Appearance
Lamella Clarifier (Inclined Plate Settler)
High-rate settling using inclined plates to provide large effective settling area in compact footprint, achieving 3-5× higher overflow rates than conventional clarifiers.
Overview
Lamella clarifiers use closely-spaced inclined plates to dramatically increase effective settling area while minimizing tank volume. They are particularly effective for secondary clarification in space-constrained sites or plant upgrades where additional capacity is needed without major construction.
When to Use
- Site expansion with limited space
- Retrofit upgrades to increase capacity
- Package plants and modular systems
- Good settling sludge (SVI <120)
- Applications requiring compact footprint
- Where rapid startup/shutdown is beneficial
Key Characteristics
- 50-70% footprint reduction vs. conventional
- Surface overflow rates: 40-80 m³/m²·d (projected area)
- Settling area based on plate surface, not tank floor
- Requires well-settled sludge (SVI <120 preferred)
- More sensitive to flow and load variations
- Higher capital cost per m² (but lower overall footprint cost)
- Excellent for plants with good primary treatment
Operating Modes
Basic Mode
- Fixed plate efficiency factor
- Simple overflow rate calculation based on projected area
- Default removal efficiency
- Single sludge hopper
Advanced Mode
- Hazen theory for inclined plate settling
- Plate angle and spacing optimization
- Flow distribution between plates
- Sludge sliding velocity calculation
- Temperature and viscosity effects
- Plate fouling factor
Mathematical Model
Governing Equation - Hazen Theory
Hazen (1904) showed that settling depends on surface area, not depth:
$$V_s = \frac{Q}{A_{projected}} = \frac{Q}{n \times L \times W \times \cos(\theta)}$$
Where:
- $V_s$ = required settling velocity (m/h)
- $Q$ = flow rate (m³/h)
- $n$ = number of plates
- $L$ = plate length (m)
- $W$ = plate width (m)
- $\theta$ = plate inclination angle from horizontal (typically 55-60°)
Effective Settling Area
The effective area is the sum of all plate surfaces:
$$A_{effective} = n \times L \times W / \cos(\theta)$$
Projected area (for SOR calculation):
$$A_{projected} = L \times W_{tank}$$
Area multiplication factor:
$$F_{area} = \frac{A_{effective}}{A_{projected}} = \frac{n \times s_{spacing}}{d \times \cos(\theta)}$$
Where:
- $s_{spacing}$ = plate spacing (typically 50-75 mm)
- $d$ = distance between plates measured vertically
Typical $F_{area}$ = 8-15
Critical Settling Velocity
Particles with settling velocity $V_{s,particle} > V_{s,critical}$ will be removed:
$$V_{s,critical} = \frac{Q}{A_{effective}} = \frac{Q \times \cos(\theta)}{n \times L \times W}$$
For activated sludge with Takács model:
$$V_s(X) = V_0 \times (e^{-r_h \times X} - e^{-r_p \times X})$$
Sludge Sliding on Plates
Sludge must slide down plates to sludge hopper:
$$v_{slide} = k_{slide} \times \sin(\theta) \times \sqrt{g \times h_{sludge}}$$
Where:
- $k_{slide}$ = sliding coefficient (0.3-0.7, depends on sludge stickiness)
- $h_{sludge}$ = sludge layer thickness on plate (mm)
Critical angle for sliding:
- θ > 50° for biofilm/sticky sludge
- θ > 45° for well-settled activated sludge
Plate Loading Rate
$$PLR = \frac{Q}{A_{plate,total}} = \frac{Q}{n \times L \times W}$$
Typical range: 5-12 m³/m²·d (plate surface area)
Equivalent to SOR (projected area): 40-150 m³/m²·d
Design Parameters
Input Parameters
| Parameter | Symbol | Unit | Range | Default | Description |
|---|---|---|---|---|---|
| Plate inclination | θ | degrees | 45-65 | 55 | Angle from horizontal |
| Plate spacing | s | mm | 50-100 | 75 | Distance between plates |
