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Sanitary Sewer System

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Did you know that wet weather conditions frequently overwhelm the Town’s sewer system and caused more than 89 public sewer system overflows from December 2004 to January 2009? These occurrences result in raw sewage overflowing onto streets, down stormdrains and directly into the San Francisco Bay. In late 2009, the California Regional Water Quality Control Board issued the Town a Cease and Desist Order under which the Town is mandated to develop a comprehensive plan to eliminate public sewer system overflows.

The Town has carefully investigated the causes of its sewer system overflows, along with options to eliminate further occurrences. These investigations revealed the following:

  • Many of the Town’s aging sewer pipelines are defective and must be replaced.
  • The capacity of the sewer pipelines and systems must be increased wherever possible to accommodate greater volumes of sewer flow that occur during wet weather.
  • Many of the pipes that connect individual properties to the main sewer system are cracked, have been punctured by tree roots, and/or are otherwise degrading. These pipes, known as sewer laterals, also allow excess water to flow into the system during wet weather conditions, and raw sewage to leak out during dry weather conditions.  This raw sewage can contaminate the surrounding ground. Blockages can also cause sewer backups in homes.

Based on these findings and in order to maintain compliance with the Cease and Desist Order, the Town is implementing an initial five-year, $25.5 million program to repair and/or replace defective sewer pipelines, while at the same time encouraging and incentivizing the voluntary repair or replacement of faulty sewer laterals by private property owners.

The sewer repair and rehabilitation plan will address three major areas: Reducing Inflow and Infiltration, Preventing Excess Stormwater in the sanitary sewer system, and repairing sewer laterals.

Inflow and Infiltration
Sanitary sewer mains and laterals often become cracked or damaged when tree roots break into the line. Inflow is then created when groundwater enters the sewer system through the damaged pipes. During wet weather events, inflow and infiltration can increase sewer flows by 300 percent, leading to SSOs and unpermitted discharges at the Burlingame and San Mateo wastewater treatment plants. That is why it is important that public mains and private laterals be inspected and maintained.

Preventing Excess Storm Water in Sanitary Sewers
Storm water entering the sanitary sewer system can cause sanitary sewer overflows (SSO) during wet weather events. While Hillsborough is working with other jurisdictions and regulators to expand treatment capacity, Town officials have identified capacity problems in the sanitary sewer collection system such as insufficent flow capacity in the Crystal Springs/El Cerrito Sewer Trunk.  This has caused SSO's on 700 block of El Cerrito every winter.  In the past 12 years more than $19 million has been spent to repair and upgrade sewer mains and trunk lines. As a result of a Cease and Desist Order (R2-2009-0020) the Town has been ordered to complete more than $13 million in capital improvements over the next 5 years. 

As part of the Cease and Desist Order the  Town in currently in the process of video inspecting every public sewer main and studying the hydraulic model of the system to determine the areas with the worst storm water issues.  Unfortunately, the video inspections are revealing numerous defects within the 100 miles of sewer mains; some require immediate repair while others can be addressed through capital improvement projects over the next 10 years.  By November 2011, the Town will submit a 5 and 10 year capital improvement plan to the Regional Water Quality Control Board. This will be subsequent to the Town's first comprehensive evaluation to the entire system.

Video of Public Main*

*It should be noted there were no above ground signs this public main was damaged.  The logitudinal crack suggests further failure of ther main will occur in the future.  This video was made during dry weather.  During wet weather it is highly likely ground water infiltration is occurring from this crack and break.  There are no roots in this main because it was treated with an herbacide and cleaned prior to video inspection.

Longer Video

As with the other video, no above ground indication that sewer main was obstructed was known in advance of video inspection.  The root masses in this main cause some surcharging within the main.  Whereever roots are found in a public main or private lateral, in wet weather inflow of storm waters will also be found.   The amount of gravel in this main suggests that an upstream section of the pipe is broken, and allowing dirt/rocks to enter the main. 

Sewer Laterals

Another key factor in solving the problem is the maintenance and repair of sewer laterals, the lines that run from each property to the sewer main. When laterals become cracked or damaged by tree roots, groundwater seeps into the sewer lines during wet weather. The system becomes overloaded and leads to sewer overflows and unpermitted discharges into the San Francisco Bay. It can also lead to sewer backups in residences. The Town is proposing to inspect sewer laterals and assist property owners with repair or replacement, when needed.

Inside a Lateral*

*This short video shows the interior of lateral in Hillsborough that has been affected by root intrusion, and subsequently ground water infiltration.  Made in summer 2010, the video is stopped because of the break in the lateral pipe and ground water leaking into the sanitary sewer.  This ground water infiltration occurs 24 hours a day, every day of the year.  Again there was no sign above ground that this defect existed, and the resident's sewer lateral was not causing any backups at the home.

If residents are having their laterals serviced to clear roots on a regular basis, they should consider having their lateral repaired or replaced. Several technologies exist where laterals can be lined with a non-corrosive material or injected with foam to prevent root intrusion. Call the Public Works Department for more information about qualified plumbing contractors at 375-7444. 

Other Ways 
You Can Help

  • Avoid putting fats, oils or egg shells down the sink or garbage disposal. Grease is a major cause of sewer stoppages.
  • If you plant trees in your yard, avoid planting them near your sewer lateral. Root intrusion damages mains and laterals. Town crews can help you located the sewer lateral on your property.
  • Do not flush plastic materials of any kind down the toilet, including feminine hygiene products or wrappers. These can clog lines, cause backups and damage pumps.
  • Flush only toilet paper down the toilet. Paper towels and other paper materials do not degrade quickly and can cause backups.

For more information about the Town sewer system, visit Sewer FAQs: /about/faqs.asp

More information is also available at


The Town of Hillsborough 1600 Floribunda Avenue, Hillsborough, CA 94010 (650) 375-7400
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