Information about Ekeby wetland, Eskilstuna, Sweden
(last updated october 10, 1999)

1. INTRODUCTION 7. Basins 14. Waterplants
2. The calculation of the wetland and the expected result 8.
Control chambers/weirs
Forest curtains
3. Hydraulic conditions 10. Collecting and distributing channel 17. Investment costs
4. The design of the wetland 11. Outlet channel 18. RESULTS
5. Connection to the wetland 12. Parshall flume/outlet 19. DISCUSSION
6. Inlet channel 13. The wetland park Back

Ekeby wetland


Eskilstuna is the chief town in Eskilstuna municipality (90.00 inhabitants) and is situated about 110 km west of Stockholm.
The biggest sewage treatment plant in Eskilstuna is Ekeby STP which treats the sewage water from the chief town and the neighboring towns (about 95% of all sewage water in the municipality is treated at Ekeby; the rest is treated in 6 smaller local sewage treatment plants).
Eskilstuna sewage treatment plant has been built step by step. It started in 1956, when the first stage - mechanical treatment was put into operation (cleaning bars, pre-airation, sand catcher and primary sedimentation). In 1965, the second stage was put into operation - biological treatment (active sludge plant with airation basins and secondary sedimentation).
Eventually the third stage was put into operation in 1974, when a chemical precipitation plant was built.
At the beginning of the 1990s a new license application was sent to the county board, since the sewage treatment plant had earlier received only a temporary permission. In the application, it was suggested that the treatment demands should be fixed to:

BOD7 < 10 mg/l
tot -P < 0,3 mg/l.

When the application was submitted, a discussion had started in Sweden, about the need of nitrogen treatment.
As early as 1989 at the Ekeby plant the full-scale experiment concerning denitrification had been carried out by means of an external coal source (starch molasses). The experiment showed that it's possible to reduce nitrogen content, but in order to reach a low content of leaving phosphorus at the same time , it would be necessary to build a final filtration plant at the sewage treatment plant.
The costs necessary were appreciated to be at least 50 MSEK. Then, even the operation costs would be very high as an external coal source had to be bought for denitrification.
Because of the high costs, no nitrogen reduction was proposed in the license application.
In 1996, eventually the final cleaning demands were set, valid for the Ekeby sewage treatment plant - BOD7 <10mg/l, tot P <0.3 mg/l and the total nitrogen content mas 15 mg as an yearly average. During the time, the license application was discussed by the authorities, careful studies of the existing process at the Ekeby treatment plant were carried out, accounted for the flow and mass balance.
In Oxelösund and Hässleholm large wetlands were built . The research showed that it should be possible to receive enough high nitrogen reduction in a newly built wetland (about 2.5 t N/ha.year) for nitrified water.
Gradually, the work was aimed at a wetland solution to reduce the nitrogen content at the Ekeby sewage treatment plant.
There were favourable conditions , a.o. that all nitrogen in the sewage water would be nitrified in the existing active sludge plant (not in the winter months), there was suitable land etc.

A proposition for a construction of a wetland was prepared and shown to the board of Eskilstuna Energi & Miljö in August 1996.
The board decided in favour of a wetland. In the autumn of 1996 the plant was designed and inquiries were made to obtain a contract. The purchase of a contract took place in May 1997 and the construction work started in the middle of June 1997. Land work was finished in October 1997 and in the winter 1997/1998 the foundation laying and concrete laying were carried out.
In the spring and summer of 1998 the water plants were planted in the ponds and in April 1999 water from the sewage treatment plant was let into the ponds.

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2. The calculation of the wetland and the expected result

The running of the sewage treatment plant is optimized so that the nitrification is carried out in the biological stage. The nitrogen in the sewage water plant will then mainly occur as nitric nitrogen and the wetland will therefore mainly work as a denitrification stage.

During the dimensioning of the wetland, it has been supposed that the supplied water is mostly nitrified. The wetland will be operated approx. 6 months per year between April/May - October.

