“Pretreatment” shall mean the reduction of the amount of pollutants, the elimination of pollutants, or the alteration of the nature of pollutant properties in wastewater thereby rendering them less harmful to the publicly owned treatment works prior to introducing such pollutants into the publicly owned treatment works. This reduction or alteration can be obtained by physical, chemical, or biological processes, by process changes, or by other means, but not by diluting the concentration of the pollutants unless allowed by an applicable pretreatment standard.