Reliable detection and control of Legionella pneumophila in wastewater

In recent years, the number of registered patients with a Legionella infection has increased in the Netherlands, with the majority of infections caused by Legionella pneumophila. L. pneumophila occurs particularly in aquatic environments when the water temperature is between 25 and 45°C. Such water temperatures occur in various man-made water systems (such as wastewater treatment plants), which have been shown to be a source of L. pneumophila infections. In the Netherlands, a Legionella management plan (‘Legionella beheersplan’ in Dutch) is the most important tool to control Legionella in these water systems. For an effective management plan, risk analysis, monitoring and application of Legionella control techniques in various water systems is important.


The current cultivation method for Legionella (ISO 11731) that is now prescribed for all water matrices has a number of disadvantages, e.g.: only Legionella species that are able to grow on the selective agar media are detected and the analysis time is 7 – 14 days. The main disadvantage of applying the current cultivation method with dirtier water samples (e.g., cooling tower water and wastewater) is that due to the growth of interfering flora on the agar plates, the lower detection limit is very high. For wastewater, the detection limit (1×105 – 1×106 cfu/l) is comparable to the level at which Legionella outbreaks can occur. This means that currently no methods are available to determine Legionella (pneumophila) numbers in wastewater that can reliably estimate consequences for public health.

For managers of wastewater treatment plants, an alternative, more reliable detection method for Legionella (pneumophila) in wastewater is, therefore, a necessary tool to be able to assess the risks to public health and to decide whether management techniques (techniques or methods that prevent Legionella growth) are needed. Several alternative detection methods for Legionella (pneumophila) in drinking water have recently come onto the market. However, it has not been investigated whether these methods are also suitable for detecting Legionella (pneumophila) in wastewater in a more reliable way than the unsuitable cultivation method.

In addition to the lack of suitable detection methods for Legionella in wastewater, there are also currently no suitable Legionella  control or mitigation techniques available for wastewater treatment plants that do not interfere with the effectiveness of the treatment process. However, several opportunities are available to design control and mitigation solutions that can potentially lead to the identification of new techniques that must then be tested in practice.

Finally, various parties conduct research into risk management of Legionella, mainly in wastewater treatment plants but also in other water systems. However, the knowledge is diffuse and not centrally available. Even though many parties are working on this, no one has ready-made answers to questions from the water boards, industries, environmental services, municipalities, and consultancies.


Within the project, in phase 1, new detection methods for Legionella (pneumophila) in wastewater from warm water streams from municipal and industrial wastewater treatment plants will be tested and compared with the standard cultivation method according to ISO 11731. As part of the project, it is expected that alternative detection methods will need to be adapted for wastewater matrices as these detection methods were often developed for cleaner water matrices such as drinking water. After the adaptations, they will be tested again for use on wastewater.

The results from phase 1 will then be used as input for phase 2, in which new control and mitigation techniques for Legionella (pneumophila) in wastewater will be identified, investigated and tested. In phase 2, the most promising combination of detection method and technological control or mitigation technique will be investigated, in order to optimize them and improve operating efficiency.

The aim of the project is to test existing Legionella detection methods and to improve the most suitable methods and make them applicable to wastewater (phase 1). This is combined with developing and testing technological control and mitigation measures of Legionella in wastewater (phase 2). The results will be shared on an online platform or knowledge base (phase 3), which the participating technology suppliers, water managers, and Legionella consultancies can turn to.


To test different detection methods in combination with control and mitigation techniques and to make results and knowledge generally available via a platform. Knowledge and skills generated are shared between various stakeholders. This increases the likelihood of the developed techniques and knowledge being implemented in practice to reduce risks of Legionella in wastewater and better safeguard public health. The information generated can be used, for example, by consultancies, administrators and control bodies to draw up substantiated risk management plans and share information. Interested parties can also find information on which alternative detection method is most suitable for their water system.