MODELING SEWAGE SOLIDS DEPOSITION PATTERNS FOR THE FIVE FINGERS ISLAND OUTFALL, NANAIMO, BRITISH COLUMBIA

A three-dimensional particle settling model has been used to predict the benthic exposure zone to sewage solids for a large marine outfall and diffuser serving Nanaimo, Canada, in an area with large variations in bathymetry. The model incorporates a range of settling velocities and a realistic treatment of tidal currents, density stratification and effluent plume trapping. Settling tests for the primary treated effluent showed that about 25% of the solids were removed in 6 hours through a process of flocculation, accompanied by discrete settling at a range of slower settling rates. Based on these tests and literature values, two fractions were used for modeling: a floc with a settling velocity of 100 m/day and a non-floc portion with a settling rate of 8.6 m/day. The predicted exposure zone was determined largely by horizontal advection, and to a lesser extent by the relative amounts of floc and non-floc solids settling at the two different rates. The results were used t0 design a two-tiered sediment sampling scheme for environmental monitoring.