An aerated grit chamber is an enlarged channel or long basin in which the cross-section is increased to reduce the velocity of the flowing sewage sufficiently to cause heavy inorganic wastewater matter such as grit, sand and gravel of size 0.2 mm, and larger wastewater suspended solids to settle, while the lighter organic matter remains in suspension. This is used to remove solids having specific gravity greater than water.
This article takes a look at 4 important factors you must consider when you design a grit chamber:
(i) Velocity Of Flow
Velocity of wastewater flow must be 0.3 m/sec. This will permit the deposition of the bulk of heavier mineral solids in wastewater while most organic matter remains in suspension. A velocity of flow in the range 0.15 – 0.3m/sec. is generally recommended. In order to keep the velocity within the desirable limits, it is usually necessary to provide two or more channels to manage fluctuations in sewage flow.
(ii) Period Of Detention
One minute (volume of the grit chamber/flow rate) is the detention time normally employed. Since sedimentation of granular solids is dependent to a large extent upon the surface area of the aerated grit chamber, their width could be kept large. A length to width ratio of 8 to 1 may be used limiting the effective depth to about 2 m.
(iii) Method Of Cleaning
Grit chambers are cleaned by hand, mechanically or hydraulically. Hand cleaning is done only in the case of smaller plants, is less hygienic and odor-free though somewhat easier for disposing off the removed material than in the case of mechanical cleaning.
(iv) Grit Storage
Storage space for grit may be provided throughout the length of the aerated grit chamber or by means of one or more pits deeper than the remainder of the basins. Concentration of grit is also useful for cleaning purposes. Channel may be provided with a frequency of cleaning of 15 days.