Appearance
Grit Removal
Removal of heavy inorganic particles (sand, gravel, cinders) from wastewater
Overview
Grit removal protects downstream equipment from abrasion and prevents grit accumulation in tanks. Types include aerated grit chambers, vortex chambers, and cyclone degritters. Removes particles >0.2 mm with specific gravity >2.65. Grit production: 15-90 mL/m³ treated.
Theory - Basic Mode
$$V_{grit} = Q \times f_{grit}$$
Where f_grit = 15-90 mL/m³ (depends on sewer type and rainfall)
Theory - Advanced Mode
Stokes' Law Settling:
$$v_s = \frac{g \times d^2 \times (\rho_p - \rho_w)}{18 \times \mu}$$
For sand (ρ=2.65 g/cm³), d=0.2mm: v_s ≈ 2.1 cm/s
Horizontal Flow Chamber:
$$\frac{L}{H} = \frac{v_h}{v_s}$$
Where L/H = length-to-depth ratio, v_h = horizontal velocity
Parameters
| Type | Velocity | Detention | Grit Size | Application |
|---|---|---|---|---|
| Horizontal | 0.15-0.40 m/s | 45-90 s | >0.2 mm | Conventional |
| Aerated | - | 2-5 min | >0.2 mm | Adjustable |
| Vortex | Variable | <60 s | >0.15 mm | Compact |
Worked Example
Given: Aerated grit chamber, Q=20,000 m³/d, detention=3 min Calculate: Volume
Volume = 20,000 / (24 × 60) × 3 = 41.7 m³
Use 2 channels × 25 m³ = 50 m³ total
References
- Metcalf & Eddy (2014)
- WEF MOP 8 (2018)