In 2021, France will have 22613 wastewater treatment plants (WWTP), representing a total load of 78.5 million population equivalent. Water treatment sludge contains organic compounds, inerts, pollutants and pathogens.

The choice of a sludge recovery or disposal process is guided by regional considerations. The average costs of these processes, per tonne of Gross Mass (GM), vary widely (AMORCE survey: EAT 05b report):

• Spreading : 23 €/t.
• Composting : 58 €/t.
• Incineration : 90 €/t.
• For co-incineration with household waste, the usual values accepted by professionals are between 70 and 120 €/t.

How to transform WWTP sludge treatment costs into revenue ?

Biochar obtained from the pyrolysis of WWTP sludge could be used to improve agricultural soils. However, the presence of metallic elements, mainly copper and zinc, would complicate its use in agriculture. In anticipation of stricter regulations, new ways of recovery are needed.

Biochar from WWTP sludge is eligible for EBC-BasicMaterials certification, for use in the cement industry. For biochar applications in concrete, the risk of heavy metals leaching into the environment is relatively low. The biochar is introduced into the matrix of the concrete material, thus avoiding any risk of leaching.

Another promising way of recovering biochar from WWTP sludge is to use it as a sustainable remediation solution for polluted soils.