Inverted siphons—the hardworking components of collection infrastructure that convey sewage or stormwater beneath streams, roadways, and other obstacles—can be challenging to access, clean, and maintain. For communities with inverted siphons that frequently fill with sediment, installing grit and gravel traps may be a wise solution for reducing the time and expense needed to keep stormwater and wastewater moving.
Most collection systems have inverted siphons that periodically require cleaning due to the buildup of sand, grit, gravel, and other materials, including fat, oil, and grease. A typical siphon cleaning operation uses a combination sewer cleaning truck positioned at the downstream outlet structure and hydro flushing passes that convey the sediment to the outlet structure where it is vacuumed out of the system. Multiple passes with the hydro flushing nozzle are required as large gravel is difficult to move upwards through the upleg of the siphon.
How often an inverted siphon needs to be cleaned is site-specific and can ranges from monthly to twice a year. Cleaning duration also varies by site—some take up to a full day to clean. To avoid this maintenance headache and save money, a grit and gravel trap can be an effective addition to a collection system. A grit trap is a simple, yet innovative structure—essentially an oversized manhole with the bottom dropped—that captures grit and gravel upstream of an inverted siphon. Grit traps are easier to clean than a large siphon, requiring only a few hours to clear out.
Two communities in California’s San Francisco Bay Area that have installed grit traps in their collections systems are examples of the benefits of this feature. The Oro Loma Sanitary District (OLSD) is located about 13 miles south of Oakland and 30 miles north of San Jose on the eastern shore of the Bay. OLSD collects sewage from approximately 46,000 households and 1,000 businesses. The district has a poorly designed inverted siphon in its system that led to grit and gravel buildup. The siphon is also located in a difficult to access area, making it very time consuming to clean and regularly maintain. To catch sediment before it reaches the problematic siphon, OLSD installed a grit and gravel trap upstream approximately 20 years ago. The district cleans the grit trap every six months.
The City of Santa Clara, California is another community that has installed grit traps to avoid clogs in its collection system’s flow. Located in Silicon Valley, Santa Clara’s water and sewer utility serves approximately 130,000 residents. In 2011, when installing a new inverted siphon in its collection system, the city also installed a new grit trap. A second grit trap was installed in 2015 upstream of an existing inverted siphon. The grit traps are inspected every six months and emptied yearly.
In both collection systems, the addition of grit traps has proved successful in keeping the inverted siphons clear of large debris, easing the maintenance for operations teams and reducing the potential for sanitary sewer overflows due to a clogged siphon. Neither agency has experienced odor issues since the grit traps were installed.
If your agency’s collection system has difficult to clean inverted siphons that frequently fill with sediment, incorporating grit/gravel traps is a solution that saves time and expenses in maintaining wastewater or stormwater flow.