Summary and conclusions, Miljøstyrelsen

Pathogens in wastewater

Summary and conclusions

This report summarises the results of a small literature study and a monitoring programme on the incidence of pathogens in wastewater and advanced after-treatment of wastewater in public wastewater treatment plants (WWTPs).

The aim of the report is to contribute to the updating of the existing knowledge 1) on the incidence of a range of microorganisms in untreated and treated wastewater from public WWTPs, and 2) on the effect of advanced wastewater after-treatment in public WWTP towards pathogens, including highly infective types of microorganims.

Originally, the aim was that the monitoring programme should involve a WWTP with ozone dosing and a WWTP with UV-desinfection. It became, however, clear during the initial phase of the project that there were no UV-disinfection plants in Denmark exhibiting a sufficiently stable and specification compliant performance to be used for the project.

Therefore, the investigation of the effect of advanced after-treatment of treated wastewater on various patogens was reduced to an introductory study of the effect of ozone dozing. The effect was tested at ozone dosing at 21.5, 53.5 and 84.8 g ozone/m³.

The microorganisms selected for monitoring in untreated and treated wastewater represent a wide spectre of microorganisms including indicator-organisms, and pathogens. Thus, it was chosen to focus on plate count at 22° C, plate count at 37° C, total coliforms, thermotolerant coliforms, enterococci, Campylobacter, Salmonella, Clostridium perfringens, spores of Clostridium perfringens, enterovirus, Giardia intestinalis and Cryptosporidium parvum. The microorganisms were chosen based on the following criteria: 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.

In the autumn of 2002, the monitoring programme and effect study on microorganisms in untreated and treated wastewater was carried out at the following three WWTPs:

Avedøre WWTP, which is a major Danish WWTP receiving wastewater from both industries, residential areas and a large hospital,

Usserød WWTP, which is a somewhat smaller plant with a predominantly residential catchment, but also a hospital, and

Kalundborg WWTP, which is a medium size WWTP with an atypical wastewater composition due to a single, major industry in the catchment. This plant is the only Danish urban WWTP to have installed a facility for after-treatment of the effluent with ozone.

Due to a number of technical difficulties and unforeseeable events during the project implementation, the originally planned sampling programme had to be modified, and resulting in the following final programme:

Three sampling rounds at Avedøre and Usserød WWTPs (inlet + outlet).

One sampling round at Kalundborg WWTP (inlet, outlet and ozone treatment at three dosage levels).

The table below shows the results of the monitoring of untreated wastewater (i.e. inlet samples). The results are stated in average number/100 ml.

Parameter	Kalundborg, number/100 ml	Avedøre, number/100 ml	Usserød, number/100 ml
Plate count, 22° C	3.8´ 10⁸	5.1´ 10⁸	5.3´ 10⁸
Plate count, 37° C	2.3´ 10⁸	4.4´ 10⁸	3.7´ 10⁸
Total coliforms	3.3´ 10⁷	5.4´ 10⁷	6.6´ 10⁷
Thermotol. coliforms	1.4´ 10⁶	3.5´ 10⁶	1.5´ 10⁷
Enterococci	3.6´ 10⁴	1.8´ 10⁶	2.3´ 10⁶
Campylobacter, jejuni/coli	<10	<10	<10
Clostridium perfringens	7,300	2.1´ 10⁴	6.8´ 10⁴
Clostridium perfr.-spores	5,900	2.0´ 10⁴	5.8´ 10⁴
Salmonella	Detected	2,800	300
Enterovirus	Detected	Detected	Not detected
Giardia intestinalis	<1	2.0 (<4)	2.0 (<4)
Cryptosporidium parvum	<1	2.0 (<4)	2.0 (<4)

The average content of the microorganisms in the untreated wastewater at Avedøre WWTP is significantly higher than at Kalundborg WWTP. The content of the microorganisms in the untreated wastewater at Usserød WWTP and Avedøre WWTP are approximately on the same level, except for Salmonella, which is about 9 times lower at Usserød WWTP. The contents of thermotolerant coliforms, Clostridium perfringens and spores of Clostridium perfr. are approx. 4.5, 3.2 and 3 times higher at Usserød than at Avedøre.

The average contents of plate count 22° C, total coliforms, thermotolerant coliforms, enterococcus, Salmonella and enterovirus determined at the three WWTPs correspond to average levels in the literature. The content of plate count 37° C is assessed to be high, whereas the content of Campylobacter, Clostridium perfringens and spores of Clostridium perfr. are assessed to be low compared to levels reported in the literature. Giardia intestinalis and Cryptosporidium parvum are not detected in the untreated wastewater to any of the WWTP.

The table below shows the results of the monitoring of treated wastewater (i.e. effluent samples). The results are stated in average number/100 ml.

Parameter	Kalundborg, MBNCD, number/100 ml	Avedøre, MBNCD, number/100 ml	Usserød, MBNCDF, number/100 ml
Plate count, 22° C	2.0´ 10⁷	3.7´ 10⁶	2.5´ 10⁵
Plate count, 37° C	2.0´ 10⁷	3.2´ 10⁶	1.5´ 10⁵
Total coliforms	3.3´ 10⁶	1.6´ 10⁵	1.0´ 10⁴
Thermotol. coliforms	4.9´ 10⁵	5.3´ 10⁴	3,000
Enterococci	1.9´ 10⁴	3.5´ 10⁴	1,000
Campylobacter, jejuni/coli	Detected	<10	<10
Clostridium perfringens	3,200	1,900	330
Clostridium perfr.-spores	<500	1,800	330
Salmonella	Detected	38	5.0
Enterovirus	Not detected	Not detected	Not detected
Giardia intestinalis	<1	2.7	0.3
Cryptosporidium parvum	<1	0.2	0.2

Among the three WWTPs studied, Usserød WWTP has in general the best outlet quality for the selected microorganisms. This is due to the sand filter, which is installed as a last treatment step before discharge at Usserød WWTP.

