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Environmental Factors and Health

12 Wastewater

12.1 Human Exposure to Environmental Factors

12.1.1 Environmental factors

In a historic perspective, the main objective of the Danish wastewater management has been hindering human contact with wastewater and hereby to avoid transference of diseases. Secondly, and especially since the 1970s there has been focus on wastewater impact on nature. Eutrification of lakes, pollution of streams and coastal waters causing fish death has been a main concern related to discharge of wastewater for the last 20 years.

Generally, the strategy has succeeded in hindering human contact with wastewater. However, some examples of human exposure exist.

The health risk from wastewater is obvious when wastewater from a damaged sewer pipe in very rare cases reaches a drinking water borehole or infiltrates the water supply system causing intestinal diseases to the consumers (examples from smaller Danish towns are Klarup 1996 and Uggerløse 1991-92). But uncertainties remain concerning the nature of the health risk from bathing in recipients contaminated by wastewater and the long-term effect from the low-level chronic exposures to heavy metals accumulated in agricultural products. Wastewater management in Denmark aims at reducing these health risks.

Reuse of raw or treated wastewater is extremely rare. However, the DEPA has investigated the potential health risk from handling raw wastewater including the separation of faeces and urine as part of its programme for Ecological Town Planning and Wastewater Treatment.

Health concern related to chemical contaminants in wastewater focuses on heavy metals and man-made chemical substances. Especially, the risk of accumulation in fish in receiving waters or the risk of transference to agricultural crops when sludge is applied as a fertiliser in agriculture.

To summarize, the potential ways of exposure to environmental factors from handling and disposal of wastewater can be divided into three categories:

1. Direct exposure to substances in wastewater (working environment) and in receiving waters (bathing waters)
     
2. Indirect exposure to substances leaching to groundwater (drinking water).
     
3. Indirect exposure to substances in sludge deposited on or adsorbed on roots of crops (foods, not covered under this project).

The following table 12.1 gives an overview of environmental factors related to human health.

Table 12.1
Summary of the origin, characteristics and potential health impacts of some critical wastewater pollutants.

Abbreviations:

12.1.2 Wastewater Management

Figure 12.1 shows wastewater management from the wastewater producer to final disposal. The wastewater producers are households, commerce and institutions, industries and urban surfaces. The run-off from rural surfaces, including agricultural areas is not considered part of the term wastewater.

Figure 12.1
Wastewater Management

Wastewater is transported in closed pipe systems except for a very few cases where holding tanks are emptied by sludge trucks. In the sewer system, minimal energy is used for pumping wastewater depending on the topography. The sewer system can pollute the surrounding soil and later on groundwater through leakage from open joints or broken pipes.

Rainwater from roofs, squares and streets are in some cases transported in a combined system together with wastewater to a wastewater treatment plant. After a heavy rainfall, overflow structures in the sewer system may discharge diluted wastewater untreated to the recipient. This can cause a dramatic decrease in the microbiological quality of the receiving water rendering the water unsafe for bathing. In this connection it is a problem that the bathing water control is based on the monitoring results from the previous year, meaning there is no reaction to the actual overflow situation (for more details please refer to chapter 8 on bathing water).

The primary wastewater impact on bathing waters is by overflow of untreated wastewater during heavy rain, but also of importance is run-off from rural areas and from scattered settlements in the countryside not being connected to a wastewater treatment plant. Today approximately 130,000 private residences and 13,000 summerhouses discharge wastewater directly from a settling tank, which with respect to micro-organisms is virtually untreated. The wastewater is discharged to drains, ditches, watercourses, lakes and to the sea giving un-hygienic conditions, especially in recipients with low water exchange.

In May 1997, the Danish Parliament agreed upon an action plan for improved wastewater treatment involving approximately 64,000 scattered settlements in the countryside. The actions will include connection to public wastewater treatment, local sand infiltration plants or small wastewater treatment plants. This action plan will contribute to the improvement of the general water quality in fjords, lakes and coastal waters and thereby also contribute to improvements of the bathing water quality.

