Frequently Asked questions
1) How should member states deal with metal-concentrations? Most effluent-data are expressed as total concentration, while the EQS is expressed as dissolved concentration
Metal concentrations in effluent data are typically reported as total values To assess the dispersion of these concentrations, a precautionary approach is advised, treating total emission data as if it were dissolved for Tiers 1 and 2 When environmental quality standards (EQS) are defined as dissolved values, comparisons should align with these standards, assuming complete partitioning in the dissolved phase However, if evidence suggests different partitioning behaviors, this can be considered in Tier 3 or, with the Competent Authority's agreement, in Tiers 1 or 2.
2) What should Member States do if there is already an exceedence of the EQS in the receiving water body?
The issue at hand pertains to consent policies rather than mixing zones, and it is essential to address it through the River Basin Management Planning process Specifically, if the water quality upstream surpasses the Environmental Quality Standards (EQS) just before the discharge point, a comprehensive review of all permits upstream may be necessary.
3) How should Member States deal with discontinuous or intermittent discharges?
When evaluating discharges, it is crucial to analyze relevant statistics thoroughly, determining whether to include the entire time period or only specific discharge events Ensuring that discharges posing a risk to the water body's overall status are not prematurely excluded is essential If Tier 2 assessments prove insufficient due to lingering uncertainties, advancing to Tier 3 allows for a comprehensive examination of the interrelationship between effluent flow, quality, and the corresponding flow and quality of receiving waters and potential receptors.
4) Is there a possibility to incorporate (bio) degradation of a substance in the model?
Biodegradation can be considered at Tier 3, and for rapidly decaying substances with established loss rates, it's feasible to integrate this loss mechanism into Tier 1 and Tier 2 assessments at the discretion of Competent Authorities In Tier 2 models like CORMIX, including such losses is relatively straightforward However, Tier 1 assessments typically focus on dilution rather than timescale Nevertheless, if the loss is solely due to chemical reactions and can be effectively characterized by mixing, an adjustment based on the well-mixed concentration may be applied at the discretion of the Competent Authority.
5) What should we do in the case of PBT-substances?
A crucial factor for a substance's inclusion on the priority list is its persistent, bio-accumulative, and toxic (PBT) properties This guidance document aims to assist the Competent Authority in evaluating all pertinent information prior to making a decision It emphasizes the importance of toxicological data sets used to establish standards, ensuring that the most relevant data is taken into account and that all potentially affected sensitive receptors are adequately protected.
6) Which variables can be set to a standard value? Not all input data are readily available
Relying solely on default or standard values can be risky, as it may not address the variability present in different cases The tiered approach aims to filter out trivial discharges early on; however, the quality of the output is fundamentally linked to the quality of the input data In situations where data is insufficient, one alternative is to determine the probable maximum and minimum values for the relevant parameter and assess the impact of the difference between these values This allows for a better understanding of the importance of accurately identifying the correct value.
7) How should we deal with multiple mixing zones?
Chapter 12 of the Guidance document on Mixing Zones addresses the impact of multiple substances in a single discharge The stringency of regulations is influenced by the ratios of [CoC] effluent to EQS, as all substances experience the same level of dilution While this principle applies at a specific moment, it is essential to consider the varying concentrations of multiple substances over time and the changing dilution in the receiving body.
Approach for dealing with natural background concentrations
Tier 0
At tier 0, the assessment begins by identifying whether the effluent from any water body contains a contaminant of concern If such a contaminant is present, the concentration is then compared against the Environmental Quality Standards (EQS) for that specific contaminant Discharges that do not exceed the EQS for any contaminant of concern are excluded from further consideration, as they do not pose a risk of exceeding the EQS in the water body.
Tier 1
At tier 1 all discharges where a contaminant of concern is present in a concentration above the EQS are checked if the discharge might have a significant impact on the receiving water body.
In Tier 1a, a significance criterion is utilized to identify non-significant discharges, defined as the allowable percentage increase in concentration after complete mixing resulting from the discharge, relative to the Environmental Quality Standards (EQS).
