Appearance
Grit Chamber
Horizontal flow sedimentation basin for removal of heavy inorganic particles (sand, gravel, grit) while keeping organic solids in suspension.
Overview
Grit chambers protect downstream equipment from abrasive wear, reduce grit accumulation in tanks and digesters, and minimize solids disposal costs by separating heavy inorganic material. The key design principle is controlled horizontal velocity to settle grit while keeping lighter organic matter suspended.
When to Use
- All municipal wastewater treatment plants
- Industrial plants with sand/grit in influent
- Combined sewer systems (high grit loads)
- Before pumps requiring protection
- Upstream of primary clarifiers and aeration tanks
- Pretreatment for anaerobic digesters
Key Characteristics
- Removes particles >0.21mm at specific gravity ~2.65
- Controlled horizontal velocity (0.15-0.45 m/s)
- Minimal organic matter removal
- Simple, reliable operation
- Requires grit washing and disposal
Operating Modes
Basic Mode
- Fixed removal efficiency (default 95% >0.21mm)
- Simplified detention time calculation
- Constant horizontal velocity
Advanced Mode
- Stokes' Law settling calculations
- Particle size distribution tracking
- Temperature-dependent viscosity
- Scour velocity verification
Mathematical Model
Governing Equation - Stokes' Settling Velocity
$$V_s = \frac{g \times d^2 \times (\rho_s - \rho_w)}{18 \mu}$$
Where:
- $V_s$ = terminal settling velocity (m/s)
- $g$ = gravitational acceleration (9.81 m/s²)
- $d$ = particle diameter (m)
- $\rho_s$ = particle density (kg/m³)
- $\rho_w$ = water density (kg/m³, ~1000)
- $\mu$ = dynamic viscosity of water (Pa·s)
Temperature Correction for Viscosity
$$\mu(T) = \mu_{20} \times \frac{1.002 \times 10^{-3}}{(1.002 - 0.02393 \times T + 0.000146 \times T^2) \times 10^{-3}}$$
Simplified: $$\mu(T) \approx 0.00179 - 0.0000535 \times T + 0.000000367 \times T^2$$
Where T is in °C.
Critical Parameters
Standard design particle:
- Diameter: 0.21mm (210 µm)
- Specific gravity: 2.65
- At 20°C: V_s ≈ 0.021 m/s (21 mm/s)
- At 10°C: V_s ≈ 0.016 m/s (reduced due to higher viscosity)
Detention Time
$$t = \frac{d}{V_s}$$
Where:
- $t$ = detention time (s)
- $d$ = effective water depth (m)
Required Length
$$L = v_h \times t = v_h \times \frac{d}{V_s}$$
Where:
- $L$ = chamber length (m)
- $v_h$ = horizontal velocity (m/s)
Surface Loading Rate
$$SLR = \frac{Q}{W \times L}$$
Where:
- $SLR$ = surface loading rate (m³/m²/h)
- $Q$ = flow rate (m³/h)
- $W$ = chamber width (m)
Design Parameters
Input Parameters
| Parameter | Symbol | Unit | Range | Default | Description |
|---|---|---|---|---|---|
| Horizontal velocity | v_h | m/s | 0.15-0.45 | 0.30 | Controlled flow velocity |
| Water depth | d | m | 1.0-2.5 | 1.5 | Effective settling depth |
| Detention time | t | s | 30-90 | 60 | At peak flow |
| Chamber width | W | m | 2.0-10.0 | 6.0 | Cross-sectional width |
| Temperature | T | °C | 5-25 | 15 | Wastewater temperature |
| Target particle | d_p | mm | 0.15-0.30 | 0.21 | Design grit size |
Output Parameters
| Parameter | Unit | Description |
|---|---|---|
| Grit removal | % | Fraction of particles >0.21mm removed |
| Chamber length | m | Required length for settling |
| Grit volume | m³/d | Volume of grit removed |
| Grit mass | kg/d | Mass of grit for disposal |
| Scour velocity check | - | Verification that organics stay suspended |
Configuration Guide
Step-by-Step Setup
Select Horizontal Velocity
- Standard: 0.30 m/s (most common)
- Low: 0.15-0.25 m/s (light grit loads)
- High: 0.35-0.45 m/s (heavy grit, combined sewers)
- Critical: Must be constant regardless of flow variation
Determine Detention Time
- At peak hourly flow: 45-90 seconds
- Typical: 60 seconds
- Colder climates: Increase 15-20% (higher viscosity)
Calculate Dimensions
- Depth: 1.0-2.0m (based on site constraints)
- Width: From flow and velocity control
- Length: From detention time and velocity
