Sludge is a byproduct of many types of wastewater treatment systems and therefore an important parameter to sample and test. It can contain MRSL substances (that have the tendency to settle down/partition into sludge during the wastewater treatment processes), metals from dyes, pigments, cotton, piping systems, and pathogens that can harm the environment and human health.

Almost all facilities generating sludge are currently required by law or permits to sample and test the sludge to determine if it is classified as hazardous or non-hazardous before disposal. However, these global law/permit requirements do not cover all parameters and limits set by ZDHC. By including sludge in the scope of the guidelines, ZDHC can help to ensure zero discharge of hazardous chemicals from the key pathways of wastewater treatment systems.