Appearance
GAC Filter (Granular Activated Carbon)
Adsorptive filtration using activated carbon granules (0.6-2.4 mm) to remove dissolved organics, micropollutants, and odor/taste compounds through physical adsorption described by Freundlich isotherms.
Overview
Granular Activated Carbon (GAC) filters combine physical filtration with chemical adsorption to remove both suspended solids and dissolved organic compounds. The highly porous carbon structure (surface area 500-1500 m²/g) adsorbs organic molecules, trace pharmaceuticals, endocrine disruptors, and residual disinfection byproduct precursors.
When to Use
- Water reuse (potable and non-potable)
- Trace organic removal (pharmaceuticals, pesticides, PFAS)
- Disinfection byproduct precursor removal (TOC, DBP-FP)
- Taste and odor control (geosmin, MIB)
- Post-ozone treatment (remove ozonation byproducts)
- Advanced wastewater treatment (indirect potable reuse)
- Chloramine removal (dechlorination)
Key Characteristics
- TSS removal: 10-20 mg/L → ❤️ mg/L (similar to sand filter)
- DOC removal: 30-70% (depends on carbon type, contact time)
- Trace organics: 50-99% removal (compound-specific)
- Hydraulic loading: 5-15 m/h (lower than sand for contact time)
- Bed life: 1-5 years before regeneration or replacement
- High capital cost, moderate operating cost if regenerated
Operating Modes
Basic Mode
- Fixed removal efficiency for DOC
- Simple TSS filtration (like sand filter)
- Time-based carbon replacement schedule
- Headloss calculation only
Advanced Mode
- Freundlich isotherm adsorption model
- Breakthrough curve prediction
- Multi-component competitive adsorption
- Bed life optimization
- Regeneration scheduling
- Biological activity in GAC (BAC mode)
Mathematical Model
Governing Equations
1. Adsorption Equilibrium - Freundlich Isotherm
The Freundlich isotherm relates equilibrium adsorbed concentration to dissolved concentration:
$$q_e = K_f \times C_e^{1/n}$$
Where:
- $q_e$ = equilibrium adsorption capacity (mg/g or mg/kg)
- $C_e$ = equilibrium concentration in solution (mg/L)
- $K_f$ = Freundlich capacity parameter (L/g or L/kg)
- $1/n$ = Freundlich intensity parameter (dimensionless, typically 0.3-0.7)
Interpretation:
- $K_f$ high → strong adsorption
- $1/n$ low (<0.5) → favorable adsorption (steep isotherm)
- $1/n$ near 1 → linear adsorption
Typical values for TOC removal:
- $K_f$ = 10-100 (L/g)^(1/n)
- $1/n$ = 0.4-0.6
2. Column Adsorption - Breakthrough Model
Solute transport through GAC bed follows advection-dispersion-adsorption:
$$\frac{\partial C}{\partial t} + v \frac{\partial C}{\partial z} = D_L \frac{\partial^2 C}{\partial z^2} - \frac{\rho_b}{\epsilon} \frac{\partial q}{\partial t}$$
Where:
- $C$ = liquid phase concentration (mg/L)
- $q$ = solid phase adsorbed concentration (mg/g)
- $v$ = interstitial velocity (m/h)
- $D_L$ = longitudinal dispersion coefficient (m²/h)
- $\rho_b$ = bulk density of GAC (kg/m³)
- $\epsilon$ = bed porosity (-)
- $z$ = depth through column (m)
- $t$ = time (h)
Simplified breakthrough time (50% breakthrough):
$$t_{50%} = \frac{L \times \rho_b \times q_e}{v \times C_0}$$
Where:
- $L$ = bed depth (m)
- $C_0$ = influent concentration (mg/L)
3. Empty Bed Contact Time (EBCT)
Critical design parameter:
$$EBCT = \frac{V_{bed}}{Q} = \frac{L}{v}$$
Where:
- $V_{bed}$ = bed volume (m³)
- $Q$ = flow rate (m³/h)
- $L$ = bed depth (m)
- $v$ = superficial velocity (m/h)
Typical EBCT values:
- TOC removal: 10-20 minutes
- Trace organics: 15-30 minutes
- Taste/odor: 10-15 minutes
- PFAS removal: 20-40 minutes
4. Bed Life Estimation
Time until carbon exhaustion (simplified):
$$t_{life} = \frac{M_{carbon} \times q_{total}}{Q \times C_0 \times \eta}$$
Where:
- $M_{carbon}$ = total carbon mass (kg)
- $q_{total}$ = total adsorption capacity (mg/g), typically 50-200 mg/g for TOC
- $\eta$ = usage factor (0.5-0.8, accounts for incomplete utilization)
Headloss (Same as Sand Filter)
Clean bed headloss (Carmen-Kozeny):
$$h_0 = f \times \frac{L}{d_c} \times \frac{(1-\epsilon)^2}{\epsilon^3} \times \frac{v^2}{2g}$$
GAC has slightly lower headloss than sand due to larger particle size.
