Risk Analysis of Decentralized Wastewater Design Flows

Decentralized wastewater treatment systems are often designed at flows of either 284 L/person/d (75 gal/person/d) or 568 L/bedroom/d (150 gal/bedroom/d). Water use data suggest that designing systems at these flow rates can lead to overly conservative designs. A study quantifying the risk of failure (exceeding a system design flow) was needed to create a design basis for future systems. The objectives of the study were to quantify the risk of failure of decentralized system design flows depending on the number of residences served by a system and to develop new guidelines for design flows of cluster systems based on quantifiable research. Data sets were from Consolidated Utility District of Rutherford County, Tennessee and contain water use information from July 2005 through July 2006 for seven subdivisions (636 residences) served by cluster systems. Water use was adjusted to wastewater production in each data set using a factor of 80 percent, and from each data set, probability distributions of average monthly flows and monthly peaking factors were made to model the variance due to residences and months, respectively. Monte Carlo simulations were conducted to simulate monthly flow distributions for differing numbers of residences, which were evaluated for risk of exceeding differing design flows. For subdivisions with thirty or more three-bedroom residences, the results show that a design flow of 25552 L/month/residence (225 gal/d/residence) limits the yearly risk of exceeding a month’s design flow to less than one percent. The results of this study can be used to design future cluster systems in similar regions.