The dimensioning data of the wetland:
Total surface ca 40 ha
Total basin surface ca 30 ha (inkl. channnels)
Total bank length 9.1 km (fit for running 5.6 km, unfit for running 3.5 km)
Waterdepth average 1 m
max ca 2 m
Volyme 300 000 m3
Waterloading* min 350 l/s (30 000 m3/d)
average 490 l/s (42 000 m3/d)
max** 1 400 l/s (121 000 m3/d)
Retention time min ca 3 days
average ca 7 days
Harvest - weed after 3-5 years
* Incl. rain water
** During the higher flows than the max. the rest of the sewage water will be overflowed into the Eskilstuna river

The wetland´s total basin surface is ca 30 ha.
The wetland is dimensioned for a reduction of about 2.5 ton of total nitrogen per ha and per 6 months operation. Retention time is ca 7 days.
In that case the wetland will reach the requirement of the authorities for max 15 mg /l of the treated sewage water, calculated as an annual average.
Not only the nitrogen reduction will take place in the wetland, but also a phosphorus reduction.
The content of the outgoing phosphorus will be probably 0,1 mg/l which means that further ca 1200 kg phosphorus will be separated per year (the present discharge is about 3800 kg a year).

Another positive side effect of wetland treatment is that the outgoing content of bacteria will be considerably reduced. The treated water will be of the same quality as the bathwater concerning the content of germs.

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3. Hydraulic conditions

The wetland is dimensioned for a max. of incoming water flow of 1400 l/s. Annual average flow is 520 l/s (incl. rain water).
Average flow per day during a dry period is about 440 l/s. Average day, max. hour (dry period) about 570 l/s.
Max. permitted water level in the inlet channel and basins 1-5 is +0.60 m (the system of height in Eskilstuna municipality)
Max. permitted water level in the distribution channel and basins 6-8 is +0.40 m.
Max. permitted water level in the outlet channel is +0.20 m.
Water level in the Eskilstuna river can vary between -0.80 - +0.00 m.
Normal water level in the river is -0.80 - -0.60 m.
Water level in the basins 1-5 (all the basins in operation) can be adjusted during an inlet of 570 l/s to an optional value between +0.20 and +0.60 m.
Water level in the basins 6-8 (all the basins in operation) and in the distributing channel can be adjusted during an inlet of 570 l/s to an optional value between +0.00 and +0.40 m (provided: max. level in the Eskilstuna river is -0.60 m).

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4. The design of the wetland

As evident from the survey below, the available surface is divided into a a number of basins that can be shut off between themselves( planted with wetland plants). The sewage water treated in the sewage treatment plant will be transferred from a final sedimentation outlet channel to an inlet channel which stretches alongside Folkestaleden and distributes water to the first basins.

Water is led from the inlet channel to the respective basin via an inlet pipe through the banks. Water flow to the respective basin is regulated individually by weirs situated in every basin ´s outlet end. The outlet pipe from the pond to the weir is made of a pipe Ø 800. These weirs have remote-controlled level regulation.
The water level is regulated so that the max level +0.60 m( the municipal height system) won´t be transgresse.
From the basins 1-5 water is collected in a common collecting/distributing channel. In the channel the water level is kept on the max 0.40 m, partly by a weir ( the collecting/distributing channel passes into the outlet channel) which is placed directly behind the passage of the Hällby ditch and partly by a weir which is in the outlet of the basins 5-8.
Usually all water from the basins 1-5 passes through the collecting/distributing channel to the basins 6-8. But in this way, it is also possible to lead everything or part of the water from the basins 2-5 directly to the outlet channel. From the basins 6-8 the water is led to the outlet channel common to the whole wetland. The channel is connected to the Eskilstuna river via a Parshall flume. There is a small test house where the flow - and test equipment is placed.

By means of a mechanically regulated weir between the outlet channel and the Mesta bank (in the vicinity of the pump station R19) it is possible to lower the level and partly drain the wetland ( the deep zones of the ponds can´t be drained). The water can be pumped into the Eskilstuna river by R19.

According to this design and by means of the chosen water levels the water can be led from the sewage treatment plant through wetland´s basins into the Eskilstuna river without any pumping at all. The existing bank pump station R19 will be kept for draining the ditch alongside E 20, Mesta ditch and if necessary for lowering the water level in the basin or for draining them.