Headloss increase with solids loading:
$$h(t) = h_0 + k_{buildup} \times \sigma(t)$$
Backwash similar to sand filter (fluidization) but less frequent due to lower TSS loading.
Design Parameters
Input Parameters
| Parameter | Symbol | Unit | Range | Default | Description |
|---|---|---|---|---|---|
| Hydraulic loading | $v$ | m/h | 5-15 | 10 | Superficial velocity |
| Bed depth | $L$ | m | 1.5-3.0 | 2.0 | GAC bed depth |
| GAC particle size | $d_c$ | mm | 0.6-2.4 | 1.2 | Effective size (d10) |
| Bed porosity | $\epsilon$ | - | 0.45-0.55 | 0.50 | Void fraction (higher than sand) |
| EBCT | - | min | 10-40 | 20 | Empty bed contact time |
| Influent DOC | $C_0$ | mg/L | 5-20 | 10 | Dissolved organic carbon |
| Target DOC | $C_e$ | mg/L | 2-8 | 5 | Effluent goal |
| Freundlich $K_f$ | $K_f$ | (L/g)^(1/n) | 10-100 | 50 | Adsorption capacity |
| Freundlich $1/n$ | $1/n$ | - | 0.3-0.7 | 0.5 | Adsorption intensity |
| Carbon density | $\rho_b$ | kg/m³ | 400-500 | 450 | Bulk density |
| Carbon life | $t_{life}$ | years | 1-5 | 2 | Time before replacement/regen |
Output Parameters
| Parameter | Unit | Description |
|---|---|---|
| Filter area | m² | Required surface area |
| Carbon mass | tonnes | Total GAC required |
| EBCT achieved | min | Actual contact time |
| Bed volumes | BV | Throughput before breakthrough |
| Annual carbon cost | £/year | Replacement or regeneration cost |
| DOC removal | % | Predicted removal efficiency |
| Headloss | m | Pressure drop |
Configuration Guide
Step-by-Step Setup
Determine Target EBCT
Selection based on removal goal:
General TSS removal only: 10 min (use sand instead) DOC removal (30-50%): 15 min DOC removal (50-70%): 20-30 min Trace organics (pharmaceuticals): 20-30 min PFAS removal: 30-40 min Taste/odor: 10-15 minCalculate Bed Depth from EBCT
$$L = EBCT \times v$$
Example: EBCT = 20 minutes = 0.333 hours v = 10 m/h (moderate loading) L = 0.333 × 10 = 3.33 m Use L = 3.0 m (practical limit), adjust velocity: v = L / EBCT = 3.0 / 0.333 = 9.0 m/h ✓Calculate Required Area
$$A = \frac{Q}{v}$$
Example: Q = 5,000 m³/d = 208.3 m³/h v = 9.0 m/h A = 208.3 / 9.0 = 23.1 m²Select Number of Cells
Similar to sand filters:
- Minimum 2 cells (allow one offline for regeneration)
- Practical maximum cell size: 50-100 m²
A_total = 23.1 m² → Use 2 cells × 12 m² each = 24 m² Cell dimensions: 3.5m × 3.5m each = 12.25 m²Calculate Carbon Mass
$$M_{carbon} = A \times L \times \rho_b$$
A_total = 24 m² L = 3.0 m ρ_b = 450 kg/m³ M_carbon = 24 × 3.0 × 450 = 32,400 kg = 32.4 tonnesEstimate Bed Life
Using Freundlich and influent DOC:
Assume: - Influent DOC = 10 mg/L - Target DOC = 5 mg/L (50% removal) - Total adsorption capacity: q_total = 100 mg/g (typical for TOC) - Usage factor: η = 0.6 (60% of capacity utilized before breakthrough) t_life = (M_carbon × q_total × η) / (Q × DOC_removed) = (32,400 kg × 100 mg/g × 0.6) / (5,000 m³/d × 5 mg/L) = (32,400,000 g × 100 × 0.6) / (5,000,000 L/d × 5) = 1,944,000,000 mg / 25,000,000 mg/d = 77.8 days ≈ 2.6 months (This is quite short! GAC for DOC removal requires frequent regeneration or high initial cost) For trace organics at μg/L levels, bed life is much longer (1-3 years)Backwash Design
