Fewer sand filters, but increasing number of mini wastewater treatment plants
Nature and the environment;Nature and the environment
avlut, Discharges and treatment of municipal waste water, sewer system, treatment plant, sludge, grey water, black water, sewage, discharges to water, nitrogen emissions, phosphorus emissions, heavy metals, organic pollutants, organic material (COF and BOF5), waste feesWater and waste water , Pollution and climate, Nature and the environment
In 2015, there were around 330 000 small wastewater facilities in Norway. Compared to 2002, this is a reduction of around 10 000 facilities, or around 3 per cent. The reduction is most evident among sludge separators with sand filters, which is down 43 per cent from 2002 to 2015.

Discharges and treatment of municipal waste water


Next update

Key figures


tonnes of phosphorus (TOT-P) is annually being discharged from wastewater facility 50 pe or larger

Municipal wastewater - discharge, treatment and sewage sludge
2015Per cent
2014 - 20152011 - 2015
Number of wastewater facilities, 50 pe or larger2 6880.4-1.9
Capacity of wastewater facilities 50 pe or larger (1 000 pe)7
Discharge phosphorus from wastewater facility 50 pe or larger (tonnes TOT-P)912.7-3.50.0
Discharge organic material from wastewater facility 50 pe or larger (tonnes BOD5)43 751-3.3.
Disposal of sewage sludge to soil improvement (tonnes dry weight)110 3946.220.3
Used in agriculture87 60611.937.2
Used in parks or green spaces4 392-40.5-56.4
Delivered to soil producers18 3960.63.0

See more tables on this subject

Table 1 
Operating expenditures, capital costs, other income and fee calculation basis (NOK million)

Operating expenditures, capital costs, other income and fee calculation basis (NOK million)
Operating expenditureCapital costsOther incomeFee calculation basis
20113 986 1211 937 628240 3685 683 381
20124 212 5151 885 465211 0365 887 303
20134 407 3672 103 831236 1266 275 071
20144 938 1102 285 467315 6066 907 975
20154 964 1002 324 459304 8976 983 662
Østfold326 513143 13912 994456 658
Akershus and Oslo1 115 990508 40460 8331 563 561
Hedmark217 65084 8133 727298 736
Oppland292 97588 08711 869369 193
Buskerud287 836158 89820 495426 239
Vestfold277 050136 0168 965404 101
Telemark217 43294 37329 350282 455
Aust-Agder141 17971 6126 558206 234
Vest-Agder183 68872 43310 124245 997
Rogaland459 606192 72942 544609 791
Hordaland424 380250 56417 238657 706
Sogn og Fjordane91 77641 7142 653130 836
Møre og Romsdal208 347102 9553 173308 129
Sør-Trøndelag225 873116 31711 694330 496
Nord-Trøndelag131 03256 2467 112180 166
Nordland174 18199 96413 519260 626
Troms - Romsa120 28473 7862 024192 046
Finnmark - Finnmárku68 30832 40940 02560 692

About the statistics

Discharges and treatment of municipal waste water is a set of statistics which describes different sides of the municipal wastewater sector e.g. fees, self cost, wastewater facilities and connection, compliance with treatment permits and sludge disposal.


Definitions of the main concepts and variables

Wastewater facility is defined in the Pollution regulation (FOR 2004-06-01 nr 931) as any plant handling wastewater consisting of one or more components: wastewaterpipelines, wastewater treatment plant or a discharge device (unofficial English translation).

Biological oxygen demand (BOD5) and chemical oxygen demand (COD)
are parameters which indirectly measure the level of organic material in wastewater. BOD5 measure the amount of oxygen consumed biochemically in water after 5 days (standard test is 20 degrees celcius over 5 days). COD also measure the amount of oxygen consumed, but this time a strongly oxidising agent is added to the water (usually containing sulphur acid and potassiumdicromat (K2Cr2O7)).

COD will normally show a higher level than BOD5, since more organic compounds will be oxidised and degraded by the added chemicals, as compared to a BOD5-test.

Operating costs constitutes the sum of administrative-, management- and maintenance costs. Data is derived from KOSTRA-form number 23: Cost absorption in water-, waste water and waste-sector’.

