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Possible Control of EU Priority Substances in Danish Waters

13 Assessment of measures against stormwater discharges

• 13.1 Introduction
• 13.2 Technical measures for retention of suspended particles in stormwater discharges
• 13.3 Assessment of treatment efficiencies for selected options
• 13.4 Economic Assessment
  • 13.4.1 General assumptions on the potential extent of the measure
  • 13.4.2 "Worst case" - Scenario B
  • 13.4.3 "Critical areas" - Scenario A/C
• 13.5 Concluding remarks on removal of priority substances from stormwater discharges

13.1 Introduction

For six out of the eight priority substances described and assessed in the preceding chapters it was found that the dominant input to surface waters in Denmark comes from the discharge of stormwater from separate systems. In some parts of Denmark the reduction factor required to comply with the EQS may for a few of the substances (nonylphenol and maybe PAH) be higher than the diluting capacity of the receiving streams at median minimum flow (summer conditions).

Further, a number of the substances are priority hazardous substances (cadmium, mercury, nonylphenol, PAH (including anthracene) and TBT) for which measures aiming to cease/phase-out pollution in accordance with the WFD must be introduced in Scenario A/C while progressive reduction and complete cessation/phase-out of discharges, emissions and losses must be achieved within 20 years in Scenario B in accordance with the draft proposal of the Daughter Directive.

Also among the other 23 priority substances not included in this study there are a number (7) of priority hazardous substances. In was concluded in the preceding chapter (Section 12.2) that five of the substances were not relevant to consider further in a Danish context while the remaining two, PeBDE and chloroalkanes, were presumably of little significance in relation to EQS compliance or being addressed already. However, all seven substances would, to the extent they occur in Danish stormwater, also be positively affected by the measures described in this chapter.

This chapter addresses the technical and economic aspects of minimizing the discharge into the aquatic environment of these and many other priority pollutants appearing in stormwater from separate systems.

Sewage can be divided into two main components: dry weather flow and wet weather flow. Dry weather flow consists mainly of domestic sewage, industrial sewage, and drains. Wet weather flow consists mainly of stormwater from paved areas in cities. Both dry weather and wet weather flows contain heavy metals as well as a large number of organic micropollutants. Figure 13-1 below indicates the main sources and processes in the "production" of wet weather flow pollution.

Figure 13-1.
Sources and processes affecting pollutants in stormwater (Göettle (1978): Ursachen und mechanismen der regenwasserverschmutzung. Ein beitrag zur modellierung der abflussbeschaffenheit in städtischen gebieten. Institut für Bauingenieurwesen V, TU Munchen. Berichte aus wassergütewirtschaft und gesundheitswesen, nr 23.).

Old sewer systems convey both sanitary sewage and wet weather runoff. In order to prevent surcharge of this unhealthy mixture a number of spills were designed that discharged into nearby surface waters, creating environmental as well as aesthetic problems. During the 1950's it became standard practice to design sewer systems as a two-stringed system; one system conveying sanitary sewage and the other system conveying wet weather runoff. Discharges from sewer systems can therefore be divided into three components:

• Dry weather discharges from wastewater treatment plants
• Wet weather discharges from combined sewer systems
• Wet weather discharges from stormwater systems

The wet weather runoff is discharged into nearby surface waters, usually without treatment. If treatment is implemented, it has in general been designed to reduce the hydraulic overloading of small rivers. In fact, with few exceptions storage facilities in relation to stormwater discharges are optimized to reduce retention of matter as much as possible thus reducing the operating and maintenance costs of the treatment facility.

During the last 10 to 20 years there has been a growing understanding that discharging wet weather runoff through separate sewer outfalls also creates environmental problems. The contents of heavy metals, PAH and a number of other substances are quite high, and studies have also shown that discharges from separate sewers are more toxic than discharges from combined sewers. Table 13-1 gives an overall indication of the importance of each of the three types of discharges into surface waters in Denmark.

Table 13-1
Comparison of discharges from separate sewers to other main types of point discharges. The assessment of the amount of discharged pollutants is quite uncertain.

1) Punktkilder 2004. Orientering fra Miljøstyrelsen nr. 9, 2005.
2) Assessment made by COWI

3) Assessment based on Arnbjerg-Nielsen et al (2002): Regnbetingede udledninger fra fællessytemer af NPO-stoffer, tungmetaller og miljøfremmede organiske stoffer. Miljøprojekt 701. Miljøstyrelsen, København.

