Appearance
Disk Filter
Rotating cloth disk filters using microporous fabric (10-40 μm) for tertiary TSS removal through surface filtration with automatic backwash, achieving <5 mg/L TSS with compact footprint.
Overview
Disk filters employ rotating cloth-covered disks partially submerged in wastewater flow. As effluent passes through the cloth media from outside to inside, solids accumulate on the fabric surface. Rotation allows periodic backwash while maintaining continuous operation, making them ideal for space-constrained sites.
When to Use
- Tertiary polishing with space constraints (50% footprint vs. sand)
- Seasonal treatment upgrades (modular expansion)
- Upstream of UV disinfection (turbidity reduction)
- Phosphorus removal with chemical coagulation
- Retrofit of existing clarifiers
- Flow equalization bypass treatment
Key Characteristics
- TSS removal: Secondary effluent 10-20 mg/L → 3-5 mg/L
- Hydraulic loading: 15-30 m³/m²·h (varies by cloth pore size)
- Backwash: Continuous, triggered by headloss or time
- Cloth pore size: 10 μm (tight), 20 μm (standard), 40 μm (coarse)
- Solids handling: 1-3 kg TSS/m² media·day
- Low energy consumption (0.01-0.03 kWh/m³)
Operating Modes
Basic Mode
- Fixed rotation speed
- Simple hydraulic loading calculation
- Constant removal efficiency
- Time-based backwash trigger
Advanced Mode
- Dynamic cloth resistance model
- Headloss-triggered backwash
- Backwash optimization (minimize water use)
- Solids loading rate calculation
- Effluent quality prediction based on cloth age
Mathematical Model
Governing Equation - Surface Filtration
Unlike depth filters, disk filters operate by surface straining:
$$C_{effluent} = C_{influent} \times (1 - \eta_{cloth})$$
Where:
- $\eta_{cloth}$ = cloth removal efficiency (-)
- Depends on pore size, influent TSS size distribution, cake buildup
Removal efficiency correlation:
$$\eta_{cloth} = \eta_{base} + \eta_{cake} \times \left(1 - e^{-k \times t}\right)$$
Where:
- $\eta_{base}$ = clean cloth efficiency (0.70-0.85 for 10-20 μm)
- $\eta_{cake}$ = additional removal from solids cake (0.10-0.15)
- $k$ = cake formation rate constant (h⁻¹)
- $t$ = time since last backwash (h)
Headloss Development
Headloss increases linearly with solids accumulation:
$$h(t) = h_0 + k_{resistance} \times m_{solids}(t)$$
Where:
- $h_0$ = clean cloth headloss (0.05-0.15 m)
- $k_{resistance}$ = specific cake resistance (m/kg)
- $m_{solids}(t)$ = accumulated solids mass per unit area (kg/m²)
Solids accumulation rate:
$$\frac{dm_{solids}}{dt} = Q \times C_{influent} \times \eta_{cloth} / A_{media}$$
Where:
- $Q$ = flow rate (m³/h)
- $A_{media}$ = total cloth media area (m²)
Backwash Triggering
Backwash initiated when:
$$h(t) > h_{trigger} \quad \text{OR} \quad t > t_{max}$$
Typical values:
- $h_{trigger}$ = 0.3-0.6 m
- $t_{max}$ = 0.25-1.0 hours (15-60 minutes)
Backwash water consumption:
$$Q_{BW} = v_{BW} \times A_{sector} \times t_{BW} \times n_{BW/day}$$
Where:
- $v_{BW}$ = backwash spray velocity (50-100 m/h)
- $A_{sector}$ = sector area (1/8 to 1/12 of disk)
- $t_{BW}$ = backwash duration per sector (10-30 seconds)
- $n_{BW/day}$ = backwashes per day
Typical: 1-3% of filtered water
Hydraulic Loading
Design loading rate based on cloth type and influent quality:
$$HLR = \frac{Q}{A_{media}}$$
| Cloth Pore Size | Influent TSS (mg/L) | HLR (m³/m²·h) | Effluent TSS (mg/L) |
|---|---|---|---|
| 10 μm | 10-15 | 15-20 | 2-3 |
| 20 μm | 15-25 | 20-30 | 3-5 |
| 40 μm | 20-35 | 25-35 | 5-8 |
Design Parameters