Contribution margin ratio refers to, in percent, how big a share of the annual wastewater related costs the municipalities actually cover by wastewater fees. Municipalities are not entitled to claim more than actual costs, neither are they obliged to claim full cost coverage.

Wastewater fees, claimed by the municipality, consist of a connection fee and an annual wastewater fee. The connection fee is collected once only - during installation - while the wastewater fee is collected every year.

Capacity and load. The capacity of a treatment plant is the amount of wastewater it is designed to handle, while the load is the amount of wastewater a wastewater plant actually receives. The unit of both capacity and load is provided in population equivalents (pe).

High-grade wastewater treatment plants are those that provide a biological and/or chemical treatment phase. Biological treatment mainly removes readily degradable organic material using microorganisms. The chemical phase involves the addition of various chemicals to remove phosphorus. Certain treatment plants also have with special phases for nitrogen removal. High-grade plants reduce the amounts of phosphorus and other pollutants in the effluent more effectively than mechanical plants.

Full cost ratio reflects the relationship between the income from fees and the fee calculation basis, and includes fund provisions and purchases from funds.

Capital cost consists of the following two costs: Depreciation of earlier annual investments and a calculated interest cost for capital goods.

Municipal wastewater facilities include all wastewater facilities and treatment plants with a capacity of 50 pe or more, also including those with no municipal ownership (for example privately owned). The facilities are generally divided into six groups: direct discharge, mechanical, chemical, biological, chemical-biological, and natural purification processes/other treatment.

Mechanical wastewater treatment plants include sludge separators, screens, strainers, sand traps and sedimentation plants. They remove only the largest particles from the wastewater, thus treatment efficiency in regards to nitrogen and phosphorus is relatively low.

The North Sea Agreements/OSPAR convention refers to the joint declarations made by the countries around the North Sea to reduce inputs of nutrients to this sea-area. One of the targets was to halve the total inputs of nitrogen and phosphorus during the period 1985 to 1995. Since Norway did not reach the nitrogen target by the end of 1995, the national time limit was extended to 2005. The North Sea Agreements applies to the areas south of the 62o N. As for the nutrient reduction targets, only the counties with drainage into the Skagerak and North Sea, from the Swedish boarder to Lindesnes, are bound by the agreement.

North Sea counties entail the following counties: Østfold (01), Akershus (02), Oslo (03), Hedmark (04), Oppland (05), Buskerud (06), Vestfold (07), Telemark (08), Aust-Agder (09) and Vest-Agder (10). Practically all land areas in these counties drain into the Skagerak and North Sea.

Population equivalent (pe) is defined as the amount of oxygen consumed in 5 days when organic material is decomposed in water. When 1 pe is defined as 60 g BOD5, it means that 1 pe will bring in daily the amount of organic material that microorganisms need 60 g of oxygen to decompose within 5 days.

Wastewater treatment plants are generally divided into three main groups according to the type of treatment they provide: mechanical, biological or chemical. Some plants incorporate combinations of these basic types. In addition, combinations of these basic types also exist.

Treatment efficiency refers to the relative change in weight of a particular chemical substance between the measuring points in and out of the wastewater treatment plant. E.g. 30 per cent treatment efficiency means the 30 per cent of the amount going in to the wastewater treatment plant are removed before discharge into the recipient.

Individual wastewater treatment facilities are designed to handle wastewater equivalent to the amount, or composition, of no more than 50 pe (generally, private plants in areas with scattered settlements). Individual wastewater treatment facilities also include those facilities, which until 2000 was named "separate wastewater facilities". Since the term "individual wastewater facilities" comprise a larger population than "separate wastewater facilities", the two cannot readily be compared.

Wastewater facilities without treatment refer to discharges originating from wastewater facilities without treatment, also commonly referred to as direct discharges. The discharge is connected to municipal pipelines, but it doesn't take place any form of treatment.

Annual cost is the sum of operating and capital costs.

Standard classifications

Classification of county.

Administrative information

Name and topic

Name: Discharges and treatment of municipal waste water
Topic: Nature and the environment

Next release

Responsible division

Division for Natural Resources and Environmental Statistics

Regional level

The figures are published on national and regional level, however in some cases the figures may also be presented according to catchment area etc.