4) Assessment based on Miljøstyrelsen (2006): Målinger af forureningsindhold i regnudledninger. Arbejdsrapport nr. 10, 2006 fra Miljøstyrelsen.

Table 13-2

Separate sewer systems, basic statistics

1) Punktkilder 2003 (revideret). Orientering fra Miljøstyrelsen nr. 1, 2005.

Table 13-2 shows the basic relation between the number of outfalls and area of paved surface connected (in hectares, ha).

Based on the data provided in the "Punktkilder 2003" ("Point Sources 2003") report (Miljøstyrelsen 2005), it is assessed that approximately 85 % of the volumetric amount of discharges from separate sewer outfalls in Denmark occurs into fresh water.

13.2 Technical measures for retention of suspended particles in stormwater discharges

So far, technical measures at separate sewer outlets have been directed towards securing an acceptable hydraulic peak loading of the surface water, implying that the treatment should be simple storage. Since the water was considered to have a low content of pollutants, the detention ponds were often designed to minimize treatment in order to minimize operation and maintenance costs. In the following various methods for retention of suspended solids are discussed. The methods have been applied in Denmark or are based on technologies that are well known from treatment of combined sewage.^[21]

Danish and international experiences are very limited with respect to measurements on treatment efficiencies of organic micro-pollutants. However, it is well known that none of the technical measures that are mentioned in the following section are able to treat the organic micro-pollutants in the sense that they are transformed into other (hopefully) harmless substances. Rather, the technical measures are based on some sort of physical removal of suspended solids. Removal of the organic micro-pollutants therefore depends on the sorptivity of the compounds. Alternatively, both water and solids can be discharged into another water body.

The treatment efficiencies with respect to the priority substances are assessed in two steps. First the removal of suspended solids is assessed for each of the relevant technical measures. Secondly, the proportion of the priority substance that will follow the suspended solids is assessed.

The technical measures can be divided into the following types of treatment:

1.           Storage and routing to wastewater treatment plant in dry weather periods;

2.           Storage in combination with treatment;

3.           Infiltration (discharging into groundwater);

4.           Filtration at point of discharge or distributed throughout the collection system.

The different types are discussed briefly below.

1. Storage and routing to wastewater treatment plant

One of the solutions to minimize the release of suspended particles from stormwater discharges is to reconnect the separate system to the treatment plant after proper storage. The technology is fully developed, but the solution has not been implemented anywhere, primarily because it implies less efficient treatment of nutrients and organic matter at the wastewater treatment plant.

The main drawbacks of this type of solution are:

• May cause more pollution at the overall scale;
• May jeopardize current treatment technology at wastewater treatment plants.

2. Storage in combination with treatment

The most well-known treatment options in relation to storage are the following:

a.          Treatment by means of optimizing sedimentation in the storage chamber;

b.         Treatment by means of filtration of the outlet of the storage chamber into the surface water.

Internationally, guidelines exist for optimized sedimentation in the storage chamber. Typically the sedimentation is enhanced by dividing the chamber into several subsections. By design, each of these subsections has a permanent water body also during dry weather flow, and each subsection also contains plants. Such storage chambers have been designed for several years by the Danish Road Directorate for treatment of road runoff at the national highways, and several municipalities have experiences with this type of treatment as well. Treatment efficiencies of suspended solids can be quite high if designed properly.

Removal of micro-pollutants will happen primarily through sorption to suspended solids, but some of the micro-pollutants will also be retained through sorption to other surfaces in the biological detention basin and uptake in the biomass. There are only few experiences with treatment by means of filtration of the outlet of the storage chamber into the surface water. The method is expected to lead to increased operating and maintenance costs, and therefore it is assumed that it will primarily be used as a second treatment step after sedimentation in the storage chamber has been obtained. Treatment efficiencies are high, up to 90 % for water receiving full treatment.

The main drawbacks of this type of solution (both options a and b) are:

• It requires extra operation and maintenance;
• Space requirements are estimated to be 2% of the connected paved area.

3. Infiltration

Infiltration of stormwater can take place at three levels

a.          Local infiltration of roof water

b.         Local infiltration of road water

c.          Infiltration trenches at collection points

Infiltration will have a removal of sorbed micro-pollutants of close to 100% with respect to surface water. However, there is a risk that groundwater may be contaminated by substances with poor sorption characteristics.

The main drawbacks of this type of solution (all three options) are:

• The system is distributed and requires regular visits;
• Local landowners must build and operate the systems (roof water).

