Appearance
Pile Cloth Filter
Continuous rotating drum filter with pile cloth media (3-5mm fiber depth) providing depth filtration through fiber matrix with self-cleaning backwash, combining features of disk filters and conventional media filters.
Overview
Pile cloth filters use a rotating cylindrical drum covered with compressible pile fabric (similar to carpet fibers). Wastewater flows from inside the drum outward through the cloth, with solids captured within the fiber matrix. As the drum rotates, a backwash spray continuously cleans a sector of the cloth, allowing uninterrupted operation. The pile cloth provides depth filtration (unlike flat woven cloth) with self-regenerating capacity.
When to Use
- Space-constrained sites (compact footprint)
- Continuous operation requirement (no offline cycles)
- Moderate TSS removal (10-25 mg/L → 5-10 mg/L)
- Retrofit of existing clarifiers or lagoons
- Seasonal treatment capacity addition
- Applications where media replacement is difficult
- Municipal and industrial wastewater tertiary treatment
Key Characteristics
- TSS removal: 15-25 mg/L → 5-10 mg/L (moderate performance)
- Hydraulic loading: 20-40 m³/m²·h (cloth surface area basis)
- Effluent quality: 5-10 mg/L TSS (not as tight as disk or sand)
- Continuous backwash: 2-5% of filtered flow
- Cloth life: 5-8 years (longer than woven cloth)
- Low headloss: 0.2-0.5 m
- Rotation speed: 0.5-2 rpm (slow, continuous)
Operating Modes
Basic Mode
- Fixed rotation speed
- Constant removal efficiency
- Continuous backwash at fixed rate
- Simple hydraulic loading calculation
Advanced Mode
- Depth filtration model through fiber matrix
- Dynamic cloth resistance based on loading
- Backwash optimization (minimize water while maintaining performance)
- Solids loading rate calculation
- Cloth aging model (performance vs. time)
Mathematical Model
Governing Equations
1. Depth Filtration Through Pile Cloth
Unlike flat woven cloth (pure surface filtration), pile cloth provides depth filtration:
$$C_{effluent} = C_{influent} \times e^{-\lambda_{fiber} \times L_{pile}}$$
Where:
- $\lambda_{fiber}$ = fiber filtration coefficient (m⁻¹)
- $L_{pile}$ = pile depth (0.003-0.005 m = 3-5 mm)
Typical values:
- $\lambda_{fiber}$ = 100-200 m⁻¹ (much higher than granular media due to fine fibers)
- $L_{pile}$ = 4 mm = 0.004 m
Example: $$\eta = 1 - e^{-150 \times 0.004} = 1 - e^{-0.6} = 0.451 = 45%$$
Additional cake filtration:
As solids accumulate on cloth surface, a filter cake forms:
$$\eta_{total} = \eta_{pile} + \eta_{cake} \times \left(1 - e^{-k \times \theta}\right)$$
Where:
- $\eta_{pile}$ = base pile filtration (40-50%)
- $\eta_{cake}$ = additional cake filtration (20-30%)
- $k$ = cake buildup rate (rad⁻¹)
- $\theta$ = angle since last backwash (radians)
Overall removal: 60-80% typical
2. Headloss Development
Initial clean cloth headloss is very low:
$$h_0 = k_{cloth} \times v$$
Where:
- $k_{cloth}$ = cloth resistance coefficient (h)
- $v$ = filtration velocity through cloth (m/h)
Typical: $h_0$ = 0.05-0.15 m for clean pile cloth
Headloss with cake:
$$h(t) = h_0 + \alpha \times m_{cake}(t)$$
Where:
- $\alpha$ = specific cake resistance (m/kg·m²)
- $m_{cake}$ = cake mass per unit area (kg/m²)
Backwash removes cake continuously, so headloss cycles:
- Maximum (just before backwash): 0.3-0.6 m
- Minimum (just after backwash): 0.05-0.15 m
- Average: 0.2-0.4 m
3. Backwash Water Consumption
Continuous backwash sprays one sector at a time:
$$Q_{BW} = v_{BW} \times A_{sector}$$
Where:
- $v_{BW}$ = backwash spray velocity (50-100 m/h)
- $A_{sector}$ = sector area being backwashed (m²)
Sector area:
$$A_{sector} = \frac{A_{total} \times \omega \times t_{spray}}{360°}$$
Where:
- $\omega$ = rotation speed (°/min)
- $t_{spray}$ = backwash spray duration per point (min)
Typical backwash flow: 2-5% of filtered flow (continuous)
4. Hydraulic Loading Rate
$$HLR = \frac{Q}{A_{cloth}}$$
Where:
- $Q$ = flow rate (m³/h)
- $A_{cloth}$ = total cloth surface area (m²)
Drum surface area:
$$A_{cloth} = \pi \times D \times L$$
Where:
- $D$ = drum diameter (m)
- $L$ = drum length (m)