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5. Connection to the wetland

The purified sewage water from the sewage treatment plant is led into the inlet channel and wetland via a conduit Ø1400. The conduit is connected to a newly built chamber at the final sedimentation outlet channel. In the both ends of the basin outlet channel there are installed mechanically regulated shut-off valves. A valve for inlet to the wetland and a valve for the old outlet will be used when the wetland is not in operation.

There is also a pump station for rain water connected to the inlet channel, which makes it possible to pump rain water into the wetland from a conduited ditch, situated below the pond 1. The conduition is connected to an open ditch just behind the pump station. Rain water comes from the ASG terminal , which is south of the wetland, on the other side of Folkestaleden.

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6. Inlet channel

The inlet channel stretches from Ekeby sewage treatment plant, alongside Folkestaleden to the end of the wetland in the north western part of the area. the channel´s total length is 1.5 km. Water is distributed from the inlet channel via a inlet tin pipe (Ø 600) through the banks to the first 5 basins. Basin 5 is north of the Hällby ditch which cuts through the whole wetland area. To reach the basin 5 the inlet channel dives below the Hällby ditch through a conduit Ø800.

The inlet channel´s bottom is -1.10 and if the max water level is +0.60, it means a depth of 1.70 m.
The breadth of the channel bottom varies between 4.0 m as far as the Hällby ditch. Behind the Hällby ditch the bredth is 2.0 m.
The channel´s embankment gradient is 1:2

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7. Basins

The wetland consists of 8 basins, divided into 2 stages. The first stage consists of 5 basins the second of 3 basins.
The basins´ bottoms are formed by the existing land(soil), no digging was necessary. In the ponds there are deeper zones which were dug. These deep zones are placed directly at the inlet to the pond, behind artificially created islands and in the outlet zone of the channel. The point with these diggings is to prevent overgrowing with weedes so that the water can be equally distributed through the basin.

The basins' surfaces and diggings are spread as follows:

Basin nr Surface (m2) Digging (m2) Digging (%)
1 26 000 7 600 29
2 31 800 6 200 19
3 38 300 8 600 22
4 24 200 4 500 19
5 32 200 11 500 36
6 30 200 3 500 12
7 38 100 8 300 22
8 29 200 14 100 48
Sum 250 000 64 300 26

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8. Banks

The banks are built in two ways.

Some of the banks (5600 m) are fit for driving and have a top bredth of 5 m. These banks are made of friction material and covered with thick clay from the digging in the ponds. On top of the banks fit for driving there is a geotextile and ca 200 mm roadbed.

The other banks (3500 m) are unfit for driving, built of clay from the ponds diggings. These banks have a top bredth of ca. 3,0 m.

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9. Control chambers/weirs

Outflow from every pond is controlled by a control chamber equipped with a weir, whose bredth is 3.0 m.
The control chamber is made of concrete and the connection to the pond is made of a tin pipe Ø 800.
See the picture below:

Besides the weirs in the 8 basins there are another 2 weirs of the same type as above. One is between collecting and distributing channel and one is at the outlet channel and works as an overflow equipment for the Mesta stream bank.

Weirs for the basins 1-5 can be regulated between +0.10 - and +0.80 m levels. the levels -0.10 - +0.60 are valid for basins 6-8 as well as for the weir between collecting and distributing channel and outlet channel.

The outlet from the outlet channel can be regulated between -0.30 and +0.40 m.

Downstreams all the weirs, the banks are covered with erosion protection (macadam) to prevent flushing and erosion of the banks.

The control of all weirs is done mechanically or manually. Remote control of the weirs is done from a central computer terminal in the Ekeby sewage treatment plant.

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10. Collecting and distributing channel

This channel takes in water from the basins 1-5 and distributes the water to the basins 6-8 and if necessary to the first part of the outlet channel, situated parallelly with the basin 6 .
The distribution of the water to the basins 6-8 is carried out in the same way as the distribution to the basins 1-5 that is through a pipe Ø 600 through the banks.