4. Filtration

Filtration systems are currently being developed that focus on sorption of micro-pollutants through different media. The systems can either be distributed throughout the separate sewer network or in a centralized treatment facility. The technology is not yet ready for large-scale implementation, and it is therefore not feasible to attempt to assess the economic aspects of implementation. The main drawback of this type of solution is:

• The technology is not ready for implementation yet

Common to all of the above solutions is the fact that they are quite expensive and require space. At many locations in city centres, it will be very difficult to acquire the physical space needed for the optimal solutions. This is the most important uncertainty factor in relation to implementing the above-mentioned measures.

13.3 Assessment of treatment efficiencies for selected options

In this study the focus will be on the two types of treatment that seems to be most suitable, i.e. option 2: Storage in combination with treatment, and option 3: Infiltration. The first of these methods aims at treating the stormwater and then discharge it into the same surface water as previously while the second of these methods aim at redirecting the water from the surface water and discharging it into the groundwater instead. As will be shown in the next section, storage in combination with treatment is relatively cost-effective when considering suspended solids. However, one of the priority (hazardous) pollutants in question - mercury - has rather poor sorption characteristics, i.e. a large proportion of the pollutant follows the water rather than the suspended solids. For mercury infiltration is a theoretically possible alternative although this technology is more expensive and also implies a risk of groundwater contamination. Other types of reducing measures are suggested for mercury (se Chapter 7).

The assessment of removal of suspended solids and the corresponding removal of micro-pollutants are described in Tables 13-3 and 13-4. For PAHs a removal of up to 80 % can be expected if applying technology type 2, Storage with treatment. For a substance like mercury the removal using this technology will in general be less than 30 % (but no need for removal from stormwater was identified for mercury).

When choosing technology type 3, Infiltration, all substances will have a removal of more than 95 % with respect to surface water. The relative cost-efficiency of the different types of technologies therefore depends significantly on the substance in question. In the specific context, technology type 2 will be the most relevant to consider for all the priority substances with the exception of mercury.

The above-mentioned two main types of solutions are considered to be the best treatment options available for full-scale implementation today. Both of these options require physical space between the connected paved area and the point of discharge. The space requirements typically correspond to 1-2 % of the paved areas connected to the treatment system. Further, land use is restricted near the location of the facilities.

If treatment at the location of the existing outlet is not feasible, other solutions must be studied, e.g. leading the surface water to another location, expropriating private property in order to recover the needed physical space, implementing novel/untested technologies etc. If the physical space is not available on site, the typical cost of moving the facility will be DKK 2,000-7,000 for every metre the facility is to be moved. The costs associated with these types of actions may, however, vary greatly. An upper limit of the costs is recovery of land by means of expropriation, which in city centres may be more expensive than the installation of the actual treatment facility, if at all politically and legally feasible.

Table 13-3.

Rough assessment of treatment efficiencies associated with implementation the most feasible technical measures. The reported treatment efficiencies are based on data from facilities that are well designed and operated. The assessment is based on treatment of approximately 95 % of the stormwater.

Table 13-4

Rough assessments of the potential reduction of substance concentration in stormwater discharge through removal of suspended solids (SS)

Source: Based on COWI expert assessment from a previous project for DEPA regarding filtered and un-filtered runoffs from roads (Miljøprojekt 355, 1997)

Among the 23 priority substances not included in this study, the majority is also believed to be reduced by more than 50 % in stormwater by the mentioned detention systems. At least the following should benefit from these systems to this extent (assessed on basis of the LogK_OW/Log K_OC, i.e. if higher than 3):

Brominated diphenylethers (PeBDE), chloroalkanes, chlorpyrifos, endosulfan, hexachlorobenzene (HCB), hexachlorobutadiene (HCBD), hexachloro-cyclohexane (HCH including lindane), octylphenol, pentachlorobenzene, trichlorobenzenes and trifluralin.

13.4 Economic Assessment

Based on various previous studies and literature sources, the overall costs and treatment efficiencies are assessed to be as presented in Table 13-5 under the assumption that physical space is available at the location without costs.

Table 13-5

Rough assessment of the costs associated with implementing the technical measures suggested in the previous section. The costs are unit costs per hectare (ha) of connected paved area. They are based on median values, and prices may vary significantly due to local conditions. Costs of land recovery are not included.

Note: *) If the cost will be financed directly by the customers, the tax distortion factor of 20 % used to calculate the welfare-economic cost may be too high.

**) At outlets without existing storage facilities, new storage facilities are necessary. In this case the high-end cost is relevant.