The action plan raises some issues regarding the groundwater protection. Approximately 30,000 local sand infiltration plants are expected to be installed at individual houses, which may imply leaching of substances to the groundwater. Especially household detergents are in focus.

Another potential source of wastewater infiltration in sewer pipes, which are never 100 percent tight. However, there are in Denmark few documented cases of drinking water contamination caused by infiltration of wastewater. The reason is probably, that leaks are sealed by the suspended matter in the wastewater.

Urban surface run-off, which is discharged separately to the recipient, causes contamination with nitrogen compounds (e.g. from rain), heavy metals (e.g. from traffic, industrial storage sites and roofs) and oil related products (e.g. traffic, gas filling stations and storage sites). The extent of contamination of surface waters with microbiological agents from direct surface run-off is unknown.

12.1.3 Wastewater Treatment

Repeating incidences in the mid-1980s of oxygen depletion in Danish fjords and coastal water led in 1987 to the Danish Action plan for the Water Environment. The action plan set minimum national standards for discharge of effluent concerning organic compounds and the nutrients nitrogen and phosphorus. The result was a reduction in wastewater impact on the water environment in Denmark (improved wastewater cleaning technology, sharpened discharge requirements, dimensioning of retention basins to minimise overflow, etc.).

For wastewater treatment plants, tertiary treatment was demanded for all plants bigger than 5,000 person equivalents (PE). The extension of the treatment plants to tertiary treatment was finalised in 1995. Today, nearly all wastewater (99%) connected to sewers in Denmark is being treated in a wastewater treatment plant. Out of these, approx. 94% is applying tertiary treatment (mechanical, biological and chemical). The treatment plants are in general designed to reduce the organic content, and the content of total nitrogen and phosphorus down to very strict demands. Occasionally, limits are set for the discharge of ammonia or nitrates.

Industrial discharges are mainly connected to the public tertiary wastewater treatment plants, and only a minor number of industries have a direct discharge to the aquatic environment of wastewater treated on their own.

The improvements of the wastewater treatment plants have led to a substantial decrease in the discharge of pathogenic micro-organisms. When supplementing secondary (biological) treatment with tertiary (chemical) treatment, the discharge of E. Coli, which is an indicator of faecal pollution, is reduced by a factor 10. However, the content of pathogens may still pose a risk to people bathing in waters affected by wastewater (please refer to chapter 8 on bathing water for more information). Effluent from Danish wastewater treatment plants typically have an E.Coli concentration of 10.000-100.000 cfu / 100 ml.

It is therefore continuously discussed whether disinfection should be added to the treatment (e.g. by UV radiation or ozone treatment). If so decided, the monitoring programmes for outlets and bathing waters will also have to be changed, because disinfection will especially decrease the micro-organisms, which today are used as indicators (E. coli etc.). Indicators have to be found for the more resistant micro-organisms.

As shown in figure 12.2, a wastewater treatment plant consumes resources and produces emissions and waste, some of which have a human health impact. Wastewater treatment plants can have their own sludge incineration facility causing air emission and the production of heavy metal loaded ash. The production of biogas from digesting sludge replaces fossil energy sources. Material in the wastewater retained on screens and in grit chambers are deposited on landfills or taken to an external incineration plant and treated together with solid waste.

Figure 12.2
Consumption and production in a wastewater treatment plant

Effluent is discharged to streams, lakes or the sea containing pathogens and heavy metals with potential human health risks if the waters are used for bathing. Sludge is after treatment used as fertiliser on agricultural land, taken to a composting plant, incinerated or deposited on a landfill.

The direct exposure to wastewater can take place when working in the sewer system or at the wastewater treatment plant. The health risk arises both from direct contact with the wastewater or by inhaling aerosols carrying pathogens or by toxic gases like hydrogen sulphides (H₂S). These risk factors primarily belong to health risks in the working environment, but neighbours may also be affected through airborne spreading of aerosols or gases.

