Grease Separator Sizing and Design Guide

Introduction

The scene: A busy downtown restaurant during Friday night dinner service. The kitchen is in full swing—fryers bubbling, dishwashers running, prep sinks draining. Suddenly, the floor drain backs up, sending greasy wastewater flooding across the kitchen floor. The restaurant must close for the night, losing thousands in revenue, while emergency plumbers work to clear the blocked sewer line.

What happened? The grease separator was undersized. It couldn't handle the peak flow rate, allowing fats, oils, and grease (FOG) to bypass the system and accumulate in the municipal sewer, eventually causing a complete blockage.

This scenario plays out in commercial kitchens worldwide when grease separators are improperly sized or maintained. Whether it's a fast-food restaurant, hotel kitchen, or hospital cafeteria, getting grease separator sizing wrong leads to costly consequences: sewer blockages, municipal fines, environmental violations, and business disruptions.

Why Grease Separators Matter

Grease separators (also called grease traps or grease interceptors) are critical devices that prevent FOG from entering municipal wastewater systems. They work by exploiting the difference in specific gravity between water (1.0) and grease (0.9-0.95), allowing grease to float to the surface while solids settle to the bottom, enabling relatively clean water to discharge to the sewer.

The consequences of failure:

• Sewer blockages: FOG accumulation causes costly emergency repairs and capacity expansion
• Environmental damage: Untreated FOG contaminates waterways and treatment plants
• Code violations: Non-compliance with EN 1825 and local plumbing codes results in fines and shutdowns
• Business disruption: Blocked drains force kitchen closures, losing revenue and customer trust

The Sizing Challenge

Proper grease separator sizing isn't just about meeting code requirements—it's about ensuring reliable operation during peak kitchen activity. The challenge involves:

• Calculating peak flow rates from meals served, sink capacity, or fixture counts
• Estimating grease loads based on establishment type and cooking methods
• Selecting appropriate separator types (conventional, automatic, or compact)
• Ensuring adequate retention time (minimum 3 minutes per EN 1825) for effective separation
• Planning for maintenance with proper grease storage capacity

Common mistakes:

• Undersizing based on average flow instead of peak demand
• Ignoring retention time requirements
• Selecting wrong separator type for the application
• Neglecting maintenance access and cleaning requirements

What You'll Learn

This comprehensive guide provides everything you need to size grease separators correctly for commercial kitchens and food service establishments. You'll learn:

• Fundamental sizing formulas per EN 1825 standards
• Peak flow rate calculations from meals served or fixture capacity
• Grease load estimation methods for different establishment types
• Separator type selection (conventional, automatic, compact)
• Installation requirements for indoor, outdoor, and underground applications
• Maintenance planning and cleaning frequency calculations

Whether you're designing a new commercial kitchen, retrofitting an existing facility, or troubleshooting grease-related sewer problems, this guide provides the engineering principles and practical methods to ensure your grease separator system protects both your business and municipal infrastructure—even during that Friday night rush.

Quick Answer: How to Size a Grease Separator?

Size grease separators based on peak flow rate and establishment type per EN 1825 standards.

Core Formula (EN 1825)

Nominal Size:

Where:

• $NS$ = Nominal size (L/s)
• $Q_{\text{peak}}$ = Peak flow rate (L/s)
• $f_d$ = Density factor (typically 1.0)

Additional Formulas

Worked Example

Reference Table

Key Standards

Sizing Formulas per EN 1825

Nominal Size Calculation

The required nominal size depends on establishment type, meals served, and movement characteristics:

Where:

• $NS$: Nominal size (L/s)
• $Q_{\text{peak}}$: Peak circulation rate (L/s)
• $f_d$: Density factor (typically 1.0 for full flow rate)

Circulation Speed Estimation

Method 1 - Meals Served:

• $N_{\text{meals}}$: Meals per day
• $f_{\text{meal}}$: Wastewater per meal (L/meal)
• $h_{\text{op}}$: Operating hours per day

Typical Wastewater per Meal:

• Restaurants: 15-20 L/meal
• Fast food: 8-12 L/meal
• Cafeteria: 10-15 L/meal
• Hotel kitchen: 20-25 L/meal

Method 2 - Sink Capacity:

• $N_{\text{sinks}}$: Number of sinks connected
• $V_{\text{sink}}$: Sink capacity (L)
• $t_{\text{drain}}$: Drain time (typically 60-90 seconds)

Separator Volume

Total separator volume includes three components:

Separation Volume:

• $t_{\text{ret}}$: Retention time (30-45 minutes typical)

Grease Storage Volume:

• $L_{\text{grease}}$: Daily grease load (kg/day)
• $D_{\text{clean}}$: Days between cleaning (30-60 typical)

Sludge Storage Volume:

What Are the Types of Grease Separators?

Conventional (Gravity) Separators

Principle: Natural gravity separation in large tank (typically >1000L).

Advantages:

• Simple and reliable operation
• No power required
• Handles high discharge rates
• Suitable for large kitchens

Disadvantages:

• Large size requirement
• Typically requires outdoor installation
• Needs regular manual cleaning

Applications: Full-service restaurants, hotel kitchens, hospital kitchens, food processing.

Sizing Range: NS 2-50 L/s, volumes 1,000-50,000 L.

Automatic (Mechanical) Separators

Principle: Mechanical skimming removes grease automatically to collection container.