Select Flow Control Method
- Proportional weir: V-notch or sutro weir at outlet
- Parshall flume: Maintains constant velocity
- Variable gates: Adjustable for flow changes
Design Grit Removal System
- Manual: Bucket elevator or screw conveyor
- Automatic: Air-lift, screw, or chain-bucket
- Washing: Classify and wash to <5% organics
Configure Multiple Channels
- Minimum 2 channels for redundancy
- Splitter box for equal distribution
- Isolation gates for maintenance
Typical Design Values
| Application | Velocity | Detention | Depth | SLR | Grit Yield |
|---|---|---|---|---|---|
| Municipal (separate sewer) | 0.30 m/s | 60s | 1.5m | 900 m³/m²/h | 4-15 L/1000m³ |
| Combined sewer | 0.35 m/s | 45s | 1.8m | 700 m³/m²/h | 20-90 L/1000m³ |
| Industrial (food) | 0.25 m/s | 75s | 1.2m | 600 m³/m²/h | 5-30 L/1000m³ |
Velocity Control Methods
Proportional Weir (Most Common):
- V-notch or sutro weir at effluent end
- Maintains constant upstream velocity
- Simple, no moving parts
- Accurate for flow variations 2:1 to 4:1
Parshall Flume:
- Critical flow section controls velocity
- Good for wider flow ranges
- More complex construction
Motorized Gates:
- Automatic adjustment based on flow measurement
- Best for very wide flow variations
- Requires controls and maintenance
Performance Criteria
Removal Efficiency
| Particle Size | Specific Gravity | Removal @ 60s Detention |
|---|---|---|
| >0.30mm | 2.65 | >99% |
| 0.21mm (design) | 2.65 | 95% |
| 0.15mm | 2.65 | 70-80% |
| 0.10mm | 2.65 | 40-50% |
| Organics (SG 1.2) | 1.2 | <10% (desired) |
UK Design Standards
| Parameter | Value | Standard |
|---|---|---|
| Horizontal velocity | 0.30 m/s | MWH, UK industry practice |
| Detention time (PHF) | 45-60s | UK industry practice |
| Water depth | 1.0-1.5m | UK operator |
| Number of channels | ≥2 | All UK water companies |
| Grit washing | <5% organics | Environment Agency |
Scour Velocity Check
To ensure organics remain suspended:
$$v_{scour} = \sqrt{8g \times d_{50} \times \frac{\rho_s - \rho_w}{\rho_w}}$$
For organic particles (d=1mm, SG=1.2): $$v_{scour} \approx 0.45 \text{ m/s}$$
Design velocity (0.30 m/s) < Scour velocity (0.45 m/s) ✓ (organics stay in suspension)
Types of Grit Chambers
Horizontal Flow (This Unit)
Advantages:
- Simple, proven design
- Easy to construct
- Minimal mechanical equipment
- Low operating cost
Disadvantages:
- Larger footprint
- Velocity control required
- Grit removal can be manual
Aerated Grit Chamber
Differences:
- Air injection creates spiral flow
- Better organic separation
- Self-washing action
Vortex Grit Separator
Differences:
- Induced vortex in circular tank
- Very compact footprint
- Higher capital cost
Troubleshooting
| Issue | Cause | Solution |
|---|---|---|
| Excessive organics in grit | Velocity too low | Increase velocity via weir adjustment |
| Detention time too long | Reduce length or increase velocity | |
| Grit not washed | Install/improve grit washing | |
| Grit carryover to primary | Velocity too high | Decrease velocity (check weir) |
| Detention time too short | Check dimensions, verify flow control | |
| High viscosity (cold) | Increase detention time for winter | |
| Grit accumulation excessive | Grit removal frequency low | Increase pumping/removal cycles |
| Combined sewer event | Normal, increase removal after storm | |
| Odor issues | Grit storage too long | More frequent removal |
| Organic content high | Improve washing, check velocity | |
| Unequal flow distribution | Splitter box issue | Check weirs, clean debris |
| Channel blockage | Inspect, remove obstruction |
Operation & Maintenance
Daily Checks
- Visual inspection of flow distribution
- Check grit removal operation
- Monitor flow velocity (if instrumented)
- Observe grit accumulation rate
Weekly Maintenance
- Measure velocity at multiple points (portable meter)
- Inspect weir condition (algae growth, debris)
- Check grit washing system
