Quantitative microbial risk assessment

The concept of QMRA (quantitative microbial risk assessment) proposed by WHO was to be tested in real waterworks in Germany. In this concept, compliance with defined health-based targets in water treatment is investigated by determining the pathogens in raw water and characterising their removal by water treatment steps using indicators.

In 2014, the German Environmental Agency recommended a procedure for a quantitative risk assessment of microbiological detects in raw water using an approach proposed by WHO (determination of pathogens in raw water and analysing the treatment efficiancy using indicators).

The aim of this project was to obtain practical experience using this approach for waterworks in Germany. Three waterworks using river water as raw water were used to test the applicability of this method.

Those three use highly contaminated river water (Neckar or Ruhr) and treat it by rapid sand filtration and underground passage as part of a pretreatment.

A monitoring programme of monthly sampling over two years was implemented at these three waterworks in the raw water as well as after the particle-removal steps, involving both indicators and pathogens.

Viral, bacterial and parasitical pathogens were detected in all three river waters. For viruses an improved enrichment technique could be applied with very high recovery rates.

The log-removal of the treatment steps was analysed by using the indicators coliform bacteria and somatic coliphages. For rapid sand filtration, the log-removal was < 1 log, for underground passage about 2 log.

QMRA was done for the pathogens present in the river waters. The infection risk per person per year was calculated and compared to a health based target of a maximum infection risk of 10-4 per person per year. This maximum infection risk was not reached for any of the detected pathogens by applying the 2 pretreatment steps. But in all three water works applying theoretical informations (literature data) on QMRA could show that the health based target was reached in the finished drinking water.


Hambsch B., Lipp P., Ho J.: Quantitative mikrobielle Risikobewertung. Veröffentlichungen aus dem Technologiezentrum Wasser, ISSN 1434-5765, TZW-Band 92 (2020)

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