The primary treatment incorporates unit operations for removal of floating and suspended solids from wastewater. They are also referred to as physical unit operations. The unit operations used are screening for removing floating papers, rages, cloths, plastics, cans stoppers, labels, etc.

The amount of grit quantities captured by aerated grit chamber ranges from 0.004-0.20 m³/10³.m³.Usually it is in range 0.015 m³/10³.m³. In terms of US unit typical value is 2 ft³/Mgal.

The headloss for a fine screen is calculated as h_L= 1/2g (Q/CA)². Where h_L is headloss through fine screen. Q is flow rate, A is effective open are of submerged screen.

The approach velocity at peak flow rates for a bar screen should not exceed 0.9m/s. This is to prevent pass-through of debris. These screens should be designed in such a way to prevent accumulation of debris or grit.

The head, pressure or energy lost by water flowing in a pipe or channel as a result of turbulence caused by velocity of flowing water, channel walls or restrictions caused by fittings.

The horizontal velocity considered while designing a grit chamber is 0.25-0.4m/sec. In most cases, it is considered as 0.3 m/sec. The setting velocity for removal of 21mm material is 1-1.3m/ sec.

The design of a horizontal flow type is such that as wastewater flows through in horizontal direction at a certain velocity (0.3m/s) particles will start to settle at channel before reaching outlet point.

Parshall flumes are installed at downstream of a bar screen. These are installed in order to control approach velocity of a bar screen. The approach velocity is a very important factor while designing screens to prevent accumulation of grit and debris.

Grit chamber is needed to protect major mechanical equipment from wear and tear including and remove inorganic particles to prevent damage to pumps and to prevent ir accumulation in sludge digesters.

Industrial wastewater treatment plant may or may not need screens. Screens must be provided even for industrial wastewater treatment plant to separate labels, stopper, cardboard, and or packing materials.