Wastewater flows through a system and is treated in some way, then that system is considered a wastewater treatment tank system. For example, oil-water separators treat wastewater by separating the oil from the water and therefore are wastewater treatment tank systems. Simply, the HTSS provides a means to collect and temporarily store sewage from a facility or dwelling, for subsequent removal and transport to an approved treatment and disposal site. The required storage capacity of a holding tank depends upon two items: daily sewage flow, and available or optimal pumping service frequency. The treatment of wastewater is part of the overarching field of sanitation. Sanitation also includes the management of human waste and solid waste as well as stormwater (drainage) management. By-products from wastewater treatment plants, such as screenings, grit and sewage sludge may also be treated in a wastewater treatment plant tank. Clarifiers are settling tanks built with mechanical means for continuous removal of solids being deposited by sedimentation. A clarifier is generally used to remove solid particulates or suspended solids from liquid for clarification and (or) thickening. Concentrated impurities discharged from the bottom of the tank are known as sludge, while the particles that float to the surface of the liquid are called scum. A city’s wastewater treatment plant has wastewater undergo five major processes: preliminary treatment, primary treatment, secondary treatment, disinfection and finally, sludge treatment. Primary and secondary treatments remove about 85% to 95% of pollutants from the wastewater in tanks before the treated wastewater is disinfected and discharged into local waterways. Sludge, the byproduct of the treatment process, is digested for stabilization inside holding tanks and is then dewatered for easier handling. The resulting material, known as biosolids which are held in another tank is then extracted and applied to land to improve vegetation or processed further as compost or fertilizers.