GAC backwashes less frequently than sand (TSS loading lower):
- Backwash rate: 30-50 m/h (lower than sand due to lighter carbon)
- Duration: 10-15 minutes
- Frequency: Every 3-7 days (or when headloss reaches limit)
- Air scour recommended to prevent carbon attrition
Carbon Regeneration or Replacement
Options:
- Replacement: Dispose spent carbon, buy new (expensive)
- Thermal regeneration: Off-site furnace treatment (800-900°C)
- Biological regeneration: Operate as BAC (see Advanced Features)
Regeneration efficiency: 70-90% (some capacity lost each cycle)
Typical Design Values
| Application | EBCT (min) | HLR (m/h) | Bed Depth (m) | DOC Removal (%) | Bed Life (months) |
|---|---|---|---|---|---|
| Post-ozone polishing | 10-15 | 10-12 | 2.0 | 30-50 | 3-6 |
| General DOC removal | 15-20 | 8-10 | 2.5 | 50-70 | 2-4 |
| Trace organics | 20-30 | 6-8 | 3.0 | 60-90 | 12-36 |
| PFAS removal | 30-40 | 5-7 | 3.0 | 80-99 | 6-18 |
| Taste/odor | 10-15 | 10-12 | 2.0 | 70-95 | 12-24 |
Performance Criteria
DOC Removal
| Influent DOC (mg/L) | EBCT (min) | Expected Removal (%) | Effluent DOC (mg/L) |
|---|---|---|---|
| 10 | 15 | 40-50 | 5-6 |
| 10 | 20 | 50-60 | 4-5 |
| 10 | 30 | 60-70 | 3-4 |
| 15 | 20 | 45-55 | 7-8 |
| 20 | 20 | 40-50 | 10-12 |
Trace Organic Removal
Highly compound-specific. Examples:
| Compound | Typical Removal (%) | Notes |
|---|---|---|
| Pharmaceuticals (general) | 60-90 | Depends on hydrophobicity |
| Carbamazepine | 70-85 | Well-adsorbed |
| Diclofenac | 80-95 | Very well-adsorbed |
| Atrazine (pesticide) | 85-98 | Excellent removal |
| PFOA/PFOS (PFAS) | 90-99 | Requires long EBCT |
| Geosmin/MIB (odor) | 90-99 | Excellent removal |
| Estrogens (endocrine) | 80-95 | Well-adsorbed |
TSS Removal
GAC also acts as depth filter:
- Influent: 10-20 mg/L
- Effluent: ❤️ mg/L (similar to sand filter)
Troubleshooting
| Issue | Cause | Solution |
|---|---|---|
| Rapid DOC breakthrough | Carbon exhausted | Regenerate or replace carbon |
| EBCT too short | Reduce flow rate, increase bed depth | |
| Influent DOC higher than design | Pre-treat, adjust replacement schedule | |
| Competitive adsorption (NOM) | Accept lower removal or use BAC mode | |
| High headloss | TSS accumulation | Backwash more frequently |
| Carbon fines from attrition | Replace carbon, improve backwash | |
| Biological growth | Chlorinate, increase backwash | |
| Channeling | Poor flow distribution | Repair underdrain, ensure even flow |
| Carbon consolidated | Fluidize during backwash | |
| Carbon loss during backwash | Backwash rate too high | Reduce to 30-40 m/h |
| Excessive air scour | Adjust air scour intensity | |
| Carbon degradation | Replace with harder carbon | |
