The Motherlode Force Main (MLFM) and downstream gravity pipeline is a nine-mile conveyance system servicing the District’s wastewater customers in portions of Placerville, Diamond Springs, El Dorado, Shingle Springs, and Cameron Park.
The MLFM conveyance system begins at the El Dorado Lift Station (EDLS) in the town of El Dorado and consists of eight miles of force main that connects to one mile of gravity pipe prior to eventually discharging at the Deer Creek Wastewater Treatment Plant (DCWWTP). The original pipe construction was completed in the late 1970’s and consisted of 12-inch asbestos cement pipe. Pipe materials such as asbestos cement and reinforced concrete are susceptible to hydrogen sulfide (H2S) corrosion in a wastewater setting, especially in a long conveyance system. The MLFM has experienced numerous failures over the years and undergone multiple replacement projects, both in emergency situations and planned repairs. To date, the District has replaced approximately 50% of the force main with polyvinyl chloride (PVC) pipe.
The upstream EDLS has three pumps within the pump station wet well, consisting of two low horsepower (HP) pumps and one high HP pump. Normal operations utilize the two low HP pumps while the high HP pump is utilized during wet weather events that produce an increase in wet weather flow to the EDLS. The EDLS has a permitted emergency overflow pond adjacent to Slate Creek for temporary emergency storage during excessive rain events as well as an unpermitted pond to the west that is a legacy asset from pre-1970’s when the facility served as a standalone treatment plant. Utilization of the permitted emergency overflow pond triggers daily regulatory permit testing requirements.
Numerous failures of the MLFM have occurred in the past. The consequence of a pipe failure on the MLFM is a halt in raw wastewater conveyance from approximately 2700 equivalent dwelling units (EDUs) for an undetermined period of time. During peak storm flows of approximately 2.5 million gallons per day (MGD) or greater, interruptions in raw wastewater conveyance requires the use of the 2 million gallon (MG) emergency overflow storage pond and performance of regulatory testing. Additionally, to avoid or minimize the potential for facility physical capacity to be exhausted, the District has found it necessary during this period of reduced pumping capacity to haul wastewater to downstream locations within the collection system where sufficient capacity exists. The District has spent over a half million dollars on these necessary services within the past two winters due to extreme storm events. Although restoring the prior pumping capacity to EDLS will not eliminate the potential for hauling during future extreme events, it will mitigate any potential future need.