In order to check if this criterion is fulfilled one has to calculate the so-called process contribution (PC) This is defined as:
( CoC Q river xQ Q eff ) PC eff eff +
In the next step, the PC as a percentage of the EQS needs to be checked with the proposed allowable increase. increase relative
If the increase falls below the significance criterion outlined in Table 8.0 of the report, the discharge will be deselected, provided there are no sensitivities nearby the discharge point.
In the text below an underpinning of the proposed allowable increase is given.
To ensure consistency, the selected criterion must guarantee that any discharges eliminated in Tier 1 would have satisfied the criteria for Tier 2 We have tested the threshold values using the Discharge Test across various water types The mixing characteristics are influenced by factors such as the flow rate, dimensions, and bottom roughness of the water body.
For the assessments the following starting point were used:
• Calculations used an assumed upstream concentration of 0,5*EQS 2 ;
• In the calculated example an EQS concentration of 3 ug/l and a MAC concentration of 9 ug/l was assumed;
• For fresh waters the assessment used Q90 net flows 3 for the water body;
Discharges must comply with the criteria of the MAC mixing zone, which is set at 0.25 times the width of the water body (with a maximum of 25 meters), and the EQS mixing zone criteria, established at 10 times the width of the water body (with a maximum of 1000 meters) during the discharge test.
In a water body or basin, multiple dischargers can be found, leading to higher upstream concentrations for those located downstream For instance, the Rhine River has over 100 significant discharges Additionally, even discharges classified as TIER 1 can still impact upstream concentration levels.
3 The flow which is exceeded during 90% of the time in a year
Tier 1a: Discharge to inland surface waters (River)
Take appropriate action or proceed to Tier 2
Water Quality No unacceptably impacted?
[CoC] ef /EQS Determine dilution factor
The EQS assessment for tidal waters utilized Q90 net flows for the water body, along with the average tidal flow associated with Q50 net flow Both net flow and tidal flow were integral to calculating accumulation, while tidal flow specifically facilitated plume mixing analysis.
• For tidal waters the MAC assessment used Q90 tidal flow 4 (for calculating plume mixing).
Table 1 presents the assessment results for freshwater bodies, including rivers and canals The study explores two scenarios: one with a low effluent flow (0.25 times the river flow per hour) paired with a high effluent concentration of 5, and another with a high effluent flow (5 times the river flow per hour) combined with a low effluent concentration of 6 Adjusting either the effluent flow or concentration impacts the overall discharge load.
Calculations indicate that low-effluent-flow scenarios are crucial, with the MAC mixing zone frequently serving as the limiting factor Consequently, the impact of upstream concentrations diminishes in comparison to scenarios where the EQS mixing zone is the constraint, resulting in a lower ratio of upstream to MAC concentrations.
In scenarios with low effluent concentrations, the Environmental Quality Standards (EQS) can be a limiting factor; however, the most stringent conditions arise in high-effluent concentration scenarios Consequently, the results from this high-effluent scenario are detailed in Table 1.
Table 1 illustrates the permissible concentration increases after complete mixing for various freshwater types, revealing that large rivers and canals permit the smallest percentage increases, with only 1.2% for very large rivers and 1.3% for very large canals To meet the Water Framework Directive (WFD) objectives at the scale of water bodies or basins, it is essential to acknowledge that larger water bodies typically have a significantly higher number of discharges compared to smaller ones.
4 The flow which is exceeded during 90% of the tidal period
5 The maximum effluent concentration where mixing zone criteria can be met, using an effluent flow of 0,25*flow-river/3600 as a starting point
6 The effluent concentration where mixing zone criteria can be met, using an effluent flow of 5*flow-river/3600 as a starting point
For tidal water a similar assessment has been carried out, with the results presented in Table 2.
Table 2 Allowable increase of concentration after complete mixing for different tidal rivers
Table 2 indicates that the permissible concentration increase after complete mixing in tidal waters is generally lower than that in freshwater This difference is attributed to the accumulation effects caused by tidal movements.
The permissible increase in water quality for both tidal and freshwater rivers is linked to the net flow of the water body Table 3 illustrates a method for determining the allowable increase following complete mixing in rivers and canals.
A distinct strategy for managing canals and rivers has been established due to the significant differences in flow ranges between the two Furthermore, the projected increase in concentration for canals at flow rates up to 100 m³/s varies from the calculated outcomes for rivers.
