Paid Engagement

Sustainability : Flanders sewage sludge treatment plant: a role model in terms of sustainability

Doosan Lentjes, Flanders
© Doosan Lentjes

The DBFMO contract was awarded in 2022 by the Flemish municipal wastewater treatment company Aquafin to FOSTER, a consortium of BESIX Group and Indaver NV. As part of the turnkey project, environmental technology specialist Doosan Lentjes will be responsible for the design, supply, installation and commissioning of the entire process, mechanical and electrical part. After commissioning, which is planned for 2026, the plant will thermally treat 65,000 tons of dry substance per year - this corresponds to two-thirds of the total sewage sludge resulting from municipal wastewater in the Flemish region.

To create the conditions for efficient recovery of valuable phosphorus, which is on the EU's list of critical raw materials, the project applies the principle of mono-combustion. The bubbling bed process used offers extensive advantages: The excellent heat and mass transfer conditions in the fluidised bed maximise combustion efficiency. This not only ensures self-sufficient plant operation, but also enables the steam generated with the waste heat from the flue gas to be used for external applications. The steam not required for plant operation is fed to the high-pressure turbine of a neighbouring industrial plant, which will use the energy as part of its steel production.

An important evaluation criterion in awarding the contract for the construction and operation of the sewage sludge treatment plant was the impact on global warming to be expected from this plant. The individual material flows and the entire facility during the period of plant operation, including start-up and shut-down processes, were assessed. The impact on global warming is related to the megagrams of dry substance treated in the plant and given in kg CO2/Mg dry substance. In this analysis, the pollutant gas emissions and in particular the so-called greenhouse gases carbon dioxide and nitrous oxide (laughing gas) have the greatest influence on global warming. In the course of further analysis, however, all measures to reduce the above-mentioned factor were examined and an economically sensible way of reducing it was found in making the entire sewage sludge treatment plant wastewater-free.

The multi-stage gas cleaning process reliably removes all relevant pollutants such as nitrogen oxides, acid gases, hydrocarbons, and heavy metals from the flue gas. The strict emission requirements according to the European BREF (Best Available Techniques Reference) conclusions are fully complied with.

Two heat exchangers are installed in the waste heat boiler to make optimum use of the residual heat contained in the flue gas. The heat extracted is used to preheat the feed water. NOx emissions are separated in the selective non-catalytic reduction (SNCR) installed in the combustor. In a further stage of flue gas cleaning, the electrostatic precipitator, most of the ash, from which phosphorus can be recovered in a further process, is separated. To comply with the required emission limits and the specification of wastewater-free flue gas cleaning, further cleaning stages are installed downstream of the electrostatic precipitator. Heavy metals, dioxins and furans are separated in the semi-dry cleaning stage, which consists of a quench reactor and a downstream fabric filter. The downstream limestone scrubber is used to separate acid gases such as SO2, HCl and HF.

At the Berlin Sewage Sludge Conference 2023, Doosan Lentjes will shed more light on the wastewater-free nature of the Flanders sewage sludge treatment plant.

In 2018, the German plant engineering company had resumed its previous involvement in the market for the thermal treatment of municipal sewage sludge. The background for this decision was the corresponding new legal regulations, which led to the expectation of a positive market development in Germany, but also in Europe. The current order from Flanders marks the successful re-entry into this business segment. In doing so, the company can draw on extensive experience: For many years, Doosan Lentjes planned and built sewage sludge treatment facilities for customers in Germany and Europe as a general contractor.

Read more about the Flanders project