13.4.1 General assumptions on the potential extent of the measure

The concentration in the surface water after initial dilution is mainly dominated by two factors: (1) the concentration of the pollutants in the stormwater runoff and (2) the magnitude of (usually less polluted) dry weather water flow in the receiving water. The two factors are discussed below.

The concentration levels in the stormwater can to some extent be predicted based on a description of the catchment. High traffic intensity usually implies higher concentrations of pollutants, and runoff from newly paved areas has a higher content of pollutants than the average. The change in the use of polluting substances is also reflected in the concentration levels, most notably in the concentrations of lead and copper.

However, there is also a significant variation in the concentration levels between wet weather events. This variation is random and in general supersedes the variation that can be described by catchment properties. Therefore, when considering possible measures, the variation of concentration levels in the stormwater runoff has relatively minor importance and can mainly be used as a guideline for deciding on which types of catchments should be treated first.

The initial dilution is large during small storms. The heavy storms nearly all occur during summer where the Danish streams and rivers have a relatively small dry weather flow (median minimum flow). The dry weather flow varies greatly throughout Denmark. Most streams in Zealand and other places in the eastern part of Denmark are characterized by low dry weather flows, and often they have a high number of discharges into the streams. In the summer, the wet weather flows in surface waters are typically about 10 times higher than the dry weather flows, i.e. in many places, it is only possible to obtain a dilution factor of about 1.1 at this time of year. In certain areas, therefore, the (summer) maximum concentration levels in the surface water are almost the same as in stormwater. In other parts of the country the ratio is substantially higher, because the level of urbanization is lower and because the dry weather flow is higher.

About 37 % of the stormwater discharges undergo some type of treatment prior to the discharge. The treatment is most often installed due to hydraulic overloading of the surface waters. Due to optimization of the operation including minimization of maintenance there is little or no retention of pollutants. Storage facilities designed near the Danish highways within the last 20-30 years are important exceptions. Facilities designed according to the guidelines provided by the Danish Road Directorate will, if properly operated, retain a significant part of the priority pollutants.

An increasing area is being paved and connected to separate stormwater runoff drainage. The development since 2001 has been used to forecast the area in 2025, and the result is presented in Figure 13-2 below.

Further, the number of separate stormwater runoffs that are connected to storage basins before discharge increases. The development in the percentage of separate stormwater runoff connected to some form of storage is shown in Figure 13-3. The rise in the percentage indicates that in general new separate stormwater discharges are constructed with retention basins and that detention basins are installed at some of the old systems as well.

Figure 13-2
Overview of the development and forecasted paved area connected to separate stormwater drainage

Figure 13-3
Proportion of areas connected to storage prior to discharge.

Note: *) Break in data series means that trends cannot be calculated for Funen and Bornholm. Instead, a discrete forecast is made.

13.4.2 "Worst case" - Scenario B

In the following, a calculation is made of the financial and welfare-economic cost of reducing the discharge of the PHSs through treatment of stormwater. Since about 85 % of the total volumetric amount of discharges from separate sewer outfalls occur to freshwater and since the water quality criteria are set as concentrations, it is assumed as a "worst case" scenario that eventually practically all separate outfalls that discharge into the freshwater environment will have to be fitted with storage facilities. This corresponds to Scenario B where ceasing/phasing out discharges, emissions and losses of priority hazardous substances to the aquatic environment must be achieved within a time frame of 20 years in accordance with the draft proposal of the Daughter Directive.

Some stormwater detention structures are already equipped with storage facilities (approx. 37%). However, the majority of these structures must be redesigned extensively in order to obtain the required functionality. Therefore, as a rough assessment, it is assumed that no existing treatment exists. Thus, treatment must be installed at discharges representing runoff from 33,000 ha impervious surface area. This is a conservative assessment.

With regard to assessing the cost of acquiring the necessary plots of land, space requirements are 2 % of the connected paved area. Further, we know that 37 % of the stormwater discharges presently undergo some type of treatment prior to the discharge. Though these treatment faciltities will have to be rebuilt, as they provide little or no retention of pollutants, the new treatment can be placed on the same area. The resulting need for additional land is around 415 ha. Of this, it is assumed that one third of the land will not have alternative use of any value and already be in the ownership of the municipality or the water company. Another third is assumed to be placed on agricultural land with an average purchasing price of DKK 150,000 per hectare. This corresponds to a welfare economic cost of just over 200,000 DKK per hectare. The last third of the land required is assumed to be located in urban areas and city centres and must be purchased and/or be acquired by expropriation. It is difficult to set an average price for such land as it will be highly dependent on local conditions. If the price is very high for land in the relevant area, moving the water to another location can be a more cost-effective solution while less space-requiring technical treatment technologies can be the most cost-effective solution at some locations. In the following estimation, an average price per hectare of land in urban areas is set to DKK 10 million. This gives a welfare-economic cost of 13.7 million DKK per hectare. In sum, the average cost of land for the basins is set to just over DKK 3.4 million per hectare in financial cost and DKK 4.6 million in welfare economic cost, which makes it the most important factor in the cost estimation.