Table 3 Proposed allowable increase in concentration after complete mixing for different water types, which can meet criteria for MAC- and EQS mixing zone.
Proposed allowable increase in concentration after complete mixing as % EQS 1 ) 2 ) 3 )
Fresh water rivers and tidal rivers
2 ) if increase in concentration after complete mixing exceeds the percentage taken up in Table 3 further assessment in Tier 2 or further is necessary.
Tier 1 serves as the initial filter in the assessment process, distinguishing between non-significant discharges that consistently meet Tier 2 discharge test criteria and other discharges While Tier 1 criteria may not eliminate discharges outright, a subsequent evaluation in Tier 2 could reveal that certain discharges fail to meet the necessary standards Therefore, adopting a worst-case approach is deemed appropriate for accurate assessment.
Table 3 outlines the discharge test criteria using the 'existing discharges' option (Tier 2), applying lower results for tidal rivers when assessing large freshwater rivers The calculated value for the upper flow-range serves as the allowable concentration increase after complete mixing across the entire flow range A high concentration scenario is considered for discharges, leading to stringent MAC mixing zone criteria compared to EQS mixing zone criteria This approach represents a worst-case scenario In new discharge situations where WFD criteria are unmet at the water body or basin level due to point source discharges, selecting the "reduced discharges" option in the Discharge Test can help minimize the EQS mixing zone size Consequently, the spreadsheet acts as a control tool to achieve WFD objectives, with the understanding that criteria derived from Tier 1 will also be affected, allowing for the consideration of smaller discharges in identifying mixing zones.
Tier 2 Assessment of mixing zone
At Tier 2, it is essential to estimate the dimensions of the mixing zone to determine the acceptability of a discharge This involves a straightforward calculation to assess the size of the mixing zone This paragraph outlines the methods used for evaluating the mixing zone in Tier 2 assessments.
Effectively managing harbours requires careful consideration, as the optimal approach may not be obvious at first The size of harbours can help define the boundaries of mixing zones, particularly for heat discharges However, it is crucial to prevent acute toxic effects in these areas, making the dimensions of the MAC-mixing zone significant in the management process.
An updated discharge test, originally distributed in December 2009, will be accessible online in spring 2010 This new version will allow for the calculation of mixing zones in both tidal and freshwater harbors, as well as in coastal waters.
A straightforward method for calculating dilution based on the distance from the discharge point can be established using Fischer equations A key consideration is that the effluent flow should be minimal compared to the flow of the water body.
At distances short enough from the discharge point (X ≤ L), the concentration at the riverbank (y=0) is solely influenced by the condition n=0, while contributions from other values of n diminish to nearly zero Consequently, the equation simplifies to u.
W = emitted load of a substance (g/s); a = depth of the receiving surface water [m]; u = flow rate of the surface water (m/s);
B = width of the receiving surface water [m];
Ky = transversal dispersion coefficient in y direction
X = distance to the point of discharge (maximum L) [m]; ϕ(x,y) = concentration at a distance x from the point of discharge and distance y from the bank of the receiving surface water. With: 10 6 , 0 max( ua
Cchezy is calculated using the formula Cchezy = 18 * log((12 * B * a) / (B + 2 * a) / k), where k represents the bottom roughness In the Netherlands, a bottom roughness value of 0.05 is applied for rivers and 0.1 for canals and lakes Additionally, the discharge test includes an option to calculate bottom roughness for various water types.
With W= Q*ϕ0 where ϕ0 is the concentration at the discharge point (x=0) and Q the flow of the emitted effluent the equation changes in: x u K a x Q y ⋅ ⋅ ⋅ ⋅ = ⋅ π ϕ ( , 0 ) ϕ 0 [3]
The dilution factor is given by ϕ0/ϕ(x,0): Q x u K a
2 0 [4] The increase in concentration ∆C at distance L, as a consequence of the discharge is given by ∆CL = C effluent / M x 2 D − plume
In Figure 1 an example of the dilution factor, the distribution pattern and concentration as a function of the distance to the point discharge, derived from the Fischer equations, is shown.