| Taste/odor breakthrough | Carbon saturated | Regenerate or replace |
| Biological growth in filter | Chlorinate, drain and clean | |
| Biological fouling | Long residence time | Accept (BAC mode) or periodic chlorination |
| High nutrient load | Pre-treat, reduce EBCT |
Operation & Maintenance
Daily Checks
- Monitor effluent DOC or UV₂₅₄ absorbance (indicator of organics)
- Record headloss for each cell
- Verify flow rate and EBCT
- Observe backwash cycles
- Check for carbon fines in effluent
Weekly Maintenance
- Measure effluent DOC (lab confirmation)
- Sample for key trace organics (if applicable)
- Check backwash flow rate
- Inspect for carbon loss
- Review headloss trends
Monthly Tasks
- Measure bed depth (check for carbon loss)
- Assess breakthrough status (plot DOC vs. time)
- Inspect underdrain during backwash
- Calibrate online UV₂₅₄ or DOC meters
- Review carbon bed life projection
Annual Tasks
- Drain filter and inspect carbon condition
- Replace carbon if exhausted or degraded
- Send carbon sample for adsorption capacity testing
- Inspect and repair underdrain
- Review performance vs. design
- Consider switch to BAC operation or regeneration
Carbon Regeneration
Thermal regeneration process:
- Remove spent carbon from filter
- Transport to regeneration facility
- Heat to 800-900°C in furnace (destroys adsorbed organics)
- Reactivate by steam treatment
- Return regenerated carbon (70-90% capacity)
- Top up with virgin carbon (10-30% makeup)
Cost: £500-1000/tonne (vs. £1500-2500/tonne virgin)
Frequency: Every 1-3 years depending on loading
Advanced Features
Biological Activated Carbon (BAC)
GAC can host biofilm that biologically degrades adsorbed organics, regenerating capacity in situ:
$$r_{biodegradation} = k_{bio} \times X_{biofilm} \times \frac{S}{K_S + S}$$
Where:
- $X_{biofilm}$ = biofilm concentration (mg/L)
- $S$ = substrate (DOC) concentration (mg/L)
- $k_{bio}$ = maximum specific degradation rate (d⁻¹)
- $K_S$ = half-saturation constant (mg/L)
Benefits:
- Extended bed life (years vs. months)
- Continuous regeneration
- Reduced operating cost
Requirements:
- No residual disinfectant (dechlorination upstream)
- Biodegradable organics in influent
- Warm temperature (>10°C)
- Dissolved oxygen >2 mg/L
- Longer EBCT (20-30 min)
Backwash: More frequent (weekly) to control biofilm thickness
Post-Ozone GAC
GAC after ozone converts ozonation products:
- Ozone breaks down large organics into smaller, biodegradable molecules
- GAC adsorbs residual organics and hosts biofilm for biodegradation
- Removes bromate (BrO₃⁻) formed during ozonation
- Synergistic treatment (better than either alone)
Trace Organic Monitoring
For potable reuse applications, monitor specific compounds:
- Pharmaceuticals: HPLC-MS/MS
- Pesticides: GC-MS
- PFAS: LC-MS/MS
- Endocrine disruptors: bioassays
Plot breakthrough curves (C/C₀ vs. bed volumes) to predict carbon replacement.
