Udledning af miljøfarlige stoffer med spildevand
Summary and conclusions
Introduction
This report presents a number of recommendations for the establishment of requirements and regulation of emission of hazardous substances with waste water. Below, the recommendations are summarised to a short guidance in the practical use of the established requirements and control.
It should, however, be emphasised that it was not the purpose of this study to propose changes to existing discharge permits as it was not the intention that the simplified methods presented in this report shall replace the more detailed calculations, which often form the basis of the establishment of requirements to existing discharges.
The presented principle of the regulation of emission of hazardous substances is based on:
- Establishment of an administratively fixed mixing zone replacing the physically defined initial dilution zone used till now
- Establishment of requirements to the concentration of the substance in the discharge
- Establishment of requirements to the maximally discharged quantity of the substance of concern
Establishment of mixing zones
This study recommends that the mixing zone replaces the previously used concept ‘the initial dilution zone’ as its application has resulted in various practical problems. The mixing zone is defined as a zone within which a certain impact on the environment is acceptable as it is a prerequisite that the discharge is made by use of the best available techniques (BAT).
Discharge of single substances requires:
 | That quality standards are met outside the mixing zone |
| That no acutely toxic concentrations occur outside the mixing zone |
The mixing zone is defined on the basis of a dilution (a dilution factor F), which the waste water achieves after discharge within a specified distance from the effluent outfall. This definition is based on a situation with a realistic but modest dilution of the waste water. The size of the mixing zone, which corresponds to a certain dilution of the waste water (F) is determined by calculation of the dilution by use of simplified models. More detailed models may be applied if the conditions of the dilution turn out to be critical as regards meeting the quality standards (Chapter 3).
The extent of the mixing zone and with that the magnitude of the dilution factor is determined on the basis of an environmentally based expert opinion taking into account the dilution capacity of the water body.
By way of an example, an area of 100 m2 may be of great importance to the conditions of a stream with a small dilution capacity whereas it will be of only local importance when discharging to the sea.
As a starting point, factors may be chosen on the basis of the below intervals. It must, however, be stressed that they are based on assessment of local conditions and that dilution factors outside these intervals may be relevant:
| · | Watercourses: | F = 2-10 times | |
| · | Lakes: | F = 5-20 times | |
| · | Sea: | F = 10-50 times |
It is, however, worth noting that a consequence of using a fixed dilution factor is that the size and location of the mixing zone depend on the discharged water volume, current conditions, water flow etc. and thus eventually change. The temporal variation is taken into account by defining the mixing zone on the basis of a realistic worst-case consideration, which may be e.g. a median-minimum water flow in a stream, a no-current situation or similar bad mixing condition for other discharges.
Establishment of requirements to and control of the discharged concentration of substance
As a fixed factor is used for defining the mixing zone, the requirement to the compliance with the water quality standards (WQS) at the limit of the mixing zone can be converted into Emission Limit Values (ELV) for single substances in the discharged waste water (ELVsubstance). Furthermore, a protection against acute adverse effects of isolated peak concentrations of the substances is ensured by fixing a maximum value for the discharged concentration of a substance (ELVsubstance,max)
ELVsubstance = WQS · F
ELVsubstance,max = WQS · a/c · F
Where:
| ELVsubstance | is the Emission Limit Value for the substance as a concentration |
| ELVsubstance,max | is the Emission Limit Value for protection against acute adverse effects as a concentration |
| WQS | is the quality standard for the aquatic environment |
| a/c | is the ratio of the acute toxicity to the chronic toxicity of the substance. The ratio is often based on literature data. If adequate data are not available, a factor of 4 may be applied as a conservative estimate of the a/c ratio. |
| F | is the dilution factor, i.e. the ratio of the concentration of the substance in the discharge to the concentration at the limit of the mixing zone. |
Continuous discharges with six or more samples per control period
As for discharges that can be assumed to result in continuous discharge of substance and for which six or more samples are taken during the control period, statistical control of compliance with the ELV in accordance with the principles of DS 2399 (Danish Standards Association) is recommended.
The control is carried out as a transport control on the basis of 24-hour flow proportional samples. In order to provide for potential extreme concentrations of substance as the result of discharges of abnormally large or small water volumes, flow-weighted concentrations form the basis of the control.
Transport control
According to DS 2399, transport control is primarily based on control of the discharged quantity. The control value is calculated as follows:
| The control value for the flow-weighted concentration (Yi in e.g. mg/L) is worked out as the product of the measured concentration (Cmeasured in e.g. mg/L) and the measured water flow (WF in e.g. L/day) divided by the average flow of all the flow measurements included in the control. |
where n is the number of measurements
| Concentrations found to be below the detection limit of the analytical method applied are fixed as half of the analytical detection limit and the control values are calculated as described above. |
The natural logarithm (Xi) is calculated for each individual control value (Yi), and the average a and the standard deviation b of the Xis is calculated.