Fig 1 Dilution, distribution pattern and concentration as a function of the distance to the discharge point.
Understanding the mixing process near discharge points is crucial for assessing potential acute toxic levels in water systems Various models, including Cormix, non-hydrostatic 3D models, and simpler methods based on Fischer equations, can effectively describe this mixing Below, we outline a straightforward approach utilizing Fischer equations.
For (initial) mixing adjacent to the point of discharge jet-mixing or plume-mixing may apply:
Typical values for Ky are 0.01-0.015 (canals/lakes) and 0.01-0.05 (rivers) Typical values used for Kz are 0.001-0.01.
Distribution pattern max concentration in m ixing zone as a function of x
0 500 1000 distance in m con cen trat ion Ce-100%
At a specific distance downstream from the discharge point, the complex three-dimensional equations governing jets or plumes can be simplified to two-dimensional plume equations for mixing purposes This transition from 3D to 2D plume mixing occurs at a defined distance, enhancing the analysis of plume behavior in environmental studies.
with w0 = the maximum velocity in the effluent stream [8]
It should be noted that when the velocity in the effluent stream increases the distance x (where jet-mixing dominates the mixing pattern), increases as well
In scenarios where jet mixing is prevalent near the discharge point, it is essential to adjust the distance x with a correction factor 'xmove' to accurately represent 2D-plume mixing behavior, as outlined in equation [4].
The distance ‘xmove”is given by: jet y
The initial mixing of effluent and surface water is influenced by several key factors, including the flow rate of the effluent stream and the surface water, as well as the horizontal and vertical positions of the discharge point Specifically, the horizontal location can be at the water body's bank or a specified distance away, while the vertical position may vary from the surface to mid-depth or the bottom of the water body.
Bed roughness significantly influences mixing processes, and while detailed data is essential for accurately assessing complex 3D models, general coefficients based on overall bottom characteristics are often sufficient for most applications.
In general the dilution factor at the maximum length (L) of the mixing zone is given by equation [4].
In scenarios where the receiving water body features a narrow channel or experiences low flow conditions, there is a significant risk that the mixing zone may encompass a substantial portion of the cross-section.
2 section which can have adverse consequences for the passage of migratory species.
To mitigate issues related to mixing zones in water bodies, regulations often restrict their width For instance, in the Netherlands, the mixing zone is capped at 25% of the water body's cross-section Additionally, discharge test criteria are established to ensure that mixing zone standards are achievable in flowing water bodies at specified distances.
L, the cross section taken by in the mixing zone, bounded by EQS, (in general) will not be greater than 25%
When assessing the mixing zone for discharges, it is crucial to consider nearby shellfisheries, drinking water sources, and other ecologically significant areas The proximity of these features to the discharge point plays a vital role, particularly if the distance is less than L or ten times the width of the water body.
The methodology for establishing mixing zones in tidal environments, such as riverine estuaries, has evolved from techniques applied in freshwater systems Tidal waters present unique challenges due to their less clearly defined boundaries and variable flow directions influenced by tidal changes Consequently, the location of the mixing zone can shift, being positioned downstream during the ebb tide and upstream during the flood tide.
Figure 3 Mixing zones in tidal flowing waters
Fig 17.2 Migration: serious problem migration may be affected migration: no problem
The extent of exceedence of concentration with potential to impact on migration success is shown by the striped sections in Figure 2 above
Tier 3 Complex asssessment
When simple assessment of the mixing zone results in an exceedence of EQS at checkpoint L, two options are possible:
• Take action to reduce the size of the mixing zone;
• Assess the mixing zone using complex models;
Various mathematical models, ranging from intricate 3-dimensional and empirical models like CORMIX to simpler 2-dimensional methods based on Fisher equations, are utilized to assess the dilution of discharged effluent relative to the distance from the discharge point The selection of the appropriate model hinges on the specific circumstances at hand Notably, complex 3D models necessitate comprehensive input data to accurately represent the situation, including factors such as bed topography, tidal influences, net river flow, and tributary interactions For modeling intricate scenarios, extensive data collection is essential, often requiring detailed information to be organized into a grid of small area units for precise calculations.
