10. Diffuser Hydraulics, Heat Loss, and Application to Vertical Spiral Diffuser

The ready availability of plume models allows analysts to predict plume behavior and performance of wastewater outfalls. Such models need inputs describing the flow rate of effluent; the number, diameter, and depth of discharge ports; and fluid properties of the effluent and receiving water and the water depth. Unfortunately, there is a crucial step in outfall design that is oftentimes neglected—hydraulic analysis of the outfall. Hydraulic analysis predicts the flows from each port of a diffuser and the hydraulic head needed to drive the diffuser for a required total effluent discharge rate. Hydraulic analysis is especially important to a relatively new class of wastewater outfall suitable for offshore oil and gas facilities, the vertical spiral diffuser. This chapter describes the rules of thumb for outfall design, the influence of the major design parameters on vertical diffuser behavior, the methodology for calculating the hydraulic performance of vertical diffusers, and an illustration of vertical spiral diffuser performance. The reconciliation of current regulatory practices in US federal waters with correct hydraulic analysis is also discussed.