The invention relates to a method for removing a chemical substance from human excreta, such as faeces and urine, comprising the following steps of: providing a toilet for a person from whose excreta the chemical substance must be removed; releasably connecting a filtering device to the toilet, wherein the filtering device comprises at least one removable cartridge with a filter; transporting the excreta by means of a transport screw from the toilet toward the filter in the cartridge, wherein pressure is exerted on the excreta by means of a plunger; filtering the chemical substance out of the excreta in situ using the filtering device; periodically replacing the at least one cartridge; and processing the replaced cartridge. The invention also relates to a filtering device for application in this method, and a cartridge for use in such a filtering device.