12.2 Level of protection

All wastewater produced in Denmark is to be collected and treated. Even single houses in the so-called "open land" are requested to have their wastewater treated (although no time limit is given in the present regulation). Both the general effluent limit values and the requested national "coverage" of treatment are stricter in Denmark compared to the present EU directives.

Although the primary objective of legislation and regulation is protection of the environment, a high level of protection of public health is achieved through:

virtually no direct access to wastewater

strict discharge limit values, which are set for water environments that are more sensitive than humans.

Table 12.2.
Examples of Quality Standards for Fresh water and Marine water

However, problems with bathing waters occur in some cases and uncertainty remains concerning the nature of the health risk from bathing in wastewater contaminated recipients, especially with respect to pathogens.

12.3 Regulatory Framework

12.3.1 Objectives and Principles

The primary objectives of the legislation and regulation of wastewater in Denmark with respect to public health are:

To prevent illness in the general population from direct contact with wastewater (discharges to sea, water courses, lakes or soil)

and from indirect contact with wastewater (through drinking water).

Application of sewage sludge to agricultural land must be performed in such a way that pathogens are not transferred to humans, neither directly (via birds and animals) nor indirectly (via crops).

Public health shall not be unacceptably impacted by contamination effects derived from wastewater influence e.g. eutrofication of surface waters or accumulation through food-chains of heavy metals and man-made chemicals in animals and humans.

In the 1970s, all Danish municipalities prepared their first wastewater master plan, which included implementation plans for the collection and treatment of all point sources. During the 1980s, the Danish counties prepared water quality plans for surface water recipients defining the quality requirements of the receiving water.

12.3.2 Legislation

The overall regulation of wastewater discharge in Denmark is regulated through the Environmental Protection Act (no. 698 of 22 Sept. 1998) and the Planning Act (no. 518 of 11 June 2000).

Planning Act

The Planning Act – and orders and guidelines prepared under the act – regulates physical planning in Denmark (see chapter 3 for more detailed information).

According to the Planning Act each county shall prepare a plan for the county (regional plan) in which guidelines for the use and state of different areas within the county are stated. With regards to waters, the quality and use of streams, lakes, and sea shall be stated. However, the requirements for discharges to these waters will normally follow the general national requirements as stated in the statutory orders of the Environmental Protection Act.

The county can set out the objective for a receiving water to comply with bathing water quality or to comply with the quality standards required to use the receiving waters as drinking water source, etc.

Environmental Protection Act

The Environmental Protection Act, chapter 4, regulates protection of surface water by outlining principles for discharge of wastewater.

Chapter 3 regulates the protection of soil and groundwater, including utilisation of sludge from wastewater treatment plants as fertiliser in agriculture.

Chapter 5 of the Environmental Protection Act regulates the polluting industries and institutions/commercial areas. It sets out principles for how a county and/or a municipality monitors and regulates non-residential discharges.

The three key statutory orders concerning wastewater are:

Statutory Order on discharge permits according to chapter 3 and 4 of the Environmental Protection Act (no. 501, 21 June 1999).

Statutory Order on quality criteria for wetlands and criteria for discharge of certain dangerous substances to streams, lakes or the sea (no. 921, 8 October 1996).

Statutory Order on use of waste products for agricultural and related purposes (no. 49 of 20 January 2000) also called the "Statutory Order on Sludge".

These statutory orders set out limits to the content of chemical substances, which are identified as being toxic, persistent or accumulating in nature. Additionally, the statutory order on sludge states hygienic requirements on the use of sludge.

According to the above acts, the municipality and the county administer the statutory orders by intervention at potential sources. In order to assist the interventions at polluting industries, the Ministry of Environment and Energy has issued a number of statutory orders aiming at special types of industries.

