Can it be Hydrogen as an abbreviation for the pH value which is the negative logarithmic value of the Hydrogen ion (H+) concentration? H = Neutralization?
Class +H plants indicate the elimination of carbon and nitrogen as well as the disinfection of the water in order to remove pathogens. While class +H is mandatory when the reclaimed water is to be discharged into water reserves, class C requirements are usually sufficient for small wastewater treatment plants used for domestic purposes.