BRIDGE 2019: SUBLIMUS – Becoming modern green urban alchemists: Gold and silver mining from Brussels urban sewage

Project Details


The objective is to develop an integrated low-cost and green process to harvest the substantial quantities of gold and silver present in urban wastewater treatment plant (WWTP) sludge residues. This project proposes a novel sustainable, circular approach for producing rare and valuable purified metals from a waste stock, which is currently not valorised in the Brussels Region. Based on data recently gathered, there is a potential annual production of 480 kg of silver and 17 kg of gold, for a value ~ 1.1 M€/year. The demand, production and price of precious metals such as gold (Au) and Silver (Ag) have increased nearly exponentially since the beginning of the 20th century. However, their global geological stock is finite and non-renewable. Although many recent studies point out the potential value of harvesting metals in domestic sludges, the step towards an integrated industrial process has not been undertaken yet. Sludge matrix is of high chemical complexity. Developing integrated low environmental impact and cost-effective methods to extract Ag and Au from this matrix remains a great challenge that implies the following steps: (1) Select the most appropriate reservoirs for recovery; (2) Develop pre-treatment and chemical extraction procedures to leach the metals from the solid matrix; (3) Establish selective metal ion separation procedures from the extraction solution. For each of these steps, SUBLIMUS identifies priority procedures to test/develop that are non (or less)-pollutant, low energy, low cost, and potentially implementable in an industrial process. The plan is to concentrate efforts on a series of innovative developments implying innovative methods to optimize extraction processes; such as the use of bioleaching with encapsulated bacteria strains to clean-up the matrix from interfering metals (Cu, Zn, Fe, …) and the utilisation of microemulsion and nanoparticle assisted ion separation. The expected results encompass a series of potentially valuable patents on the newly developed procedures, and a complete flow chart for precious metal extraction from wastewater treatment residues based on green technologies. The approach proposed represents a pioneering study, which serves as a base to design and set up a pilot scale production plant, with a significant potential to attract private investors. For the latter, SUBLIMUS mentors SBGE and Aquiris (as part of Veolia group) will certainly play a key role. The setup of such an extraction plant represents a world première for the valorisation of common domestic wastewater treatment plant sludges. Being the first metropolitan area where this circular economy project is tested and implemented will put the Brussels Capital Region in the spotlight as a top sustainable city. If successful, this initiative will undoubtfully attract investors. The establishment of a precious metal extraction plant will positively impact job creation and bring significant benefits for the Brussels Region.

Effective start/end date1/03/1930/06/22


  • Domestic sludge
  • bioleaching
  • Circular economy
  • Precious Metals
  • Nanoparticles

Flemish discipline codes in use since 2023

  • Other (bio)chemical engineering not elsewhere classified
  • Inorganic green chemistry


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