The design of a grit chamber must follow these principles:

(4) The grit amount for municipal sewage can be calculated as 30 m3 of grit per 100,000 m3 of sewage, with a moisture content of 60% and a bulk density of 1500 kg/m3.

(2) The design flow rate should consider phased construction:

(a) When sewage enters by gravity flow, it should be calculated based on the maximum design flow of each phase;

(b) When sewage is pumped in, it should be calculated based on the maximum combined flow of the operating pumps in each phase;

(c) In a combined sewer system, it should be calculated based on the design flow during rainfall.

(3) The grit and impurities removed by the grit chamber are mainly particles with a specific gravity of 2.65 t/m3 and a particle size above 0.2 mm.

(1) Municipal sewage plants should generally be equipped with grit chambers; the number of units or compartments should be no fewer than 2, designed for parallel operation.

(5) The hopper volume should be calculated based on 2 days of grit accumulation; the angle between the hopper wall and the horizontal plane should not be less than 55 degrees, and the grit-discharge pipe diameter should be no less than 0.3 m.

(6) The freeboard of the grit chamber should not be less than 0.3 m.

(7) Grit removal should generally use mechanical methods. When gravity discharge is used, the grit chamber and the sand-drying bed should be located as close as possible to shorten the grit-discharge pipe length.

Grit chambers come in four forms: horizontal-flow, aerated, vortex, and Dorr. Because the aerated grit chamber offers small footprint, low energy consumption, and low civil-construction cost, it is the most commonly used. An aerated grit chamber has a row of air diffusers installed on the side wall of a horizontal-flow grit chamber, causing transverse flow in the sewage and forming a spiral rotation. Its structural features consist of an inlet device, outlet device, settling zone, aeration system, and sludge-discharge device. The water-flow section of an aerated grit chamber is an elongated channel; along the entire length on one side wall, 0.6-0.9 m above the tank bottom, air diffusers are installed, with a grit-collection trough provided and a bottom slope of i = 0.1-0.5 to ensure grit slides in. The aerated grit chamber overcomes the drawback of horizontal-flow grit chambers where grit entrains 15% organic matter, increasing the difficulty of subsequent grit treatment. The Dorr grit chamber has a square top and circular bottom; its grit-removal mechanism is similar to that of a horizontal-flow grit chamber. It is usually designed based on surface hydraulic load, with a very shallow depth, typically less than 0.9 m. Influent is evenly distributed into the chamber through a flow regulator, then discharged over an overflow weir. Sand particles are scraped into a collection pit by a center-driven sand scraper and discharged by a screw sand washer together with the separated organic matter. The Dorr grit chamber has not yet found known users in China, and little information is available about it.