A method of treating wastewater wherein the biomass associated with the wastewater is subjected to simultaneous nitrification and denitrification in a single reactor. In some embodiments, an apparatus for characterizing a magnetic field in a substrate processing system may include a carrier having a form substantially similar to a substrate to be processed in the substrate processing system. When the printing device can be operated in a first gear-shifting mode or a second first gear-shifting mode, the clutch transmission mechanism coaxially transmits the driving force.