Frequency and timeliness

Key figures are published annually in connection with Municipal-State-Reporting (KOSTRA) on the 15 th of Mars (un-revised) and 15 th of June (revised). In addition, a report is being published each year, usually in the end of the year, summarising the wastewater treatment situation in Norway.

International reporting

The statistics is reported to EUROSTAT/OECD and EFTA Surveillance Authority (ESA).


The datamaterial is saved as text files. Revised dataset are saved in Oracle databases with Statistics Norway, but also with the Norwegian Environment Agency (Miljødirektoratet).


Background and purpose

The purpose with the statistics is to give an overview of the status and development in the municipal wastewater sector. Among other things, it provides information about number of wastewater plants, treatment capacity, type of treatment, degree of people connected, sludge disposal, discharges, economical figures in wastewater sector (investments costs and income), together with illustrating, on a municipal level, the relationship between revenues from wastewater fees, annual costs, and wastewater tariff.

Data collection was carried out sporadically in the 1980s, however since 1990 there's been a data collection on an annual basis. First it was administered under the name SSB-avløp, a co-operation between county administrations and Statistics Norway; secondly there was SESAM (1998-2001); and now finally, there's the Municipality-State-Reporting (KOSTRA) (from 2002).

Users and applications

Important user groups of the wastewater statistics are the different national and regional public administrations/governments, mainly the Ministry of Environment, Norwegian Environment Agency, the different county administrations and municipalities. To various degrees, other potential user groups are the media, business and industry and NGOs.

Equal treatment of users

Not relevant

Coherence with other statistics

The statistics is related to other statistics concerning discharge and pollution of water. The discharge figures of the wastewater sector, together with the similar figures from agriculture, industry and aquaculture, add up to a national account on nitrogen and phosphorous discharges for Norway. Norwegian Institute for Water Research (NIVA) publishes the account every year in their so-called TEOTIL-report.

Legal authority

Data are collected by Statistics Norway on behalf of the Climate and Pollution Agency with legal basis in the Pollution Control Act (13th Mars, 1981, no. 6) . Statistics Norway can make use the collected data for official statistical purposes with legal basis in the Statistics Act. (16 th June 1989, no. 54, §2-2).

EEA reference

Council Directive of 21 st May 1991 concerning urban wastewater treatment (91/271/EEC).



All municipalities (complete collection).

Data sources and sampling

The Municipal-State-Reporting system, Kostra, constitutes the prime data source.

Notably, since 2001, all municipalities have reported their accounts electronically with KOSTRA, and this created, together with the new account scheme, a way of utilizing the accounts as a powerful information source to financial statistics within municipal wastewater sector.

However, by collecting data directly from the municipality’s accounts, information from various organisation structures will not be collected because they are not visible in the account for the municipality. Therefore data for cost absorption since 2006 is collected via KOSTRA questionnaire 23 (Kostnadsdekning i vann, avløps- og avfallssektoren). The data reported must be limited to the instruction for calculation of full cost.

Physical data: Complete collection. All the municipalities are reporting yearly on behalf of more than 2 700 wastewater facilities (approved for 50 population equivalents (pe) or more) and around 300 000 individual wastewater facilities, the latter most commonly situated in scattered settlements.

Financial data: Complete collection. Annual reporting for all municipalities.

Collection of data, editing and estimations

Data is collected electronically once every year, and the information should be in hands of Statistics Norway by 15 th of February.

The electronic forms contain built-in consistency checks and logical tests. When the data arrive Statistics Norway, they will be checked automatically and manually.

Unrevised data will be published on 15 th of Mars. There's a second deadline on the 15 th of April for municipalities to correct possible errors in the figures just published.

The KOSTRA control and revision process is constantly adjusted and improved.

The information is collected with legal basis in the Pollution Control Act. Norwegian Environment Agency finances part of the work, and are allowed to comment on the different figures before they're being publishing.

The most updated and detailed theoretical framwork and methodology applied in wastewater statistics is published in Norwegian language only, see the report Municipal wastewater: Expenditures, investment, discharges, treatment and disposal of sewage sludge. Wastewater fees. A brief summary of the methodology is however presented below, while the remaining details is only available in Norwegian.

Calculation of discharges of phosphorous and nitrogen

Due to the fact that only part of wastewater facilities undertake direct analysis of pollution loads and discharges, some level of estimation and the use of standard factors needs to be applied in the calculations. Thus, total discharge and treatment efficiency figures of nitrogen and phosphorus are in part based on estimations (see below for further details).

