Catenary screens have an open design. Due to this construction, re is a possibility that y might emit odour. These screens also remove large objects.

The typical headloss through microscreens is 75-150mm. In case headloss exceeds 200 mm n bypasss should be provided. The mesh is generally made up of stainless steel.

Reciprocating rake is a type of mechanically driven screen. This can handle very large particles. This has relatively low operating and maintenance cost.

The quantity of screening depends on type of rack or screen used as well as sewer system and geographic location. The quantity of screening removed by bar screen is 0.0035 to 0.0375 m3 / 1000 m3 of wastewater treated (Typical value = 0.015 m3 /1000 m3 of wastewater).

The detention time for aerated grit chambers is 2-5 mins. Usually, in most cases, a detention time of 3 mins is considered. The release of VOC is one application of se aerated grit chambers.

Comminutors are usually installed in plants which are <0.2 m3/sec. These are installed in order to shred material. These require high operating and maintenance cost.

The head loss for fine screens would be 0.8-1.4m. In many applications fine screens is limited to plants where head loss through screens is not a problem. Water sprays are supplied in order to keep screens clean continuously.

The settling velocity of discrete particles can be determined using an appropriate equation depending upon Reynolds number. Stoke’s law holds good for Reynolds number, Re below 1.

Screens can be broadly classified into two types depending upon opening size provided as coarse screen (bar screens) and fine screens. There are manually cleaned screens and mechanically cleaned screens based on cleaning operation.

While designing an aerated grit chamber width is assumed in range 2-5 m. When expressing same in US unit it is 7-16 ft. The release of VOC by ses aerated grit chambers is one thing which we have to consider and take proper measures to reduce same.