NSW ocean outfalls: using near field models to investigate hydrodynamic performance

Abstract

Currently 35 licensed Sewage Treatment Plants discharge various grades of treated sewage to the ocean via shoreline, extended or deep submerged outfalls along the New South Wales coast. Outfalls range from single pipes located in beach foredunes to multi-port gas-burner type diffusers located 3–4km offshore at depths of 60–80m.The relative hydrodynamic performance of NSW coastal sewage discharges was examined using the United States Environmental Protection Agency’s hydrodynamic mixing zone model CORMIX (Cornell University’s expert mixing zone model). Examination of near field model predictions demonstrates that NSW ocean outfalls fall into three broad categories: 1) hydrodynamically efficient multi-port diffuser type outfalls, 2) moderately hydrodynamically efficient single port shoreline outfalls, and 3) hydrodynamically inefficient single port shoreline outfalls. From the results of this study, multi-port diffuser outfalls are the most efficient in dispersion of sewage-derived pollutants. In some situations, however, a well-designed shoreline or nearshore discharge can achieve moderate to high dilutions. Such outfalls are generally less expensive to construct, operate and maintain compared with multi-port diffuser type outfalls.