The channel is ca 600 m and its bottom level is -1.60. Max. water level is +0.40, which means that the max. depth is 2.0 m. The bottom bredth is 2.20 m and the embankment gradient is 1:3.

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11. Outlet channel

The outlet channel runs alongside the Eskilstuna river and the Mesta stream´s bank and collects all the water that passes through the basins 6-8 sw well as the water which passes through the first part of the outlet channel, which runs alongside the basin 6.

The first phase of the outlet channel is about 300 m and the second, the final outlet channel is ca 450 m. It ends at a flume at the Ekeby sewage treatment plant.

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12. Parshall flume/outlet

In the end of the outlet channel there is a Parshall flume, where the flow measurement is carried out by means of a sonic-depth finder. All the water which passes through the wetland must pass the flume before it flows into the river.The Parshall flume is made of concrete.

The flume is connected to the existing conduit (Ø1600) that leads into the Eskilstuna river. Before the wetland was built, the purified sewage water was released from Ekeby sewage treatment plant into the Eskilstuna river via this conduit. The conduit will be used when the wetland isn´t in operation.
In connection to the flume there is a testing house where the water testing takes place.

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13. The wetland park

In the wetland, a small area has been used to build a wetland park. This area has been earlier used as a sludge deposit. The sludge has been deposited in the banks and new banks have been built later. The area has been covered and tightened with clay and a drainage system surrounds the area to prevent possible leakage to reach the wetland. The drainage system is connected to the incoming conduit at the Ekeby sewage treatment plant.

In the wetland park a more organised vegetation with artificial streams and water steps is planned, which will turn it into a park rather than to a wetland.

There are 3 small ponds, whose water is from the river, not the sewage water. There are also 2 fairly large groves in the area.

We also plan to build a visitors´ house in the park. The house will be used for study visits and exhibitions. Different associations and organisations (i.e. ornithological organisation and the society for the conservation of nature) may use the house.

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14. Waterplants

The vegetation in the ponds with reed(phragmites communius) and reed sweet grass(glyceria maxima) is a good source of coal for a a microbiological breaking down process . They grow fast and create a large attaching area for the bacterias in the water.

Different floating leaves species such as coontail (ceratophyllum demersum), frogbit (hydrocharis morus-ranae), spiked water-milfoil (myriophyllum spicatum), different pond weed (potamogeton sp), duckweed (lemna) and yellow water lily (nuphar lutea) thrive in the nutritious nitrogenous basins.

The natural shore arts auch as iris, marsh marigold and dock also provide a basis for a rich insect life in a good blooming period and a varying bird life in Ekeby wetland.

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15. Forest curtains

Alongside E 20 and Folkestaleden a vegetation curtain has been planted, 50 m broad alongside E 20 and 15 m alongside Folkestaleden. Totally about 5 ha have been aforested.

The vegetation consists of mixed forest and bushes such as sloe, rosehip, alder, birch, poplar, spruce, ash, oak, bird.cherry, willow, mountain ask, dogwood a.o.

In the middle of the forest curtain alongside E20 there are artificial ditch leading into the existing ditches.

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16. Construction

In 1996 we started to plan a new wetland to reach the new nitrogen requirements and in August 1996 the board of Eskilstuna Energi & Miljö decided that a wetland should be built.

Investigation and planning started in the autumn of 1996 and on March 12, 1997 inquiries have been sent to some contractors.

The purchasing lasted during the spring of 1997 and in May, we made a contract with NCC that tendered the best bid.

On June 25, 1997 the work started and the first step taken was to change the existing optocable which crossed the whole wetland area.

Between July and September most of the ground work was carried out. It took only 3 months depending on the fact that the summer was warm and dry, which was necessary to remove the clay.

During the autumn of 1997 and winter/spring 97/98 we went on building concrete constructions and rebuilding the banks. In the spring of 1998 we started to plant the vegetation in the ponds. This work was carried out by AMA and Park & Idrottskontoret, Eskilstuna kommun.

The wetland was finally inspected in August 1998 and in the autumn the ponds were filled with the rain water. The wetland was taken into fully operation in the mid April 1999.

