Hydraulic modelling of purging in sea outfall

Abstract

The presence of seawater in a sea outfall can reduce the effluent discharge from a land-based treatment plant or domestic and industrial areas. In the longterm, it can damage the sea outfall. To avoid the presence of seawater and to anticipate a varying effluent discharge, a minimum effluent discharge is needed to purge the outfall system. -- The objective of this study was to investigate various ways of decreasing the minimum effluent discharge needed to purge a sea outfall. The effect of port size and an increase in mixing between saltwater and effluent are considered in this study. In tests on the effect of port size, three different types of tests were undertaken. There included risers without caps, risers with caps having one port of diameter 2.54 cm (one inch), and risers with caps having two ports of diameter 2.54 cm (one inch). Attempts were made to increase mixing in the vertical risers using small water jets located below the risers, water jets located upstream of the risers, air jets located below the risers and a barrier located at the top of tunnel upstream the risers. -- The maximum reduction in the purging discharge was obtained using water jets (1.001/min) located below all risers. The purging discharge using these jets was only 48% of that without water jets. This is a significant reduction.