The following calculation procedure has been applied to municipal wastewater facilities, capacity of 50 population equivalents (pe) or more:

  1. If information on discharge quantity, expressed as kilogram per year, has been reported, then this information will be used directly.

  2. If no information as specified in point 1 above has been reported, but outflow concentrations and average water quantities have been provided, then discharge quantity is calculated in kilos per year from the following equation:

    Concentration (mg/l) x average water quantity (m 3 /year) / 1000

  3. If neither point 1 nor 2 above can be calculeted, then discharge quantities will be estimated by multiplying the number of people connected to the wastewater facility with a standard factor for average discharge per person and another factor for standard treatment efficiencty. These factors are listed below:
Average discharge per person per day:

1.8 gram (phosphorus)

12 gram (nitrogen)

Standard treatment efficiencies of different types of treatment plants (per cent):

Type of treatment















Natural purification processes/other



Discharge of nitrogen and phosphorous per year at the outlet of wastewater facility can then be estimated from the following equations:

Phosphorous: ((number of persons connected x 1.8 x 365) / 1000) x average treatment efficiency

Nitrogen: ((number of persons connected x 12 x 365) / 1000) x average treatment efficiency

Small wastewater treatment facilities (less than 50 pe), usually individual plants, make use of the same set of standard factors on average discharge of nitrogen and phosphorous per person per day, as mentioned above. However catogories for type of treatments and treatment efficiencies differs somewhat. Therefore, treatment efficiencies for small wastewater treatment plants, in percentage, are specified below:

Type of treatment



Direct discharges



Sludge separator



Sludge separator with infiltration



Sludge separator with sand filter



Mini wwtp, biological



Mini wwtp, chemical or chemical-biological combined



Sealed tank*



Sealed tank for blackwater**



Biological toilet**



Constructed wetland**



Sealed tank for black water, filter for grey water**



Biological toilet, filter for grey water**



Other treatment***



* Wastewater from sealed tanks are normally delivered to other wastewater plants and treated elsewhere, thus, discharge and treatment efficiency calculations are included with them.

** Standard factor set up in collaboration with Bioforsk.
*** Standard factor set up in collaboration with Norsk Vann.

Calculation of discharges of organic material, heavy metals and persistant organic pollutants

The calculations are mainly built on the method suggested by Blytt og Storhaug (2008) . The method was originally designed for heavy metals and persistant organic pollutants, but it has now also been extended to cover also organic material (biological oxygen demand (BOD 5 ) and chemical oxygen demand (COD)). The statistics presented here covers only discharges from treated wastewater to the water recipient. In addition the statistics, so far, only covers discharges from wastewater treatment facilities 50 pe or larger.

The following parameters are included in the statistics:
- Arsenic (As)
- Cadmium (Cd)
- Crom (Cr)
- Copper (Cu)
- Mercury (Hg)
- Nickel (Ni)
- Lead (Pb)
- Zink (Zn)
- Biological oxygen demand (BOD 5 )
- Chemical oxygen demand (COD)

A. Annual discharge from wastewater facilities reporting annual discharge data
Calculation of discharges from these wastewater facilities are relatively straight forward. The discharges constitute the total sum of calculated discharges from all these facilities (either reported as (1) kg discharge per year or (2) the combination of the volume of water treated and average concentration of the parameter).

According to the Forurensningsforskriften ("Pollution regulation", FOR 2004-06-01 no 931), chapter 11, wastewater treatment plants larger than 20 000 pe are required to carry out analyses for content of heavy metals, while wastewater treatment plants larger than 50 000 pe in addition also are required to analyse for a selection of organic pollutants. That means large wastewater treatment plants directly or indirectly report actual discharge data which can be used for statistical purposes, while discharges from smaller treatment plants are largely unknown.

B. Annual discharge from wastewater treatment plants with no reported discharge data
Calculation of discharges which are not covered by the Pollution regulation, or for some other reason are missing, will be estimated for. The estimations are based on standard factors - discharge per person connected. The data source to establish these factors are based on actualy data from treatment plants in "point A" above. Data on people connected to wastewater facilities exist in most cases, even if they don't carry out analyses, and thus, this informatio is the used to upscale discharges to national level.

