Is It Time to Rethink Daily Water Use Numbers?

Low-flow toilets and education about conservation are driving down water use per capita. So is it time to change the way septic systems are sized?
Is It Time to Rethink Daily Water Use Numbers?
Is It Time to Rethink Daily Water Use Numbers?

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Is household water use decreasing?

A couple of months ago, I read an interesting article in Pumper that referred to a study on household water use, stating it had (on average) decreased since 1999 by about 22 percent. It’s worthwhile to discuss some of the implications this study may have for our industry. So, I looked up the executive summary of the study conducted through the Water Research Foundation.

In the design of any onsite sewage treatment system, two important estimates drive the overall system component sizes: the estimated daily water usage and the long-term acceptance rate of the soil. Understanding both of these numbers and how they interact is the key to good system design.

A couple of notes are important to keep in mind. First, a 1999 study was used to establish the water-use comparisons and reductions. The 2016 study involved twice as many households and a broader array across the country. The households looked at were on water utilities: the numbers were derived from their data on households. The study did separate out outdoor versus indoor use through a survey subset.


When we have looked at household water use in the past to estimate daily flows, one of the questions has always been: How much of the total flow numbers are due to outdoor use such as lawn watering and car washing? This study made that distinction.

The range of household water use was 120 to 479 gallons per day. Indoor water use per household fell from an average of 177 to 138 gallons per day — a reduction of 22 percent. The average number of persons in the household fell from 2.77 to 2.65 people. As always, looking at averages can sometimes be misleading because there are both higher and lower numbers. If median figures are used, it means that half the numbers are higher than the median and half are lower.

Of more interest to me were the per capita (per person) numbers, which showed a reduction of 69.3 to 58.6 gallons per person, a 15 percent drop from 1999 to 2016. This improvement was attributed to increased efficiencies for clothes washers and toilets. There was a 36 percent reduction for clothes washers from 1999, a reflection of the mandated increased efficiency in the 1990s.

For the toilet, the average number of flushes per day of five remained the same, but gallons per flush went from 3.65 to 2.6 — a decrease of 29 percent. Interestingly, the toilet continues to be the highest single water-using device at 24 percent of daily flow. From my perspective, this is significant because two decades ago, the toilet comprised about 40 percent of the daily flow. So, the efforts to reduce the gallons per flush seem to finally be paying off.


Hopefully it also means people are not using their toilet to dispose of extraneous items like cigarette butts, condoms and sanitary products. I have taught a lot of classes over the years and have said since 1991 that I was hopeful we would see the toilet contribution drop due to better low-flush toilets. That seems to be happening.

One other aspect of the study was faucet use, and one key point was that the amount of faucet use did not change whether or not a dishwasher was present. To me, this indicates that if people have a dishwasher, they are prewashing the dishes before they go into the dishwasher. While the dishwasher use per cycle has also dropped, the prewash or rinse has not. This is an area where educating your homeowner about water use habits could make an impact to reduce use.

From a water use estimation to determine system design, these numbers are also interesting. One common method (though, not the only one) that was established by the U.S. Environmental Protection Agency is to use 75 gallons per person per day along with the number of bedrooms to come up with an estimated daily flow. An important assumption of this method is that two people occupy each of the bedrooms; the per bedroom flow estimate is 150 gallons per day. The estimate for a three-bedroom house is 450 gallons per day.

In 1999, the per capita average of 69 gallons per day is right in the ballpark with the 75, so it may not be a bad number to use. The per capita average has dropped to under 60 gallons, so maybe now is the time to reassess the per capita number for design purposes.

The average number of people per household, though, in both 1999 and 2016 was less than half the six people that are used in the design method. Again, this is average, so there are a number of households where there are in fact six people present. Since the number of bedrooms and the per capita numbers work together, if the per capita number is dropped to 60 gallons per day and we stick with the two persons per bedroom, then the design flow number drops from 450 to 360 gallons per day for a three-bedroom house. This would affect both tank sizing and size of the soil treatment area.


As an industry, we should work with EPA, state and local regulators to see if now is the time to make some changes to our water use estimates.


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