For many years it has been attempted to recycle sludge from wastewater treatment plants on farmland as a fertiliser. However, difficulties in reaching the required criteria for the sludge and boycotts by farmers have made it necessary to dispose part of the sludge as landfill or incinerated. Incineration, composting or disposal as landfill is covered by the legislation on waste.

Sludge from wastewater treatment plants can be used as fertiliser and soil improver in agriculture, horticulture and forestry providing regulatory conditions are fulfilled. Such uses of waste products are regulated by the Statutory Order no. 49 of 20 January 2000 on use of waste products for agricultural and related purposes (the "Statutory Order on Sludge"). Herein maximum levels of 7 heavy metals (Cd, Cr, Cu, Hg, Ni, Pb, Zn) and 4 organic chemical contaminants (DEHP, LAS, NPE, PAH) as well as maximum application amounts are stipulated. Furthermore, the statutory order contains soil quality criteria for farmland, that must not be exceeded if the utilisation of sludge on the farmland shall be permitted (these soil quality criteria are referred in chapter 6).

If farmland soil and sludge fulfil the above criteria, certain hygienic based limitations are imposed depending on the treatment of the sludge. If the sludge has undergone a process of hygienisation and therefore contains less than 100 faecal streptococci/g and no salmonella, no restrictions are imposed on the use. If the sludge has been through a stabilisation or composting process, the sludge may not be placed on farmland with directly consumable products. Further requirements for the use of composted or stabilised sludge include e.g. ploughing the sludge into ground within a given number of hours, closing the forest for public entrance etc. If the sludge has not received any treatment it can not be used as fertiliser on farmland.

12.4 Instruments

12.4.1 Regulatory Instruments

Norms

To prevent the risk of leakage of wastewater from sewers, the installation of sewers are required to comply to certain norms: Danish Standard (DS).

Wastewater master plans

In accordance with the Environmental Protection Act, Art. 32 the municipality shall prepare a wastewater master plan for the municipality. The wastewater master plan shall be in accordance with the county regional plan. The master plan shall contain a complete description of wastewater handling including information on the entire sewer network and treatment facilities as well as a schedule for repair and extension.

Today, the contamination of bathing water with microbiological agents through wastewater mainly originates in stormwater overflows on combined sewer systems. On the regional plan, a county may set out the maximum number of discharges per year allowed for a specific receiving water. The municipality will in its wastewater master plan describe how this is achieved, either by building larger retention basins, by separation of wastewater from rainwater, or by direct infiltration of roof rainwater through fascines.

Even though wastewater planning seeks to avoid contamination of bathing waters through improvement of treatment facilities, dimensioning of retention basins and location of discharge points, problems prevail in some areas.

Discharge Permits

In order to discharge wastewater (treated or not treated) it is required to obtain a discharge permit from the proper environmental authority. Based on the Environmental Protection Act, the regulation of discharge permits is stated in the statutory orders no. 501 of 21 June 1999 and no. 921 of 8 October 1996. In general, the county gives permission to discharge waste-water to streams, lakes or the sea and the municipality gives permission to connect wastewater to public sewerage systems.

The statutory order no. 921 of 8 October 1996 on quality criteria for wetlands etc. is aiming at minimising the contents of substances dangerous to nature or health in discharges to the recipients. Directly relevant for health is the accumulation of heavy metals and other toxic substances in fish, which are caught and consumed in a non-commercial activity. The requirements in form of effluent criteria in the statutory order no. 501 of 21 June 1999 are mainly focusing on treatment of organic matter and nutrients, which are of environmental concern, but in this context not relevant to human health.

The requirements for more extensive treatment of nutrients, however, have an effect on the cleaning efficiency of the wastewater treatment plant for the pathogenic micro-organisms. Treatment of wastewater involving removal of only organic compounds (mechanical, biological treatment) will for instance remove approximately 90% of the E. Coli, whereas adding removal of nutrients (including nitrification, denitrication, chemical removal of phosphorous) will remove approximately 99% of the faecal bacteria. However, depending of the start concentration, the concentration may still be too high for discharge into bathing waters.