Thus, for every wastewater facility in "point A" above with reported discharge data, we will first calculate a specific discharge for each chemical component (unit: microgram / person connected):

Spesific discharge = microgram discharge per year / number of people connected

This is carried out on every wastewater facility containing actualy data for every component being analysed. Then these wastewater facilities are divided into two categories based on their type of treatment:

  • Treatment category I: direct discharges, mechanical and other treatment
  • Treatment category II: chemical, biological, chemical-biological and natural purification

For each of these treatment categories, the median value for each heavy metal and each organic pollutant will become the overall specific discharge factor. "People connected" in the sense applied here means permanent residents who are being served by the wastewater facility through their pipeline system.

For example cadmium, there will be two spesific discharge factors, one for treatment category I and another for treatment category II. The same applies to all the other components in the statistics. The factors are updated each year based on the most recent data reported.

Please note! For BOD 5 and COD, a 5 per cent trimmed average instead of median value has been used when establishing the spesific discharge factors. This deviates from the original method suggested by Blytt and Storhaug (2008). The reason for this adjustment in method is that this better explains the variation in discharges for these particular components. Spesific discharge factors for the different chemical components:


Treatment category I

Treatment category II

Unit per person and year

Arsenic (As)

136 512 96 670


Cadmium (Cd)

15 484 5 921


Crom (Cr)

457 808 152 672


Copper (Cu)

4 905 495 1 078 586


Mercury (Hg)

6 580 1 978


Nickel (Ni)

618 918

492 906


Lead (Pb)

588 715

82 188


Zink (Zn)

10 092 180 4 711 163



734 024 137 049










Source: `KOSTRA 2013-data (reported Spring 2014).

When the spesific factores have been established, they are then applied to the wastewater facilities with no discharge data (in point B). For example if a treatment facility belonging to treatment category I has 1 500 people connected, the estimated discharge is then 1 500 people multiplied with the discharge factor for treatment category I.

In this way, by partly using data based on actual analyses and partly by using estimations/factors, the statistics on heavy metals and organic pollutants, one can then calculate a national discharge for the whole wastewater sector.

Additional calculation and adjustments for BOD 5 and COD
For BOD 5 and COD there exist an additional calculation compared to what is the case with heavy metals and persistand organic pollutants. It only applies in those cases when it has been reported (1) data on BOD 5 , but not COD, or (2) data on COD, but not BOD 5 . Both BOD 5 and COD measure organic material, and in order to avoid too much estimations, discharges in these instances are calculated from a theoretical BOD 5 /COD-relationship (source: KOSTRA 2012-data):

  • Treatment category I: 0.38
  • Treatment category II: 0.27

For example the BOD 5 /COD-relationship in treatment category I of 0.38 means that the amount of BOD 5 in average is 38 per cent of what is the case with COD. The BOD 5 /COD-relationships above have been established based on wastewater facilities that reports both BOD 5 and COD, and thus makes it possible to establish a more general BOD 5 /COD-relationship. The relationship is then applied to those wastewater facilities that misses either BOD 5 or COD in their reporting (but not both!). For example if a wastewater facility belonging to treatment category II is reporting 200 tonnes of BOD 5 , but data on COD is missing in their reporting, then this facility will have a COD discharge of 200 tonnes / 0.27 = 740 tonnes. Opposite, if a wastewater facility belonging to the same treatment category reports 200 tonnes of COD, then the BOD 5 discharge will be estimated to 200 tonnes * 0.27 = 54 tonnes.

Consequently, discharge from these "one-sided" reportings (wastewater facilities) will be calculated based on part factor (BOD 5 /COD-relationship) and part discharge data and actual analyses (either BOD 5 or COD). This is considered a "second best solution" for estimating discharges for these facilities as apposed to a complete factor calculation. 

Calculation of use of sewage sludge
The 2005-data-reporting introduced a new way of reporting sewage sludge compared to earlier years. Gross sewage sludge and its associated percentage dry weight are now included in the reporting compared to earlier it was only asked for dry weight of sewage sludge. In order to calculate the dry weight now, the information of percentage dry weight is essential. In cases where percentage dry weight is missing, then the following method has been applied to the data:

1. If percentage dry weight of the produced sludge product has been reported, but not for the amounts of sludge used, then this percentage has been applied.

