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Hygiejnisk kvalitet af spildevand fra offentlige renseanlæg

Summary and conclusions

The present report summarises the result of a literature study and a monitoring programme concerning the hygienic quality of wastewater. The report is an update of the knowledge for the period 1995-2000 concerning the waterborne microbial contamination of the aquatic environment.

The report describes the incidence of the chosen microorganisms in raw and treated wastewater and the treatment efficiencies for the chosen microorganisms in public wastewater treatment plants (WWTP). The results shall contribute to identify the significant sources of microbial contamination of the wastewater.

The report contains a statement of the magnitude of the literature study and the problems that have arised from the literature in proportion to the application of information's and data in the present study.

The spreading of pathogens in wastewater and the problems that can arise according to this is described together with a statement of the sickness cases related to wastewater and drinking water. Furthermore there is given a brief description of the chosen microorganisms, including a description of infectious dose and survival in the aquatic environment.

It is chosen to focus on the bacteria's, total coliforms, E. coli, faecal enterococcus, Campylobacter and Salmonella, the protozoa's, Cryptosporidium parvum and Giardia intestinalis, and the virus type, enterovirus, in order to assess the health risks related to wastewater. The microorganisms are chosen based on the following criteria's: 1) Low infectious dose, 2) Large excretion from humans and animals and therefore expected in large numbers in the wastewater and 3) Contagious through water. Salmonella and Campylobacter are primarily chosen because they are considered to be most significant causes to diarrhoea among humans in Denmark.

The sewer areas for the two investigated wastewater treatment plants (Egå and Marselisborg) are described, and the significant sources of pathogens in wastewater are identified theoretically. Both sewer areas contain hospitals, food producing industries and tourist industries, all considered to be important pathogenic sources to contamination of the wastewater. Today preventive and protective measures related to the wastewater are rarely taken in hospitals, which represent a significant source of infectious microorganisms. According to current knowledge, authority regulation of the industries and institutions in the form of requirements to the content of pathogens do not exist, as it is known for example with nutrients, synthetic substances and heavy metals.

Based on the literature study it is found that both the water consumption and the wastewater production per person have decreased with approximately 25 percent from 1990 to 1999. The content of pathogens in raw wastewater depend on several factors such as the epidemically situation in the area, the season and the discharge of wastewater from hospitals and slaughterhouses etc.

There is given a simple theoretical arithmetical example of the excretion of microorganisms from one healthy person. A healthy person will excrete approximately 104-107 bacteria's per litre, 105 viruses per litre and 103-104 protozoa per litre raw wastewater. An infected person will excrete a larger number of pathogens. The pathogens will be reduced (die-off) through the sewer system to the inlet of the wastewater treatment plant. The magnitude of the reduction in the sewer system is however unknown.

The table below shows the results of the executed monitoring programme. The shown numbers are the average content in the untreated wastewater.

There is furthermore detected enterovirus in 8 of a total of 12 samples. The untreated wastewater is not analysed for the protozoa's becaused the used method could not be used for untreated wastewater.

The measured levels of pathogens in the inlet of Egå and Marselisborg WWTP correspond generally to the levels given in the studied literature for foreign WWTP's. The occurrence of microorganisms in untreated wastewater from Egå and Marselisborg WWTP is immediately assessed to be rather high compared to other Danish public WWTP due to the fact that the sewer areas accommodate both hospitals, slaughterhouses etc.

A simple assessment of the exposure dose for untreated wastewater, treated wastewater and filtered wastewater is given. The exposure dose corresponds to the quantity of wastewater, which must be consumed in order to be likely to cause disease. The rough estimation can not be used as a real risk assessment. The rough estimation of the exposure dose is based on the theoretical infectious dose and the measured content of microorganisms in the wastewater. It is assumed that all the significant factors, which have to be combined to cause disease, is fulfilled. The interesting microorganisms are, thus, assessed on an overall basis. Theoretically, the quantity of untreated wastewater needed to cause disease varies from a few millilitres to 5 millilitres. All the microorganisms studied, i.e. Salmonella and Campylobacter, are as could be expected, interesting in relation to human health risk.

The results of the monitoring programme and the literature study regarding the hygienic quality of treated wastewater are presented. The table below shows the results as average values from the monitoring programme at the two WWTP's.

The measured levels of pathogens in the outlet of Egå (before filter) and Marselisborg WWTP correspond generally to the levels given the studied literature for foreign and Danish WWTP's. Enterovirus is not detected in the outlet from the two WWTP's, but the literature shows that enterovirus can go through treatment plants and to the recipient. The occurrence of Cryptosporidium parvum in the treated wastewater from Egå WWTP is regarded to be in the high level of the levels given in the literature. The occurrence of Giardia intestinalis and Cryptosporidium parvum in the treated wastewater indicates that there have been a too low reporting of human giardiasis in the sewer areas as the number of detected Giardia intestinalis was significant larger than detected Cryptosporidium parvum. This can relate to the fact that Giardia isn't under the duty to report.

In relation to an assessment of the presence of viable Cryptosporidium oocysts a viability test was carried out on several samples taken before the sandfilter at Egå WWTP. All the samples contained viable oocysts, which indicates that they will be infectious and thus can be a potential health risk if spread in the nature.

