of sludge produced per year per inhabitant in France
Volume of sludge
The primary and secondary treatment of wastewater treatment plants generates sludge which is generally dehydrated and then stabilized before composting or spreading. For wastewater treatment plants of more than 50,000 inhab. the methanisation of this sludge is increasingly implemented, making it possible to reduce the volume of sludge produced by an average of 30%, while at the same time recovering the organic matter in biogas, which is then transformed into biomethane or, via cogeneration, heat and electricity.
Concerning the management of this sludge, the discussions carried out in the “Confidence Pact” and the legislative changes introduced by the “EGALIM” law (adopted on 30 October 2018) are raising concerns, particularly with regard to the sustainability of the agricultural recovery of sludge. Indeed, the joint treatment of sewage sludge and green waste is questionned while preventing any mixture containing urban sludge from gaining “product status”. These two elements present a real obstacle to the development of sewage sludge composting. Thus, local authorities would have to reorient themselves either towards direct spreading (which involves a loss of fertilizer quality as well as an increasingly difficult societal acceptance), or towards incineration (which in turn generates much higher treatment costs).
The current health crisis highlights the complex regulatory context associated with sludge recovery. ANSES recently recommended not to spread sewage sludge produced during the epidemic episode without prior hygienisation, as current knowledge does not allow to precisely define the level of contamination for untreated sludge, nor to specify a storage period beyond which the virus would be inactivated (Anses opinion n° 2020-SA-0043). Thus, most of the sludge will be directed towards incineration, which is a costly outlet.
The methanisation of sludge, beyond providing a solution to the volume of sludge, is fully integrated into the policy of renewable energy production. France was supposed to reach 23% of renewable energy in its overall energy mix by the end of 2020. This rate has increased from 9.5% in 2004 to 16.3% in 2017, but the objectives have not been met. More specifically, in 2015, the roadmap of the law on energy transition set the share of green gas at 10% by 2030, reduced to 7% in the multi-annual energy program published in January 2019, which implies multiplying annual production by a factor of 30.