| Plate length | L | m | 1.0-3.0 | 2.0 | Flow path length |
| Number of plates | n | - | 20-200 | - | Dependent on tank width |
| Tank depth | H | m | 2.5-4.0 | 3.0 | Overall depth |
| Plate material | - | - | - | PVC/PP | Corrosion resistance |
| Flow distribution | - | - | - | Perforated | Inlet manifold type |
| SVI | - | mL/g | <150 | 100 | Sludge settling (critical!) |
Output Parameters
| Parameter | Unit | Description |
|---|---|---|
| Projected SOR | m³/m²·d | Based on tank floor area |
| Plate loading rate | m³/m²·d | Based on total plate area |
| Effective settling area | m² | Sum of all plate surfaces |
| Area multiplication factor | - | Effective area / projected area |
| Effluent TSS | mg/L | Predicted performance |
| Sludge sliding velocity | m/h | Ensures sludge doesn't accumulate |
| Hydraulic retention time | min | Detention time in tank |
Configuration Guide
Step-by-Step Setup
Assess Sludge Characteristics
Critical: Lamella clarifiers require good settling sludge.
SVI <100: Excellent candidate SVI 100-120: Good candidate SVI 120-150: Marginal (reduce loading) SVI >150: Not recommended (use conventional clarifier)Select Plate Configuration
Inclination angle:
- 55-60° preferred for activated sludge
- 60° for sticky biofilm sludge
- 50° for well-flocculated sludge
Spacing:
- 50 mm: Higher capacity, more fouling risk
- 75 mm: Standard for secondary clarification
- 100 mm: Lower capacity, less fouling
Calculate Required Projected Area
Use higher SOR than conventional due to plate multiplication:
For SVI = 100: SOR_projected = 60-80 m³/m²·d (average) SOR_projected = 100-120 m³/m²·d (peak) A_projected = Q_peak / SOR_peakDetermine Plate Dimensions
Example: Q = 5,000 m³/d, SOR = 100 m³/m²·d A_projected = 5,000 / 100 = 50 m² If tank width W = 5 m: Tank length L_tank = 50 / 5 = 10 m Plate length L = 2.0 m Plate spacing s = 0.075 m Number of plates n = W / s = 5 / 0.075 = 67 plates A_effective = 67 × 2.0 × 5 / cos(55°) = 1,166 m² Area factor = 1,166 / 50 = 23.3Verify Plate Loading
PLR = Q / A_effective = 5,000 / 1,166 = 4.3 m³/m²·d ✓ (Target: 4-8 m³/m²·d for activated sludge)Check Sludge Sliding
θ = 55°, g = 9.81 m/s² v_slide ≈ 0.5 × sin(55°) × √(9.81 × 0.003) = 0.070 m/s = 252 m/h This is >> sludge accumulation rate, so plates will self-clean ✓Design Flow Distribution
Critical for plate performance:
- Perforated inlet header spanning full width
- Velocity through openings: 0.3-0.5 m/s
- Equal flow to all plate channels
- Stilling zone upstream of plates
Configure Sludge Collection
- Central hopper beneath plates
- Steep hopper walls (60° minimum)
- Continuous or intermittent sludge withdrawal
- RAS pumping similar to conventional clarifiers
Typical Design Values
| Application | SVI (mL/g) | Projected SOR (m³/m²·d) | Plate Loading (m³/m²·d) | Plate Angle | Spacing (mm) |
|---|---|---|---|---|---|
| Excellent settling AS | 80-100 | 80-100 | 6-8 | 55° | 75 |
| Good settling AS | 100-120 | 60-80 | 5-7 | 55-60° | 75 |
| Moderate settling AS | 120-140 | 40-60 | 4-6 | 60° | 75-100 |
| Trickling filter effluent | 60-100 | 60-80 | 5-7 | 50-55° | 50-75 |
| Industrial biofilm | 80-120 | 50-70 | 5-6 | 60° | 75 |
Performance Criteria
Achievable Effluent Quality
| Sludge Type | SVI (mL/g) | Plate Loading (m³/m²·d) | Expected TSS (mg/L) |
|---|---|---|---|
| Excellent AS | 80-100 | 6-8 | 5-10 |
| Good AS | 100-120 | 5-7 | 10-15 |
| Moderate AS | 120-140 | 4-6 | 15-25 |
| Poor settling | >140 | <5 | >25 (not recommended) |
Capacity Comparison
| Parameter | Conventional Circular | Lamella | Benefit |
|---|---|---|---|
| Projected SOR (m³/m²·d) | 20-25 | 60-100 | 3-4× higher |
| Footprint (m²) | 400 | 100 | 75% reduction |