The simple assessment of the exposure dose of wastewater for identification of the interesting microorganisms in health risk relations shows that the theoretically exposure dose to cause disease is approx. 5 millilitres treated wastewater. The relevant microorganisms in treated wastewater regarding health risks are Salmonella and Campylobacter, as expected. Furthermore, Giardia intestinalis and Cryptosporidium parvum are interesting as they have low infectious doses, even though that the exposure doses are approx. 1,2 to 3,3 litres.

The reduction efficiencies for the chosen microorganisms in the two WWTP's and the efficiency of the sand filter at Egå WWTP are stated. The results from the literature study are presented. A combination of exposure of sunlight, sedimentation and filtration is expected to reduce the occurrence of bacteria's and viruses in great numbers. The protozoa's however will sediment slower and will often be detected in the treated wastewater.

The results of the monitoring programme has as expected shown that the removal of bacteria's and viruses follow the removal of organic matter and suspended matter. Furthermore the results has shown that the removal of bacteria's relate to the retention time in the clarifier, as a low retention time caused by heavy rainfall results in lower reduction efficiencies for both the bacteria's and the traditional wastewater components.

The table below shows the average reduction efficiencies at Egå and Marselisborg WWTP. It is noticed that the large efficiencies are not synonymous with that pathogens can't be detected in the treated wastewater.

The reduction efficiencies for bacteria's and viruses in MBNKD-plants are big (varies between 99,6-99,9 percent for the bacteria's), and with installation of a sand filter (MBNCDF-plant) the reduction is improved (varies between 99,8-99,99 percent for bacteria's). The literature gives efficiencies for enterovirus varying between 30-99,5 percent in an activated sludge plant. The literature gives efficiencies in activated sludge plants between 83-99,3 percent for Giardia intestinalis and 90,7-96,8 percent for Cryptosporidium parvum.

The sandfilter at Egå WWTP has a lower reduction effect towards Cryptosporidium parvum than Giardia intestinalis. This can be related to that the size of the Giardia cysts is larger than the Cryptosporidium oocysts and therefore better can be retained in the sandfilter.

The occurrence of microorganisms in treated sandfiltered wastewater measured in the present monitoring programme is presented in the table below as average values.

Enterovirus is not detected after the sandfilter at Egå WWTP. It is noticed that there has been measured 10-100 Campylobacter per 100 ml treated and sandfiltrated wastewater.

The presence of viable Cryptosporidium oocysts was investigated on the samples taken before and after the sandfilter at Egå WWTP. The average percent of dead and viable oocysts of the total number of detected oocysts respectively was 35 percent and 18 percent for samples taken before the filter. The average percent of dead and viable oocysts of the total number of detected oocysts respectively was 41 percent and 4 percent for samples taken after the filter. The viable oocysts will be infectious and thus be a potential health risk if spread in the nature.

The simple assessment of the exposure dose of wastewater for identification of the interesting microorganisms in health risk relations shows that the theoretically exposure dose to cause disease by Campylobacter is approximately 0,5 litres of treated sandfiltrated wastewater.

The results of the literature study regarding to the occurrence of the microorganisms in recipients, groundwater and drinking water are presented. The occurrence of microorganisms in the recipients has not been monitored in this project. It has not been possible to sort out the results from the literature study into background levels for the different recipient types. Several of the results were found in surface water, which is contaminated with raw, untreated wastewater, treated wastewater, livestock and stock of game, and, at the same time, is used for recreational purposes and surface water abstraction for drinking water purposes.

At the moment bathing is not allowed at the outfall of the Egå, which is recipient of Egå WWTP. The Municipality of Århus assesses that the combined sewer overflows at the Egåen are the main cause for not being able to meet the bathing water requirements.

The combined sewer overflows is described and it is assessed whether these represent human health risks for the bathers. The content of the pathogens in the overflow water will vary during the rainfall period. The content will be biggest in the first flush, because the composition will be affected by the runoff from paved surfaces and tearing off and suspension of biofilm in the sewer system.

The literature indicates that the content of pathogens in recipients, both bacteria's and protozoa's, is increasing during rainfall due to discharges from combined sewer overflow. It is concluded that rainfall related discharges constitute a health risk for the bathers even though the wastewater will be diluted. The occurrence of microorganisms in the wastewater will still be high after the dilution.

On basis on the present literature study and the executed monitoring programme it is assessed that wastewater discharged from activated sludge plants with or without filtration to aquatic environments can constitute a health risk. Installation of a sandfilter is assessed to contribute to a further reduction of the discharge of pathogens from wastewater treatment plants. Furthermore it is assessed that rainfall related discharges (combined sewer outflows) can constitute a health risk for the bathers.

It is suggested that further investigations involve:

	further assessments concerning the health risks for the bathers due to discharge of wastewater from WWTP and combined sewer outflows
	assessments of the existing technologies for advanced treatment of wastewater
	assessments of the risks by using alternative wastewater treatments and treatment technologies.

Further, it is proposed that it is considered to develop tools for characterising significant sources of microbial contamination of wastewater, and, thus, guidelines for issuing permits of connection to public sewer for special types of wastewater, and guidelines for discharge approvals for WWTP's. It is furthermore proposed to further investigate the health risk of wastewater related infections of wastewater workers during operation and maintenance of sewer systems and wastewater treatment plants.

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