May 2013, Vol. 5, No.25

Operator essentials

What every operator should know about sanitary sewer overflows

Jose Rodrigues and James Mcpherson

Click here to download a PDF of this article .  




Practical considerations  

Sanitary sewer overflows (SSOs) 

An SSO is any overflow, spill, release, discharge, or diversion of untreated or partially treated wastewater from a sanitary sewer system. SSOs often contain high levels of suspended solids, pathogenic organisms, toxic pollutants, nutrients, oil, and grease. SSOs pollute surface waters and groundwaters, threaten public health, adversely affect aquatic life, and impair the recreational use and aesthetic enjoyment of surface waters. Typical consequences of SSOs include closures of beaches and other recreational areas, inundated properties, and polluted rivers and streams. 

Check with your state’s environmental protection agency or water resource board for any updated requirements. 


Take pictures and compile an SSO report that includes a clear chronology and description of events and actions taken. 


This report is incorporated into the SSO event record for data capture and trend tracking. The report also should include a spill-volume estimation worksheet and any service requests or other field documentation. 

Ensure that all required documentation forms are available on all response vehicles. 

SSO estimation — measured volume method 

The volume of many small SSOs that have been contained can be estimated using this method. The shape, dimensions, and depth of the contained wastewater are needed. The shape and dimensions are used to calculate the area of the SSO, and the depth is used to calculate volume.  

Use geometric shapes (e.g., rectangles, circles, triangles, etc.) when determining the size of the spill. 











SSO estimation — duration and flow rate method 

Calculating the volume of larger SSOs, where it is difficult or impossible to measure the area and depth of the collected water, requires a different approach. In this method, separate estimates are made of the duration of the SSO and the flow rate. This method can be used only when an overflow is observed in progress.  

Agencies should develop handouts that show pictures of how different flow rates look pouring from the style of manhole lids in use in the area. 

SSO estimation — eyeball method 

To use this method, imagine the amount of water that would spill from a jug, bucket, or barrel. A jug contains 3.8 L (1 gal), a bucket contains 19 L (5 gal), and a barrel contains 209 L (55 gal). 


If the SSO is larger than 209 L (55 gal), try to break the standing water into barrels and then multiply by 209 L (55 gal). This method is useful for contained SSOs up to approximately 760 L (200 gal).  

Agencies should train employees on the visual difference in spills ranging between 3.8 and 209 L (1 and 55 gal). 

Secure SSOs from the public 

To secure an SSO area, install barricades, cones, or lighted barricades to keep the public out of the area. 


If the SSO enters surface waters, signs may have to be posted prohibiting the public from water contact. 


Cordon-off the area with caution tape. 

Advise any unauthorized people to shower or seek medical attention if they were exposed to any significant amount of wastewater. 


Containing an SSO may include but is not limited to the following actions: 

          Plug any catch-basin outlets, or use rubber mats to cover
   the catch-basin inlet.

          Use sandbags or other containment barriers. 

          Excavate to establish containment, if necessary. 

          Initiate containment in downstream storm drains, and plug the
   downstream storm drain outlet to capture SSO.

          Install air plugs on storm lines, whenever possible, to contain
   the SSO.

          Use a vacuum truck to assist in containment. 

Ensure that responders wear proper personal protective equipment. 


Ensure that all equipment and tools can be accessed easily by employees. 


SSO sites must be thoroughly cleaned after a spill so that no identifiable residue remains. Solids and debris must be collected and disposed of properly. 


Actions that should be taken may include the following: 

Apply absorbent material. 

Remove contaminated soil and used absorbent. 

Flush the SSO site with fresh water. 

Recover as much of the SSO as possible and return it
   to the sanitary sewer.


Dispose of all wash-down water into the sanitary sewer. 

Ensure that containment is not compromised during cleanup. 


Jose Rodrigues is a collections systems worker II, and James Mcpherson is the technical training coordinator at the Union Sanitary District in Union City, Calif.