The dimensions of the model area are crucial, as emissions must have a negligible impact at its boundaries Typically, the region near the discharge point is modeled in detail using a fine grid, while areas further away can be represented more generally If the discharge's influence remains significant at the model's boundary, multiple models may be required, increasing both computational time and costs These principles apply to modeling mixing zones across various water types, including rivers, tidal rivers, lakes, and estuaries.
When dealing with tidal waters, it is crucial to consider the density differences caused by variations in salinity and temperature between the discharged water and the receiving surface water Additionally, the effects of tidal movements must be factored in, as these elements can significantly influence the distribution patterns of the water.
When considering discharges at harbours, it is essential to account for other discharges, as they can impact the streaming velocity within the harbour Additionally, ship movements play a crucial role in influencing the size and location of mixing zones in these areas.
Tier 4
For the purposes of this guidance ‘Investigative Studies’ can include a wide range of activities including: a) Chemical concentrations (of PS/PHS or other determinands under consideration)
Bathymetric data, sediment characteristics, water velocity, and water level are essential for the setup, calibration, and validation of modeling efforts Additionally, receptor characterization should emphasize the biological aspects of receiving waters, including the bed, banks, and water column biology, as these factors fluctuate over time within the projected impact zone of the discharge and throughout the broader water body Furthermore, assessing the impairment of receptors is crucial, focusing on the extent of biological changes linked to discharge operations.
A bed topography card for a tidal river is essential for comparing biological zones affected by discharge with control zones, which may either be the same area before the discharge event or a separate, valid control location Additionally, conducting literature reviews or new laboratory-based ecotoxicity studies is crucial for identifying specific important receptors when applicable or useful proxy data is not readily available.
Investigative studies, although classified as Tier 4, can also provide valuable contributions to Tiers 0-3, as any available information can assist competent authorities in their decision-making processes This guidance encourages the collection and utilization of relevant site-specific data, which enhances the regulatory database and informs better decision-making It is often the responsibility of the discharger to present such data; otherwise, the competent authority may view proposed exceedances of Environmental Quality Standards (EQS) as unacceptable.
From INERIS guidance 2007 prepared for DG Env Working Group E.
In the United States, water quality models play a crucial role in the process of issuing discharge permits These models are utilized to forecast future water quality, ensure adherence to Environmental Quality Standards (EQS), and, when needed, establish additional emission limit values (Ragas, 2000).
The United States is unique in having specific recommendations for the development and use of mixing zones in various guidance documents, although these recommendations are not mandatory According to the Clean Water Act, "allocated impact zones" (AIZ) are defined in relation to mixing zones, but the establishment of a mixing zone policy is left to individual States' discretion As a result, the implementation of mixing zones varies significantly across States, ranging from advanced mixing zone models to simple dilution factor calculations Regardless, any State policy must align with the Clean Water Act and receive approval from the Regional EPA administrator.
Water quality criteria are enforced at the edges of designated impact zones, allowing for temporary exceedances within these areas, provided that acute toxicity is avoided.
The design should feature a straightforward configuration that is easily identifiable within the water body and does not interfere with ecologically significant regions Additionally, it is essential to steer clear of shore-hugging plumes to minimize environmental impact (US-EPA, 1994).
The area or volume of impact should be minimized to the smallest practical size to avoid disrupting designated uses and the established aquatic life community (US-EPA, 1994).
12 Ragas, A (2000) Uncertainty in Environmental Quality Standards Doctoral dissertation University of Nijmegen. http://webdoc.ubn.kun.nl/mono/r/ragas_a/unceinenq.pdf
13 see: http://www.epa.gov/waterscience/standards/mixingzone/
14 US-EPA (1994) Water Quality Standards Handbook: Second Edition US-EPA, Office of Water, Washington, DC. http://www.epa.gov/waterscience/standards/handbook/
15 Application of the CORMIX model is predominantly preferred.
States typically regulate mixing zones in streams and rivers by imposing limits on their widths or cross-sectional areas, while lengths are assessed individually For lakes, estuaries, and coastal waters, some states define the surface area impacted by discharges, which includes both the water column and benthic region In situations where specific dimensions for mixing zones are not provided, their shape and size are usually determined through individual negotiations.
