing tank operations, water quality data should always be used as the basis for determining the extent of impacts of tank operations on water quality.