The purpose of influent pumping is to lift the incoming untreated sewage from the terminus of the several interceptor sewers up and into the headworks from where the sewage can flow by gravity through the other treatment processes. Approximately 97% of the influent sewage is pumped at the influent pump station; the remaining 3% is discharged directly into the headworks from the Calle La Cruz pump station in the Carmel Meadows subdivision.
Five major interceptor or trunk sewers (two Carmel trunk sewers, the Hatton Fields sewer, the Carmel Valley sewer and the Pebble Beach Interceptor) join together at a point on the northerly bank of the Carmel River directly across from the Treatment Facility Influent Pump Station. The sewage then flows through a 24-inch sewer line that runs under the Carmel River to the influent pump station. The influent pump station is a wet-well dry pit type pump installation with the incoming sewage sump (wet well) and pump room (dry pit) located below grade. Three influent sewage pumps are located in the station along with a sump pump for the dry pit, in an air compressor for the level sensing bubbler, ventilation fans, and a standby engine-generator. The influent sewage pumps are driven by 50 horsepower adjustable frequency drive electric motors which are controlled by the sewage level in the wet well. Each pump is rated at capacities ranging from 800 gpm (1.2 MGD) to 3,500 gpm (5.0 MGD) at speeds ranging from 575 to 1,200 RPM. The influent pump station is designed to pump at flow rates up to 8.0 MGD when two pumps are operating. The third pump serves as a standby and is activated when one of the duty pumps fails or when peak wet weather flows exceed the capacity of the two duty pumps operating at maximum speed. During power outages, a standby generator automatically provides backup power to all three influent pumps. There is always at least a 7-day supply of fuel onsite to power the standby generator.