Emptying sewage from the valve-pit sump through service lateral to vacuum main

Abstract

A simple hydraulic model of liquid and gas flow from a collecting sump via vacuum service lateral to a vacuum main has been presented. The model was formulated and validated on experimental data and CFD simulations. The standard gas (air) to liquid (sewage) volumetric ratio is roughly equal to the ratio of subsequent admittance times of these two phases, provided that the liquid plug is not fully sucked from the service lateral to the vacuum main during the valve open time. A longer air suction time is needed only when the service lateral is too short to provide enough air to transport the sewage past the nearest downstream lift on the vacuum main. Sizing properly the active sump volume and valve open time for a given service lateral length, one can provide the required air to liquid ratios along the vacuum main, thus minimizing the energy consumption by the vacuum pumps.