Typical loading: 20-40 m³/m²·h (higher than disk filters due to depth filtration)
Design Parameters
Input Parameters
| Parameter | Symbol | Unit | Range | Default | Description |
|---|---|---|---|---|---|
| Flow rate | $Q$ | m³/h | 50-2000 | 400 | Average flow to treat |
| Hydraulic loading | HLR | m³/m²·h | 20-40 | 30 | Design loading rate |
| Drum diameter | $D$ | m | 2.0-3.5 | 2.6 | Standard drum size |
| Drum length | $L$ | m | 2.0-6.0 | 4.0 | Axial length |
| Pile depth | $L_{pile}$ | mm | 3-5 | 4 | Fiber depth |
| Influent TSS | $C_0$ | mg/L | 10-30 | 18 | Secondary effluent |
| Target effluent TSS | $C_e$ | mg/L | 5-10 | 7 | Design goal |
| Rotation speed | $\omega$ | rpm | 0.5-2.0 | 1.0 | Drum rotation rate |
| Backwash pressure | $P_{BW}$ | kPa | 150-400 | 250 | Spray nozzle pressure |
| Submergence | $f_{sub}$ | % | 30-40 | 35 | Fraction of drum submerged |
Output Parameters
| Parameter | Unit | Description |
|---|---|---|
| Cloth surface area | m² | Total filtration area |
| Number of drums | - | Parallel drum units |
| Backwash flow | m³/h | Continuous backwash requirement |
| Backwash % | % | Percentage of production |
| Solids loading | kg TSS/m²·d | Solids captured per cloth area |
| Power consumption | kWh/m³ | Energy for rotation + backwash |
| Cloth replacement cost | £/year | Amortized capital for cloth media |
Configuration Guide
Step-by-Step Setup
Select Hydraulic Loading
Based on influent quality and effluent target:
Good influent (10-15 mg/L TSS): HLR = 35-40 m³/m²·h Standard influent (15-20 mg/L): HLR = 28-35 m³/m²·h High loading (20-30 mg/L): HLR = 20-28 m³/m²·hCalculate Required Cloth Area
$$A_{cloth} = \frac{Q}{HLR}$$
Example: Q = 8,000 m³/d = 333.3 m³/h HLR = 30 m³/m²·h A_cloth = 333.3 / 30 = 11.1 m²Select Drum Configuration
Standard drum: D = 2.6m, L = 4.0m $$A = \pi \times 2.6 \times 4.0 = 32.7 \text{ m² per drum}$$
Submerged area (35%): $$A_{effective} = 32.7 \times 0.35 = 11.4 \text{ m² per drum}$$
Number of drums = A_cloth / A_effective = 11.1 / 11.4 = 0.97 Use 1 drum (11.4 m² cloth) ✓Verify Loading
Actual HLR = 333.3 / 11.4 = 29.2 m³/m²·h ✓ (Within design range)Design Backwash System
Backwash spray configuration:
- Spray bar: Full length of drum (4.0m)
- Nozzle spacing: 75mm
- Number of nozzles: 4000 / 75 = 53 nozzles
- Nozzle flow: 0.3-0.5 m³/h each
- Total spray flow: 53 × 0.4 = 21.2 m³/h
Backwash as % of flow:
Backwash % = (21.2 / 333.3) × 100 = 6.4% (Typical 2-5%, so this is slightly high—reduce nozzle flow to 0.25 m³/h each) Adjusted: 53 × 0.25 = 13.25 m³/h Backwash % = 13.25 / 333.3 = 3.98% ✓Rotation Speed Selection
Backwash frequency requirement:
- Each point on drum should be backwashed every 15-60 minutes
- Faster rotation → more frequent cleaning → better performance but higher wear
For 1 rpm rotation: Time per revolution = 60 minutes Each point backwashed every 60 min ✓ For 0.5 rpm (slower): Time per revolution = 120 minutes (conservative, longer runs between cleaning)Select: 1.0 rpm (standard)
Solids Handling
Daily solids captured = Q × (C_influent - C_effluent) = 8,000 × (18 - 7) / 1000 = 88 kg TSS/day Backwash flow = 13.25 m³/h × 24 h = 318 m³/day Backwash TSS concentration = 88,000 mg / 318 m³ = 277 mg/L (Low concentration—can return to headworks or discharge directly to sludge treatment)Power Consumption
Drum rotation motor: 0.5 kW × 24 h = 12 kWh/day Backwash pump: 2 kW × 24 h = 48 kWh/day Total: 60 kWh/day Specific energy = 60 / 8,000 = 0.0075 kWh/m³ = 0.75 Wh/m³ (very low)
Typical Design Values
| Application | HLR (m³/m²·h) | Effluent TSS (mg/L) | Backwash % | Rotation (rpm) |
|---|---|---|---|---|
| Standard tertiary | 28-35 | 5-8 | 3-5% | 0.8-1.2 |
| High-quality effluent | 20-28 | 5-7 | 3-5% | 1.0-1.5 |
| High-rate polishing | 35-40 | 7-10 | 4-6% | 0.5-0.8 |
Performance Criteria
TSS Removal Efficiency
| Influent TSS (mg/L) | HLR (m³/m²·h) | Expected Effluent TSS (mg/L) | Removal (%) |
|---|---|---|---|
| 10-15 | 30 | 5-7 | 53-60% |
| 15-20 | 30 | 6-8 | 55-65% |
| 20-25 | 28 | 7-10 | 58-68% |
| 15-20 (with coag) | 25 | 3-5 | 70-82% |
Note: Pile cloth performance is moderate—better than clarifiers but not as tight as sand/disk filters.