| Depth (m) | 4-5 | 3-4 | Shallower |
| Capital cost per m² | Lower | Higher | But smaller overall area |
| Sensitivity to SVI | Moderate | High | Requires better sludge |
| Maintenance | Low | Moderate | Plate cleaning needed |
Limitations
- Not suitable for bulking sludge: SVI >150 will cause plate fouling
- Sensitive to shock loads: Flow/load variations affect performance more than conventional
- Requires good primary treatment: Grease/FOG can foul plates
- Periodic plate cleaning: Every 1-3 years depending on conditions
- Limited denitrification capacity: Short detention time
Troubleshooting
| Issue | Cause | Solution |
|---|---|---|
| High effluent TSS | Sludge bypassing plates | Check flow distribution, repair plates |
| Poor sludge settling (high SVI) | Reduce loading, address bulking | |
| Plate fouling | Clean plates, check for grease | |
| Plate clogging | Grease/FOG accumulation | Improve primary treatment, hot water wash |
| Filamentous sludge | Address filamentous bulking at source | |
| Algae growth (if exposed) | Cover tank, reduce light | |
| Uneven flow distribution | Blocked inlet ports | Clean inlet manifold |
| Poor header design | Redesign distribution system | |
| Sludge accumulation on plates | Angle too shallow | Increase to 60°, high-pressure wash |
| Very sticky sludge | Increase wash frequency, consider conventional | |
| RAS concentration low | Short-circuiting | Improve inlet distribution |
| High withdrawal rate | Reduce RAS pumping if possible | |
| Rising sludge (denitrification) | Nitrate + low DO in plates | Increase sludge withdrawal, reduce blanket |
Operation & Maintenance
Daily Checks
- Visual inspection of effluent clarity
- Monitor RAS/WAS flow rates
- Check for visible sludge on plates (if accessible)
- Verify inlet flow distribution
- Observe any plate damage or deflection
Weekly Maintenance
- SVI measurement
- Check sludge hopper levels
- Inspect inlet header for clogging
- Verify equal flow through all plate channels
- Clean scum baffles
Monthly Tasks
- Plate inspection (visual or camera)
- Measure effluent TSS
- Check for plate fouling or damage
- Verify RAS concentration
- Review performance trends
Quarterly/Annual Tasks
- High-pressure plate washing: Remove biofilm/grease buildup
- Full plate inspection
- Inlet distribution system cleaning
- Sludge hopper cleaning
- Structural inspection
- Plate replacement if damaged
- Performance evaluation vs. design
Advanced Features
Automated Plate Washing
Some systems include:
- Fixed spray nozzles along plate length
- Periodic high-pressure water wash (daily or weekly)
- Automated sequencing to wash sections without taking unit offline
Flow Equalization
Lamella clarifiers benefit from steady flow:
- Upstream flow equalization tank
- Variable-frequency drive (VFD) on feed pumps
- Storm flow bypass to conventional clarifiers
Multiple Stages
For very high rates:
- Two-stage lamella (primary + secondary plates)
- First stage removes fast-settling solids
- Second stage polishes effluent
Temperature Control
Performance improves with warm water (lower viscosity):
- Cover tanks in cold climates
- Heat tracing for extreme cold
- Temperature effect: ~2% per °C on settling velocity
Worked Example
Given
- Flow: 8,000 m³/d (average), 12,000 m³/d (peak)
- MLSS: 3,200 mg/L
- SVI: 105 mL/g (good settling)
- Site constraint: Maximum footprint 80 m²
- Conventional clarifier would require: 12,000/30 = 400 m² (not feasible)
Solution
Step 1: Select Lamella Loading
SVI = 105 → SOR_projected = 75 m³/m²·d (average), 110 m³/m²·d (peak)Step 2: Calculate Projected Area
A_projected = 12,000 / 110 = 109 m²
This exceeds site constraint of 80 m² !
