iTSS - inert inorganic Total Suspended Solids

Because the nonbiodegradable particulate COD (nbpCOD) is organic material, it will also contribute to the VSS concentration of the watewater and mixed liquor in the activated-sludge process, and is refered to here as the nonbiodegradable Volatile Suspended Solids (nbVSS). The influent wastewater will also contain nonvolatile influent suspended solids that add to the MLSS concentration in the activated-sludge process. These solids are influent inert TSS (iTSS) and can be quantified by the difference in influent wastewater TSS and VSS concentrations, by:

iTSS = TSS -VSS

source: Metcalf and Eddy page 668