Distribution water quality: the final frontier
Nick McElvain, P.E., Water Operations Support Manager, Lincoln Water System, Lincoln, Nebraska
Eric C. Lee, Assistant Superintendent of Water Production & Treatment - Operations, Lincoln Water System, Ashland, Nebraska
Andrew Hansen, Project Engineer, Black & Veatch, Kansas City, Missouri
Historically, the water quality in the distribution system was perceived to be a direct function of the water treatment process. Drinking water regulations were developed and implemented with focus on reducing or eliminating contaminants during treatment. With the promulgation of the Total Coliform Rule in 1989, the focus has now started to shift toward a two-pronged approach of treatment quality and distribution quality. The intent of the Total Coliform Rule, which requires public water systems to sample and monitor water for the group of bacteria referred to as Total Coliforms and Fecal Coliforms, was to ensure that from a microbiological standpoint, the public was receiving safe, potable water.
The most common test for coliforms was the membrane filter test. Water was considered safe to drink if it contained less than four coliform colonies per 100 milliliters and showed no evidence of fecal organisms. This standard applied until the limit was changed in 1990 to zero colonies per 100 ml. In 1992 USEPA approved the so-called definitive substrate technologies for detection of the presence/absence of coliforms. This ruling caused a significant change in the coliform testing and monitoring world by making it possible to test for the presence or absence of total coliforms and E. coli in one 24-hour procedure. Most utilities have embraced this change, but not without a great deal of controversy.
The Total Coliform Rule will soon undergo review by USEPA, which may result in changes, one of which could be the replacement of the presence/absence of Total Coliforms by the presence/absence of E. coli. All affected utilities should be aware of the impending review and utilize the comment period offered by USEPA to provide their input to the rulemaking process.
In order to provide protection against coliforms in the water delivered to consumers, the current trend is to maintain a higher disinfectant residual concentration in the distribution system. Although this has alleviated the risk of microbial contamination, it has aggravated the problem of disinfection by-products (DBPs).
Improved analytical techniques and increased efforts in toxicological testing have raised concerns about DBPs and their possible cancer-causing properties and other adverse health effects. These factors will be considered by USEPA during development of future regulations in an attempt to strike a balance between adequate removal and inactivation of microbial contaminants and the reduction or elimination of DBPs. The two most common methods of reducing DBP formation have been to lower the concentration of naturally occurring organic matter in source water before adding a disinfectant, and to substitute chloramines for free chlorine for maintaining a disinfectant residual in finished water throughout the distribution system. Using both of these measures reduces the amount of regulated DBPs in the finished water delivered to the customers' tap.
Increased attention to the water quality in the distribution system has led to modifications of treatment processes at the plant to ensure a safe and high quality product. After modifications are made at the plant, the focus shifts to maintaining the quality of the finished water in the distribution system. Utilities want their water to be as biologically stable as possible, which means that the water should contain only low numbers of "background" bacteria and should not undergo changes that would encourage growth of bacteria.
The quality of water in the distribution system can be evaluated by a water quality monitoring program. The need for such a program cannot be overemphasized. It should start with a few basic tests to track the microbial activity in the system. Residual chlorine concentration, heterotrophic plate count (HPC), water temperature, alkalinity and pH, and nitrite concentration are key indicators of conditions such as nitrification, stagnant water, and excess nutrients that encourage bacterial growth and regrowth. Test parameters such as Assimilable Organic Carbon (AOC) and Biodegradable Organic Matter (BDOM) can be effective means of assessing the biological stability of water. Assessing the sustainability of the chlorine or chloramines residual concentrations throughout the distribution system is another determination that provides valuable information.
Many of the system variables are analogous to the shelf life of food products at home or at the grocery. Clearly, the condition of the container and the storage environment make a big difference in how long a food product will last. According to research funded by the American Water Works Association Research Foundation, more than a dozen pipe materials have been used for distribution of water, with each having a unique reaction with the water being transported. Unlined cast iron pipe, for example, is prone to forming iron oxide tubercles, which harbor bacteria that deplete the disinfectant residual, which in turn creates an environment conducive to more bacterial growth. An increase or a change in flow rate, or water hammer caused by system utilization, can stir up the normally hidden bacteria, causing in many cases localized positive results that are unrelated to the quality of the water delivered to the affected area. Understanding how such system variables affect disinfectant residuals and possible positive coliform testing is a challenge for most distribution system operators. Increasing disinfectant residuals for the entire distribution system in many cases may not correct a trouble spot in the system.
Impacts of hydraulic modeling
Until recently, running a hydraulic model of just the skeletonized distribution piping on a PC was a task that took many hours. The models were run for maximum day and maximum hour conditions to determine the improvements necessary for providing adequate distribution pressures and fire flows. Today the models can perform extended simulations, taking into account the velocity and direction of flow in every pipe of the system. The model determines the transport route "container" and transport time "shelf life" of the product being delivered to the customers.
The utility now possesses two valuable pieces of information: sampling feedback and hydraulic model output. This information can best be visualized and interpreted using Geographic Information Systems (GIS). Evaluation of a distribution system by these methods makes it possible to locate trouble spots, and to plan remedial procedures such as unidirectional flushing. It also provides valuable information for developing operational modifications and capital improvement projects that will ultimately lead to a better product.
As drinking water professionals continue working to supply adequate amounts of safe water to their customers, knowing and understanding the dynamics of water quality within a distribution system will be of growing importance. Investing in non-regulatory water quality testing to gain a better understanding of an existing system will be another choice. Using today's technology to determine where and how portions of the existing piping system may be degrading water quality will become a more important topic in the water industry. Investment in supply and treatment facilities has been the primary focus of water quality capital funding in the past. The "final frontier" will require an equal focus on maintaining or enhancing water quality through improvements to the delivery system.
Nick McElvain can be reached at (402) 441-7571 or at email@example.com; Eric C. Lee can be reached at (402) 944-3306 or at firstname.lastname@example.org; and Andrew Hansen can be reached at (913) 458-3417 or at email@example.com.