Can Conserving Water on Rainy Days Prevent Sewer Overflows? Part 2 of 2

June 26, 2015—In Part 1, Vincent Bergl, P.E., discussed whether or not conserving water on rainy days could help prevent overflows in separated sewer systems. Here he explores combined sewer systems, which were primarily constructed in the early twentieth century when the goal was to convey both untreated sewage and stormwater to the nearest river or stream as quickly and cheaply as possible.

OVRFLW005_lowBy design, combined sewers are significantly larger than sanitary sewers, so in a well-functioning combined sewer system, pipeline capacity is rarely the primary constraint. Rather, it’s the capacity of downstream treatment facilities that’s easily exhausted. Most urban treatment plants were built prior to the Clean Water Act.… Read the rest

iStock_447677_Overflow_low

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.

WATER USE

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.

Can conserving water on rainy days prevent sewer overflows? Part 1 of 2

Joseph Cotton, PE, ENV SP, of RJN Group, Inc. (RJN), will co-host a technical breakout session at the upcoming Texas Water 2015 on Friday, April 17 at 9:30 am. In this session, “How to Clean and TV Siphons at One of the Busiest Intersections in Texas During Construction Season”, Mr. Cotton and his co-presenters, Jerome Iltis, PE, and Dennis Laskowski, PE, both of San Antonio Water System (SAWS), will highlight their applicable experiences, challenges, and successes with work conducted in February through April, 2014.

San Antonio’s River Walk and Downtown are premier tourist attractions. People from across the nation and the world go to San Antonio to visit the sites that make San Antonio a vacation hot spot. SAWS was faced with the logistical challenge of maintaining its sewer mains and the associated siphons in the busiest intersection in the Downtown area adjacent to the River Walk.

SAWS hired RJN to prepare construction specifications for this project, with the focus to catalog and assess the existing conditions of various single and multiple barrel siphons within the downtown area. Some of the siphons had been CIPP lined over a quarter of a century ago and most had not been cleaned, televised or assessed within the recent past. This project required the coordination of both pedestrian traffic and an increased volume of vehicle traffic around the local tourist landmarks. Another factor that impacted the project was area renovations by the San Antonio Convention Center. Terra Contracting Services from Kalamazoo, Michigan was the chosen by SAWS to complete this project.

SanAntonio Pictures

Photos from the San Antonio project.

 

 

Attendees of Texas Water 2015 are invited to listen how RJN, SAWS, and Terra Contracting Services worked seamlessly with each other and area stakeholders to complete the project with no major issues or concerns. Attendees will glean insights on how coordination and communication are pivotal to any successful project, especially those in a congested downtown area of a metropolitan city under construction.

RJN Group to Present at Texas Water 2015

Zach Matyja Awarded Young Leader Award

Zach Matyja, P.E., a Client Manager at RJN Group, Inc. (RJN), recently received the 2015 Young Leader Award from the Suburban Branch of the American Public Works Association (APWA) Chicago Metro Chapter. He is the current Past-President of the chapter, previously serving as Secretary in 2012 and as Vice President and President in 2013 and 2014, respectively.

Zach Matyja Receives Award

Zach Matyja (left) receiving the Young Leader award at the APWA Awards Luncheon.

The Young Leader Award is given to recognize and encourage young APWA members who have demonstrated an initial commitment to the profession and the association, and show potential for future growth within the association.… Read the rest