Input Parameters
| Parameter | Symbol | Unit | Range | Default | Description |
|---|---|---|---|---|---|
| Flow rate | $Q$ | m³/h | 100-5000 | 500 | Average flow to treat |
| Hydraulic loading | HLR | m³/m²·h | 15-35 | 25 | Design loading rate |
| Cloth pore size | $d_{pore}$ | μm | 10-40 | 20 | Fabric mesh opening |
| Number of disks | $N_{disk}$ | - | 4-24 | 8 | Disks per module |
| Disk diameter | $D$ | m | 2.0-3.6 | 3.0 | Standard disk size |
| Submergence | $f_{sub}$ | % | 30-40 | 35 | Fraction of disk submerged |
| Influent TSS | $C_0$ | mg/L | 10-30 | 18 | Secondary effluent |
| Target effluent TSS | $C_e$ | mg/L | 3-5 | 4 | Design goal |
| Rotation speed | $\omega$ | rpm | 0.5-2.0 | 1.0 | Disk rotation rate |
| Backwash trigger | $h_{trigger}$ | m | 0.3-0.6 | 0.4 | Headloss setpoint |
Output Parameters
| Parameter | Unit | Description |
|---|---|---|
| Total media area | m² | Required cloth area |
| Number of modules | - | Parallel disk filter modules |
| Backwash frequency | cycles/day | Typical backwashes per day |
| Backwash water % | % | Percentage of production |
| Solids loading | kg TSS/m²·d | Solids captured per media area |
| Power consumption | kWh/m³ | Energy for rotation + backwash |
Configuration Guide
Step-by-Step Setup
Select Cloth Pore Size
Based on application and influent quality:
Tight effluent (<3 mg/L): 10 μm cloth Standard tertiary (3-5 mg/L): 20 μm cloth Coarse polishing (5-8 mg/L): 40 μm cloth With chemical coagulation: 10 μm cloth (excellent performance)Calculate Required Media Area
$$A_{media} = \frac{Q}{HLR}$$
Example: Q = 10,000 m³/d = 417 m³/h HLR = 25 m³/m²·h (for 20 μm cloth) A_media = 417 / 25 = 16.7 m²Determine Number of Disks
Standard disk area (both sides):
- 3.0m diameter disk: A = 2 × π × (1.5)² × 0.35 (submerged) = 4.95 m²/disk
Number of disks = A_media / A_disk = 16.7 / 4.95 = 3.4 → 4 disks minimumSelect Module Configuration
Modules typically contain 4-24 disks:
- Small modules: 4-8 disks (20-40 m² media)
- Standard modules: 8-12 disks (40-60 m² media)
- Large modules: 12-24 disks (60-120 m² media)
For A_media = 16.7 m², use: - Option 1: One 4-disk module (19.8 m² media) ✓ - Option 2: Two 2-disk modules (2 × 9.9 m² = 19.8 m²) Select Option 1 for simplicityDesign Backwash System
Backwash spray specifications:
- Pressure: 150-300 kPa
- Flow per spray bar: 10-30 m³/h
- Spray nozzle spacing: 50-100 mm
- Coverage: One sector at a time (typically 1/8 disk)
Disk area per sector = 4.95 / 8 = 0.62 m² Spray flow = 20 m³/h Backwash time per sector = 15 seconds Backwash volume per cycle = (20/3600) × 15 × 8 sectors = 0.67 m³ Assuming 24 cycles/day (hourly backwash): Daily backwash = 24 × 0.67 = 16.1 m³/day Backwash % = 16.1 / 10,000 = 0.16% ✓ (excellent efficiency)Configure Control System
Backwash triggers:
- Headloss: Differential pressure > 0.4 m (primary)
- Time: Maximum 60 minutes since last backwash (backup)
- Effluent quality: Turbidity spike detection (optional)
Rotation control:
- Constant speed: 1 rpm (simple, reliable)
- Variable speed: 0.5-2 rpm based on headloss (optimize backwash)
Solids Handling
Backwash solids concentration: 1,000-5,000 mg/L
Daily solids captured = Q × (C_influent - C_effluent) = 10,000 × (18 - 4) / 1000 = 140 kg TSS/day Backwash solids flow = 16.1 m³/day Average backwash TSS = 140,000 mg / 16.1 m³ = 8,696 mg/L Return to headworks or thickening
Typical Design Values
| Application | Cloth Size (μm) | HLR (m³/m²·h) | Effluent TSS (mg/L) | Backwash % |
|---|---|---|---|---|