- Review grit disposal records
Monthly Tasks
- Clean proportional weir
- Verify detention time calculations
- Check channel for debris/accumulation
- Inspect grit removal equipment
- Test flow splitting between channels
Quarterly Tasks
- Drain channel and clean (one at a time)
- Measure actual grit depths
- Inspect structural condition
- Check for erosion or corrosion
- Calibrate flow measurement
Annual Tasks
- Performance evaluation (particle size analysis)
- Verify removal efficiency
- Update O&M procedures
- Plan major repairs/upgrades
Grit Handling
Grit Washing
Purpose: Reduce organic content from ~30% to <5%
Methods:
- Detritor (cyclone separator + classifier)
- Grit washer (screw with spray water)
- Hydrocyclone
Performance:
- Organic reduction: 80-95%
- Moisture content: 20-30%
- Classification: Removes <0.15mm fines
Grit Disposal
| Disposal Method | Requirements | Notes |
|---|---|---|
| Landfill | Washed to <5% organics | Most common |
| Land application | Clean, inert | Agricultural use |
| Construction aggregate | Washed, graded | Beneficial reuse |
| Incineration/WTE | If not washed | Avoid if possible (low value) |
Grit Production Estimates
| Source | Grit Yield |
|---|---|
| Separate sanitary sewer | 4-15 L/1000m³ |
| Combined sewer (dry weather) | 10-30 L/1000m³ |
| Combined sewer (wet weather) | 50-200 L/1000m³ |
| Industrial (food processing) | 5-30 L/1000m³ |
Worked Example
Given
- Flow: Average 20,000 m³/d, Peak 50,000 m³/d
- Temperature: 12°C (winter design)
- Target: Remove 95% of particles >0.21mm (SG 2.65)
- Two channels for redundancy
Design
Step 1: Calculate Settling Velocity at 12°C
Viscosity at 12°C: μ ≈ 0.00124 Pa·s
$$V_s = \frac{9.81 \times (0.00021)^2 \times (2650 - 1000)}{18 \times 0.00124} = 0.0163 \text{ m/s}$$
Step 2: Select Design Parameters
- Horizontal velocity: v_h = 0.30 m/s
- Water depth: d = 1.5m
- Detention time: t = d/V_s = 1.5/0.0163 = 92s ✓ (conservative)
Step 3: Calculate Length $$L = v_h \times t = 0.30 \times 92 = 27.6 \text{ m}$$
Round to L = 28m per channel
Step 4: Determine Width
At peak flow, one channel (allow one offline): $$Q_{peak,single} = \frac{50,000}{24 \times 3600 \times 1} = 0.579 \text{ m}^3/\text{s}$$
$$W = \frac{Q}{v_h \times d} = \frac{0.579}{0.30 \times 1.5} = 1.29 \text{ m}$$
Round to W = 1.5m per channel
Step 5: Verify Surface Loading Rate $$SLR = \frac{Q}{W \times L} = \frac{50,000/24}{1.5 \times 28} = 49.6 \text{ m}^3/\text{m}^2/\text{h}$$
Convert: 49.6 × 20 = 992 m³/m²/d (acceptable for grit chambers)
Step 6: Estimate Grit Production
Assume 10 L/1000m³ for sanitary sewer:
- Grit volume: 20,000 × 10/1000 = 200 L/d = 0.2 m³/d
- After washing: ~0.15 m³/d at 25% moisture
- Mass: 0.15 × 2000 kg/m³ = 300 kg/d
Step 7: Design Weir for Velocity Control
Proportional weir to maintain 0.30 m/s from 50-100% of design flow. Use sutro weir or parabolic weir (design by hydraulic specialist).
Advanced Features
Automated Grit Removal
- Continuous screw conveyor
- Airlift systems
- Chain and bucket elevators
- Programmable cycle timing
Flow Measurement and Control
- Ultrasonic flow meters
- Automated gate control
- SCADA integration
- Alarm on velocity deviation
Grit Washing Integration
- Inline grit washer/classifier
- Continuous organic separation
- Wash water recirculation
Related Units
- Vortex-Grit-Separator - Alternative compact design
- Aerated-Grit-Chamber - Alternative with air injection
- Coarse-Screen - Upstream protection
- Fine-Screen - Upstream or parallel
- Primary-Clarifier - Downstream treatment
References
- Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed. McGraw-Hill.
- WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8.
- Camp, T.R. (1946). "Sedimentation and the Design of Settling Tanks." Transactions of ASCE, 111.
- Tchobanoglous, G. et al. (2003). Wastewater Engineering: Treatment and Reuse, 4th Ed.
- MWH (2005). Water Treatment Principles and Design, 2nd Ed.