June 12, 2015—Project Engineer Vincent Bergl, P.E. recently caught up with an old college friend who mentioned she was refraining from using water during heavy rains so that she wouldn’t contribute to overloading the sewer system. It sparked an impromptu lesson on urban hydrology (lucky her), and after hearing similar concerns expressed elsewhere, Vinnie decided his thoughts were worth sharing:
No one welcomes beach closings or basement backups, but can water conservation during storms prevent sewer overflows? The answer is maybe sometimes, but not really, depending on context. Let me explain.
SEPARATED VS. COMBINED SEWERS. Let’s start by distinguishing separated from combined sewer systems because the difference affects the answer. Combined systems collect both wastewater and stormwater runoff in one pipe, whereas separated sewer systems have a pipe dedicated for stormwater runoff, and another for wastewater. Even in separated systems, as much as 10% of rainfall enters the wrong pipes in the form of inflow and infiltration (I/I).
If you live in a community with separated sewers, the primary risk of an overloaded collection system is that the pipes will surcharge. This is most likely to happen during heavy rain when I/I overwhelms the system and builds pressure causing wastewater overflows into basements and out the tops of manholes. These surcharges are called sanitary sewer overflows (SSOs).
REALITY CHECK. But can too much domestic water use also cause an SSO during dry weather? Generally, no, because sanitary sewers are designed to convey a minimum of three (and more likely, five or 10) times the peak dry-day flow. Even the smallest public sewer—an 8-inch-diameter pipe at 0.4% slope–has a full-pipe capacity of 330 gallons per minute (gpm). To put that in perspective, the average residential household produces around 250 gallons per day (gpd) of wastewater, which equates to about 0.2 gpm.
Of course, domestic water consumption isn’t a constant, and the amount of wastewater produced by a single household varies greatly by source and by time of day. Even at the extreme, let’s say you’re doing laundry and washing dishes while your spouse is taking a (probably very uncomfortable, temperature-erratic) shower, you are only kicking out 15 gpm of wastewater – roughly 5% of the capacity of that 8-inch pipe. Now, if every neighbor on your block was engaging in water-intensive tasks at that very same time, the sewer main running below your street could have some issues. In reality, it doesn’t happen this way.
THE REAL CULPRIT. In communities with severe I/I, it’s not uncommon for peak wet-weather flows to exceed wastewater flows by 10, 20, even 30 times. That’s because I/I sources, such as deteriorated lateral connections and illegally connected downspouts, can dump more than 10 gpm into the sanitary system, dwarfing the 0.1 to 0.5 gpm average that you and your neighbors are collectively producing at any given time. While the I/I component will always be several times larger than the wastewater component in most SSO events, wastewater flows representing even a twentieth of a sewer’s capacity are significant enough that concerted conservation during wet weather could lower the risk of an SSO.
BOTTOM LINE. While there’s no reason to eliminate water use during storm events, if you live in a community that experiences SSOs, you could be doing your neighbors a favor if you postpone laundry, bathing and flushing.
Next time we’ll explore this question for communities with combined sewers.