For the three WWTPs on average, the contents of thermotolerant coliforms, enterococcus and enterovirus correspond to average levels given in the literature. The content of total coliforms is assessed to be high, whereas the content of Campylobacter, Salmonella, Giardia intestinalis and Cryptosporidium parvum are assessed to be low compared to levels reported in the literature.

The content of total coliforms, faecal coliforms and enterococcus in treated wastewater from the three WWTPs exceed the existing required levels in bathing water and proposed new requirements. However, decay and dilution will occur in the recipient. The outlet levels from Usserød WWTP come closest to obtain the required levels in bathing water due to the sand filter.

The table below presents the average reductions (%) from inlet to outlet at the three WWTPs.

Parameter	Kalundborg, MBNCD, %	Avedøre, MBNCD, %	Usserød, MBNCDF, %
Plate count, 22° C	94.7	99.3	99.9
Plate count, 37° C	91.3	99.2	100
Total coliforms	90.0	98.5	100
Thermotol. coliforms	65.0	98.6	100
Enterococci	47.2	97.2	100
Campylobacter, jejuni/coli	n.a.	n.a.	n.a.
Clostridium perfringens	56.2	89.1	98.5
Clostridium perfr.-spores	95.8	91.5	98.9
Salmonella	n.a.	99.0	95.0
Enterovirus	To detection limit	To detection limit	n.a.
Giardia intestinalis	n.a.	53.6	84.5
Cryptosporidium parvum	n.a.	86.2	97.5

n.a.: Not assessed because that both results is under the detection limit, or that the results is alike for the two monitoring stations.

It should be noticed that reductions at 100 % are not tantamount to that there are no microorganisms in the treated wastewater (ref. the previous table).

Due to its final sand filtrating step, Usserød WWTP shows, as expected, the highest reduction efficiencies of the three WWTPs while Avedøre WWTP generally has the second highest reduction efficiencies.

The reduction efficiencies for Avedøre WWTP correspond in general to the efficiencies given in the literature for WWTP’s with MBNCD-structure while the reduction efficiency for total coliforms, termotolerant coliforms and enterococci for Kalundborg WWTP are low for WWTPs with this structure. Usserød WWTP shows reduction efficiencies, which correspond to the efficiencies for WWTPs with MBNCDF-structure.

The results indicate that a sand filter can further reduce the content of pathogens in the treated wastewater.

Ozone has a highly oxidising effect on pathogens in wastewater. The oxidation process degrades the cells or cell materials, which results in a disinfection of the wastewater. There are a number of references, in which ozone has been used to disinfect wastewater. Dosing, contact time and type of microorganism are important factors when trying to optimise reduction efficiency. In particular, the reduction of vira, spores and cysts can be limited if the right dose or contact time is not applied.

As shown in the table below, the after-treatment of the effluent at Kalundborg WWTP by ozone results in further removal of all the microorganisms.

Parameter	Inlet/Outlet, %	Inlet/Ozone-20, %	Inlet/Ozone-50, %	Inlet/Ozone-80, %
Plate count, 22° C	94.7	96.1	98.4	98.7
Plate count, 37° C	91.3	95.7	97.3	97.0
Total coliforms	90.0	99.3	99.97	100
Termotol. coliforms	65.0	90.7	99.2	99.99
Enterococci	47.2	97.2	(97.2)	(97.2)
Clostridium perfringens	56.2	80.8	93.2	87.7
Clostridium perfr.- spores	(91.5)	(91.5)	(91.5)	(91.5)
Enterovirus	To detec.	To detec.	To detec.	To detec.

(): Modified calculated reduction effect.

The effect of ozone on Salmonella, Campylobacter, Giardia intestinalis and Cryptosporidium parvum can not be assessed because the levels are at or below the detection limit already at the inlet to the ozone reactor.

Ozone treatment of treated wastewater from WWTPs results in a large reduction of the number of indicator bacteria. Some indicator bacteria are reduced to (below) the detection limit.

The literature reports reduction efficiencies for ozone plants at approx. 99.8-100 % for thermotolerant coliforms. This reduction effect is obtained at Kalundborg WWTP at the highest dose tested; 84.8 g O₃/m³. The specific reduction effects of the ozone dosing at 21.5, 53.5 and 84.8 g O₃/m³, i.e. in the outlet compared to the inlet to the ozone ractor, are 25-95, 69-99.7 and 66-100 %, respectively.

Increasing the ozone dosing from 21.5 g O₃/m³ to 53.5 and 84.8 g O₃/m³ results in increased patogen reduction, which is particularly apparent for the coliforms bacteria. The numbers of total coliforms and termotolerant coliforms are reduced approx. 300 and 1,300 times, respectively, at an ozone dose of 84.8 g O₃/m³ compared to one at 21.5 g O₃/m³.

In conclusion, based on the observed levels of microorganisms it is assessed that treated and even treated + sand filtered wastewater can constitute a health risk to bathers. However, installation of a sand filter improves the reducing effet of a WWTP further. After-treatment of wastewater with ozone results in a high reduction of indicator bacteria. Some of the indicators are even reduced to or below the detection limit. Even after after-treatment with ozone the levels of microorganisms is so high that a health risk to bathers cannot be dismissed.

It is suggested that possible further investigations should include:

Characterisation of pathogens in untreated and treated wastewater from different public WWTP in order to obtain a wider knowledge of the incidence.

Further assessments concerning the existing methods/technologies for advanced treatment of wastewater, for example UV and ozone, and the effect of microorganims and pathogens.