Homeowners need to understand their septic system has a finite capacity and is directly impacted by their water use
There are two aspects to management of onsite systems and they need to go hand in hand.
There is the management of the individual system that serves a residence, and then there is the regulatory management program, which covers managing all systems in an area, township, county, etc.
The regulatory program is only as good as the worst systems being managed. If systems are not “good” from the perspective of site location, proper system type for the area or there are installation problems, the management program will not meet the objectives of having long-lasting systems that take care of water use from the residence and adequately treat the wastewater to protect the environment and public health.
Management begins then at the point of estimating the daily sewage flow from the residence (using the assumption that the system serves a home, so the wastewater generated will be domestic waste). If some other type of waste will be generated because there will be some type of business operated from the residence, both the quantity and type of waste will need to be evaluated and appropriate adjustments made to the size and possibly the type of system installed. The designer is dependent on the homeowner for the information on potential water use.
Most state codes and local codes provide for estimation of daily sewage flow from residences and other establishments. Since we are focusing here on residences, those processes are relatively straightforward, often based on numbers of bedrooms and water-using devices to be installed. Water-using devices are things like toilets, sinks, dishwashers, washing machines and others. A common method was to use the number of bedrooms, assume for design purposes that two people occupied each bedroom and each of them generated 75 gpd of wastewater, therefore, a three-bedroom home would be estimated to generate 450 gpd.
That worked relatively well, but it wasn’t long before it was recognized that not all three-bedroom houses were the same. A way to compensate was to adjust estimates based on square footage of houses and water-using devices. All of this can be justified from a management and regulatory perspective. Calculations also recognize that in a lot of cases there will not be two people in each bedroom, and a number of research studies show that per capita water use is actually around 60 gpd. The 75-gallon figure is a conservative estimate, which in my view is good for design purposes, and making adjustments based on additional information makes sense. Cutting the estimate too close can quickly result in undersized systems that will struggle to perform.
Most homeowners will tell you that they are different and do not actually use 60 gpd, but research seems to bear out they do. Homeowner education needs to stress they can help or hurt their system by how much water they use, and in my opinion this means having water meter data available to monitor water use. If they are using more than system capacity they can see what they need to cut back, or make the decision to enlarge their system to accommodate the flows. They need to understand the system has a finite capacity that cannot be exceeded on a regular basis.
About the author
Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program and is an emeritus professor in the university’s Department of Soil Water and Climate. Send him questions about septic system maintenance and operation by email to firstname.lastname@example.org.
This article is part of a series about designing and installing onsite septic systems with future management in mind.