Principles and Technical Application of Mixing Zones for Wastewater Discharges to Freshwater and Marine Environments
2. Effects of Wastewater Discharges
2.1. Effects on the Benthos and Water Column
- The characteristics of the effluent (flow, volume, and chemistry);
- The discharge regime (frequency and duration);
- The dispersion and dilution of the effluent after discharge;
- The physical, chemical, and biological characteristics of the receiving environment.
2.2. Effects on Human Health
3. Defining a Mixing Zone
3.1. Effluent Mixing
3.2. Information Requirements
- The quality and quantity of the effluent discharged;
- The health of the receiving environment before the effluent is mixed (for new discharges, these background characteristics are best determined prior to consenting);
- The proximity of the discharge to ‘sensitive’ receptors, including protected habitats and other natural resources and human uses of the environment;
- The hydrodynamics of the water body (including those within the mixing zone);
- The physical, chemical and biological interactions between the discharge and the receiving environment;
- The capacity of the receiving environment to assimilate the contaminants in the discharge.
4. International Regulations on Mixing Zones
- Best management practices for preventing or limiting harmful impacts to the environment should be applied;
- Best available technologies have been considered in the proposed discharge activity. A DMZ should not be used as an alternative to reasonable and practical treatment of effluent or effluent stream;
- Effluent discharge and water quality within the DMZ should not be acutely toxic to aquatic life;
- Contaminants of potential concern should not bioaccumulate to levels harmful to receptors as a result of conditions within a DMZ;
- Contaminants of potential concern should not accumulate to acutely toxic levels in the water or sediments of the DMZ;
- Conditions within a DMZ should not attract aquatic life or wildlife, causing increased exposure to contaminants of potential concern;
- Negative aesthetic qualities or other nuisance conditions in the receiving waters (e.g., odor, color, scum, oil, floating debris) should not occur as a result of the discharge and/or DMZ;
- Dominance of a nuisance species should not occur as a result of conditions within the DMZ that are due to the discharge;
- Use of a DMZ should not impair the integrity of the water body as a whole.
- A DMZ should be as small as possible to minimize the extent of the receiving environment potentially exposed to chronic toxicity levels;
- A DMZ should not adversely affect sensitive aquatic habitats (e.g., spawning, hatching, rearing areas for fish, overwintering habitats for fish or migratory waterfowl, areas used for aquaculture, etc.);
- A DMZ should maintain adequate zones of passage for migrating fish that do not deter the fish from passing through, do not affect their sense of orientation, and do not pose health risks to migrating species;
- A DMZ should not result in an adverse effect at the edge of the zone on designated water uses in the area (livestock watering and irrigation, drinking water and recreation, etc.);
- A DMZ should not be sited near drinking water intakes or food harvesting areas (e.g., shellfish beds or Indigenous Peoples’ traditional harvesting locations);
- A DMZ should consider setbacks from sensitive areas;
- A DMZ should avoid highly frequented recreational water use areas (e.g., public beach);
- At the edge of the DMZ, water quality should not result in short-term or long-term effects to aquatic life;
- DMZs for adjacent authorized effluent discharges should not overlap with each other;
- The effluent plume within the DMZ should not contact the shoreline of a water body in any manner that would prevent effective mixing and/or result in accumulation of contaminants of potential concern in the sediments;
- Diffusers used to discharge effluent into a DMZ should be designed to maximize mixing.
4.2. United States of America
- Mixing zones do not impair the designated use of the waterbody as a whole;
- Pollutant concentrations within the DMZ are not lethal to organisms passing through the zone (lethality is considered a function of the magnitude of a pollutant concentration and the duration an organism is exposed to that concentration);
- Pollutant concentrations within the mixing zone do not cause significant human health risks considering likely pathways of exposure;
- Mixing zones do not endanger critical areas such as breeding or spawning grounds, habitat for threatened or endangered species, areas with sensitive biota, shellfish beds, fisheries, drinking water intakes and sources, or recreational areas.
- Allocated Impact Zone: in effect, the same as a DMZ. The term has been more commonly used since the publication of guidance to determine environmentally acceptable size of mixing zones around point source discharges into freshwater and marine environments ;
- Legal Mixing Zone: the mixing zone in a regulatory sense, i.e., the dimensions of the mixing zone as the State authority defines them as opposed to the mixing that naturally occurs in a stream ;
- Toxic Dilution Zone: a sub-zone within the DMZ that attempts to limit the exposure of aquatic flora and fauna to toxic substances . Two regulatory criteria for toxic substances are recommended by the USEPA: a criterion of maximum concentration for protecting against acute or lethal effects and a criterion of continuous concentration for protecting against chronic effects ;
- Zone of Initial Dilution: regularly shaped area around the discharge structure that encompasses the regions of pollutant concentrations exceeding the relevant standard(s) under design conditions .
- Determine the need for zone;
- Establish the boundaries of the waterbody;
- Analyze current and future discharge data;
- Analyze ecosystem data;
- Develop environmental mapping;
- Assign relative values;
- Determine level of protection;
- Select mixing zone procedure;
- Allocate DMZ;
- Specify quality within DMZ.