Comparison to Other Tertiary Filters
| Filter Type | Effluent TSS (mg/L) | Footprint | Backwash Water | Complexity |
|---|---|---|---|---|
| Sand filter | 2-4 | Large | 2-3% | Medium |
| Disk filter | 3-5 | Medium | 1-2% | Low |
| Pile cloth | 5-10 | Small | 3-5% | Low |
| Dual-media | 2-3 | Large | 1-2% | Medium |
Pile cloth niche: Simple, compact, continuous operation, but moderate effluent quality.
Troubleshooting
| Issue | Cause | Solution |
|---|---|---|
| High effluent TSS | Cloth damage or tears | Inspect and patch or replace cloth |
| Excessive hydraulic loading | Reduce flow or add drums | |
| Cloth blinding (fouling) | Chemical clean or replace cloth | |
| Poor rotation (areas not cleaning) | Check drive motor, increase speed | |
| High headloss | Cloth fouling | Chemical wash (acid or caustic) |
| Excessive solids loading | Reduce HLR, improve pre-treatment | |
| Backwash ineffective | Increase spray pressure, check nozzles | |
| Poor backwash cleaning | Low spray pressure | Increase pump pressure to 250+ kPa |
| Clogged nozzles | Clean or replace spray nozzles | |
| Worn cloth (compressed pile) | Replace cloth media | |
| Uneven cloth wear | Flow distribution poor | Check inlet design, add baffling |
| Localized high loading | Improve mixing upstream | |
| Drum rotation issues | Motor or bearing failure | Repair drive system |
| Excessive torque (solids buildup) | Increase backwash frequency/intensity | |
| High backwash water use | Spray flow too high | Reduce nozzle size or pressure |
| Spray bar leaks | Repair spray manifold | |
| Cloth delamination | Age-related degradation | Replace cloth (typical 5-8 year life) |
| Chemical attack | Check backwash chemical compatibility |
Operation & Maintenance
Daily Checks
- Monitor effluent turbidity/TSS online
- Record headloss across drum
- Observe drum rotation (smooth, continuous)
- Check backwash spray operation (visual)
- Verify influent TSS quality
- Inspect cloth for visible damage
Weekly Maintenance
- Measure effluent TSS (lab confirmation)
- Inspect cloth condition during rotation
- Check backwash spray nozzles (clean if clogged)
- Verify spray pressure
- Clean inlet screens
- Review headloss trends
Monthly Tasks
- Detailed cloth inspection (look for tears, wear, compression)
- Measure backwash water usage
- Inspect drive system (motor, gearbox, bearings)
- Calibrate headloss transmitter
- Review solids loading data
Annual Tasks
- Chemical cleaning of cloth (acid for Fe/Ca, caustic for organics)
- Inspect drum structure and mounting
- Overhaul drive system (lubricate, replace bearings)
- Review performance vs. design
- Assess cloth life remaining
- Plan for cloth replacement (5-8 year life)
Cloth Replacement
Indicators for replacement:
- Pile compression (depth <3mm)
- Multiple tears or patches
- Persistent blinding despite chemical cleaning
- Increasing headloss trend
- Declining effluent quality
Cost: £5,000-15,000 per drum (installed)
Frequency: Every 5-8 years
Advanced Features
Chemical Coagulation Integration
Upstream coagulant dosing improves performance:
- Dose ferric or alum 30-60 sec upstream
- Flocculation occurs in pipe/channel (orthokinetic)
- Pile cloth captures flocs effectively
- Effluent TSS: 3-5 mg/L (vs. 5-10 mg/L without coagulant)
Variable Speed Control
Adjust rotation based on headloss or time:
- Normal operation: 0.8-1.0 rpm
- High loading: 1.2-1.5 rpm (more frequent cleaning)
- Low loading: 0.5 rpm (minimize wear)
