Risk assessment of overflow dredging: circular use or disposal?

Overflow dredging is sludge that is removed from ditches and canals close to a sewer overflow. This type of sludge is often more polluted than in other places. This is because sewage from the overflow directly enters the surface water during heavy rainfall without being treated. In the Netherlands the general policy of most authorities that manage waterways is therefore to dredge overflow sludge separately and to transport it to a landfill site. In this way any contaminants that may be present in this dredging material can not cause any risks. Risks can arise, for example, when this overflow sludge is deposited on an embankment, bringing cattle that graze on the fields bordering the waterway into contact with the pollution.

But what are actually the risks? Unfortunately, this is usually not investigated. Separate dredging and removal of the overflow sludge costs the waterboards a considerable amount of money, causes inconvenience for local residents and creates extra CO2 emissions.

Our approach: insight through research

If there are really (major) risks of using the overflow dredge on site, then separate disposal is of course necessary. But if these risks do not exist or are at an acceptable level, then disposal is not really a sustainable solution. That is why Waterboard Noorderzijlvest asked Bioclear earth to develop a research method to investigate these risks. In cooperation with a several partners, we set up a research protocol based on various analyses, including DNA identification. 

What are the potential risks associated with overflow dredging? In particular, medicine residues and pathogens (micro-organisms that are pathogenic for humans or livestock) may end up in the surface water and thus also in the dredging spoil. In addition, a combination of antibiotics and pathogens can lead to bacteria that are resistant to antibiotics. The use of, for example, the contraceptive pill can give sewage water (and therefore overflow dredging spoilage) hormone disrupting properties.

What do we assess?

We have investigated the above-mentioned risks in both the overflow dredge and in non-suspect dredge (further upstream in the same watercourse). During the research we used various innovative analysis techniques:

  • Chemical analysis for medicine residues.
  • Culture methods for measuring antibiotic resistance, among other things.
  • DNA analyses for identification and quantification of pathogens.
  • Effect measurements for measuring the degree of endocrine disruption.

In order to identify and quantify relevant pathogens in the dredge, our laboratory (Microbial Analysis) developed qPCR tests for this project. This technique uses DNA to recognise pathogens and determine their quantity.

Safe circular reuse is the future

We have now conducted this research at four different locations. By conducting research into the presence and risks of medicine residues and pathogens in both overflow and non-suspect dredging, knowledge of the actual risks will grow and so will the willingness to formulate a risk-based policy.

No risks? Then sustainable reuse is perfectly possible. Moreover, this can significantly reduce the costs in comparison to disposing of the dredged material. If unacceptable risks from (overflow) dredging are to be expected, measures must be taken (disposal or decontamination for example). Bioclear earth helps you to understand the risks and the possible solutions and we are also happy to contribute to the development of risk-based policies.

Photo of pilot depot with overflow dredging: Waterboard Noorderzijlvest.

If you want to know more about risk assessment of overflow dredging, please contact Adri Nipshagen or Cirsten Zwaagstra.

Adri Nipshagen
Senior consultant soil
Cirsten Zwaagstra
Consultant soil & water