The completely mixed conditions are usually defined as the location where the concentrations across a transect of the water body differ by less than 5% (US-EPA,
In practice, the US-EPA accepts circular mixing zones defined by dimensions equal to the horizontal water depth from any point of the discharge installation, as long as they comply with other mixing zone restrictions (Jirka et al., 1996).
Mixing zones must be carefully designed to avoid harming non-motile benthic and sessile organisms, as well as protecting free-swimming and drifting species The establishment of mixing zones is prohibited in ecologically critical habitats, such as feeding and breeding areas, shellfish growing zones, salmonid spawning sites, shoreline habitats, and near drinking water supplies and recreational areas Denial of mixing zones is warranted when discharges contain harmful substances that are bioaccumulative, carcinogenic, mutagenic, or teratogenic Additionally, effluents that attract organisms, such as warmer discharges, should also be considered for denial It is essential to prevent visible slicks and maintain aesthetic quality in these environments.
The US-EPA designates a smaller area known as the "zone of initial dilution" around the outfall, where acute effect standards may be exceeded, nested within the larger chronic allocated impact zone The boundary of this toxic dilution zone is defined by the criterion maximum concentration (CMC), which aligns with the EU's maximum admissible concentration (MAC) for preventing acute toxicity Compliance with one of four specified conditions is required for the zone of initial dilution (Jirka et al., 1996).
- meet the CMC within the pipe itself, or
- meet the CMC within a short distance from the outfall by applying specific geometric restrictions 19 , or
The User's Manual for Cormix, developed by Jirka, Doneker, and Hinton in 1996, serves as a comprehensive guide for utilizing the hydrodynamic mixing zone model and decision support system designed for assessing pollutant discharges into surface waters This resource, published by the DeFrees Hydraulics Laboratory at Cornell University's School of Civil and Environmental Engineering, provides essential insights for environmental engineers and researchers For more information, you can access the manual at the EPA's official website.
17 US-EPA (1991) Technical Support Document for Water Quality-based Toxics Control US-EPA, Office of Water, Washington,
Schnurbusch, S (2000) developed a comprehensive guidance document on mixing zones for the Oregon Department of Environmental Quality This project was part of the requirements for obtaining a Master of Environmental Management degree from Portland State University The document serves as a critical resource for understanding the standards and regulations associated with mixing zones in environmental management For further details, the guidance can be accessed at [EPA's website](http://www.epa.gov/waterscience/standards/mixingzone/files/OR_MZ_Guidance.pdf).
19 These geometric restrictions should ensure a sufficient dilution under all possible circumstances, including situations of severe bottom interactions and surface interaction:
- meet the CMC within 10% of the distance from the edge of the outfall installation to the
- it should be demonstrated that drifting organisms will be exposed to the CMC for less than 1 hour no more than once every 3 years on average, or
Acute effects should be avoided inside the AIZ, but exceedance of chronic EQS is allowed
Zone of Initial Dilution (ZID)
Acute and chronic effects are allowed inside the ZID
Outside the AIZ, chronic EQS mut be met
Figure 5: Regulatory mixing zones in the USA
The small red circle represents the Zone of Initial Dilution (ZID); the bigger blue circle represents the Allocated Impact Zone (AIZ)
Discharge characteristics
The discharge characteristics play a crucial role in determining the effluent trajectory and the extent of mixing near the outfall Turbulent mixing is essential for maximizing initial dilution and is influenced by factors such as discharge flow rate, velocity, and fluid density, as well as the design elements of the effluent outfall, including outlet diameter, diffuser configuration, and the positioning and orientation of the flow as it enters the surrounding watercourse.
The discharged flux is mainly characterised by 3 physical parameters:
The flow rate of effluent refers to the mass of fluid discharged per unit of time, making it a crucial metric for evaluating contamination levels.
The exit velocity of effluent, which measures the speed of discharge over distance and time, is different from flow rate A higher discharge velocity compared to the surrounding ambient velocity promotes turbulent mixing.