2. If percentage dry weight of neither the produced sludge product nor the amounts used has been reported, a standard factor of 25 per cent dry weight has been applied.

Calculation of heavy metal content in sewage sludge

The calculation of average heavy metals in sewage sludge constitutes a weighted average of the reported figure; that is the content of heavy metals is weighted based on the amount of sludge reported for the different wastewater treatment plants. Thus, figures reported from larger treatment plants generally influence the final average figures more compared to smaller ones.

In the same way, maximum heavy metal content in sludge constitutes a weighted average figure of the values reported as maximum (as above, weighted against the amounts of sewage sludge disposed).


Calculation of number of small wastewater treatment plants (less than 50 pe) and their corresponding inhabitants connected.
Due to the fact that there is less reliable information available on smaller treatment plants, some estimation is necessary. Two estimation techniques may occasionally be applied:

1. Certain municipalities lack information on the distribution of inhabitants on type of treatment for smaller wastewater treatment plants. If the municipality has provided information on total amount of inhabitants connected, in addition to number of small treatment plants, the inhabitant distribution on type of treatment is estimated. The inhabitants are then distributed proportionally based on the number of treatment plants. For example lets say the municipality reports totally 3000 inhabitants connected to small treatment plants, 400 sludge separators, and 500 sludge separators with infiltration, but no distribution of inhabitants on types of treatment. Then the inhabitants connected is estimated to be 3 000x(400/(400+500)) = 1 333 inhabitants to sludge separator and 3 000x(500/(400+500)) = 1 667 inhabitants to sludge separators with infiltration, totally 3 000 inhabitants.

2. If the municipality lacks information on number of small treatment plants, but has information on the number of inhabitants connected, then this information may be used for estimation purposes. A theoretical connection factor based on the reported material is being applied. In 2012, this factor was 2.4 inhabitants per wastewater treatment plant (median value). For example if a municipality has the following information: 1 000 inhabitants connected to sludge separator, the estimated figure turns out to be 1 000/2,4 = 417 small treatment plants.

Economic data
Due to missing or incomplete data, estimates are made for some municipalities. This is done because figures for the entire country are to be estimated. For municipalities which do not report figures for operating expenditures and capital costs figures have been estimated. The estimates are based on the data from the municipalities that have reported data. The basis for the estimation is that there are a correlation between the variable and an explanatory variable (population in the municipality). The estimates for the individual municipality are not published.

Seasonal adjustment

Not relevant


The main rule governing confidentiality is that any number in a table based on less than 3 units should not be published, but should be aggregated.

Comparability over time and space

Due to the fact that various well-established wastewater plants have not entered the database before more recent years, comparison over time may be limited. This applies mainly to the smaller wastewater facilities with no treatment (direct discharges). The change in reporting system from SSB-avløp to SESAM (1997/98) and later from SESAM to KOSTRA (2001/02) may also limit the comparability before and after those years.

A general problem still existing is that some wastewater plants, without notice, may report in under a different "id-number" than the one they've been assigned. However, this is however now in later year not such a big problem any more, now that now that the electronic forms contain more pre-filled information and become more user-friendly.

In connection with introduction of new regulation on wastewater treatment the 2007/12 report made some changes to the definition of capacity. Earlier the concept of hydraulic capacity (PE) was used, but now capacity is defined according to Standards Norway and the new wastewater regulation as population equivalents (pe). The counties have been involved in the updating process of capacity, partly assisted by the municipalities. The capacity from 2004 and earlier is however still hydraulic capacity in the official statistics.

Use of sewage sludge has been reported in different ways after KOSTRA took over. The 2004 reporting was carried out on aggregated municipality level, but this was changed back again to reporting on sludge treatment plant level in the 2005 data collection.

The category "deposited" was not included as a separate use-category in 2003 due to a ban on deposition of wet organic waste, including sewage sludge (Regulation on deposition of waste (Department of Environment 2004). Because it is not totally face out the category was re-introduced again in 2004 in order to cover these amounts in the official statistics. The changes in the reporting, in addition to that the reported data is of various quality, result in some uncertainty in the statistics.

