Wastewater Lift Station Rehabilitation

This project provided a reliable code-compliant wastewater conveyance system from Muir Woods to the Tamalpais Community Service District located about a mile and a half away. The system pumps effluent several hundred feet above the location of the existing lift stations and discharges into the Sausalito-Marin City collection system for treatment at the Sausalito-Marin City Wastewater Treatment Plant. The project evaluated several alternatives for the rehabilitation of the two existing lift stations, consolidating them into one new replacement lift station, removing double pumping of the wastewater. Facilities to be connected included three public comfort stations, two employee/concessions private restrooms, and one permanent employee residence.

The major system component was the design of a new 20,000 gallon wet well with adjacent pump station housed inside a building outside the flood plain of Redwood Creek. New piping alignment minimized excavation and impacts to natural resources during construction. This was essential for the reach of sewer main from the new manhole adjacent to the lift station to the existing sewer main pipeline. The two replaced lift stations pumped wastewater that turned septic in a force main wit an approximate length 7,400-feet before dumping into the gravity system without the ability to drain back. The force main’s large volumetric capacity caused effluent to sit in the pipe between pumping cycles creating significant odor problems. The new lift station was designed to incorporate a gravity flow drain-back into the wet well after each pump cycle to produce self-cleansing scour velocities not achieved during the pump cycle, utilizing a coarse bubble aeration system inside the wet well to help reduce odors between pump cycles.

The project included the installation of a grinder system for the wastewater entering the wet well. The lift station facility was designed as a concrete substructure housed within a conventional wood frame structure for ease of construction. Two blowers were installed on the main level with oxygen sensors to keep the wet well aerated. The project also addressed the need to acquire a new power service drop from the electric utility company (Pacific Gas and Electric- PG&E) relocating the supervisory control and data acquisition from the existing Lift Station No. 2 into the new station. The structure was designed to blend in with other structures within Muir Woods, with provisions for an upcoming parking lot expansion project to incorporate a new comfort station adjacent to the new lift station with the source of water, power and wastewater disposal incorporated into project design. Additionally, the project provided traffic, landscaping and erosion control in a highly congested and natural resources sensitive area. Total cost of the project was approximately $2.75 million.

Services Provided by HECO Engineers:

• Civil engineering
• Title II Design
• Construction Documents
• Project management
• Condition assesment