Multi-Layer Design
Stratified carbon types:
- Top layer: Coarse GAC (1.5-2.4 mm) for pre-filtration
- Bottom layer: Fine GAC (0.6-1.2 mm) for polishing
- Reduces headloss, improves efficiency
Worked Example
Given
- Flow: 8,000 m³/d (average)
- Influent DOC: 12 mg/L (post-secondary effluent)
- Target DOC: 5 mg/L (for potable reuse)
- Trace pharmaceuticals: 500 ng/L total
- Application: Indirect potable reuse (IPR)
Solution
Step 1: Select EBCT
For DOC removal (50-60%) and trace organics: EBCT = 25 minutes = 0.417 hoursStep 2: Select Hydraulic Loading
For long contact time: v = 8 m/hStep 3: Calculate Bed Depth
L = EBCT × v = 0.417 h × 8 m/h = 3.33 m
Use L = 3.0 m, adjust v = 3.0 / 0.417 = 7.2 m/h ✓Step 4: Calculate Area
Q = 8,000 m³/d = 333.3 m³/h
A = Q / v = 333.3 / 7.2 = 46.3 m²
Use 4 cells × 12 m² each = 48 m² (3.5m × 3.5m cells)Step 5: Calculate Carbon Mass
M_carbon = A × L × ρ_b = 48 × 3.0 × 450 = 64,800 kg = 64.8 tonnesStep 6: Predict DOC Removal
Using Freundlich with typical values:
K_f = 50 (L/g)^(1/n), 1/n = 0.5
At C_e = 5 mg/L:
q_e = 50 × 5^0.5 = 50 × 2.236 = 111.8 mg/g
DOC removed = 12 - 5 = 7 mg/L
Removal = 7/12 = 58.3% ✓Step 7: Estimate Bed Life (DOC basis)
q_total = 100 mg/g (typical for TOC)
Usage factor η = 0.6
t_life = (M_carbon × q_total × η) / (Q × DOC_removed)
= (64,800,000 g × 100 × 0.6) / (8,000,000 L/d × 7)
= 3,888,000,000 / 56,000,000
= 69.4 days ≈ 2.3 months
For trace organics (ng/L), bed life is 12-24 months (limiting factor)
Use 12-month replacement cycle ✓Step 8: Economic Analysis
Virgin carbon cost: £2,000/tonne
Initial carbon = 64.8 tonnes × £2,000 = £129,600 (capital)
Annual replacement (assume regeneration):
Regeneration cost: £800/tonne
Annual cost = 64.8 tonnes × £800 = £51,840/year
Specific cost = £51,840 / (8,000 m³/d × 365 d) = £0.0177/m³ = 1.77 p/m³Step 9: Consider BAC Operation
If operated as BAC (no disinfectant, biodegradation active):
- Bed life extends to 3-5 years
- Annual cost reduces to carbon makeup only (10-20% per year)
- Backwash frequency increases (weekly vs. monthly)
- Total cost reduces by 50-70%Related Units
- Rapid-Sand-Filter - Simpler TSS filtration
- Ozone-Disinfection - Upstream pre-treatment for GAC
- UV-Disinfection - Downstream disinfection
- Nanofiltration - Alternative for trace organics
- Reverse-Osmosis - More complete removal but higher cost
References
- Crittenden, J.C. et al. (2012). MWH's Water Treatment: Principles and Design, 3rd Ed., Wiley, Ch. 17.
- AWWA (2011). Water Quality and Treatment: A Handbook on Drinking Water, 6th Ed., McGraw-Hill, Ch. 14.
- WEF (2020). Advanced Wastewater Treatment Systems, WEFMOP 35.
- Freundlich, H. (1906). "Over the Adsorption in Solution." Z. Phys. Chem. 57: 385-470.
- Sontheimer, H. et al. (1988). Activated Carbon for Water Treatment, 2nd Ed., DVGW, Karlsruhe.
- EPA (2020). Potable Reuse Compendium, EPA 810-R-20-002, Ch. 6.
- WRF (2015). Framework for Direct Potable Reuse, Water Reuse Foundation, WRF-14-20.