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17. Investment costs

The construction of Ekeby wetland has cost 16 million SEK distributed as follows

Contract 11,5 MSEK
Purchase of the ground 1,3 MSEK
Consultants 0,4 MSEK
Costs of planning 0,6 MSEK
Vegetation 0,25 MSEK
Internal costs 1,7 MSEK
Other costs 0,25 MSEK

The total investment sum of the nitrogen removal project is ca 20 million SEK (US$ 2,4 M) including all the steps taken at Ekeby sewage treatment plant (changing of the blowers, blowersystem etc.)

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The wetland was taken in operation on April 12 1999 when the water was let in from the final sedimentation in the sewage treatment plant.
On Friday April 16 the basins were filled and the water began flowing out of the outlet channel.
Below in enclosure 1 some results concerning phosphorus, nitrogen, BOD and bacteria are presented in figures.
The sample values are presented in enclosure 2.

Figure 1. Total Phosphorus, all values
Figure 2. Total Phosphorus, week samples
Figure 3. Total Phosphorus, monthly average
Figure 4. Total Phosphorus, quarterly average
Figure 9. Total Phosphorus, monthly average (mg/l)
Figure 10. Total Phosphorus, monthly average (t)
Figure 11. Phosphorus reduction
Table 1. Total Phosphorus, all values
Table 3. Phosphorus, water from the final sedimentation
Table 4. Phosphorus, water from the wetland

The reduction of phosphorus (tot-P) in the wetland is very good. The mean value to the wetland during the period April-August is 0,17 mg/l tot-P and the leaving mean value is 0,08 mg/l tot-P. This is a reduction of 53%.
During July-August total phosphorus from the wetland is lower than 0,05 mg/l (figure 1). As can be seen from figure 11 the phosphorus reduction increases and the mean value reduction is 0,019 t/ha year.


Figure 5. Total Nitrogen, all values
Figure 6. Total Nitrogen, week samples
Figure 7. Total Nitrogen, monthly average
Figure 8. Total Nitrogen, monthly average, in and out
Figure 9. Total Nitrogen, monthly average (mg/l)
Figure 10. Total Nitrogen, monthly average (t)
Figure 11. Nitrogen reduction, different fractions of nitrogen
Table 1. Total Nitrogen, all values
Table 2. Total Nitrogen, to and from the wetland
Table 3. Nitrogen, water from the final sedimentation
Table 4. Nitrogen, water from the wetland

As you can see from figure 9 the nitrification of nitrogen in the activated sludge plant slowly starts in May and is nearly total in July and August. At present (September) the nitrogen reduction is 5-6 mg/l. This is not enough to reach the demand - a year average less than 15 mg/l as the wetland is closed in the period of November- mid-March.
An experience from the studies in Linköping is that the nitrogen reduction in the first year of operation is not so good due to lack of accessible carbon in the system. The following years show much better results.
As can be seen from figure 11 the nitrogen reduction is increasing (except July) and the mean value reduction is 0,62 t N/ha year. To reach the demands it's necessary to reduce nitrogen content with about 2,5 t N/ha year.


Figure 12. BOD7, week samples
Figure 13. BOD7, monthly average
Figure 14. BOD7, quarterly average
Table 2. BOD7, to and from the wetland
Table 3. BOD7, water from the final sedimentation
Table 4. BOD7, water from the wetland

The BOD7 leaving the wetland is very low, below 3 mg/l. No increase in the BOD7 can be noticed as is the experience in Hässleholm when the water passes the wetland. An explanation to this can be that in Hässleholm floating algea can leave the wetland over the weirs. This is not possible at Ekeby wetland due to the construction of inlet pipes and control chambers with weirs. No floating substances can leave the wetland.


Figure 15. Coliform bacteria
Figure 16. E Coli
Figure 17. Fecal Streptococci

Coliform bacteria, ecoli and fecal streptococci have been examined. The reduction is very good and has been reached already in the first stage (samples taken in the distribution channel). The water from the wetland is of the same quality as in the recipient - the Eskilstuna river.

