Appearance
Grease Trap
Gravity separation device for removing fats, oils, and grease (FOG) from wastewater through buoyancy-driven separation.
Overview
Grease traps (also called grease interceptors) use the density difference between water and FOG to achieve separation. As wastewater flows through the device at low velocity, lighter FOG floats to the surface where it is retained, while heavier solids settle to the bottom. Clarified water exits through a submerged outlet.
When to Use
- Food service establishments (restaurants, kitchens)
- Food processing facilities
- Industrial sites with FOG in wastewater
- Municipal WWTPs with high FOG influent
- Upstream of biological treatment (protects biomass)
- Before sewer discharge (prevent blockages)
Key Characteristics
- Simple gravity separation (no power required)
- Removes >90% of free FOG
- Requires regular pumping/cleaning
- Can cause odor if poorly maintained
- May need heating in cold climates
Operating Modes
Basic Mode
- Fixed FOG removal efficiency (default 90%)
- Simplified detention time
- Constant temperature
Advanced Mode
- Stokes' Law for grease particle rise
- Temperature-dependent viscosity
- Emulsified vs. free oil modeling
- Solids settling concurrent with FOG rise
Mathematical Model
Governing Equation - Stokes' Rising Velocity
$$V_r = \frac{g \times d^2 \times (\rho_w - \rho_f)}{18 \mu}$$
Where:
- $V_r$ = terminal rise velocity (m/s)
- $g$ = gravitational acceleration (9.81 m/s²)
- $d$ = droplet diameter (m)
- $\rho_w$ = water density (1000 kg/m³)
- $\rho_f$ = FOG density (kg/m³, typically 900-950)
- $\mu$ = dynamic viscosity (Pa·s)
Typical FOG Properties
| Property | Value |
|---|---|
| Density | 900-950 kg/m³ |
| Specific gravity | 0.90-0.95 |
| Viscosity @ 20°C | 0.03-0.08 Pa·s (30-80 cP) |
| Viscosity @ 40°C | 0.01-0.03 Pa·s (10-30 cP) |
Detention Time
$$t = \frac{H}{V_r}$$
Where:
- $t$ = minimum detention time (s)
- $H$ = effective water depth (m)
For droplet d = 0.15mm (150 µm) at 20°C:
- V_r ≈ 0.0002-0.0004 m/s (varies with oil type)
- For H = 1.0m: t = 2500-5000s (40-85 min)
Required Volume
$$V = Q \times t$$
Where:
- $V$ = trap volume (m³)
- $Q$ = flow rate (m³/s)
- $t$ = detention time (s)
Surface Area Requirement
$$A = \frac{Q}{V_r}$$
Where:
- $A$ = surface area (m²)
- Critical for FOG to reach surface
Design Parameters
Input Parameters
| Parameter | Symbol | Unit | Range | Default | Description |
|---|---|---|---|---|---|
| Flow rate | Q | L/s | 0.1-50 | 5 | Design flow |
| Detention time | t | min | 10-60 | 30 | At design flow |
| Water depth | H | m | 0.5-2.0 | 1.0 | Effective depth |
| Temperature | T | °C | 10-40 | 20 | Wastewater temperature |
| FOG concentration | C_FOG | mg/L | 50-2000 | 300 | Inlet FOG |
| Target droplet size | d | mm | 0.10-0.25 | 0.15 | Design particle |
Output Parameters
| Parameter | Unit | Description |
|---|---|---|
| FOG removal | % | Free FOG captured |
| Trap volume | L or m³ | Total capacity |
| FOG storage | L | Accumulated FOG layer |
| Solids storage | L | Settled solids capacity |
| Cleaning frequency | days | Recommended pumping interval |
Configuration Guide
Step-by-Step Setup
Estimate Peak Flow Rate
- Restaurants: 0.05-0.15 L/s per seat
- Commercial kitchens: 0.1-0.3 L/s per fixture
- Food processing: Measure or estimate
Select Detention Time
- Minimum: 10 minutes (small traps, pre-treatment)
- Standard: 20-30 minutes (most applications)
- High efficiency: 40-60 minutes (heavy FOG loads)
Calculate Required Volume $$V = Q \times t \times 60$$
Example: Q = 5 L/s, t = 30 min V = 5 × 30 × 60 = 9,000 L = 9 m³
Determine Dimensions
- Length-to-width ratio: 2:1 to 4:1 (reduces short-circuiting)
- Depth: 1.0-1.5m typical
- Surface area: Critical for FOG rise
Configure Inlet/Outlet
- Inlet: Submerged, with baffle to reduce turbulence
- Outlet: Submerged, with tee or baffle to retain FOG
- Inlet-to-outlet distance: >2/3 of length
Add Baffles
- Inlet baffle: Distribute flow, prevent jetting
- Outlet baffle: Prevent FOG escape
- Internal baffles: Improve plug flow (optional)
Plan Maintenance Access
- Manhole(s) for pumping access
- Sampling ports
- Ventilation
Typical Design Values
| Application | Flow (L/s) | Detention (min) | Volume (L) | Frequency (weeks) |
