Primary treatment is the simplest form of wastewater treatment, which targets the physical removal of solids.  The first part of primary treatment, which is called skimming, removes plastics and other floatables, which are then sent to landfills for disposal.  Removing additional solids occurs by passing water through a series of screens, grit removal, and settlement, where wastewater is held in a clarifier tank for several hours during which time, the more dense solids settle to the bottom of the tank and the lighter ones float to the top.  The solids that are removed during primary treatment are known as biosolids or sludge, and the remaining wastewater is then known as effluent.