The adjustment factor kn is estimated. kn provides for the achievement of 95% probability of acceptance of a water quality equivalent to the requirement.
The natural logarithm to the control unit C is estimated by use of:
ln Ccontrol = a + kn × b
Ccontrol is compared with the ELVsubstance.
If Ccontrol is smaller than or equal to ELVsubstance, the emission standard is met.
Ccontrol £ ELVsubstance
The control unit for ELVsubstance is calculated as the average of the flow weighted concentrations in the control period. Thus it is controlled if the emission of substance on an average basis exceeds the ELVsubstance. For control of ELVsubstance, max a control unit for a critical fraction of 10% is used, in order to evaluate if the ELVsubstance, max is exceeded more that 10% of the time.
Continuous discharges with less than six samples per control period
At many discharges, less than six samples are taken per control period for the control of emission of hazardous substances. In these cases, it is recommended that the emission standard is that the mean value of the flow-weighted concentrations (Ymean ) must be smaller than or equal to the quality standard ELVsubstance in order to obtain compliance:
Ymean £ ELVsubstance
In order to ensure the protection against acute adverse effects of single samples with a high concentration it is moreover required that, all flow-weighted concentrations are smaller than or equal to the quality standard ELVsubstance,max:
Yi £ ELVsubstance,max
Non-continuous discharges
Non-continuous discharges are not suited for a statistically based control if e.g. the effluent fluctuates heavily within a control period due to alternating production, batch production etc. In these cases, sampling must be made at hours representing a normal high discharge of substance. It is recommended to use 24-hour flow proportional samples.
Furthermore, it is recommended to require that the mean value of the flow-weighted concentrations must be smaller than or equal to the quality standard ELVsubstance:
Ymean £ ELVSsubstance
In order to ensure the protection against acute adverse effects of single samples with a high concentration, all flow-weighted concentrations should also be smaller than or equal to the quality standard ELVsubstance,max:
Yi £ ELV,max
Establishment of requirements and control of the discharged quantity of substance
Requirements to the discharged quantity should be established in accordance with the application of the best available techniques and should ensure that the total discharge of a substance to a water body does not cause unacceptable adverse effects. Furthermore, it must be ensured that, eventually, discharges of the substances that Denmark is under international obligation to ensure discharge reduction or phase out, are reduced.
The best way to obtain knowledge regarding the already existing concentration of a substance in a water body is direct monitoring. Often, these data are not available and it may be expedient to estimate the yearly input on the basis of available information on discharges from various sources.
Estimation of the resulting concentration of substance in various compartments demands dynamic modelling estimating the fate and transformation of the substances in the environment. A very rough estimate of the average concentration in the water phase, in which the transformation of the substances and their elimination from the system are not taken into account, may be considered as the input of substance divided by the supplied water volume:
Where:
| Cregion | is the resulting regional concentration at total mixing after input of substance from all sources |
| Ci | is the concentration of the substance in the point source i, including streams flowing into the water body (mg/L) |
| C0 | is the concentration of the substance in the surrounding water bodies (average) (mg/L) (i.e. the already existing concentration of the substance not including the load contribution from the point sources i) |
| Vi | is the water volume supplied from the point source i (m3/day) |
| V0 | is the water volume supplied from the surrounding water bodies (m3/day) (i.e. the water exchange in e.g. an estuary/creek or the water flow of a watercourse). |
Other sources of emission of dangerous substances to the same water body should be included in the setting of limit values for a particular discharge. This can be done by including the estimated regional concentration (Cregion) in the derivation of the ELV. It should be noted, however, that the contribution from the discharge itself should not be included in the calculation of Cregion.:
ELVsubstance = F·(WQS-Cregion)
Because ELVsubstance is controlled as the flow-weighted average concentration this also secures compliance with the maximum acceptable emitted amount of substance.
Establishment of requirements and control of the toxicity of the whole effluent
Regulation of single substances in complex mixtures of waste water must be concentrated on the substances of which Denmark is under international obligation to reduce or discontinue the discharge in relation to EU legislation
Furthermore, it must be considered whether other substances with properties potentially causing considerable risk to the aquatic environment are present in the discharge. As regards these substances, it must be estimated whether the discharge of concern is essential, i.e. whether there is a risk of effects, or whether the safety margin to critical concentration levels or discharged quantities is adequate.
In order to protect against toxic effects, requirements to the toxicity of the whole effluent may be established as a supplement to this regulation of single substances in complex mixtures of waste water.
According to current practice, regulation based on the toxicity of the whole effluent is applied in situations in which an impact zone with reduced quality objectives has been defined around a discharge. With reference to the Danish EPA’s Guidance on recipient quality planning (Danish EPA 1983), it is a requirement that no chronic toxicity must occur outside the impact zone and that, after initial dilution, the waste water must not have any acutely toxic effects.