The statistics on individual wastewater treatment plants covered until the year 2000 received wastewater which in amount and quality corresponded with 7 houses or vacation homes. After that the definition shifted and only plants less than 50 pe were covered in the definition of individual wastewater treatment plants. Hence, the statistics may therefore not be readily comparable before and after that time.

In recent years, a few re-calculations has been carried out in the earlier published statistics, these are presented in the table below:

Type of data

Year, dataset

Date of recalculation


Number of individual wastewater treatment plants
(< 50 pe)


March 2006

Re-calculation of number of individual wastewater treatment plants based on updated information of certain municipalities.

Discharge per inhabitant (Nitrogen (N) and phosphorous (P))


Mach 2006

"Discharge per inhabitant" re-calculated from discharge per inhabitant actually living in the county to inhabitant connected to wastewater facility.

Capacity of wastewater facilities


February 2007

Capacity of wastewater facilities was adjusted in the transition between the SESAM and KOSTRA reporting system.

Use of sewage sludge


February 2007

Re-calculation of use of sewage sludge based on updated and corrected information of certain sewage sludge facilities.

Nitrogen, discharge data


February 2007

Re-calculation of discharge of nitrogen from wastewater facilities equal or larger than 50 pe based on updated and corrected information.

Use of sewage sludge

2008 December 2010

Re-calculation of use of sewage sludge based on updated and corrected information of certain sewage sludge facilities.

 Number of wastewater facilities, capacity, people connected, discharge of TOT-P and TOT-N

2002-12 December 2014

 Comprehensive re-calculation for statistics on wastewater facilities 50 pe or more. The purpose was to establish a more coherent time serie and fix possible missings in the reporting.

Last version of recalculated statistics can always be found in Statbank (Nature and the environment => Water and wastewater => Discharges and treatment of municipal wastewater).

From the reporting year 2006, the KOSTRA-questionnaire for cost coverage is changed. This is done to obtain figures for the municipalities' income from fees, and the fee calculation basis for the areas that are subjected to the principles for full cost. The reason for the changes is that the previous key figure calculations partly were based on data with origin in the reported annual accounts, and partly based on data reported in the form for cost coverage. Due to the different ways to organise the municipal services (e. g. intermunicipal cooperation, municipal undertaking), it is not possible to find all the data needed in the annual accounts form the municipalities.

Accuracy and reliability

Sources of error and uncertainty

For treatment plants where standard factors constitute the sole basis for calculation of discharge, the resulting figure may over- or underestimate the true discharge. The use of standard factors is necessary in those cases where direct measurements have not been carried out or exist (not part of the discharge permit). This applies in particular to individual wastewater treatment plants, in addition to several relatively small wastewater plants in the west and north of Norway. Normally the discharge permits set fewer restriction on discharge of nitrogen compared to phosphorous, and thus data and direct measurements are more available for phosphorous. The use of standard factors is therefore more commonly applied in the statistics for nitrogen compared to phosphorous.

In addition, some uncertainty is attached to the actual measurements undertaken by the treatment plants; the level of accuracy may vary. Overall, the measurements undertaken in on North Sea counties are generally more accurate compared to the rest of the country. The reason is more stringent regulations and pollution ceilings for this particular area.

The highest level of uncertainty is associated with possible errors and incomplete data sets as provided by the municipalities. Certain information on various wastewater plants and municipalities is missing, and it may sometimes be difficult to detect and correct all errors or inconsistencies in the reported data set. Some uncertainty is also associated with start-up year, capacity enlargement/reduction and closing down of certain plants, which again may affect the quality of aggregated county figures. This may lead to uncertainty whether the wastewater plants are still running.

Non-reporting of wastewater plants or specific parameters can lead to uncertainty in the final statistics. In certain cases, a wastewater plant not being reported can mean that the plant is closed down or that is still running but has not been reported. Therefore, continuous effort is put into removing plants shut down from the statistics in addition to adding plants that is still running but has not been reported in KOSTRA like they should.

Possible errors may arise during the revision process. It is a necessary quality-assuring tool, but errors and uncertainty may also arise because of the process. In particular, this applies to instances where previously reported material is imputated for non-reported data in the current year, but also other alterations to the material during the process.

The statistics of individual wastewater treatment plants (less than 50 pe) is often more uncertain compared to wastewater treatment plants equal ot or larger than 50 pe.


Not relevant