- edge of the AIZ in any spatial direction,
- meet the CMC within the distance: 50 cross sectional area of the discharge outlet
- meet the CMC within distance of 5 times the local water depth in any horizontal
The document titled "A Mixing Zone Guidance Document" was prepared for the Oregon Department of Environmental Quality by Schnurbusch in 2000 This project was submitted as part of the requirements for obtaining a Master of Environmental Management degree from Portland State University.
The revised US 1991 Technical Support Document for water quality-based toxics control advises that new discharges should achieve a minimum exit velocity of 3 meters per second to ensure adequate mixing.
Buoyancy refers to the tendency of effluent flow to either rise (positive buoyancy) or fall (negative buoyancy), influenced by the gravitational force's acceleration This phenomenon is primarily driven by the density difference between the effluent and the receiving water, which can fluctuate based on environmental conditions such as seasonal temperature variations that alter density profiles in the receiving waters.
The physical mixing at the jet outfall is primarily driven by the entrainment of ambient water into the discharge jet, with a defined area known as the zone of flow establishment (ZOFE) extending about six jet diameters from the orifice, where no ambient water is entrained Beyond the ZOFE, high entrainment rates lead to turbulent mixing, as the faster-moving jet accelerates the surrounding water This interaction creates viscous shear between the two fluids, facilitating the replacement of ambient water with the discharged effluent Turbulent eddies at the edge of the jet further enhance the mixing process, resulting in quicker dilution of the effluent (Schnurbusch, 2000).
Drifting organisms are unlikely to be fully entrained into the Zone of Final Effluent (ZOFE), as they may not encounter 100% of the effluent However, due to the entrainment phenomenon, these organisms are often drawn into the turbulent mixing area The duration of their exposure in this turbulent zone can be estimated by considering the entrainment velocity, which is typically around one-tenth of the jet velocity (Schnurbusch, 2000) Additionally, this entrainment velocity should be compared to the swimming speeds of fish and their capacity to evade the discharge.
Design and construction of outfalls
The design of the effluent outfall may indirectly modify the discharge characteristics:
- The outlet diameter triggers the flux velocity.
The arrangement of diffusers plays a crucial role in enhancing mixing efficiency Multiport diffusers, featuring several submerged ports along a diffuser line, are engineered to promote turbulent and rapid mixing, improving overall performance.
The position of the port significantly influences the interaction of the discharge plume with the surrounding environment, including surface boundaries, pynoclines, and the bottom of the water body Rapid mixing is more likely to occur when discharges happen away from these boundaries, and this is also affected by the port's elevation above the bottom Notably, surface discharges, especially those occurring at the shoreline, are the least effective for initial mixing, potentially leading to elevated concentrations of pollutants.
When there is a significant density difference between ambient water and mixed effluent, vertical spreading occurs due to buoyancy, leading to convection motion A buoyant effluent will rise until it reaches a vertical boundary, such as the water surface or bottom, causing the plume to spread laterally and thin out Submerged discharges are often preferred for rapid mixing; however, they may not be suitable for shallow water bodies or areas prone to dredging and sediment changes Maximum dilution typically occurs when the discharge is positioned at mid-depth, and the location of the discharge point, such as in the center of a river, can further enhance mixing efficiency.
The orientation of a port significantly affects the flow direction of water entering the surrounding watercourse By optimizing the velocity difference between the discharged fluid and the ambient water, it is possible to create a strong shearing action that enhances turbulent mixing.
Mandate for Drafting Group on Mixing Zones
This mandate was presented to (and ratified by Water Directors at their Paris meeting on 24- 25th November 2008.
Authors: DG ENV, the Netherlands, United Kingdom, France
Background: On the 17 th of June 2008 a second reading agreement between the European Parliament and the Council on the Environmental Quality Standards (EQS) Directive was achieved
The Council has officially approved the text 24, which is currently undergoing legal and linguistic finalization The publication in the Official Journal is anticipated by the end of this year at the latest During the recent Working Group E meeting in March 2008, discussions highlighted the necessity for a comprehensive guidance document on mixing zones regarding priority substances.
- Endorsement by the Water Directors during their meeting on 24-25 November 2008.