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It is important to the economy that a suitable land is accessible in the vicinity of the sewage water which will be treated. Long way transportation pipes which need pumping will considerably increase investment costs as well as operative expenses.
The soil where the wetland will be established must be of suitable quality partly topographically, partly geologically and geotechnically.
The soil should be properly inclined in order to avoid water pumping. The soil should be dense to avoid leakage and possible pollution of ground water.
The area should be adapted for water plants, too. The soil should, therefore, consists of dense clay covered by a stratum of top-soil.
Eventually the soil should be stable (from geotechnical point of view) so that there will be no risk of landslide.
The wetland should be formed by transferring of soil in the area. In this way, the costs will be kept down. It can be necessary to supply extra friction material to get banks fit for driving
The construction can be realized only if the weather conditions are good that is - dry weather so that there will be no problems for caterpillars, bulldozers and wheel loaders/lorries.
When the banks will be laid, it is necessary to take the settlement of the soil into consideration; they must be laid higher than necessary depending on the subground.
Concrete constructions for e.g. control weirs, measuring flumes etc. should be piled in order to avoid settlements.
Certain banks must be fit for driving for lighter vehicles so that the maintance can be done.
The channels must be dimensioned for low hydraulic friction losses.

Water plants

In order to reach fast vegetation the sowing and plantation of suitable water plants should take place a year before the wetland will be operated.
The irrigation will be done either by natural rainfalls or by small amounts of sewage water.
Suitable water plants are plants whose leaves are big under water , are easily decomposed and will create easily accessible coal for the denitrification bacteria. The water plants should be adapted to the local climate.Even if there is no plantation, a spontaneous vegetation will take place if the soil is suitable and especially if the used soil has earlier been an old natural wetland that left a seedbank.


The leaving content of SS is less than 5mg/l for the water which passed the wetland. This content is approx. the same as that which is normally absorbed by the wetland. Since the incoming content consists mainly of particular phosphat-phosphorus and the leaving total content of phosphorus is very low, the leaving content has changed its composition.
Another phosphorus reduction, more powerful, takes place in the wetland. A leaving total phosphorus content of less than 0.05 mg/l is the result. The low phosphorus content can limit the growth of the plants.
The amount of chemicals for the reduction of phosphorus at the sewage treatment plant can be reduced.
No higher leaving phosphorus content could be found at the start of the wetland. Perhaps, this can depend on the fact that many plants have been established the year before the wetland was operated and used the possible surplus of the phosphorus in the soil.
In the activated sludge of the sewage treatment plant a complete nitrification of ammonium-nitrogen into nitrite- nitrogen takes place. Nitrification starts in May when the water temperature raises.
A basis for a good reduction of nitrogen in the wetland is a good nitrification that has already taken place in the sewage treatment plant. In that case, denitrification will take place in the wetland and nitrite-nitrogen will be transformed into gaseous nitrogen.
Sufficient denitrification (and sufficient nitrogen reduction) haven't been yet found in the wetland. However, this is in accordance with the experiences from a trial operation in Linköping and with the full scale operation in Hässleholm (south Sweden).
The reduction of nitrogen increases considerably in the 2:nd year and the following years. It depends on the fact that the bottom sediment increases and that there is more and more easily accessible coal in the system. Good bacteria reduction is reached in the wetland to 1/100 - 1/1000.
There were no smell problems and no mosquitos.
So far about 60 different species of birds have been observed in the wetland. There is fish in the ponds. A fairly great number of fish is in the outlet channel; they got there from the recipient - the Eskilstuna river.


The wetland in Eskilstuna is very cost-efficient plant for the reduction of nitrogen, phosphorus and bacteria.
The wetland has fairly low investment cost and operation expenses.
An alternative addition with denitrification in the sewage treatment plant needs among other construction of a new final filter plant to reach the required purifying result.
Such a plant needs about 3-4 times higher investment cost, that is 50-60 MSEK. Since an external source of coal must be added, too, the running expenses will be much higher (about 10 times) than for a wetland.


By constructing a wetland located not far from the center of Eskilstuna , an interesting environment has been created for environmental friends and ornithologists.
The plants is also very important for good publicity.

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