Biosorption for heavy metal removal from sewage sludge

A new sewage sludge ordinance has been in force in Germany since October 2017. This states that from 2023, water treatment companies and sewage plant operators must submit at least one concept on the planned and initiated measures to ensure phosphorus recovery, soil-related recycling or other sewage sludge disposal in accordance with the Circular Economy Act (KrWG). The measures must be implemented in just a few years’ time, depending on the (expansion) size of the wastewater treatment plant.

Sewage sludge: the end of the sewage chain

Sewage sludge is the mixture that is at the end of a wastewater treatment chain. It consists mainly of harmless organic substances, but also contains pharmaceutical residues, heavy metals, pathogenic organisms and various anthropogenic micro- and nanoscale components (2018 report by the Federal Environment Agency, UBA). It also contains valuable plant nutrients such as phosphorus, nitrogen, iron, calcium and magnesium. The application of sewage sludge as a fertilizer in agriculture has not proved successful. Not only does too much phosphate and nitrogen end up on the field and thus in the groundwater, but toxic heavy metals also find their way into nature.

The new sewage sludge ordinance not only aims to end the spreading of sewage sludge for fertilization purposes, but also to stimulate the recovery of phosphorus and other important raw materials and nutrients all in the spirit of a sustainable circular economy.

Since the new sewage sludge ordinance does not define the type or methodology of phosphorus recovery, many different phosphorus recovery processes are currently being developed and tested in practice. The problem is that in most chemical recovery processes, the phosphorus-containing products still have too high a concentration of heavy metals.

Hard to prevent: heavy metals in wastewater

Where do the heavy metals in sewage sludge come from? On the one hand, they enter the wastewater from households and commercial enterprises, on the other hand, iron compounds in particular are added to the wastewater as precipitants in the wastewater treatment plant and finally accumulate in the sewage sludge (Oliva et al. 2009). Since heavy metals lead to health risks and environmental damage, they must be removed and properly disposed of. According to the 2018 report of the Federal Environment Agency, the content of heavy metals in the recyclates depends largely on the respective recovery process and the sewage sludge used.

Microbial Toolbox for BioSorption: something for everyone

BRAIN has developed a biotechnological solution for the recovery of metals from wastewater and other liquids. Using "functional biomass", desired metals can be separated by means of biosorption. In the process, the metals are bound to the surface of microorganisms through chemical interactions. However, not every microorganism is capable of this.

In its comprehensive collection of microorganisms ("BioArchive"), BRAIN has identified and characterized microorganisms that can bind different metals very efficiently and selectively. This has led to the "Microbial Toolbox for BioSorption", which contains around 80 microorganisms with different binding profiles for metals. These include microorganisms that bind heavy metals at extremely low pH values or those that bind heavy metals at extremely high pH values. Depending on the application, the microorganisms are prepared in such a way that they can be used similarly to a chemical ion exchange material. For example, the "biogranulate" is passed through a cartridge with the metal-containing solution and retains the desired metal. The metal is then either washed off the biogranulate again or it is released by ashing the carrier material.

The following heavy metals can, for example, be separated using organisms from the "Microbial Toolbox for BioSorption":

• base metals such as iron, calcium, magnesium, aluminum;
  
• precious metals such as gold, platinum or palladium;
  
• toxic heavy metals such as lead or cadmium.
  

Depending on the problem, suitable organisms are selected and optimized for the respective purpose. Experience shows that the Toolbox has something for just about every application.

References:

Umweltbundesamt (UBA): Klärschlammentsorgung in der Bundesrepublik Deutschland. Stand Oktober 2018

Oliva, J. et al.: Klärschlamm-Materialien zur Abfallwirtschaft; Umweltbundesamt AT; Report; Klagenfurt 2009