Methods for treating substrates are described. The methods comprise the steps of flowing an aqueous liquid medium through a flow channel and at least one outlet slit onto a substrate to be treated and exposing the aqueous liquid medium to UV-radiation of a specific wavelength at least in a portion of the flow channel immediately adjacent the at least one outlet slit and after the aqueous liquid medium has flown through the outlet opening towards the substrate and thus prior to and while applying the aqueous liquid medium to the surface of the substrate to be treated. In one method, the electrical conductance of the aqueous liquid medium is adjusted to be in the range of 20 to 2000 μS, by the addition of an additive to the aqueous liquid medium, the aqueous liquid medium prior to the addition of the additive having an electrical conductivity below 20 μS, prior to or while exposing the same to the UV-radiation. Additionally, the pH of the aqueous liquid medium may be adjusted to a range of 8 to 11 or 3 to 6 prior to or while exposing the same to the UV-radiation. The adjustments may lead to a shift in an equilibrium of reactive species generated in the aqueous liquid medium by the UV-radiation towards preferred species.