4.4. European Union and United Kingdom
4.5. Aotearoa New Zealand
- “The production of conspicuous oil or grease films, scums or foams, or floatable or suspended materials;
- Any conspicuous change in color or visual clarity;
- Any objectionable odor;
- The rendering of freshwater unsuitable for consumption of farm animals;
- Any significant adverse effects on aquatic life.”
5. Case Studies on the Application of Mixing Zones to Different Types of Discharges
5.1. Effects of Municipal UV-Disinfected Effluent on Intertidal and Subtidal Benthic Communities
5.2. Effects of Municipal Secondary-Treated Effluent on Recreational Water Quality
5.3. Effects of an Industrial Discharge on Physical and Chemical Water Quality
6. Conclusions and Recommendations
- The purpose(s) for which the effluent is managed;
- The characteristics of the discharge, including types, concentrations, and volumes of contaminants;
- The location of the outfall and timeline for improvements;
- The characteristics of the receiving environment, including the available dilution and dispersal and the proximity of the discharge to areas of ecological, recreational, cultural or economic value;
- The proposed method of wastewater treatment and timeline for improvements;
- The need to keep the mixing zone as small as possible and to confine any ‘significant’ effects within the mixing zone boundaries.
- Determine the factors that determine incomplete mixing;
- Characterize the bio-transformations of persistent organic pollutants and bio-accumulative chemicals in environments receiving discharges;
- Develop a framework for cumulative effect assessments to contextualize discharge effects with those associated with other contaminant inputs;
- Determine the costs and benefits of DMZ versus alternative advanced treatment options for a range of discharges.
Institutional Review Board Statement
Informed Consent Statement
Conflicts of Interest
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|Type of Treatment||Short Outfall||Long Outfall|
|Primary (including septic tanks)||HIGH||LOW|
|Secondary plus disinfection||LOW||VERY LOW|
|Tertiary plus disinfection||LOW||VERY LOW|
|Type of Information||Description||Useful References|
|Mixing zone||Description of why the mixing zone is necessary||[36,37]|
|Characteristics of the effluent||Volume, flow rate, and discharge frequency|
Concentrations of contaminants
Contaminant concentrations in the effluent relative to those in the receiving environment
Evidence that discharge volume and quality have been optimized to mitigate any effects on the receiving environment
Discussion of any potential contaminant bioaccumulation and/or toxicological effects on marine organisms
|Outfall/diffuser||Geographical location and design (single/multi-port diffuser, depth in relation to water surface and bed of water body)|
Anticipated performance (best-/worst-case scenarios)
|Physical mixing of the effluent||Type of mixing zone model (water quality, particle tracking, hydrodynamic) and/or field dilution studies|
List of model input parameters
Model calibration/validation/sensitivity testing
List of data/metadata records
|Characteristics of the receiving environment||Type of waterbody|
Water temperature, salinity, dissolved oxygen conditions (ranges, seasonality)
Background water quality
Designated water uses and sensitive/protected habitats and resources
Relevant water quality criteria/limits/standards/goals
|Assessment of ecological effects of the discharge||Nature of the receiving environment, including presence of ecologically, culturally, or economically important species|
Species at risk
Seasonal changes in water quality or presence of migratory species
Exposure of aquatic species to contaminants in the discharge
|Assessment of human health effects of the discharge||Microbiological/chemical hazards |
Relevant health-based guidelines/targets
Recreational, fishery, aquaculture, or other uses (e.g., domestic, industrial, agricultural water supply)
Risk of illness from exposure to contaminated waters
|Waterbody depth||11 m|
|Discharge depth||11 m|
|Ambient current (from drogue studies)||0.18 m/s|
|Effluent density||1000 kg/m3|
|Seafloor roughness||0.025 Manning’s n|
|Discharge data (alternating staged diffuser)|
|Total number of ports||10|
|Distance between ports (same side)||4 m|
|Port diameter||0.25 m|
|Distance to start of diffuser||350 m|
|Distance to end of diffuser||368 m|
|Port height off bottom||0.5 m|
|Effluent flow rate||9000 m3/day|
|Receiving water density||1025 kg/m3|
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Campos, C.J.A.; Morrisey, D.J.; Barter, P. Principles and Technical Application of Mixing Zones for Wastewater Discharges to Freshwater and Marine Environments. Water 2022, 14, 1201. https://doi.org/10.3390/w14081201
Campos CJA, Morrisey DJ, Barter P. Principles and Technical Application of Mixing Zones for Wastewater Discharges to Freshwater and Marine Environments. Water. 2022; 14(8):1201. https://doi.org/10.3390/w14081201Chicago/Turabian Style
Campos, Carlos J. A., Donald J. Morrisey, and Paul Barter. 2022. "Principles and Technical Application of Mixing Zones for Wastewater Discharges to Freshwater and Marine Environments" Water 14, no. 8: 1201. https://doi.org/10.3390/w14081201