Determination of the Pressure Regimes in the Design of a Portable Faecal Slurry Evacuation System

Abstract

The evacuation of latrines and septic tanks often requires the design
of portable machines that can be moved to location to pump out the
faeces slurry. This paper explores the process of determining the
pressure regime in the vacuum suction system used for the
evacuation. These are the pressures in the vacuum chamber and
pump for the suction and blow flow of the faeces slurry. The suction
and blow pressures in the suction chamber were determined using
Herschel-Buckley model equation for non-Newtonian fluids and the
Bernoulli’s energy equation while the pressure in the water-ring
vacuum pump used gas flow equations. The pressures determined for
the 100l vacuum chamber for a 33.4mm (1¼ in) pipe were suction
pressure of 37,302 Pa vacuum and blow pressure of 187,756 Pa
gauge. The critical pressure for the continuum gas flow in the waterring pump using a 12.5 mm (½”) nozzle was 68,900 Pa but has to be
reduced to 37,302 Pa before the flow will seize