ABSTRACT
A typical marine wastewater system, which includes a treatment plant and an outfall, is shown in Figure 17.1. The outfall is a pipeline or tunnel, or combination of the two, which terminates in a diffuser whose purpose is to efficiently mix the effluent in the receiving water. Outfalls generally discharge onto coastal shelves with a gradual slope, typically less than 1%, so the horizontal flow extents are much greater than vertical ones. Most outfalls range from 1 to 5 km long and discharge into waters 20-70 m deep. Some may lie outside these ranges, for example shorter outfalls when the seabed slope is very steep and deep water is readily available, or longer when the slope is very gradual. The overall disposal system can be thought of as the treatment plant, outfall, diffuser, and also the region round the diffuser (known as the near field) where rapid mixing and dilution occurs.