|---|---|---|---|---|
| Small restaurant (50 seats) | 2.5-5 | 20 | 3,000-6,000 | 4-8 |
| Large restaurant (200 seats) | 10-20 | 30 | 18,000-36,000 | 2-4 |
| Commercial kitchen | 5-15 | 25 | 7,500-22,500 | 4-6 |
| Food processing (small) | 10-30 | 40 | 24,000-72,000 | 1-2 |
Regulatory Requirements
UK/EU:
- EN 1825: Grease separators standard
- Minimum 200L volume (very small installations)
- Required where FOG >50 mg/L to sewer
USA:
- PDI G101: Grease interceptor sizing standard
- Varies by jurisdiction (1-3× flow per minute common)
- Some cities require traps >3,000 gallons (11,000L)
Performance Criteria
Removal Efficiency
| FOG Type | Removal Efficiency |
|---|---|
| Free floating oil/grease | 85-95% |
| Coalesced droplets (>150µm) | 80-90% |
| Fine emulsion (<50µm) | 10-30% |
| Solidified fats (cold) | 90-99% |
Note: Grease traps remove free/floatable FOG, NOT emulsified or dissolved FOG.
Factors Affecting Performance
| Factor | Effect on Efficiency |
|---|---|
| Low temperature | Reduces (FOG solidifies, higher viscosity) |
| High flow/turbulence | Reduces (re-entrainment, short-circuiting) |
| Detergents/surfactants | Greatly reduces (emulsifies FOG) |
| Long detention | Improves (more rise time) |
| Regular cleaning | Improves (maintains volume) |
UK Design Standards
| Parameter | Value | Standard |
|---|---|---|
| Minimum detention | 10 min | EN 1825 |
| Typical detention | 20-40 min | Building regulations |
| Depth | 0.8-1.5m | EN 1825 |
| FOG layer alarm | 25% of depth | Good practice |
| Cleaning frequency | FOG+solids <50% volume | EN 1825 |
Types of Grease Traps
Passive Gravity (This Unit)
Configuration:
- Underground tank
- Inlet and outlet pipes
- Baffles for flow distribution
- No power required
Advantages:
- Simple, reliable
- No operating cost
- Long service life
Disadvantages:
- Large size requirement
- Regular pumping needed
- Odor potential
Small Under-Sink Traps
Configuration:
- Compact (20-200L)
- Installed under fixtures
- Easy access for cleaning
Use:
- Small cafes, bars
- Individual fixtures
- Supplemental to main trap
Limitations:
- Frequent cleaning (weekly)
- Lower efficiency
- Not suitable as primary treatment
Automatic Grease Recovery Units (AGRU)
Configuration:
- Mechanical skimming
- Heated surfaces
- Automated FOG removal
Advantages:
- Continuous FOG removal
- Smaller footprint
- Less frequent service
Disadvantages:
- Higher capital cost
- Power requirement
- Maintenance complexity
Troubleshooting
| Issue | Cause | Solution |
|---|---|---|
| FOG in effluent | Trap full (needs pumping) | Pump out, increase frequency |
| High flow/turbulence | Reduce flow or increase volume | |
| Detergents emulsifying FOG | Eliminate detergent use, educate staff | |
| Short-circuiting | Add/adjust baffles | |
| Cold temperature | Insulate, add heating | |
| Odor problems | Anaerobic conditions | Pump more frequently |
| Solids accumulation | Full cleanout (not just FOG layer) | |
| Poor ventilation | Improve venting | |
| Long detention | Reduce retention time slightly | |
| Blockage/overflow | FOG layer too thick | Emergency pumping |
| Solids blocking outlet | Full cleanout, check outlet baffle | |
| Structural failure | Inspect, repair/replace | |
| Slow separation | Temperature too low | Heat influent or trap |
| Emulsified FOG | Prevent detergent use | |
| Turbulence | Improve inlet design |
Operation & Maintenance
Daily Checks (Staff)
- Visual check for overflows
- Observe inlet flow (no unusual color/odor)
- Check for leaks
Weekly Checks
- Measure FOG layer thickness (dipstick)
- Check inlet/outlet for blockages
- Verify ventilation
Monthly Maintenance
- Detailed inspection via manhole
- Measure solids layer
- Check baffles and pipes
- Review pumping records
Cleaning/Pumping
Indicators for Pumping:
- FOG layer >25% of depth
- FOG + solids layer >50% of depth (EN 1825)
- Every 1-3 months typical for restaurants
Procedure:
- Pump out FOG layer
- Pump out water (middle layer)
- Remove settled solids
- Flush and clean interior
- Dispose of waste properly (licensed hauler)
- Refill with clean water
Important: Never use enzymes or biological additives without approval. These can emulsify FOG, causing pass-through to sewer and downstream problems.
