Following screening and grit removal, the wastewater flows very slowly through large tanks called primary clarifiers, where most of the fine settleable solids settle down to the tank bottom. Scrapers move the settled solids (sludge) to sumps at one end of the tank. From there the sludge is pumped to fermenters for the production of volatile fatty acids required for biological treatment. The remaining clarified liquid, containing mostly dissolved materials, flows to the secondary treatment stage. Scum floating on the surface is removed by a skimmer and sent to digesters for treatment.