Shallow water outfalls for brine disposal from desalination plants

Abstract

Submerged outfalls consisting of multiple, closely spaced jets are often used to discharge industrial effluents in coastal waterbodies. Examples of such effluents include heated water from thermal power plants, treated wastewater effluent from sewage treatment plants, and reject brine from desalination plants. At locations with shallow water depth, the interaction between adjacent jets is enhanced and can affect mixing. The mixing of submerged outfalls in shallow water is studied in this thesis with particular emphasis on discharge of dense treated brine from desalination plants. Treatment options for brine involve blending it with less saline effluents or its concentration, and can have significant effect on the design of outfall and its mixing. The effect of shallow water depth on dilution of submerged outfalls is determined first for quiescent conditions, and a unified theory is developed for single and multiple jets discharging in shallow water.

The effect of shallowness is shown to be characterized by a non-dimensional parameter, which depends on the receiving water depth and the effluent momentum and buoyancy fluxes. The effect of brine treatment processes, which affect both discharge momentum and buoyancy, on the dilution of various contaminants is determined next. The effect of brine treatment on outfall design is also explored, and optimum outfall design variables are calculated for a range of conditions. In the presence of a crossflow, the mixing dynamics of multiple port outfalls are quite different, and can give rise to complex jet interactions and even reversing flow close to the upstream jets. Laboratory experiments, in which discharge and ambient parameters are varied, have led to an improved empirical expression for dilution. In addition to the strength of crossflow, outfall length and spacing of jets are also found to significantly affect dilution.

A numerical model, capable of modeling the discharge of multiple jets in a crossflow, is developed and shows significant improvement over existing models.