Wastewater contains valuable resources, like metals and minerals. Classical treatment technologies are mainly focused on producing clean water, while resource recovery has received little attention. As a result, wastewater treatment not only results in clean water, but also in sludge which often is considered a waste rather than a resource. Recovering these valuable components solves these waste-related issues, while reusing the recovered resources contributes to closing cycles.

In this research theme we aim for efficient recovery of metals and minerals from wastewater using biological crystallization processes. Precipitated crystals can easily be separated and recovered for reuse, thereby closing material cycles and supporting the circular economy.


An effective way to recover metals and minerals is by stimulating the crystallisation of these resources. Crystals have several important benefits. They are pure, relatively easy to separate from the waste stream and suitable for recycling. This reduces the need for mining and the inevitable environmental damage, and eliminates the need for costly waste management.

The Biocrystallisation theme of ETE uses biological processes, using microorganisms present in nature, to enhance resource crystallization. This has the advantage that no extra chemicals are needed and the reaction can usually be carried out at lower temperatures and pressure than chemical processes.

Research areas

  • Recovering metals from waste water by crystallisation, using sulfur. At high temperature (80 °C) and low pH (< 3) microorganisms convert sulfur into hydrogen sulfide (H2S). This chemical may efficiently form a crystal with different metals, including copper, zinc, lead, and cadmium. These crystals consequently precipitate and can be retrieved.
  • Sulfur recovery from biogas. In biogas, hydrogen sulfide (H2S) is efficiently converted into sulfur by bacteria. However, the sulfur crystals formed are very small, and therefore difficult to recover. This research aims to increase crystal size by changing process conditions, like microorganisms and gas flow rate.
  • Bio-recovery of selenium from wastewater. Selenium is present in several waste streams from the industry, for example in emission gases from coal plants. These gases are washed before release, and as a result, selenium ends up in waste water. It’s toxicity requires removal. This research aims to develop a biological process to recover selenium from waste water. Challenges are to produce crystalline selenium that is pure and suitable for reuse. In addition, recovery should be efficient, with residual selenium concentrations in wastewater being extremely low, in the microgram per liter range.