About contaminated soil and sustainable landfills Julia Gebert is the new Professor of Waste Management and Material Cycles

“I work on research topics that at first glance do not seem to belong together,” says Professor Julia Gebert. The biologist and soil scientist, who previously worked at TU Delft in the Netherlands, is the new Professor of Waste Management and Material Cycles at Leichtweiß Institute for Hydraulic Engineering at TU Braunschweig. Her research combines waste management with the navigability of waterways and coastal protection. In an interview with Bianca Loschinsky and Heiko Jacobs, she explains why landfills need to be stabilised, what mature dredged material is and what motivated her to conduct research in this area.

You have been working at TU Delft in the Netherlands for the past eight years and are now moving to Braunschweig. What attracts you to TU Braunschweig?

What appeals to me about TU Braunschweig is the opportunity to work with many related disciplines, as they are all relatively close together on campus. I am a scientist with a strong interdisciplinary focus, so it is important and enjoyable for me to have colleagues from the natural and environmental sciences in close proximity and with scientific connections. In addition, I hope to be able to apply my research more than before, with private or public partners, without losing the basic scientific aspect. I think Braunschweig is a good place for this!

With which neighbouring disciplines would you like to collaborate?

There are many points of contact, especially, of course, in my own institute, in the fields of hydrology, hydraulic engineering and coastal protection. Furthermore, there are many opportunities for collaboration with environmental geochemistry or soil science, as well as, of course, urban water management.

Your professorship is based at the Leichtweiß Institute for Hydraulic Engineering (LWI), which is not a matter of course in the denomination. You spent your professorship in coastal cities like Hamburg and Delft. Do you see any links to LWI departments there?

Good waste management always helps to protect ground and surface water. In addition, there are many waste dumps around the world in close proximity to the coast. In this respect, the connection of the department to LWI is logical and the link to other LWI departments is given.

Another example of a topic that links all four departments, and which is also the subject of my research, is aquatic sediments. Sediments are a natural part of a water body, but they can also hinder the use of a water body as an economic route if they limit the water depth after deposition. In our neighbouring federal states with large ports such as Bremen or Hamburg, but also in the seaport of Emden, a lot of dredging is carried out for precisely this reason. In Hamburg, for example, around eleven million tonnes of sediment have to be removed from the fairway and harbour basin every year to keep the harbour navigable. Some of this sediment is contaminated as a result of industrial activities in the Elbe catchment area and cannot be relocated in the water, but must be treated and disposed of as waste on land. If there is no possibility of recycling, the dredged material is deposited in landfills. As with other waste sites, these landfills have to meet the requirements of waste legislation and be monitored and treated accordingly – and the bridge from sediment to waste management is built!

In Delft, you have also worked on projects for coastal cities in Germany. What are they about?

The rise in sea levels associated with climate change, the increase in storm surges or extreme weather events, and land subsidence are creating a huge demand for materials to widen and raise our dikes, especially in coastal regions. That is why another of my research priorities is how we can turn dredged sediments into soil material for coastal and flood defence dikes. This means that sediments are used as a resource and at the same time natural soils, which would otherwise be used as earthen material in dike construction, are protected.

Mature dredged material is used in this process. What is mature dredging spoil? How is it used in practice?

On the river bed, the sediment rests in a water-saturated state under anoxic conditions and has very different geotechnical, chemical and biological properties than soil material that would normally be used in dike construction. What we want to achieve with this soil maturation is that this sediment, which has previously been transported in the river, undergoes biological, chemical and physical maturation processes to transform it into a soil-like material. It is not enough to simply dewater it. We are currently investigating this in a project funded by the Dutch Research Council. The research sites are also in Bremen and Hamburg, because the ports are interested in finding ways to use their dredged material. We are conducting research on a very large scale to find out what controls these maturation processes and how they can be accelerated and optimised to quickly provide flood and coastal protection with larger quantities of geotechnically suitable and biochemically stable material to strengthen our dikes.

You also work on the decontamination and stabilisation of landfills. Why is it necessary to stabilise landfills in the first place?

Waste is reactive – due to the microbial degradation of the organic matter in the waste and the geochemical processes that take place in the waste body, hazardous substances can be released into the environment via landfill leachate and landfill gas. In order to prevent this, and thus protect our soils, groundwater, surface water and climate, landfills have safety, containment and treatment systems. However, biogeochemical processes in a waste body can continue for long periods of time, requiring monitoring of the structure and environment for decades to centuries. This is contrary to the principle of sustainability, as the costs and risks are borne by future generations. The aim is therefore to stabilise the waste body, which means making it less reactive, so that the residual risk to the environment and human health is minimised, the landfill can be safely reused and long-term monitoring is no longer necessary.

The methodical approach to landfill stabilisation is based on the initial stimulation of the biological degradation process under controlled conditions, thereby accelerating the achievement of a stable state. This is done by aerating the landfill or adding water. Ideally, this is done over a period of ten to 25 years, depending on the specific characteristics of the landfill (such as the type of waste).

The Department of Architecture is very interested in renewable resources and the recycling of building materials. How do you envision working with the Faculty of Architecture in general?

There are a lot of different approaches, for example in the research area “City of the Future”. Here it is interesting to build houses that are greened from the outside, so that the cooling effect of vegetation is directly integrated into the structure. The same goal is pursued by unsealing surfaces, which allows urban areas to return to the water cycle. There are also some great designs in Delft, for example on the campus. Not only from a waste management perspective, but also from a soil science perspective, there is great potential for good joint research and design applications.

What motivated you to do research in the field of waste management in the first place?

I originally studied biology and soil science. Soil science led me to landfills, because at the time I was looking at the requirements for soils used as cover layers on landfills. Biological processes take place in these soils that are important, among other things, for climate protection – for example, the greenhouse gas methane contained in landfill gas can be broken down by bacteria in landfill cover soils optimised for this purpose, thus reducing emissions into the atmosphere.

Waste management is a highly interdisciplinary field that combines approaches and methods from the geosciences, biology and engineering, and includes both basic and applied research.
I find the different cultures, languages and approaches of the scientific disciplines involved interesting and enriching. I feel very comfortable in this niche, both scientifically and personally, and appreciate the diversity of topics. Complex issues require different expertise and the integration of different approaches. I would like to contribute to that.

How would you describe your working day in three words?

Communication, science and education.

Thank you very much!