Contacts: madalina.david@ec.europa.eu; gerrit n iebeek@rws.nl; John.batty @ environment- agency.gov.uk; Yvan.AUJOLLET@developpement-durable.gouv.fr
The new EQS Directive introduces the concept of "mixing zones," which permits certain concentrations of substances to exceed established EQS levels near outfalls However, it is essential that these allowances do not impact the overall compliance of the surrounding surface water with the relevant standards.
INERIS has developed a background document for WG-E that reviews existing methods for delineating administrative mixing zones This document aims to guide Member States in protecting aquatic life and human health while ensuring compliance with Environmental Quality Standards (EQS) for the entire water body.
24 http://register.consilium.europa.eu/pdf/en/08/st10/st10732.en08.pdf (under legal/linguistic finalisation)
The Drafting Group's objective is to develop activities related to mixing zones, which will aid the Working Group on Priority Substances and contribute to the Common Implementation Strategy of the Water Framework Directive 2000/60/EC.
The Drafting Group is set to create a technical guidance document (TGD) aimed at identifying mixing zones, as mandated by Article 4(4) of the EQS Directive This document will provide strategies to minimize the size of these mixing zones and will be evaluated across various water categories with distinct hydromorphological features.
The initial meeting occurred on May 7th in Brussels, hosted by DG ENV, to outline the essential principles for drafting guidance Subsequently, a follow-up meeting was held on September 22nd-23rd, 2008, in Paris, organized by France's Ministry of Ecology, Energy, Sustainable Development, and Territorial Development, to review the first draft of the Drafting Group's mandate and the initial guidance document.
The Drafting Group on mixing zones, comprising representatives from the UK, Netherlands, France, Denmark, and DG ENV, will be led by John Batty from the United Kingdom, with Gerrit Niebeek from the Netherlands and Madalina David from DG ENV serving as co-leads.
Technical meetings of the Drafting Group will be decided on an ad hoc basis or scheduled in advance
The group leader will share updates during the WG E meetings, followed by the presentation of progress reports to the SCG, Water Directors, and the Article 21 Committee.
A participant list will be created for those actively contributing to the Drafting Group, while all members of Working Group E on Priority Substances are encouraged to provide feedback on the draft guidance and reports.
VI Links with other activities
The Drafting Group on Mixing Zones will operate under the Working Group on Priority Substances (WG E), maintaining strong connections with Chemical Monitoring Activities It is essential to consult similar expert groups associated with international marine conventions, such as OSPAR, Helsinki, and Barcelona, as well as river conventions like the Danube and Rhine, alongside other relevant Community legislations, including REACH, the IPPC Directive, and the Pesticide/Biocide Directive.
VII Type and intensity of the work
The drafting of technical guidance will involve ad hoc meetings and email exchanges for experience sharing, information collection, text correction, and discussion of specific issues The Commission’s consultants will assist the Drafting Group, and members of WG E are encouraged to participate in this collaborative effort.
Presentation of approach to WG-E October 2008
SCG and Water Directors agree mandate November 2008
Drafting group 1 st working draft to DG Environment December 2008
Revisions and consultation with WG E February 2009
Presentation of consolidated 1 st draft to WG-E meeting March 2009
Start the testing of the guidance March 2009
John.batty @ environment- agency.gov.uk
Gerrit Niebeek Centre for Water
Management; NL gerrit n iebeek@rws.nl
Madalina David DG ENV/D2 Madalina.DAVID@ec.europa.eu
Yvan Aujollet Ministry of Ecology, FR Yvan.AUJOLLET@developpement- durable.gouv.fr Jứrgen Jứrgensen Ministry of the
Environment, DK jojoe@rin.mim.dk
Dju Bijstra Centre for Water
Management, NL dju.bijstra@rws.nl
UK norman.babbedge@environment- agency.gov.uk
Edwige Duclay Ministry of Ecology, FR Edwige.DUCLAY@developpement- durable.gouv.fr Hubert Verhaeghe French Water Agency in
H.verhaeghe@eau-artois-picardie.fr
Jean-Marc Brignon INERIS, FR
Jean-Marc.BRIGNON@ineris.fr
Amend document to reflect Member States written comments
Final version agreed at WG-E October 2009
SCG and Water Directors adopt final version November 2009
Article 21 Committee's endorsement November 2009/Spring 2010