The municipal wastewater treatment plant will have to apply for a discharge permit at the county, which will require compliance with the effluent criteria as stated in the statutory order 501 or better. The general effluent criteria for public wastewater treatment plants depend on load of the treatment plant calculated as the number of personal equivalents (PE).

In special cases, an industry can be allowed to discharge directly to the recipient, and the statutory order no. 501 1999 and statutory order no. 921 also states the general requirements for this kind of discharge.

Connection permits

All buildings situated in a public sewer catchment area are in general required to connect to the public sewer unless they are located in an area where the municipality accepts local discharge (typical rural areas). Local discharge will - according to the statutory order no. 501 of 21 June 1999 - also be subject to requirements of treatment as described in statutory order no. 500 of 21 June 1999 on mini-treatment plants (5-30 PE).

In Denmark almost all industries and residential areas are localised within a public sewer catchment area. Most wastewater is therefore connected to a public sewer and treated in municipal wastewater treatment plants prior to discharge to the recipient. When connecting to the public sewer, the municipality will prepare a connection permit, which in case of polluting industries, may involve requirements to the quality and amount of wastewater.

When interpreting the statutory order no. 501, 1999, the local authorities can use the guideline to the order: Guideline no. 5, 1999 and a special guideline for the connection of industrial wastewater to the public sewer: Guideline no. 6, 1994.

The municipality shall, when stating the requirements in a connection permit, ensure the safe transport of the sewage to the wastewater treatment plant, taking the health of sewer workers into consideration as well as the risk of corrosion of the sewer pipes. Furthermore, it must be ensured that the function of the wastewater treatment plant is not endangered and that the quality of the sludge is not reduced.

Depending on the quality of the wastewater from an industry, the municipality may require pre-treatment at the industry in the connection permit.

Discharge of hazardous substances

The statutory order no. 921 for Wastewater Discharge to Recipients, Annex 1 lists hazardous substances, which are to be regulated in wastewater discharge. DEPA has in its Environmental Project no. 250, 1994 described how this list should be understood. A county must in its work with a discharge permit for an industry consider two issues: (1) best available technology and (2) quality criteria for the receiving water. Two strategies are therefore combined (the combined approach) and whichever of the two leads to the strictest effluent criteria will be applied. This means that if discharge based on the best available technology results in quality criteria, which are lower than the one defined in the counties regional plan, the discharge should be based on the best available technology.

A county is guided in its work with discharge permits through a number of publications by DEPA (so-called Environmental Projects) like no. 260, 1994, Environmental toxicity of industrial wastewater and no. 188, 1992 Ecotoxicological evaluation of industrial wastewater.

The monitoring of discharge permits will be adapted to individual local conditions. The discharge permits are therefore set depending on the acute or chronic effect or potential bioaccumulation of relevant substances.

Acute effect is considered by setting standards to maximum concentration, whereas chronic effects are considered by setting average concentration over a certain period in combination with maximum volume of water discharged. By combination of the latter indirect standards are set for the amount of substance that can be discharged over a given period.

Monitoring

The discharges from industries and commercial activities are monitored according to a programme agreed between DEPA and the Danish Society of Municipalities , implying that the municipalities have to inspect industries at a certain frequency. Major industries are requested to self-monitor and report their discharge regularly and the municipalities will receive the results. The analyses of the industrial effluent discharge to the public sewer are carried out by a registered laboratory. Furthermore, at their own initiative the municipality can perform analyses of wastewater in the effluent from industries or in the sewer network.

Municipalities register new industrial or commercial activities through a number of public databases, e.g. Købmandsstandens Oplysningsbureau (the Danish information agency for retailers).

