The presently disclosed subject matter is directed to an apparatus for the preparation of a cleaning, sanitizing, or sterilizing solution. In some embodiments, the apparatus comprises a modular component, such as a cartridge. The cartridge may contain chemical precursors to allow the generation of chlorine dioxide. The apparatus has a fluid inlet, and separates the fluid into a first flow path, which fills a reservoir. It also has a second flow path, which is heated before passing through the cartridge to create the desired gas. The fluid output from the cartridge is then fed into the reservoir.