Background

Located in Riverside County, the Eastern Municipal Water District (EMWD) provides water, wastewater and recycled water to a 555-square mile service area in western Riverside County. As part of its sewer collections system, EMWD owns and operates the Day Street Lift Station (DSLS), located in the City of Moreno Valley. The DSLS has a pumping capacity of 3.4 million gallons per day (MGD) and was constructed in 1995. Due to high corrosive soils and sewer gasses, the pipe and fittings inside the DSLS wet well were showing signs of corrosion and nearing the end of their useful lives. In order to provide reliable services to its rate payers, EMWD determined that the replacement of the corroded piping and fittings was required.

Situation

In 2019, a constructability review was performed by LEE + RO to evaluate construction alternatives for the rehabilitation of the aging DSLS. Two alternatives were deemed viable; (1) bypass the existing lift station to repair elements in need of replacement, or (2) construct a new lift station. LEE + RO recommended bypassing the existing lift station to reduce overall construction costs and duration.

The constructability review determined elements of the rehabilitation should include replacing the corroded pump discharge piping and discharge elbows, repairing the wet well interior wall liner, and installing flexible joints on the discharge piping to isolate vibrations from pump operation. To implement the rehabilitation elements recommended, EMWD retained LEE + RO to provide the design of these improvements.

Challenges

The objective of this project was to provide improvements to critical components of the DSLS that were recommended in the constructability review. Performing the improvements and maintaining sewage collection services from the tributary area of the manhole was critical. Bypassing the DSLS without surcharging of the upstream sewer manholes and collection infrastructure, including sewer house connections, was one of the most challenging aspect of the project.

SolutionS

1. Bypass Pumping and Surcharging

In order to replace the corroded wet well discharge piping, it was necessary to bypass the existing wet well. In lieu of installing temporary bypass pumps at an upstream manhole, a permanent bypass manhole and permanent bypass wet wells were designed. To divert the incoming sewage to the new bypass wet wells, a weir structure with a removable stop gate was designed for the bypass manhole. During bypass operations, the outlet from the bypass manhole is plugged and the stop gate is removed to allow sewage to flow by gravity to the new bypass wet wells. In addition to preventing surcharging during bypass operations, the bypass manhole and wet wells provide additional operational flexibility, as they can also be utilized for emergency storage purposes if necessary.

2. Existing Wet Well Liner Condition

The existing wet well liner appeared to be in good condition. However, EMWD expressed interest in assessing the condition and repairing or, if necessary, replacing the liner. To accomplish this, it is necessary to perform bypass pumping for testing and repairs. The bypass manhole and wet wells facilitate safe working conditions within the existing wet well to perform the low voltage spark testing and repair of the existing liner.

3. Vibrations on Discharge Piping

During operation of the lift station pumps, EMWD staff observed noticeable levels of vibrations in the discharge piping. This is caused by the rigid connection of the pipes, which stiffens the pump discharge piping and propagates vibrations. Flexible connections were selected to connect the wet well discharge piping to the existing force main piping within the pump station building in order to mitigate vibrations from pump operation.