An emerging and promising method for the recovery of critical metals from waste and wastewater are Acidic Deep EutecticSolvents (ADES). Within the EU-funded project WaterProof, these ADES are synthesised from the base chemical formicacid. This formic acid is a renewable feedstock as it is gained by electrochemical conversion of CO₂ emissions from waste andwastewater treatment plants.ADES are a type of ionic solvents that consist of a hydrogen bond donor (formic acid in the case of WaterProof) anda hydrogen bond acceptor1. This way, ADES dissolve metals present in wastewater sludge and waste incineration ashby binding to these chemical elements via hydrogen bonds and formation of metal-ADES complexes that can be separatedfrom the residual waste material. Important features of ADES are their often low toxicity, biodegradability, customisable properties and high metal selectivity2,3, which enable the precise recovery of valuable metals from complex waste streams under mild and energy-efficient conditions.The development of ADES has high potential as a sustainable and pioneering method, and might offer the solution for lowconcentration recovery due to its high efficiency and selectivity for different metal types. Methods and aproaches are presented in this factsheet.