Can You Work Around Daily Estimated Sewage Flow Numbers?

System sizing rules become an issue when homeowners invariably focus on reducing onsite system costs during a building project.

Recently we were asked about estimating daily sewage flows. The questioner wondered if there is an accepted method to reduce the estimate from published regulatory numbers. The person was concerned that the numbers for their situation would require a system that was too large and expensive.

We thought it would be a good idea to spend a couple of columns discussing where those numbers come from and elaborate on different approaches that can be used to estimate average daily sewage flows.

The onsite system design process starts with estimating average daily sewage flows, and that — along with the estimated loading capacity for soils at the site — determines system size. Every state code we have looked at over the years has some type of table or listing of daily sewage flow for residences as well as other establishments.

For residences, a typical first-level approach is to determine a per-person per-day figure based on various research studies and published numbers in engineering manuals. In Minnesota, the number used to estimate flow is 75 gallons/day/person. For design purposes, it is assumed each bedroom will be occupied by two people. This means the estimated average daily sewage flow is 150 gallons/bedroom multiplied by the number of bedrooms. For example, a four bedroom residence would have an estimated flow of 4 bedrooms x 150 gallons/day/bedroom = 600 gallons/day.


As we’ve traveled around the country, we have seen variations to this formula. For instance, in Arizona the base number is 80 gallons/day/person, so 5 gallons per day a person higher than the Minnesota estimate. In other places we have seen lower numbers based on water-use studies and published data. The 150-gallon-per-bedroom number has been in use in Minnesota since the 1970s. Arizona’s rule was published in 2005.

In general, the studies these numbers are based on have occurred during the past 30 years and show typical average daily flows between 50 to 70 gallons/person/day. Not unexpectedly, there is a lot of variability within each study and variation between different areas of the country. In addition, some small studies show the numbers are somewhat smaller in predominantly rural areas served only by individual wells versus more suburban areas using public water.

One of the most comprehensive studies published in 2000 indicated per person per day water use was 54-67 gallons. For design purposes, building in a safety factor is an accepted good engineering practice. Our observation has been that while 75 or 80 gallons/person/day may be a little high based on these numbers, it is certainly in an acceptable range for a design factor. An Arizona study found in the Phoenix area an average daily water use of 77 gallons/person/day. This, at least on the surface, would indicate their 80 gallons/person/day figure is not far off the average and does not provide much of a safety factor.

The question posed to us involved allowable variation to these numbers based on additional information or mitigating factors. The answer depends on what is written into your state code and whether there is an opportunity to prove your design numbers are accurate for the intended use and type of system to be installed.


Using the same two states as examples, the opportunity for using different numbers was addressed in two different ways.

In Minnesota, we addressed this issue in the 1970s and established four dwelling classifications that were determined by overall size of the dwelling, number of bedrooms and expected water-using appliances. Water-using appliances are washing machines, dishwashers, bathtubs with greater than a 40-gallon capacity, garbage disposal and self-cleaning furnace humidifiers. This approach remains in effect. Beyond this, there is still opportunity to make a case to the local regulatory authority or the state to use a different number based on data or published studies.

Arizona addressed this question in two ways. There is the opportunity to estimate daily sewage flows based on water fixtures and water-using devices. This gives an opportunity to make the case for reduced flows and allows a way to calculate estimated flows for atypical residences, something probably everyone should think about incorporating in their codes if they haven’t already. They also provide the opportunity to make the case to the state regulatory agency for a different basis for calculation.

The direct answer to their question is yes, there is usually the opportunity to make a case for calculating estimated daily flow for a residence in a different way. Each state approaches the question differently based on the same studies and data. So the bottom line is that it’s the responsibility of the designer to provide the supporting data for a change in the estimate.

It is not going to be enough to say to the state regulator, “I don’t think we use this much water.”  Monitoring data or a reference to published studies will be required to make your case.

Next month we will take a closer look at the issue for nonresidences and other establishments.


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