It is recommended to introduce a de minimis limit to the toxicity of the effluent, i.e. no requirements are established to the toxicity of the whole effluent if it below the de minimis limit. The de minimis limit should correspond to what is empirically considered to be low acute toxicity, e.g. a realistic low toxicity of effluent from Danish well-functioning municipal wastewater treatment plants. The data material on the toxicity of effluent from municipal WWTPs is limited but more tests are expected to be carried out within the next few years.
Waste water is considered to have a low acute toxicity when the 50% effect concentration is above 500 mL waste water/L (i.e. LC/EC50 > 500 mL/L) or when no significant acute effects are seen in test of the waste water at a dilution corresponding to 300 mL/L (i.e. LC/EC10 > 300 mL/L).
de minimis based on the acute toxicity of the waste water
The assessment of whether a waste water succumbs to this de minimis limit may be made on the basis of a screening of the toxicity of the waste water with the so-called base-set of tests (short-term tests with algae, crustaceans and fish). If the most sensitive test has a LC/EC50 above 500 mL/L or LC/EC10 is above 300 mL/L, it is estimated that the application of ecotoxicological test methods for monitoring of the discharge is not necessary.
As regards waste water with a toxicity above the de minimis limit, protection against chronic adverse effects outside the mixing zone can be ensured by defining ELVs for the chronic (ELVtox, chron) and acute effects (ELVtox, acute) of the wastewater.
Where:
| ELVtox, chron | is the quality standard of chronic toxicity. Please note that the ELV depends on UFPNEC, chron (see below). |
| ELVtox, acute | is the quality standard of chronic toxicity. Please note that the ELV depends on UFPNEC, acute (see below). |
| F | is the dilution factor of the actual discharge corresponding to the extension of the mixing zone. If an impact zone has been defined, F corresponds to the dilution of the effluent at the border of this zone. |
| UFPNEC ,chron | is the assessment factor applied in the extrapolation from the lowest measured effect concentration in mL/L to the predicted chronic no effect concentration. The magnitude of the factor depends on amount of the available data on the toxicity of the waste water forming the basis of the permit. As an example, UFPNEC, chron is 20 if tests corresponding to the basic set (short-term tests with algae, crustaceans and fish) have been performed, but it can be reduced to 10 if data are available from tests of a total of five groups of organisms (Pedersen et al. 1994). |
| UFPNEC ,acute | is the assessment factor applied in the extrapolation from the lowest measured effect concentration in mL/L to the predicted acute no effect concentration. The magnitude of the factor depends on amount of the available data. |
The control of ELVtox, chron and ELVtox, acute is performed as in the case of control of ELVsubstance. A control unit C is calculated on the basis of the flow-weighted toxicity unit in stead of the flow-weighted concentration. The control parameters Yi and Ccontrol is calculated as described for single substances and the conditions for statistical or non-statistical evaluation of the compliance are the same. The toxicity for the wastewater in Toxic Units (TU) is calculated as:
Where,
| TOX | is the measured toxicity of the wastewater, e.g. EC50 in mL/L. |
| 1000 | is the conversion factor between mL/L and the dilution, which is the unit of a TU. |
The joint toxicity of substances in complex mixtures of waste water often is additive. Assuming that the quality standards of the individual substances are known, an assessment of the joint effect of the substances discharged to the same water body can be made by estimating the sum of the contributions of the individual substances. The risk of the individual substance causing effects in the environment can be estimated as the concentration of substance in a given point outside the mixing zone (Ci) divided by the predicted no effect concentration (PNEC) of the substance. As regards a mixture of many substances, the joint risk can be estimated as follows assuming additivity:
S (i=1 .. n) (Ci/PNECi)
Where:
| Ci | is the concentration of the substances i = 1..n |
| PNECi | is the predicted no effect concentration of the substance i = 1..n . |
A joint risk quotient of more than 1 indicates a risk of effect on organisms and its magnitude indicates the probability of the occurrence of this effect.
Similarly, the toxic contributions of the substances (estimated as toxic equivalents (TEQ)) in a complex mixture of waste water can be summed up in order to achieve an expression of the total quantitative discharge of toxic and persistent substances weighted according to their dangerousness expressed by their PNEC value - or even better - as the water quality standard, which also takes into account the potential of substance to bioaccumulate; i.e.:
S TEQi = S i=1..n (Ci · Vi /WQSi)
As TEQi is the discharge of the individual substance i estimated as toxic equivalents, i.e. the discharged quantity per time unit (e.g. one year) divided by the water quality standard. The unit of the toxic equivalent is a water volume per time unit corresponding to the exact dilution that must take place in order that no exceeding of the WQS (at total mixing) occurs.