Mixing Zone Evaluations

Mixing zones, limited areas or volumes of water allowed for initial effluent dilution, are often employed in discharge situations where effluent quality does not meet surface water quality standards they are typically developed for effluent parameters such as metals and toxic compounds when determining water quality based effluent limits. Ambient concentrations of water quality constituents may exceed acute or chronic water quality standards in a mixing zone. A typical mixing zone is comprised of two parts: an acute and a chronic zone. The acute mixing zone is the area of initial dilution, sometimes referred to as the Zone of Initial Dilution (ZID). Acute criteria can be viloated within the zone but must be met at it's edge. The zone is sized for quick mixing and preventing lethality to passing organisms. Beyond the acute mixing zone and of larger area is the chronic mixing zone where, at the edge of this zone, chronic criteria must be met. Both mixing zones typically have maximum size and location restrictions and are sized to minimize impact upon the environment.

Estimating initial dilution can either be accomplished through mathematical modeling (initial dilution models) or through dye studies, where the latter is only valid for existing discharges. HydroQual, Inc. has extensive experience with near-field modeling programs supported by the U.S. Environmental Protection Agency such as PLUMES and CORMIX. In addition, HydroQual has employed various other modeling approaches to analyze initial dilution for mixing zones based on steady state and time-variable, far-field models and coupling with hydrodynamic, sediment transport, and receiving water quality evaluations. HydroQual has completed numerous mixing zone studies around the United States and overseas for single- and multi-port submerged diffusers and surface (bank) discharges; assessing dilution and developing mixing zones for existing and proposed discharges. These studies have been completed for conventional and toxic substances and also for thermal discharges. In many studies, HydroQual has also been involved in the design of proposed diffusers that included iterations between dilution and diffuser hydraulics for determining additional pumping requirements at treatment facilities.

Mixing Zone/Initial Dilution Analyses

  • Design and implementation of field survey programs,
  • Data analysis and interpretation,
  • Mixing zone determination (acute and chronic),
  • Studies in tidal and non-tidal, stratified and non-stratified, fresh and marine waterbodies,
  • Evaluation of long-term pollutant build-up for tidal waterbodies,
  • Coupled near-field and far-field modeling,
  • Thermal discharge analyses,
  • Support for NPDES permitting,
  • Regulatory assistance, and
  • Biotic Ligand Model (BLM) and/or metal translator studies.

Engineering Solutions

  • Siting and design of diffuser structures,
  • Diffuser design (number of ports, orientation, alignment, etc.), and
  • Hydraulic analysis for head loss and pumping requirements.

Initial Effluent Dilution

Initial dilution is the process of forced entrainment of ambient water into a discharge plume through both jet- (momentum) and buoyancy-induced turbulent and shear processes. As the discharge plume propagates into the ambient environment it loses kinetic energy and entrains water that dilutes the discharge. Through the entrainment process, the plume density approaches the ambient density (neutral buoyancy) and depending on the location at which this occurs, the plume can either reach the surface or become trapped at some intermediate level. The spatial extent of this mixing region is sometimes referred to as the "zone of initial dilution" (ZID) and is where a majority of initial dilution is achieved. After the zone of initial dilution, ambient mixing processes tend to control further dilution of the plume and additional dilution is usually minimal.

Plume dilution can be controlled to some extent within the two stages with the first stage primarily controlled by the diffuser design and the second stage controlled more through outfall siting. The first stage is dominated by plume jet (momentum) and buoyancy forces and can be controlled by adjusting various diffuser characteristics such as port diameter, spacing, number of ports, and port orientation. A majority of the initial dilution is achieved in this first stage and therefore proper design of the diffuser characteristics will optimize the initial dilution. Ambient mixing processes dominate the second stage and therefore diffuser design has little impact on the additional dilution in this stage. Practical considerations can be weighed to increase dilution in the second stage such as outfall siting in an area of high mixing and maximum water depth. In general, shallow water depths, sharp vertical stratification, low current velocity conditions, and high background concentrations inhibit dilution in receiving waters.