It hasn’t taken long for some of the most obvious unintended consequences to appear: More people are complaining that their shower experience is deteriorating due to lower flow rates, and shower times are increasing as people have to shower longer to wash the shampoo out of their hair. Others complain of having to flush a toilet multiple times to get human waste down the drain. While the plumbing industry continues to try to respond to users’ needs, the advances are now hitting against the limits of physics and science, especially in regard to piping systems.

The back-end first

While the impact of water reductions is certainly an issue on domestic water supplies, we need to make sure not to lose sight of the effect lower volumes of water is having on sanitary piping systems. The concerns of reduced flow in distribution piping is often spoken of at length, but the issue of reducing the flow of water that is going to our sewer systems also has much documentation.  

First some context: Since the 1940s, the way in which sanitary piping systems have been engineered and installed remains largely unchanged. Based on the number of plumbing fixtures (toilets, sinks, etc.) and how much the sanitary pipe slopes (i.e., how steep a pipe is installed), a pipe size can be determined. Originally, back in 1940, pipe sizing was predominantly based on achieving “scouring” action. Scouring in sewer piping occurs when the water flows at a certain speed at which sediment will not settle at the bottom of the pipe. The general rule of thumb in plumbing engineering is a minimum velocity of 2 feet per second is needed to maintain the scouring action. However, as discussed before, the volume of water being used by plumbing fixtures since 1940 has been massively reduced, but the method of how sanitary piping systems are sized has not. Consequently, we’ve seen an influx in sanitary sewer systems that are beginning to experience waste building up.  

The first issue is probably the simplest to understand, so it is a wonder that this wasn’t considered: Sanitary-sewer (or waste) drainage issues. The traditional sewer system, which is common in the United States, is sloped so that gravity carries the human waste away. The most common slope in a gravity sewer system is between 1% to 2% (this equates to a 1/8-inch drop in elevation for every linear foot, to a 1/4-inch drop in elevation for every linear foot). As you can tell, this does not provide much gravity to help propel the waste down the pipe. But when there is a lot of waste, it can move it rather quickly.  

An example to help illustrate this point: Grab a glass and fill the bottom of it with just enough water to cover the bottom. Then dump it on your kitchen table. If your table is reasonably level, the water will likely sit in a puddle on the table and not go anywhere. Now, refill the glass halfway with water and dump the water on the kitchen table. You won’t do it? Why? Because you know that you’ll end up with water off the counter and on the floor and cabinets! Many children learn this lesson the hard way when they first are learning to pour themselves beverages or cereal. The concept also applies to plumbing: The counter is a slightly sloped pipe. The half-full glass of water indicates what the flow rates and volumes of waste that flowed through sewer systems used to be, while the barely-covered-bottom glass is in essence what current flow rates are. However, unlike the glasses of water, less water waste, in this case, is making more of a mess, as waste is no longer propelled through sewer systems as it once was.

In theory, if we reduce sewer pipe sizes, we should be able to decrease toilet and other plumbing fixture flow rates continuously. However, in practice, we are operating on a bell curve in which as we decrease flow rates, physics seems to tell us that smaller pipe sizes make no difference. We may not be able to go any lower in our flow rates without negatively impacting public health and safety.

In 2012, the Plumbing Efficiency Research Coalition (PERC) developed a study titled “The Drainline Transport of Solid Waste in Buildings,” which was followed up by a second phase report in 2016. The PERC studies sought to understand how low-flow rates impacted sanitary-waste systems. These reports reached some interesting conclusions:

  • A PERC study showed some minor differences in an experiment system made of plastic (very smooth piping) but the real world has shown more and more backups in sewer systems as a result of lower and lower flow rates going into sanitary systems;
  • “The effect that toilet fixture designs have on drain line transport in long building drains has been found to be minimal. Instead, flush volume, toilet paper, and pipe slope were found to have a large effect upon drain line transport of solid waste;” and
  • Smaller pipe size was shown to have “no significant impact on drain line carry.”  

So what does this all mean? That we have potentially hit a wall in water conservation. In theory, if we reduce sewer pipe sizes, we should be able to decrease toilet and other plumbing fixture flow rates continuously. However, in practice, we are operating on a bell curve in which as we decrease flow rates, physics seems to tell us that smaller pipe sizes make no difference. We may not be able to go any lower in our flow rates without negatively impacting public health and safety.   

The front end — domestic water

While sanitary systems engineering may be hitting the wall of physics, domestic water is another story. David LaFrance, CEO of the American Water Works Association, once said, “People think that water is simple, but it is highly complex.” Domestic water — because of the complexity of distribution systems’ layout, water quality and performance needs, among other requirements — means there is more opportunity in right-sizing water systems. The problem has mostly been that these complexities were not even accounted for when water usage rates were reduced.  

A similar method was created in the 1940s to size domestic water systems. This method had remained largely unchanged for the past 80 years. In essence, it treated every building in the U.S. as if it were a sports stadium at halftime — assuming long queuing lines for the restrooms and numerous plumbing fixtures being “on” simultaneously. At the time, this methodology was groundbreaking.

However, in the past 80 years, as water conservation concerns have increased, no updates were made to this process. The volume of water stored in the pipe remained the same while the flow rates were reduced. This increased the duration of time water was in piping, which led to numerous negative side effects. Water age increased as flows decreased, leading to such negative second-order effects, such as:

  • Hot water delivery times increased (decrease in energy inefficiency);
  • Disinfectants in the water decreased while disinfectant byproducts increased (decrease in safety);
  • Waterborne pathogens such as Legionella pneumophila increased (decrease in safety); and
  • Corrosion of water piping systems increased (decrease in reliability).

It has gotten to the point that even the National Academy of Sciences, Engineering, and Medicine (NASEM) has weighed in. In its 2019 consensus report on the management of Legionella in buildings NASEM stated:

“Low-flow fixtures should not be allowed in hospitals and long-term care facilities because of these buildings’ high-risk occupant populations. Low-flow fixtures have been promoted to conserve water and, in some cases, energy. Because of their low flow, however, these fixtures, primarily low-flow faucets but also showers, increase water age and restrict disinfectant levels, including the disinfection provided by elevated water temperatures. As such, low-flow fixtures present a greater risk for Legionella development in the plumbing systems that feed them.”

To have NASEM come out with this kind of statement should catch your eye. For a governmental body to implicate that water conservation is at odds with water safety is no small matter. This shows the complexity and need for research and investment in plumbing system design and construction, particularly in pipe sizing. Fortunately, this research is happening.

Editor’s Note: This column is part of a series. Catch up with Part 1 and Part 2.