| Standard tertiary | 20 | 25 | 3-5 | 1-2% |
| Tight effluent | 10 | 20 | 2-3 | 2-3% |
| With coagulant | 10 | 18 | 1-2 | 2-3% |
| High-rate polishing | 40 | 30 | 5-8 | 1-2% |
| Seasonal upgrade | 20 | 20-35 (var) | 3-6 | 1-3% |
Performance Criteria
TSS Removal Efficiency
| Influent TSS (mg/L) | Cloth Size (μm) | Expected Effluent TSS (mg/L) | Removal (%) |
|---|---|---|---|
| 10-15 | 10 | 2-3 | 80-85% |
| 15-20 | 10 | 3-4 | 75-82% |
| 15-20 | 20 | 3-5 | 75-80% |
| 20-25 | 20 | 4-6 | 70-78% |
| 20-30 | 40 | 6-10 | 60-70% |
Phosphorus Removal (with Coagulant)
When dosed with ferric chloride or alum:
- Total P removal: 60-85% (with 10 μm cloth)
- Effluent TP: <0.5 mg/L achievable from 2-4 mg/L influent
- Metal dose: 10-30 mg Fe³⁺/L or 20-50 mg Al³⁺/L
Turbidity Reduction
- Influent: 3-8 NTU (secondary effluent)
- Effluent: 1-2 NTU (20 μm cloth, no coagulant)
- Effluent: 0.5-1.0 NTU (10 μm cloth with coagulant)
Troubleshooting
| Issue | Cause | Solution |
|---|---|---|
| High effluent TSS | Cloth damage or tears | Inspect cloth, replace damaged panels |
| Excessive hydraulic loading | Reduce flow or add modules | |
| Cloth blinding (permanent fouling) | Acid wash or replace cloth | |
| Poor influent quality | Improve secondary treatment | |
| Frequent backwash | High solids loading | Check influent TSS, increase media area |
| Backwash trigger too sensitive | Increase headloss setpoint | |
| Cloth fouling | Chemical cleaning required | |
| Poor backwash efficiency | Low spray pressure | Check pump, increase pressure to 200+ kPa |
| Clogged spray nozzles | Clean nozzles, improve filtration of BW water | |
| Worn cloth | Replace cloth media | |
| High headloss | Cloth blinding | Chemical wash (acid for Fe/Ca, caustic for organics) |
| Excessive solids loading | Add media area | |
| Cold water (high viscosity) | Normal, adjust trigger for temperature | |
| Uneven cloth wear | Flow distribution issues | Check inlet baffling, ensure even submergence |
| Localized TSS loading | Improve mixing upstream | |
| Disk rotation issues | Motor or gearbox failure | Repair drive system |
| Excessive torque (solids buildup) | Emergency backwash, check headloss | |
| High backwash water use | Too frequent backwash | Increase headloss trigger |
| Long backwash cycles | Optimize spray time per sector |
Operation & Maintenance
Daily Checks
- Monitor effluent turbidity/TSS online
- Record headloss and backwash frequency
- Observe disk rotation (visual check for smooth operation)
- Check backwash spray operation
- Verify influent TSS quality
- Inspect for cloth damage or tears
Weekly Maintenance
- Measure effluent TSS (lab confirmation)
- Inspect cloth condition during rotation
- Check backwash spray nozzles for clogging
- Verify spray pressure
- Clean inlet screens or grids
- Review headloss trends
Monthly Tasks
- Detailed cloth inspection (look for holes, abrasion)
- Measure backwash water usage
- Inspect drive system (motor, gearbox, bearings)
- Calibrate headloss transmitter
- Review solids loading data
- Check chemical dosing accuracy (if used)
Annual Tasks
- Replace worn cloth panels (typical life: 3-5 years)
- Inspect disk structure and mounting
- Overhaul drive system (lubricate gearbox)
- Chemical cleaning of cloth (acid/caustic soak)
- Review performance vs. design
- Optimize backwash strategy
Chemical Cleaning
Acid wash (for mineral scaling):
- Citric acid or HCl (2-5% solution)
- Soak cloth for 2-4 hours
- Removes Fe, Ca, Mg precipitates
