34 HEALTH AND SANITATION
Low flow rates impact sanitary piping and domestic water systems
David Viola , IAPMO CEO
In the spirit of the sharing of unique experiences that shape the plumbing industries in our respective nations , the following article is the last of a three-part series looking at the formation of Hunter ’ s Curve , which attempted to predict peak water demand in a building . Written by IAPMO Vice President of Strategic Initiatives Christoph Lohr , P . E ., it originally ran in PM Engineer magazine and is the next in a regular series of similar articles that will run in this magazine . Dave Viola IAPMO CEO
By
Christoph Lohr
Misunderstanding Roy — Part 3
PERC studies sought to understand how low-flow rates impacted sanitary-waste systems .
Unintended consequences 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 must 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 regarding piping systems .
The back end first While the impact of water reductions is certainly an issue for domestic water supplies , we need to make sure not to lose sight of the effect which lower volumes of water are having on sanitary piping systems . The concerns of reduced flow in distribution piping are 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 : Sanitarysewer ( 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 barelycovered-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 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 These reports reached some interesting conclusions :
• A PERC study showed some minor differences in an experimental system made of plastic ( very
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