Mixing Zone Analysis for Coastal Wastewater Discharge

A case study of the conceptual design and mixing zone analysis for an ocean outfall to discharge a dense wastewater is summarized. The recommended diffuser consists of upwardly inclined nozzles that discharge the effluent as high-velocity jets resulting in high dilution. Mixing by oceanic diffusion increases the dilution further as the wastefield is transported to the edge of the mixing zone. Local currents are highly variable, resulting in intermittent transport of the wastefield to any location. This is analyzed by computing the spatial variation of the “visitation frequency,” and the minimum and harmonic average dilutions of the wastefield. The visitation frequency decreases quickly and the average dilution increases rapidly with distance from the diffuser, indicating that any environmental effects of the discharge will be confined to a small area. Maximum and average values of chemical concentration at the mixing zone boundary were computed to assess compliance with acute and chronic water quality criteria. It was concluded that all requirements of the California Ocean Plan and other international standards would be met.