DEPA launched at the end of the 1990s a national surveillance programme of the water environment (NOVA 2003) with the purpose of monitoring discharge of heavy metals and anthropogenic substances to groundwater and surface water. This implies that major wastewater treatment plants in Denmark now regularly have wastewater samples analysed for the content of heavy metals and anthropogenic substances. Besides providing a national overview it also provides vital information to the operators of the wastewater treatment plants and the characteristics of the wastewater they are receiving. In general, the surveillance programme will improve targeting and regulating sources of heavy metals and anthropogenic substances.

Lead used to be a problem for many wastewater treatment plants in Denmark rendering the sludge unusable for application on agricultural land according to the Statutory Order on Sludge. Part of the exhaust from cars ended on the road surface and was washed with the rain into the public sewers accumulating lead in both the effluents from treatment plants and in the sludge. This problem has been drastically reduced by the introduction of lead free fuel.

In general, many of the interventions against discharge of heavy metals and anthropogenic substances to the public sewer are executed in co-ordination with improved control of industrial and commercial solid waste. Programmes for cleaner technology have among others had the aim of reducing the discharge of heavy metals. At times industries have been forced or convinced to substitute raw material with substances that was a problem to the public sewer system or wastewater treatment.

12.4.2 Economic Instruments

The economic instruments used to control wastewater discharge are national wastewater taxes on the amount of discharged nutrients and organic matters.

A sewer fee on water consumption covers all investment costs and operation and maintenance costs for the wastewater sector. A municipality can further impose special taxes for discharge of wastewater to the public sewer, when the wastewater has organic matter and nutrients in higher concentration than normal domestic wastewater.

These economic instruments are not related to public health issues.

12.5 Actors

The public environmental and planning responsibilities concerning wastewater mainly rest with the local and regional authorities. However, some issues are, due to the general or crosscutting character, primarily attended to at state level. This applies for instance for research and the provision of guidance for management procedures and quality criteria.

The primary actors concerning regulation of wastewater are listed in table 12.3 For general descriptions of the mentioned actors please refer to chapter 3.

Table 12.3
Actors, roles and responsibilities concerning wastewater 

12.6 Evaluation

The overall health objective of the wastewater regulation is to avoid any harmful impact on public health, whether it be directly (caused by outlet to surface water or soil) or indirectly (leaching to groundwater or inflow to drinking water installations, through application on farmlands or through effects on eco-systems etc.).

The existing wastewater regulation in Denmark has generally succeeded in limiting the public exposure to wastewater. An indicator hereof is that the occurrence of waterborne diseases is much lower in Denmark than in the rest of Scandinavia. The primary reason, however, is mainly due to Danish drinking water supply being based on groundwater, whereas it is primarily based on surface water in the rest of Scandinavia. Only a few cases of drinking water contamination with wastewater have been recorded in recent years in Denmark, and they were caused by flow of wastewater into the water supply network. They did however result in a number of affected persons (as described in chapter 7).

The primary health concern of wastewater is exposure through bathing water. Even though there has been a large reduction in wastewater impact on the water environment for the last ten years (improved wastewater cleaning technology, sharpened discharge requirements, dimensioning of retention basins to minimise overflow etc.), some bathing waters are still influenced by wastewater. The primary impact is caused by overflow of untreated wastewater during heavy rain, but also by run-off from rural areas and from scattered settlements in the countryside not being connected to a wastewater treatment plant.

The Danish Parliament in May 1997 agreed on an action plan for improved wastewater treatment for the scattered settlements in the countryside. The actions will include connection to public wastewater treatment, local sand infiltration plants or small wastewater treatment plants. This action plan will contribute to improvement of the general water quality in fjords, lakes and coastal waters and thereby also contribute to improvements of the bathing water quality.

Important challenges for the wastewater regulation are to improve wastewater treatment for the scattered settlements in the countryside with respect to bathing water quality without creating new problems with groundwater contamination from infiltration of wastewater. Also control and reduction of overflow of untreated wastewater during heavy rain must be in focus.

