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Coastal dispersal of sewage effluent from the Tarumi Sewage Treatment Plant located in Osaka Bay, Japan, was investigated with a quadruple-nested high-resolution ROMS model coupled with a 3-D passive tracer model. The study area is quite famous for seaweed farming in the nation, which has been claimed considerably affected by the effluent from the plant, particularly in fall when the seaweed spores are most vulnerable to contamination that counteracts their favorable growth and productivity. We applied the model to three different discharge scenarios to explore possible optimal operations that could reduce the influences. If the discharged volume rate is simply decreased at ~16.7% relative to the baseline case that represents the actual operation, the effluent accumulating in the farm is reduced at ~25.4% showing a significant nonlinearity. A tracer flux analysis demonstrated that the transient component accounting for fluctuating tides and eddies dominates over the time-averaged linear contribution to the effluent accumulation in the cross-shore effluent transport to the farm. By contrast, in the along-shore direction, the linear component leads to increased influx near the surface at the western transect of the farm, while the transient component is more essential to the overall reduction of the incoming tracer flux to the farm. On the other hand, density adjustment of the effluent for suppressing surfacing of buoyant effluent plume released from the bottom-mounted diffusers does not work well as expected, because pronounced far-field dilution takes over the near-source dilution affected by the density adjustment.

A three-dimensional numerical assessment on the influences of treated sewage effluent on an adjacent seaweed farm with different discharge operations