Diffuser Design and River Modeling
Christiano, Paul D.
Loudermilk, Daniel M.
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Permitted wastewater discharges from industrial facilities are required by state and federal environmental regulations to assimilate into the receiving stream within a reasonable distance downstream of the discharge. Diffusers are currently the most common device employed to achieve this assimilation. This paper will explore diffuser Point of Discharge (POD) selection, design methodology, the use of diffusers for various discharge scenarios. Ideally the diffuser should be designed to maximize near-field dynamic mixing under a variety of stream and production water flow rates. The physical configuration should be such that it can be constructed and installed in a timely and cost effective manner. Analytical modelling should be employed to assess the design's effectiveness in reaching adequate dilution within the mixing zone defined by the regulatory agencies. The physical characteristics of the diffuser manifold can vary greatly depending on the particular needs of the operator. A desirable diffuser will have a single or multiple ports sized such that the effluent discharge exits the port at a relatively high velocity during all expected flow rates. This high momentum of the discharge relative to the receiving water causes the discharge stream to entrain ambient water into the jet plume in the "near-field." The objective in the near-field is sufficient ambient water entrainment prior to the loss of jet momentum to prevent the persistence of slug flow and/or significant wastewater/ambient flow density differential in the "far-field." The horizontal and vertical angles of the ports as well as the configuration of the port itself are also critical components of design. The density of the effluent relative to the ambient water body is one of the most important determining factors in the configuration and orientation of the diffuser ports. In addition to the characteristics of the wastewater stream, the physical constraint imposed by both the river morphology and the regulatory constraints must be taken into account. These include river bathymetry, stream bottom structure (sand or rock), stream flow variability, allowable mixing zones, and instream toxicity limits. This paper presents a review of the analytical methods for assessing in-stream diffusion characteristics as well as design alternatives for accomplishing effective in-stream assimilation.