Even though Danish wastewater treatment plants are very efficient and supply tertiary treatment, they still discharge pathogenic micro-organisms. Therefore it is continuously discussed whether disinfection should be added to the treatment (e.g. by UV-radiation or ozone treatment). If so decided, the monitoring programmes for outlets and bathing waters will also have to be changed, because disinfection will especially decrease the micro-organisms used as indicators today (E. coli etc.).

Another trend to observe is an increasing interest in ecological wastewater treatment with the aim of minimising resource consumption, recirculate nutrients and increase groundwater formation by local infiltration. These techniques should be carefully examined with respect to health impacts.

There is a need for new knowledge in several fields:

Potential health impacts from effluents from tertiary wastewater treatment.

Efficiency and consequences of disinfection at wastewater treatment plants.

Ecological wastewater treatment and health impacts hereof.

Consequences of new bathing water standards on wastewater management.

Occurrence of man-made chemicals in wastewater and potential health impacts.

12.7 References

DEPA (1994): "Environmental Project no. 25, Økotoksikologiske kvalitetskriterier for overfladevand" (Ecotoxilogical Quality Criteria for Surface Water).

DEPA (1994): "Environmental Project no. 260, Industrispildevands miljøfarlighed" (Environmental toxicity of industrial wastewater).

DEPA (1994): "Environmental Project no. 188, Økotoksikologisk vurdering af industrispildevand" (Ecotoxilogical evaluation of industrial wastewater).

DEPA (2000): "Ecological town planning and wastewater treatment Publication no. 10, Risk of infection from handling of urine, faeces and wastewater."

DEPA (1999): "Environmental Project no. 476, Introduktion til kildesporing af miljøfremmede stoffer i kloaknet" (Introduction to trace man-made chemical substances in sewers).

DEPA (1999): "Environmental Project no. 475, Kildesporing af miljøfremmede stoffer i kloaknet" (published only on DEPA's homepage) (Tracing of man-made chemical substances in sewers).

DEPA (1993): "Environmental Project no. 245, Tilslutning af industrispildevand til kommunale renseanlæg" (Connection of industrial wastewater to municipal wastewater treatment plants).

DEPA (1994): "Environmental Project no. 278, Miljøfremmede stoffer i renseanlæg" (Man-made chemical substances at wastewater treatment plants).

DEPA (1991): "Spildevandsforskning fra Miljøstyrelsen no. 21, Hygiejnisk kvalitet af spildevand fra renseanlæg" (Sanitation quality of wastewater from wastewater treatment plants).

DEPA (1993): "Spildevandsforskning fra Miljøstyrelsen no. 51, Hygiejnisering af renset spildevand" (Sanitation in treated wastewater).

Stenström, T.A. (1996): "Sjukdomsfremkallande mikroorganismer i avloppssystem - riskvärdering av traditionella och alternative avloppslösninger". Naturvårdsverket. Rapport, 1996.

Bukhari, Z, Smith, H.V., Sykes, N., Humphreys, S.W., Paton, C.A., Girdwood, R.W.A., Fircker, C.R. (1997): "Occurence of Cryptosporicium spp oocysts and Giardia spp cysts in sewage influents and effluents from treatment plants in England". Wat. Sci. Tech., 35, 11-12, 385-390.

Nordisk Ministerråd (1994): "Vattenburna infektioner i Norden", TemaNord 585, ISSN 0908-6692. (Waterborne infections in the Nordic Countries).

The Planning Act: Act no. 518 of 11 June 2000.

The Environmental Protection Act: LBK no. 698 of 22 September 1998.

Statutory Order on discharge permits according to chapters 3 and 4 of the Environmental Protection Act (no. 501, 21 June 1999).

Statutory Order on quality criteria for wetlands and criteria for discharge of certain dangerous substances to streams, lakes or the sea (no. 921, 8 October 1996).

Statutory Order no. 49 of 20 January 2000 on use of waste products for agricultural and related purposes (the "Sludge Order").

Guideline for discharge of industrial effluent to municipal wastewater treatment plants, no. 6/1994.

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