Try higher loading: SOR_peak = 120 m³/m²·d (aggressive for SVI=105)
A_projected = 12,000 / 120 = 100 m² (still too large)
Try SOR_peak = 150 m³/m²·d (very aggressive, will require excellent operation)
A_projected = 12,000 / 150 = 80 m² ✓ (exactly at constraint)Step 3: Tank Geometry
Use two tanks in parallel for redundancy:
Each tank: 40 m² projected area
Tank width W = 4 m (standard plate module)
Tank length L_tank = 40 / 4 = 10 mStep 4: Plate Configuration
Plate angle θ = 60° (for SVI ~100)
Plate spacing s = 0.075 m
Plate length L = 2.5 m
Number of plates per tank: n = W / s = 4 / 0.075 = 53 plates
Effective area per tank:
A_eff = 53 × 2.5 × 4 / cos(60°) = 1,060 m²
Total effective area (both tanks): 2,120 m²
Area multiplication factor = 2,120 / 80 = 26.5Step 5: Verify Plate Loading
PLR = 12,000 / 2,120 = 5.66 m³/m²·d ✓ (within 5-7 target)Step 6: Average Conditions
SOR_avg (projected) = 8,000 / 80 = 100 m³/m²·d ✓
PLR_avg = 8,000 / 2,120 = 3.77 m³/m²·d ✓Step 7: Expected Performance
At SVI = 105, PLR = 5.66 m³/m²·d:
Expected effluent TSS ≈ 12 mg/L (good)
Critical: SVI must remain <120 for acceptable performance
If SVI rises to 130, performance will degrade significantlyStep 8: RAS System
Similar to conventional clarifier:
RAS ratio R = 0.75 (for X_MLSS = 3200, X_RAS = 10,000)
Q_RAS = 0.75 × 8,000 = 6,000 m³/d (average)Summary
- Two 40 m² lamella clarifiers (10 m × 4 m each)
- 53 plates per tank @ 60° angle, 75 mm spacing
- Projected SOR = 100 avg, 150 peak m³/m²·d
- Plate loading = 3.8 avg, 5.7 peak m³/m²·d
- Expected TSS < 12 mg/L with SVI <120
- Total footprint: 80 m² (vs. 400 m² conventional = 80% savings)
Related Units
- Secondary-Clarifier - Conventional circular alternative
- Rectangular-Clarifier - Conventional rectangular alternative
- Activated-Sludge-Basin - Upstream biological treatment
- DAF-Thickener - Alternative for thickening function
References
- Hazen, A. (1904). "On Sedimentation." Trans. ASCE 53: 45-88.
- Yao, K.M. (1970). "Theoretical Study of High-Rate Sedimentation." J. Water Pollut. Control Fed. 42(2): 218-228.
- WEF (2018). Clarifier Design, WEF Manual of Practice FD-8, 3rd Ed.
- Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed., pp. 1045-1055.
- Parker, D.S., et al. (2001). "Assessment of Secondary Clarification Design Concepts." Water Sci. Technol. 43(11): 7-14.
- UK Water Industry Research (2015). High-Rate Clarification Technologies.