A First Order Guideline to Assess the Feasibility of Gravitating Sewage

Author(s): T.K Bowman, J. Petzer, R. Eardley
WISA 2016 Biennial Conference (Water Institute of South Africa), MAY 15-19, 2016


The sustainability of infrastructure is an increasing challenge as the operation and maintenance of these facilities becomes more expensive each year. This is especially true in municipal infrastructure designed and built for the conveyance of waste water. Electrical driven systems are also increasingly at risk with power supply constraints and there is an increasing need to reduce operational costs. With rising energy costs, and the resultant impact on operating costs, there can be cost advantages to gravitating sewage through tunnels between catchments, rather than pumping. The use of Tunnel Boring Machines, TBM’s, makes it feasible to tunnel through many different geotechnical conditions. TBM’s have been used successfully in South Africa for hard rock tunnels, as well as tunnels in sand and clay beneath sea level. 

This paper investigates the project lifecycle present worth of costs of pumping sewage between adjacent catchments or gravitating sewage via a tunnel for various flows, lengths and pumping heads. The results provide a first order tool to indicate whether tunnelling may be preferable to overland pumping.