Prof. Dr. Gertjan Medema

Chair on Water & Health

The research field of Water & Health explores the spread of infectious diseases through water. It studies the behaviour and fate of pathogenic micro-organisms (viruses, bacteria, parasites) in water, aerosols, sludge, soil and water treatment processes. The research aims to quantify human exposure to pathogenic micro-organisms through water (drinking, bathing water, wastewater, sludge, aerosols) and translate this to human health risks by Quantitative Microbial Risk Assessment.

Relevance for science

The field is an internationally active research area in which several trends guide the research:

  1. There will always be new pathogens that may be distributed through to the (water) environment (SARS, bird flu virus). This demands knowledge of the behaviour of these organisms and the risk of spread through water and how to prevent that.
  2. The rapid development of molecular methods creates numerous possibilities for developing rapid, specific detection methods, sensors and source tracking of pathogenic micro-organisms in water.
  3. Developments in "Quantitative Microbial Risk Assessment (QMRA) make it increasingly possible to convert knowledge on pathogens in water into a quantitative health risk. The data on pathogens in water, transmission routes, exposure and dose response are not stand-alone, but can be combined to an integral risk analysis, using probabilistic methods.

Relevance for the society

Safe water is essential to the functioning of our society. Protection against the spread of infectious diseases through the environment plays an important role in the safety of society and the perception of the safety of our water systems in the people. The emergence of new pathogens (in humans and farm animals) emphasizes the vulnerability of our densely populated society for infectious diseases. It is important to know about the pathogens, their possible pathways and effective management. In our densely populated country, the burden on the environment with pathogenic micro-organisms is high. In domestic wastewater levels are the viruses, pathogenic bacteria and parasites in the order of 10^(5-7) per m3, in secondary effluent are usually still 10^(4-6) per m3. New concepts for dealing with water in our society (reuse, water in the cities, etc.) provide new niches and exposure routes for pathogens. Research into the extent of human exposure through the (water) environment and the health risks of information to make informed decisions and the people to manage these risks as possible.

Secondary employment

Gertjan Medema is chief science officer and principal microbiologist at KWR Watercycle Research Institute (formerly: Kiwa Water Research). As chief science officer he coordinates the joint research program of the Dutch water utilities (BTO) and the research strategy and environment at KWR.

Research field

The focus of his research at KWR and TUDelft is the provision of a scientific basis for microbial risk management for the Dutch drinking water utilities and other stakeholders in the water sector. This encompasses:

  • development of methods for detection/typing/source tracking of faecal pathogens in water, soil, aerosols
  • studying the occurrence of these pathogens in the water cycle (wastewater, surface water, groundwater, drinking water, aerosols)
  • studying the transport of pathogens in water bodies (modelling)
  • studying transport of micro-organisms in the subsurface (bank filtration, infiltration, groundwater abstraction)
  • assessment of the removal of micro-organisms by water treatment processes, both conventional and advanced (membrane filtration, UV, advanced oxidation)
  • incorporating this knowledge into quantitative risk assessment/health impact assessment
  • advising risk management

Website KWR Watercycle Resaerch Institute (
Website Microrisk (

Professional experience and expertise

At KWR Watercycle Research Institute and Kiwa (1996-now)

He started as leader of the Biology section at Kiwa (staff of 8). His research focussed on the barriers in the drinking water systems to prevent transmission of pathogens. So design, operation and control of safe water systems. We provided building blocks for the water utilities to assess the safety of their drinking water supplies. Also here, the research framework was the scientific basis for quantitative microbial risk assessment in drinking water. The research results help the water utilities to perform risk assessment and risk management. The collaboration within the EU on microbial risk assessment (MICRORISK), World Health Organisation and IWA/Health Related Water Microbiology group were very stimulating and helped promote the Dutch approach internationally. With the broadening of KWR's scope to the watercycle, the research is looking into the health risk of other water systems (bathing water, water in the city etc.). He is vice-chair of the Health-related Water Microbiology Specialist Group of the International Water Association ( He served on the Dutch Health Council committee on safe bathing water and the WHO microbiology working group. He is associate editor of Water Science and Technology and is on the editorial board of the Journal of Water & Health. He helped to organize international scientific conferences and national dedicated workshops and conferences.

At the National Institute of Public Health and the Environment (RIVM; 1988-1996)

He started as project scientist to study the inactivation of Aeromonas by chlorine dioxide at the Laboratory for Water and Food Microbiology, National Institute of Public Health and the Environment, Bilthoven. He grew to project manager and later manager of water microbiology research team (staff of 8). His research field was the sources, occurrence and fate of pathogens in surface, drinking and recreational water and their health significance. His research focussed on the occurrence and risk of pathogenic bacteria (Campylobacter, Aeromonas, Plesiomonas), protozoa (Cryptosporidium, Giardia, the subjects of his PhD research) through drinking water and recreational water. He has also been involved in similar research on viruses. He has been co-responsible for the epidemiological studies on the health effects of bathing in fresh water. He was part of the team that developed the quantitative microbial risk assessment approach for (drinking) water and applied it to Cryptosporidium, Giardia and Campylobacter in drinking water and bathing water. The team collabotared with the World Health Organisation.

At TU Delft (1986-1988)

Research of the microbial ecology of waste water bacteria, mixotrophic growth of Thiospaera pantotropha.


Ph. D. Microbiology (Utrecht University). Thesis: Cryptosporidium and Giardia: new challenges to the water industry.
M.Sc. Microbiology (Leiden University). Majors: microbiology, biotechnology.

Gertjan Medema

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