Table 13-6 shows the financial and welfare-economic results for the situation where measures have to be implemented for 85 % of all runoffs in Denmark (Scenario B). As mentioned, the focus is on the two types of treatment that seem to be most suitable, i.e. options 2a and 2b: Storage in combination with treatment by sedimentation and by filtration respectively. The welfare-economic results are shown with both a 3 % and 6 % discount rate.

It must be noted that the estimations are made on the basis of unit costs based on the assumptions mentioned earlier in this chapter. Handling all runoff stormwater currently running to separate sewer systems would in reality imply a number of special measures that could both increase or decrease the cost significantly compared to the results below.

Table 13-6

Financial and welfare-economic costs in worst case (Scenario B) of retention of sediments in stormwater runoff.

Note: Results have been rounded up or down.

The results in Table 13-6 show that in the worst case the welfare-economic cost with a deadline 2025 (Scenario B) is DKK 5.6 billion for storage chambers with sedimentation and DKK 13.9 billion if treatment of outlet is added (discount rate of 3 % is used). The cost of land acquisitions less than 1 billion in both cases whereas the cost of investment and the operating and maintenance cost of the technology is DKK 4.6 and 12.7 billion respectively.

The total financial cost is DKK 3.7 billion for storage chambers with sedimentation and DKK 7.8 billion if treatment of outlet is added (discount rate of 6 % is used). The cost of investment and operating and maintenance cost of the technology alone is DKK 3 and 7.1 billion respectively.

13.4.3 "Critical areas" - Scenario A/C

The "critical areas" scenario addresses the situations where compliance with the MAC-EQS appears not to be possible to achieve at present and, thus, in this respect it is relevant not only to Scenario A/C but also to Scenario B. The scenario only considers modification of outlets in the critical parts of the country where the MAC-EQS compliance problem exists. In this report these parts are identified as those where typically only a dilution factor of less than 3 can be achieved in the summer season - the approximate reduction factor that is needed for the most critical substance, nonylphenol, in stormwater to comply with the MAC-EQS value (see section 9.1.5).

This scenario is considered also to represent Scenario A/C (but not B) with regard to the objective of the WFD of aiming at ceasing or phasing out priority hazardous substances

A forecast of the geographical distribution of the areas connected to some type of storage in 2025 is given in Table 13-7 below as well as a very crude assessment of where treatment might be necessary and where installation of treatment facilities is technically feasible. Some of the existing treatment facilities can be renovated to meet demands of higher treatment efficiency. However, it must be expected that a substantial part of the existing detention ponds can only be used with large modifications. Therefore, the economic benefit of reusing the existing ponds is limited, and it will not affect the overall assessment of costs.

Table 13-7

Forecasted assessment of connected areas in 2025

* Existing storage is sufficient because treatment measures are already implemented.
** Assessment of the percentage of areas that may require treatment measures in Scenario A/C
*** Percentage of areas where new measures may be necessary in Scenario A/C.

Based on the general assumptions in Section 13.4.1 above and the values in Table 13-7, it is roughly assessed that the "critical areas" scenario (Scenario A/C) should include detention/treatment of about 40 % of the volume of stormwater from separate systems (including the vast majority with existing detention systems as these generally need to be strongly modified to fulfil the objective (retention of suspended solids). The same assumptions on the need for additional land are used in this scenario as in Scenario B. The size of this land is around 195 ha in Scenario A/C.  For at better estimate of the volume of stormwater from separate systems representing “most critical areas” a more in-depth analysis of the issue is recommened.

The "critical areas" scenario includes only the outlets for which the initial dilution that can be obtained under summer dry weather flow (median minimum flow) in the streams/rivers is less than a factor of 3. Only nonylphenol (NP) requires such a high reduction factor to comply with the EQS while cadmium requires a reduction factor of 1.6. Presumably, also the PAH levels require reduction prior to discharge, but it is not possible to state an exact reduction factor as currently no MAC-EQS has been established for PAH.

