Sludge is a by-product of many types of wastewater treatment systems and therefore an important parameter to sample and test. Almost all facilities generating sludge are currently required by law or permit to sample and test the sludge to determine if it classifies as hazardous or non-hazardous prior to disposal. 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.