Caustic wash (for organic fouling):
- NaOH (1-3% solution)
- Soak cloth for 2-4 hours
- Removes oils, greases, biofilm
Frequency: 1-2 times per year or when headloss increases significantly
Advanced Features
Coagulant Dosing Integration
Upstream ferric or alum dosing enhances performance:
- Dose point: 30-60 seconds upstream of disk filter
- Rapid mixing: G = 100-300 s⁻¹
- No flocculation required (orthokinetic flocculation on cloth)
- Metal dose optimization by jar testing
Variable Speed Operation
Adjust rotation speed based on loading:
- Low flow: 0.5 rpm (minimize wear)
- High flow: 2.0 rpm (more frequent backwash passes)
- Headloss-based control (increase speed as headloss rises)
Multi-Stage Filtration
Coarse cloth pre-filter + fine cloth polishing:
- Stage 1: 40 μm cloth (remove bulk solids)
- Stage 2: 10 μm cloth (final polishing)
- Reduces fouling of fine cloth, extends life
Online Monitoring
- Headloss transmitters (trigger backwash)
- Turbidity meters (influent and effluent)
- TSS analyzers (optional, for critical applications)
- Flow meters (verify hydraulic loading)
- SCADA integration (optimize across multiple modules)
Worked Example
Given
- Flow: 15,000 m³/d average, 22,500 m³/d peak (3-hour duration)
- Influent TSS: 18 mg/L (secondary effluent)
- Target effluent: <4 mg/L TSS, <1.5 NTU
- Application: Tertiary polishing before chlorination
Solution
Step 1: Select Cloth
For standard tertiary (3-5 mg/L target): 20 μm cloth
HLR = 25 m³/m²·h at average flowStep 2: Calculate Media Area
Q_avg = 15,000 m³/d = 625 m³/h
A_media = Q_avg / HLR = 625 / 25 = 25 m²Step 3: Select Disk Configuration
Use 3.0m diameter disks:
A_disk = 2 × π × 1.5² × 0.35 = 4.95 m²/disk
Number of disks = 25 / 4.95 = 5.05 → 6 disks
Total media area = 6 × 4.95 = 29.7 m²Step 4: Select Modules
Use one 6-disk module (29.7 m² media) ✓Step 5: Verify Peak Conditions
Q_peak = 22,500 m³/d = 937.5 m³/h
HLR_peak = 937.5 / 29.7 = 31.6 m³/m²·h
(Acceptable for short-duration peak with 20 μm cloth)Step 6: Predict Performance
Cloth efficiency (20 μm, typical): η = 78%
C_effluent = C_influent × (1 - η) = 18 × 0.22 = 4.0 mg/L ✓
(Meets target of <4 mg/L)Step 7: Backwash Design
Spray flow per bar: 20 m³/h
Sectors per disk: 8
Backwash time per sector: 15 seconds
Backwash volume per cycle = (20/3600) × 15 × 8 × 6 disks = 4.0 m³
Assume hourly backwash (24 cycles/day):
Daily backwash = 24 × 4.0 = 96 m³/day
Backwash % = 96 / 15,000 = 0.64% ✓Step 8: Solids Handling
Daily solids captured = 15,000 × (18 - 4) / 1000 = 210 kg TSS/day
Solids loading = 210 / 29.7 = 7.1 kg TSS/m²·d ✓
Backwash TSS concentration = 210,000 mg / 96 m³ = 2,188 mg/L
(Return to primary clarifier or waste directly to sludge thickening)Step 9: Power Consumption
Disk rotation: 0.5 kW per module × 24 h = 12 kWh/day
Backwash pump: 2 kW × (15 sec × 24 cycles / 3600) = 0.2 kWh/day
Total = 12.2 kWh/day
Specific energy = 12.2 / 15,000 = 0.0008 kWh/m³ ✓ (very low)Related Units
- Rapid-Sand-Filter - Alternative depth filtration
- Pile-Cloth-Filter - Similar rotating cloth design
- Chlorination - Downstream disinfection
- Chemical-Dosing - Coagulation for enhanced removal
- 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.
- Parkson Corporation (2020). DynaSand and AquaDisk Technical Manual.
- Nordic Water (2019). Discfilter Technology for Tertiary Filtration.
- Tchobanoglous, G. et al. (2003). Wastewater Engineering: Treatment and Reuse, 4th Ed., McGraw-Hill.
- EPA (2021). Tertiary Treatment Technologies for Nutrient Removal, EPA 832-R-21-006.