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Grit removal

The goal of grit removal is to separate gravel and sand and other mineral materials down to a diameter between 0.2 and 0.1 mm. Grit chambers are provided to (a) protect downstream moving mechanical equipment from abrasion (b) reduce formation of heavy deposits in pipe line and (c) reduce the frequency of digester cleaning caused by excessive accumulation of grit.

There are three general types of grit chamber:

  1. horizontal-flow – rectangular configuration

  2. horizontal-flow – square configuration

  3. aerated; (see figure 3 and 4)

The use of vertical flow chambers have shown an insufficient separation of very fine grained sand fraction.

The quantity of removed grit will vary depending on the type of sewer system, the characteristics of the drainage area, etc. The amount of removed gravel is different plant by plant.

Figure 3: Example for an aerated grit chamber (Source: Entwässerungsbetriebe Mainz)

Figure 4: Aerated grit chamber (Crites and Tchobanoglous, 1998)

In aerated grit chambers, grit is removed by causing the wastewater to flow in a spiral pattern, as shown in figure 4. Air is introduced in the grit chamber along one side, causing a perpendicular spiral velocity pattern to flow through the tank. Heavier particles are accelerated and diverge from the streamlines, dropping to the bottom of the tank, while lighter organic particles are suspended and eventually carried out of the tank.

In the following table 3.2.1 some typical values by different authors for combined sewer systems are shown.

Tab. 3.2.1 Typical amounts of collected grit (combined sewer)

Total amount of mineral material inside influx of grit chamber is between 10 and 60 g per m³ of wastewater (grain size ranges between 0.09 – 3.0 mm). The use of a sieve in upstream decreases mass of separated solids in grit chamber. (ATV-Handbuch, 1997).

In case of combined sewer, over a dry period (low flow velocity) sand can settle down in the sewer pipes: a hard rain causes an increasing flow speed and the sand is flushed into wastewater treatment plant. Thus, the highest amounts of sand reaches the plant with stormwater.

In case of separate sewer, the quantity of grit will be less than that expected for combined sewer (typical value is 0.5 l m-3 of wastewater). The moisture content of the collected grit ranges between 15 e 40 %; volatile content, on dry basis, ranges between 20-50 %.

In table 3.2.2 a typical grit chamber operation sheet is reported; the reporting frequency depends on the WWTP size; details on routine inspections, lubrication and adjustment, performed by the operator, should be reported as well.

Tab 3.2.2 Grit chamber operation sheet

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