The use of a gel consisting of a colloidal solution comprising, preferably consisting of: 5% to 30% by weight, preferably 5% to 25% by weight, more preferably 8% to 20% by weight relative to the weight of the gel, of at least one inorganic viscosifying agent; a mineral base selected from among hydroxides of alkaline metals, hydroxides of alkaline-earth metals and the mixtures thereof, said mineral base being present in a proportion of 0.05 to 10 mol/L of gel, preferably in a proportion of 0.1 to 5 mol/L of gel; an oxidizing agent stable in a basic medium selected from among permanganates, persulfates, ozone, hypochlorites and the mixtures thereof, said oxidizing agent stable in a basic medium being present in a proportion of 0.05 to 5 mol/L of gel, preferably 0.1 to 2 mol/L of gel; 0.1% to 2% by weight relative to the weight of the gel, of at least one surfactant; and a solvent; the gel not containing any super-absorbent polymer, to remove a biofilm present on a surface of a solid substrate.