Summary and conclusions, Miljøstyrelsen

Investigation of pathogens in selected waterworks

Summary and conclusions

The purpose was to investigate the occurrence of selected pathogens in untreated and treated drinking water from a number of waterworks. The intention was merely to search for investigation areas with a high risk for occurrence of the pathogens than investigating waterworks representative for Danish water supply. Surface water was included to elucidate the microbial quality of surface waters as a raw water source.

Twelve waterworks were selected, and the principle in their water treatment process was described in each case. The treatment ranged from no treatment, over traditional, simple water treatment including aeration and rapid sand filtration, over advanced water treatment including granular activated carbon filtration and UV-radiation to chemical-microbial treatment of surface water. The catchment area for each waterworks was described briefly geologically, and the degree of protection of the aquifers ranged from less than 0.5 m above a chalk aquifer to a 46 m moraine till above the aquifer. The production of the waterworks varied from less than 4,000 m³/year with 83 consumers to 3,204,00 m³/year supplying 30,000 persons. The water types also varied: from aerobic aquifers to anaerobic aquifers with methane, sulphide, ammonia, iron or manganese water, which should be removed from the water. Since these compounds are excellent substrates for microorganisms, high numbers were expected in the water supplies.

Two of the waterworks abstracted aerobic water, with elevated concentrations of nitrate, which indicated influence from agricultural activity. Four of the waterworks abstracted groundwater contaminated by pesticides, which also indicated a certain risk for contact with the surface.

At least two waterworks had substantial microbial problems (heterotrophic plate count at 21°C and 37°C in the treated water). Generally, the elevated bacteria concentrations occurred at waterworks with strongly reduced raw water with methane and sulphide. UV-radiation at one of the waterworks seemed insufficient to control the microbial growth.

Generally were the surface water samples more microbial contaminated than the groundwater samples, because coliform bacteria were detected in all the surface water samples, including thermotolerant coliform bacteria in 6 out of 7 samples. Thermotolerant coliform bacteria were only detected in one of the investigated 35 groundwater samples and only in one out of the 46 investigated drinking water samples.

Campylobacter jejuni /coli or Salmonella were not detected in any of the samples.

Giardia was found in 5 out of 7 investigated surface water samples (4-5 cysts pr. 10 L), but not in any of he drinking water samples. Giardia was found in 1 out of 31 groundwater samples, but neither a subsequent analysis of a water sample collected together with the positive sample nor analysis of samples collected later found Giardia.

Cryptosporidium parvum was found in 2 of the 7 surface water samples (2 oocysts pr 10 L), but not in any of the other samples.

In conclusion: None of the investigated pathogens were found in any of the investigated 80 groundwater and drinking water samples, collected during 4 sampling rounds during the year from 12 waterworks with suspected microbial problems.