FOG Disposal
Disposal Options
| Method | Requirements | Notes |
|---|---|---|
| Rendering | Collected FOG to renderer | Best option, beneficial reuse (biodiesel) |
| Anaerobic digestion | High-strength organics | Co-digest with biosolids |
| Landfill | Solidified, mixed with absorbent | Least preferred |
| Incineration | Energy recovery | Only if no other option |
Regulatory Compliance
UK:
- Controlled waste (licensed carrier required)
- Duty of care (waste transfer notes)
- Waste hierarchy (prefer reuse/recovery)
Rendering/Biodiesel:
- Clean FOG has value
- Contamination reduces value
- Regular collection preferred
Worked Example
Given
- Restaurant: 120 seats
- Operating hours: 12h/day (lunch & dinner)
- Peak flow estimate: 0.10 L/s per seat
- Design detention time: 30 minutes
Design
Step 1: Calculate Design Flow
$$Q = 120 \times 0.10 = 12 \text{ L/s}$$
Step 2: Determine Required Volume
$$V = Q \times t \times 60 = 12 \times 30 \times 60 = 21,600 \text{ L} = 21.6 \text{ m}^3$$
Round to V = 22 m³ (22,000L)
Step 3: Select Dimensions
Choose depth H = 1.5m Surface area A = V/H = 22/1.5 = 14.7 m²
For L:W = 3:1 rectangle:
- L × W = 14.7
- L = 3W
- 3W² = 14.7 → W = 2.2m, L = 6.6m
Use: 6.5m long × 2.2m wide × 1.5m deep = 21.45 m³ ✓
Step 4: Estimate Cleaning Frequency
Assume:
- FOG production: 50 g/seat/day
- FOG density: 950 kg/m³
Daily FOG: 120 × 50 = 6,000 g = 6 kg/d Volume: 6/950 = 0.0063 m³/d = 6.3 L/d
FOG layer limit: 25% of 1.5m = 0.375m Volume available: 14.7 × 0.375 = 5.5 m³
Days to fill: 5,500/6.3 = 870 days (unrealistic)
More realistic with solids and safety factor:
- Combined FOG + solids: ~20-30 L/d
- Pump at 30% full (safety margin)
- Frequency: (22,000 × 0.30)/25 = 264 days
Practical: Pump every 2-3 months (60-90 days) to prevent odor and ensure performance.
Step 5: Verify Rise Velocity
FOG droplet d = 0.15mm, SG = 0.92, T = 20°C, μ_w = 0.001 Pa·s
$$V_r = \frac{9.81 \times (0.00015)^2 \times (1000-920)}{18 \times 0.001} = 0.00098 \text{ m/s}$$
Time to rise H = 1.5m: $$t = 1.5/0.00098 = 1531 \text{ s} = 25.5 \text{ min}$$
Detention provided: 30 min ✓ (adequate)
Best Practices
Installation
- Install as close to source as practical (shorter drains)
- Provide easy access for pumping trucks
- Ensure proper ventilation
- Insulate in cold climates
- Install FOG level alarm (optional but recommended)
Operation
- Train kitchen staff on FOG prevention (scraping plates, dry wiping)
- No food waste, paper, plastics into drains
- No detergents or degreasers into trap
- No hot water flushing (emulsifies FOG)
Maintenance
- Regular pumping schedule (don't wait for blockage)
- Full cleanout (not just FOG layer)
- Inspect baffles and pipes
- Keep records (regulatory requirement)
Advanced Features
Monitoring Systems
- FOG level sensors (ultrasonic/capacitance)
- Temperature monitoring
- Flow meters
- Alarms and telemetry
Enhancements
- Heating coils (winter operation)
- Automated sampling
- Flow equalization
- Pre-screening
Related Units
- Coarse-Screen - Upstream solids removal
- Dissolved-Air-Flotation - Advanced FOG/TSS removal
- Primary-Clarifier - Municipal-scale separation
- Aerated-Grit-Chamber - Pre-aeration benefits
References
- EN 1825-1 (2004). Grease Separators - Part 1: Principles of design, performance and testing.
- EN 1825-2 (2002). Grease Separators - Part 2: Selection of nominal size, installation, operation and maintenance.
- PDI G101 (2007). Grease Interceptors Standard. Plumbing & Drainage Institute.
- WEF (2008). Prevention and Control of Sewer System Overflows, 3rd Ed.
- Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed.
- Water UK (2017). Sewers for Adoption, 8th Ed.