A pressure filtration system for cleaning residual quenching water of a gasifying device that gasifies carbon-containing fuels under elevated pressure is provided. The soot water from the quencher is passed to a filter system, which has pressure filter chambers operating alternately in filtering mode under gasifying pressure or in cleaning mode. The filtrate from these chambers is passed to a quenching water reservoir, from which the quencher is fed with quenching water. Only small temperature and pressure losses with respect to the residual quenching water leaving, and only a small additional amount of energy has to be expended to overcome the remaining pressure difference to bring the filtrate that is to be returned back to the gasifying pressure. The residual quenching water is cleaned substantially under gasifying pressure in a pressure filter, avoids flash evaporation of the residual quenching water into the vacuum area, with vapour cooling and a subsequent increase in pressure and reheating.