Contact: Mark Williams, General Manager
Las Gallinas Valley Sanitary District
Heavy rainstorm contributes to capacity problems at Las Gallinas Valley Sanitary District wastewater treatment plant
San Rafael (December 5, 2012) — The strongest rainstorm of the season contributed to capacity problems at the Las Gallinas Valley Wastewater Treatment Plant and caused a small sewage spill on Las Gallinas Avenue when a manhole overflowed into a nearby storm drain.
Rainfall at the Las Gallinas plant totaled 3 inches for the 24-hour period ending at midnight Dec. 2.
Sunday morning’s downpour caused a surcharge in one of the plant inflow channels, a situation where inflow exceeds the capacity, and resulted in an unauthorized bypass of 25,524 gallons of wastewater within the plant. The bypassed flow received preliminary treatment of screening and grit removal, and was then recombined with the remainder of the treated plant flow. The combined flow was disinfected prior to being discharged to Miller Creek via the District’s outfall.
A quick response by District staff minimized the bypass to a period of about 90 minutes. The District has spent millions of dollars in upgrades at the plant during the past seven years aimed at mitigating the effects of this type of storm on the plant’s hydraulic capacity. The District is completing an investigation into the cause of the bypass event and is taking steps to prevent a reoccurrence of this type of incident.
The Las Gallinas Avenue event occurred at 8:18 a.m. when heavy rains caused the sewer system to become overloaded. About 2,000 gallons of sewage overflowed from a manhole into a nearby storm drain, which empties to Santa Margarita Creek. Required signage has been posted and water sampling is being conducted.
The required regulatory notification procedures were completed the day of the events and the District is in the process of completing the water quality analysis to meet its regulatory requirements.