The year 2035 is set as the deadline for implementation of the necessary measures in Scenario A/C. This deadline has been selected arbitrarily, but can illustrate the difference in cost of delaying the actions against these substances compared to Scenario B having a mandatory deadline in 2025. In the discussion of the economic results, the interpretation of the deadlines is further discussed. The starting point of the implementation is set to 2006, even though programmes to implement the WFD do not have to be made operational until 2012. This means that the cost may be slightly overestimated as delaying the investments will reduce the cost in net present value. The results are the net present value of the cost of land acquisition (based on the same cost model as described for Scenario B in Section 13.4.2), the investment cost plus the operation and maintenance costs over the time period^[22]. The net present value discounts expenses made in the future back to the value of that expense today in order to be able to compare different investment packages.

As mentioned, the focus is on the two types of treatment that seem to be most suitable, i.e. options 2a and 2b: Storage in combination with treatment by sedimentation and by filtration respectively. The results are calculated both as a financial cost, which illustrate the cost of the project to the contractors. Another relevant result is the welfare-economic cost, which includes the cost of publicly financing of the investment package - that is a policy action to implement the directives.

Table 13-8 shows the same results for the "critical areas" scenario comprising 40 % of the volume of separate stormwater discharges in Denmark. The welfare-economic cost is DKK 2.0 billion for storage chambers with sedimentation and DKK 4.7 billion if treatment of outlet is added (discount rate of 3 % is used). Some of the cost is due to land acquisitions whereas the cost of investment and operating and maintenance costs of the technology is DKK 1.6 and 4.4 billion respectively.

The total financial cost is DKK 1.2 billion for storage chambers with sedimentation and DKK 2.6 billion if treatment of outlet is added (discount rate of 6 % is used). The cost of investment and operating and maintenance cost of the technology alone (with no cost of land) is 1.0 and 2.4 billion DKK, respectively.

Table 13-8 Financial and welfare-economic costs in a scenario of implementation of retention of sediments in 40 % of all stormwater runoff

Note: Results have been rounded up or down.

In both cases - the "critical areas" scenario (A/C) and the "worst-case" scenario (B) - the costs (reported in Table 13-8 and 13-6 respectively) are considered to be high compared to the typical level of public spending on environmental policy in Denmark for purposes having the degree of specificity as these.

13.5 Concluding remarks on removal of priority substances from stormwater discharges

It is possible to significantly reduce or, in some situations, even practically remove the pollution with priority substances from stormwater, but at a high cost compared to the general level of public spending on the environment. The technology proposed is a well-known (though not currently not often applied in Denmark) general technology which reduces or removes suspended solids to which priority (hazardous) pollutants (and many other harmful chemical substances) tend to be bound and therefore will be retained to a significant degree along with the suspended particles.

It is assessed that in general the concentrations of priority substances in discharges of stormwater from separate systems are so low that to comply with the MAC-EQS values only a very modest, if any, reduction by initial dilution in the receiving stream is necessary. However, in some parts of Denmark (corresponding to about 40 % of the volume of the stormwater discharges) the natural flow in the streams is so low under realistic worst-case conditions (summer median minimum flow) that for a few substances (in particular nonylphenol and maybe PAH) the required reduction in concentration appears not to be achievable in this situation.

With focus on environmental protection the mentioned 40 % scenario is at the same time considered to represent the reasonable requirements for ceasing/phasing out the priority hazardous substances in Scenario A/C while the establishment of detention systems at all stormwater outlets to fresh water systems, i.e. 85 % of the volume of all separate stormwater outlets, represents the "worst-case" scenario, Scenario B, for these substances.

Implementing stormwater retention arrangements in Scenario A/C solely with the purpose of achieving the reduction target of cessation for priority hazardous substances is, however, assessed by the DEPA to be unrealistic. This is taking into account that the reduction target is of non-legally binding character, and that the environmental benefits obtained by establishing retention arrangements are small and thus disproportional to the very high costs.

Footnotes

^[21] With the exception of technology No. 4. This is elaborated below in connection with each technology.

^[22] The following assumptions and arithmetic assumptions are made: The time horizon for the calculation is 30 years, the price level is 2005 prices, linear depreciation of the assets is assumed to calculate scrap values, the discount rate used is 3 % according to the guidelines for economic project evaluation by the Danish Ministry of the Environment, investments are assumed to be made at the end of the year for the purpose of the net present value calculation, the 2005 net present value is calculated.

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