Preliminary treatment of wastewaters includes screening, grit removal, odor control and flow measurement. The removal of debris in the screening area and the removal of sand, rocks, gravel, and other in organics in the grit removal system protect downstream treatment processes. Should the screening or grit removal fail, other downstream treatment processes may also fail.


Coarse screening removes large solids and trash that could otherwise interferewith the operation of a treatmentplant.Coarse screens are classified as either bar racks or bar screens. Bar racks have clear spacing of between 2.0 and 4.0 inches allowing larger debris size objects to accumulate on the screens. At certain intervals, the debris accumulated on the screens is removed for disposal, either to a landfill or incinerator.

Wastewater grit materials are generally nonputrescible, have a settling velocity greater than that of organic putrescible solids, and consist of discrete particles. Such materials include sand, cinders, rocks, coffee grounds, cigarette filter tips, and other large-sized, relatively nonputrescible organic and inorganic substances.

Grit removal, which is an essential element, protects equipment by:

• Reducing clogging in pipes;
  
  
• Protecting moving mechanical equipment and pumps from abrasion and accompanying abnormal wear;
  
  
• Preventing accumulations of materials in aeration tanks and sludge digesters that result in loss of usable volume; andReducing accumulations at the bases of mechanical screens.
  
  
• A velocity of 1 ft/sec (0.3 m/s) is used in most grit removal systems to separate grit from the organics.
  
  
• Aerated grit chambers provide a period of wastewater detention to trap grit through air-induced rotation of the wastewater at approximately 1 ft/sec (0.3 m/s). Compressed air, usually from the plant’s process air supply, enters the grit chamber through diffusers at a controlled rate to induce the correct wastewater velocity near the floor for settling the grit.