The Minnesota Department of Transportation (MnDOT) State Wide Rest Area Program is composed of a coordinated system of public rest areas and waysides that are intended to help motorists travel safer. In 1979, MnDOT collected data at rest areas to refine assumptions and improve techniques for design of rest area water supply and sewage treatment designs. They found on average that with water conserving devices people used 2.8 gallons, while non-water-conserving devices used 4.5 gallons. In the following years, water use charts have been informally altered, but no formal study has been conducted. This study evaluated the accuracy of MnDOT design charts and formulas based on people counts and water flows. A total of twelve rest areas were included in the study, with each site belonging to one of two building categories, interstate or non-interstate. The study included data collected beginning Spring 2015 to Fall 2016. None of the sites had continuous data for the study period and half the sites began data collection Fall 2016. Water use estimations were obtained electronically by calculating averages from hourly people counts and hourly water flow data. When electronic data were not available, manual daily water flow was used. Site specific average water use per person ranged from 0.9 gallons to 4.6 gallons. A difference was found between the two building types, interstate and non-interstate, with interstate visitors averaging 2.7 ± 0.6 gallons and non-interstate visitors averaging 1.8 ± 0.7 gallons. The difference between building types was not easily explainable, however it is theorized fewer visitors at non-interstate sites results in less water needed for cleaning or water treatment. The results of this study indicate that the original design values are still valid. However, due to the wide variation of water use per site, maximum water demands and usage trends should be estimated when designing a new septic system. This will ensure the most appropriate septic system is installed, resulting in the successful treatment of waste water and the fulfillment of expected system lifespans without additional maintenance costs. Water use studies similar to this should be conducted over longer time periods for better understanding of water use trends and peak usage over many years.
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University of Minnesota (Minneapolis, Minnesota)
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