The health of a water distribution system is commonly gauged by a statistic known as Unaccounted for Water (UAW). It is normally expressed as a percentage of the total volume withdrawn during a given year. The volume lost is the difference between the volume withdrawn and the volume that is accounted for.
The easiest way to account for water is through billing. All of the servcies in Topsfield are metered and we can quickly calculate the total volume of water billed in a year. The billing cycles for two-thirds of our customers do not match the calendar year but the differences occur in the winter time when water use is generally lower and more likely to be consistent from year to year. The Department of Environmental Protection allows us to also account for water use that can be 'confidently estimated'. Estimated use includes water used for fire fighting & training, hydrant flushing , water main breaks, drain cleaning and street sweeping.
We know the volume pumped, billed and estimated so where does the rest of the water go? This is where the problem lies since there are a number of areas where the water is lost. Here is a list for starters:
- Master meter inaccuracies - The master meters are located at our sources and measure the volume of water pumped into the distribution. These meters are calibrated annually but any error will have a dramatic affect on UAW.
- Customer meter inaccuracies - These meters must read within 2% when they are new but lose accuracy as they age. They typically under-register as they wear out. We replaced all of our customer's meters to improve the system's overall accuracy. We are required to replace or test meters every ten years.
- Theft - Water that is stolen is likely a small volume in our system but it does contribute to UAW. Theft includes illegal connections made prior to meters, the removal of meters or the register and businesses that fill tanks from fire hydrants without prior approval.
- Leakage - This is the main cause of high UAW because many of the smaller leaks do not surface and can go undetected. We conduct leak detection surveys to try and minimize the amount of leakage. Small leaks that are found and repaired are not included in the 'confidently estimated' volume.
Unaccounted for Water expressed as a percentage is a common benchmark used to compare losses for a system over multiple years or between multiple systems. It is the industry standard but it can be problematic. You may want to read this article to learn more about the pitfalls of using UAW as a percentage.