Declines in Household Water Use Are Good News

Average daily flow changes through more efficient toilets and washing machines will impact future onsite system designs

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When we conduct a basic training workshop covering individual sewage treatment systems, we start with a discussion about the amount and characteristics of flow. This is done for two reasons. First, before a site evaluator, designer or installer can make decisions about the system to be installed, they must know the characteristics of the material they want to treat. Second, flow is something the homeowner or business owner can control — unlike the soil and site conditions, which are natural and do not change dramatically if left in their natural condition.

There has been a continuing effort on the part of industry and manufacturers to make water-using devices more efficient and to use less water. Is less water being used by homeowners? And if so, how does this impact domestic or commercial systems that rely on soil for the final treatment and dispersal medium?

In 2016, a study looked at residential water-use changes from 1999 to 2016. While this study focused on data from 23 water utilities across the country and the use of averages can sometimes be misleading, the results probably indicate in general what is going on with our customers as well.


Overall, they found average household water use declined by 22% and average per-person use declined by 15%. The average household decline is positive from a water conservation perspective and because our systems will receive lower wastewater volumes. The difference between household- and individual-use declines can be explained by the interesting fact that as the number of residents increases, their daily average per-person use declines. This probably makes sense if you think of an individual still using the dishwasher, washing machine and other water-using devices somewhat less efficiently (not full loads). But with more people in the house, these usage activities are shared.

The largest reduction in flow occurred in the washing machine, with a 36% change. In 1999, very few households (2%) indicated they had a high-efficiency clothes washer. In 2016, two-thirds reported having a high-efficiency machine. Back in the early 1990s, we would estimate the average clothes washer typically used 50 gallons per wash. The 1999 study indicated the average was 41 gallons per load; in 2016, this was down to 31 gallons per load — quite a flow reduction over 30 years. Average number of loads per day did not change, which means the reduced water use is due to the higher-efficiency machines and the ability to adjust water use by load size.

Water usage for toilets — one of our personal favorites because we long have encouraged installers and service providers to work with customers to limit extraneous flushes for things that should not be put down the toilet — decreased 23%. To which we said: “Fantastic, people are finally not flushing as much!” The celebration ended when we read that flushing frequency was unchanged. Again, the reduction in flow is due to the increased use of low-flow toilets. So it looks like we still need to talk about limiting unnecessary flushes. One final toilet-related thought: The toilet still is the largest single water-using device in the home. Even with the numbers coming down, there is still work to be done.

One depressing aspect of this study involved the presence of plumbing leaks. If the home is on municipal sewer and water, leaks have consequences of higher individual sewer and water charges. As we have discussed numerous times, leaks can have disastrous consequences for onsite systems, resulting in hydraulic overload and system failure, which can cost homeowners a large amount of money to fix.

The study indicated that only 5% of households did not have leaks, while 63% had leaks of less that 10 gallons per hour. If the leak is a gallon per hour, 24 gallons of unnecessary water are added to and treated in the system; probably not a big deal in most cases. However, at 10 gallons per hour, 240 gallons are added. If the family has a 450-gpd system and is already using 250 gallons per day on average, this level of leakage is enough to put the daily amount of water delivered above system capacity. It will ultimately fail if this continues indefinitely.

Another 32% of the households had leaks larger than 10 gallons/hour! In these cases, failure of their septic system will occur in a very short time — a matter of days or weeks instead of months. Bottom line: Check for leaks on your service calls and stress the importance of fixing them as soon as possible.


The study concludes that these changes are due primarily to use of more efficient devices and not changes in behavior. Newer homes exhibited the highest reductions because they are equipped with the higher-efficiency units. The trend is expected to continue, so these changes should be factored into future system design planning.

For onsite systems, this means we can expect average flows to continue to decrease over time and they could be incorporated into the flow estimate part of the equation for system sizing. However, system size reductions can only occur as a part of an overall analysis of soil and site conditions, flow volume and wastewater characteristics. So size reduction may not be as obvious or straightforward as homeowners would like. We will explore the volume/wastewater-strength issue in an upcoming column.


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