The original Yosemite West WWTP facility was constructed in the 1960s to support the needs of a small residential community in the high elevation of Yosemite Valley. The facility was upgraded to some degree in the 1980s and again in the 1990s. The drinking water supply is of high, pristine mountain quality, low in minerals, hardness, and alkalinity. Ironically, the purity of the drinking water supply has a significant detrimental impact on the biological treatment of wastewater.
The current facility was designed to treat about 0.1 million gallons per day (MGD) or approximately 105,000 gallons per day (gpd). The build out capacity of the WWTP service area is 96,400 gpd. The Central Valley Regional Water Quality Control Board issued a notice of Violation to Mariposa County (County) on August 30, 2012. The violation was related to surfacing of effluent from the leach fields. In response, the County evaluated options for mitigating the violation. Specifically, the County investigated the causes for effluent surfacing and considered increasing the number of leach fields to increase the plant capacity and reduce the over impact of the existing leach fields.
We prepared a report summarizing the analysis for Yosemite West Wastewater Treatment Plant (WWTP) capacity with the addition of potential leach fields in the area surrounding the plant. We reviewed prior documentation that identified and analyzed the three alternate leach field areas. Our analysis showed that using a slightly less conservative disposal application rate (median test result value) the potential to reach the build out capacity of 96,400 gpd was feasible. While using a conservative disposal application rate for design is an industry standard, making a financial decision such as a property purchase based on conservative design practices could result in a large portion of property that is not used.
Scope of services:
- Project management
- Develop Process Diagram for the WWTP
- Hydraulic analysis including capacity calculations
- Hydraulic Analysis Results Report preparation