A process for preparing acetylene and synthesis gas by partial oxidation of hydrocarbons with oxygen, in which a first input stream comprising one or more hydrocarbons and a second input stream comprising oxygen - are separately preheated, mixed in a ratio of the mass flow rates of the second input stream to the first input stream corresponding to an oxygen ratio λ of less than or equal to 0.31, the oxygen ratio λ being understood to mean the ratio of the amount of oxygen actually present in the second input stream to the stochiometrically necessary amount of oxygen required for the complete combustion of the one or more hydrocarbons present in the first input stream, - supplied via a burner block to a combustion chamber in which the partial oxidation of the hydrocarbons takes place - to obtain a cracking gas which is quenched to 80 to 90 degrees Celsius downstream of the combustion chamber by injection of an aqueous quench medium, to obtain - a process water stream I liq and - a product gas stream I g which - is cooled in a cooling column by direct heat exchange with cooling water to obtain - a process water stream II lig as the bottom stream, - a product gas stream II g as the top stream and - a side stream which - is depleted of soot in an electrostatic filter to obtain a process water stream III liq in the electrostatic filter, and - process water streams I liq, II liq and III liq are combined and passed through soot channels with surface particulate precipitators to obtain a combined process water stream IV liq, which comprises subjecting the combined process water stream IV liq to a cleaning operation by partial vaporization in a one-stage flash vessel, the combined process water stream IV liq being vaporized in a proportion of 0.01% by weight to 10% by weight, based on the total weight thereof, to obtain a cleaned process water stream V liq, which is recycled into the process. - are separately preheated, mixed in a ratio of the mass flow rates of the second input stream to the first input stream corresponding to an oxygen ratio λ of less than or equal to 0.31, the oxygen ratio λ being understood to mean the ratio of the amount of oxygen actually present in the second input stream to the stochiometrically necessary amount of oxygen required for the complete combustion of the one or more hydrocarbons present in the first input stream, - supplied via a burner block to a combustion chamber in which the partial oxidation of the hydrocarbons takes place - to obtain a cracking gas which is quenched to 80 to 90 degrees Celsius downstream of the combustion chamber by injection of an aqueous quench medium, to obtain - a process water stream I liq and - a product gas stream I g which - is cooled in a cooling column by direct heat exchange with cooling water to obtain - a process water stream II lig as the bottom stream, - a product gas stream II g as the top stream and - a side stream which - a process water stream I liq and - a product gas stream I g which - is cooled in a cooling column by direct heat exchange with cooling water to obtain - a process water stream II lig as the bottom stream, - a product gas stream II g as the top stream and - a side stream which - is depleted of soot in an electrostatic filter to obtain a process water stream III liq in the electrostatic filter, and - process water streams I liq, II liq and III liq are combined and passed through soot channels with surface particulate precipitators to obtain a combined process water stream IV liq,