- Headloss-triggered speed increase
Dual-Stage Configuration
Two drums in series:
- First drum: Coarse pile (5mm depth), removes bulk TSS
- Second drum: Fine pile (3mm depth), polishing
- Reduces fouling of second stage, extends cloth life
Enhanced Backwash
Compressed air assist:
- Air jet nozzles alongside water spray
- Dislodges solids more effectively
- Reduces water backwash requirement
Chemical backwash:
- Periodic citric acid or NaOH spray (weekly/monthly)
- Prevents organic and mineral fouling
- Extends cloth life
Monitoring and Control
- Headloss transmitters (variable speed control)
- Turbidity meters (influent and effluent)
- Flow meters (verify HLR)
- SCADA integration (optimize rotation speed and backwash)
Worked Example
Given
- Flow: 12,000 m³/d average, 18,000 m³/d peak
- Influent TSS: 20 mg/L (secondary effluent)
- Target effluent: <8 mg/L TSS (moderate requirement)
- Application: Tertiary polishing before chlorination, compact site
Solution
Step 1: Select HLR
For 20 mg/L influent, target <8 mg/L: HLR = 28 m³/m²·hStep 2: Calculate Cloth Area
Q_avg = 12,000 m³/d = 500 m³/h
A_cloth = 500 / 28 = 17.9 m²Step 3: Select Drum Configuration
Standard drum: D = 2.6m, L = 4.0m
A_drum = π × 2.6 × 4.0 × 0.35 (submerged) = 11.4 m²/drum
Number of drums = 17.9 / 11.4 = 1.57 → Use 2 drums
Total cloth area = 2 × 11.4 = 22.8 m²Step 4: Verify Loading
Actual HLR = 500 / 22.8 = 21.9 m³/m²·h ✓
(Conservative—will achieve better than 8 mg/L target)Step 5: Verify Peak Conditions
Q_peak = 18,000 m³/d = 750 m³/h
HLR_peak = 750 / 22.8 = 32.9 m³/m²·h
(Acceptable for short peak with 2 drums)Step 6: Predict Performance
At HLR = 22 m³/m²·h, empirical removal ≈ 65%
C_effluent = 20 × (1 - 0.65) = 7.0 mg/L ✓
(Meets <8 mg/L target)Step 7: Backwash Design
Spray flow per drum: 12 m³/h (typical)
Total backwash: 2 × 12 = 24 m³/h continuous
Daily backwash = 24 × 24 = 576 m³/day
Backwash % = 576 / 12,000 = 4.8% ✓Step 8: Solids Handling
Daily solids = 12,000 × (20 - 7) / 1000 = 156 kg TSS/day
Backwash TSS = 156,000 mg / 576 m³ = 271 mg/L
(Return to headworks)Step 9: Power Consumption
2 drums × 0.5 kW rotation = 1 kW × 24h = 24 kWh/day
Backwash pump: 3 kW × 24h = 72 kWh/day
Total: 96 kWh/day
Specific energy = 96 / 12,000 = 0.008 kWh/m³ = 0.8 Wh/m³ ✓Step 10: Footprint
Each drum: 2.6m diameter × 4.0m length = 10.4 m² footprint
Total footprint: 2 × 10.4 = 20.8 m² (plus access, controls)
vs. Sand filter: ~60 m² (3× larger)Step 11: Capital Cost Estimate
Pile cloth drum: £30,000 per drum (installed, with controls)
Total: 2 × £30,000 = £60,000
Cloth replacement (every 6 years):
£10,000 per drum = £20,000 / 6 years = £3,333/year
Specific cost = £3,333 / (12,000 × 365) = £0.00076/m³ (negligible)Related Units
- Disk-Filter - Similar rotating cloth concept, woven media
- Rapid-Sand-Filter - Higher quality, larger footprint
- Dual-Media-Filter - Higher quality alternative
- Chlorination - Downstream disinfection
- UV-Disinfection - Alternative disinfection
References
- WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Ed., Ch. 16.
- Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed., pp. 1290-1310.
- Andritz Separation (2020). Fiber Disc Filter Technology Manual.
- Huber Technology (2019). Rotamat Fleece Filter Technical Documentation.
- EPA (2021). Tertiary Treatment Technologies for Nutrient Removal, EPA 832-R-21-006.
- Tchobanoglous, G. et al. (2003). Wastewater Engineering: Treatment and Reuse, 4th Ed., McGraw-Hill, Ch. 11.