Advantages:

• Reduced cleaning frequency
• Compact design
• Better separation efficiency
• Indoor installation possible

Disadvantages:

• Requires electrical power
• Higher initial cost
• Mechanical maintenance needed

Applications: Restaurants with limited space, facilities requiring minimal maintenance, high-end establishments.

Sizing Range: NS 1-10 L/s, volumes 200-5,000 L.

Compact (Under-Sink) Separators

Principle: Small unit installed directly under individual sinks.

Advantages:

• Very compact design
• Easy installation
• Suitable for small facilities
• Lower initial cost

Disadvantages:

• Very frequent cleaning required (weekly)
• Limited capacity
• Not suitable for high-volume operations

Applications: Small cafes, coffee shops, bakeries with minimal cooking, bar sink drainage.

Sizing Range: NS 0.1-1 L/s, volumes 40-200 L.

Installation Locations

Indoor Installation

Requirements:

• Adequate ventilation (odor control)
• Floor drain for overflow
• Easy access for pumping trucks (if applicable)
• Heat control (prevent grease solidification)

Advantages: Protected from weather, no freeze risk, easier to monitor.

Codes: Some jurisdictions prohibit indoor installation for large separators (>5000L).

Outdoor Above-Ground

Requirements:

• Weatherproof housing
• Insulation in cold climates
• Secure cover (prevent unauthorized access)
• Proper drainage around unit

Advantages: Easy access for maintenance, no excavation cost, ventilation not required.

Outdoor Underground

Requirements:

• Watertight construction
• Traffic-rated cover if in vehicle areas
• Proper depth (access for cleaning)
• Air circulation to surface

Advantages: Out of sight, no space footprint, suitable for any climate.

Common Type: Most large conventional separators (NS >10 L/s).

Grease Load Estimation

Calculation Method

Daily grease load depends on establishment type and cooking methods:

Where:

• $f_{\text{grease}}$: Grease factor per meal (0.05-0.15 kg/meal)
• $k_{\text{type}}$: Establishment type factor

Establishment Type Factors ($k_{\text{type}}$):

Example Calculation

Restaurant serving 200 meals/day:

Cleaning and Maintenance

Cleaning Frequency

Regulatory Maximum: When grease layer reaches 25% of separation volume or 75mm thickness (EN 1825).

Recommended Schedule:

• High-volume restaurants: 2-4 weeks
• Medium restaurants: 4-8 weeks
• Low-volume cafes: 8-12 weeks
• Compact under-sink: 1-2 weeks

Cleaning Procedure

1. Pump Out: Remove all grease, sludge, and water (do NOT skim grease only - this violates codes)
2. Inspection: Check for damage, corrosion, proper operation of baffles
3. High-Pressure Wash: Clean all surfaces
4. Refill: Add clean water to restart bacterial action
5. Documentation: Record date, volume pumped, observations

Maintenance Items

Monthly:

• Check for odors (indicates needed cleaning)
• Inspect access covers (ensure sealed)
• Verify stream is normal (not backing up)

Quarterly:

• Measure grease layer thickness
• Check inlet/outlet pipes for blockage
• Lubricate mechanical components (automatic separators)

Annually:

• Full structural inspection
• Replace gaskets and seals
• Test alarms (automatic separators)

What Are the Design Considerations for?

Baffle Design

Purpose: Prevent turbulence, retain grease, prevent scum carryover.

Inlet Baffle: Submerged 300-450mm below water level, directs amperage downward.

Outlet Baffle: Submerged 450-600mm, positioned at neutral zone to collect clearest water.

Temperature Considerations

Optimal Range: 20-30°C for best separation.

High Thermal value (>60°C): Grease remains liquid, poor separation. Solution: Install cooling trap upstream.

Low Degree (<10°C): Grease solidifies prematurely. Solution: Indoor installation or insulation.

Sampling and Compliance

Many jurisdictions require FOG monitoring:

Sampling Location: Downstream of separator (to verify performance).

Parameters:

• FOG concentration (<100 mg/L typical limit)
• pH (6-10 typically)
• Heat level (<40°C)

Material and Construction

Common Materials

Interior Coating

Food-grade epoxy or polyurethane coating required for concrete/steel units. Must withstand pH 2-12 (grease becomes acidic).

Standards and Regulations

EN 1825

European standard for grease separators. Specifies:

• Nominal size (NS) classification system

• Minimum retention time (30 minutes)

• Separation performance requirements (>95% for particles >10 microns)

• Testing and certification procedures

• Marking and documentation requirements

• Installation depth and access

• Earthquake resistance (Turkey-specific)

• Material specifications for local conditions

Local Plumbing Codes

Most municipalities have FOG ordinances requiring:

• Grease separator for any food service facility
• Minimum sizing (often NS $\geq$ 1 L/s or 200L minimum volume)
• Regular cleaning (maximum 90 days)
• Maintenance documentation
• Sampling and compliance monitoring

Common Design Mistakes

1. Undersizing: Using number of sinks without considering actual movement leads to undersized separators. Always compute peak circulation based on actual usage.

2. Dishwasher Connection: NEVER connect commercial dishwashers to grease separators. High temp and detergents destroy separation.

3. Sanitary Fixtures: Do NOT connect toilets, urinals, or floor drains to grease separator (dilution reduces effectiveness).

4. No Cooling: Hot wastewater (>60°C) prevents separation. Install air conditioning trap if necessary.

5. Poor Access: Installing separator where pumping trucks can't reach requires expensive manual pumping.

6. Inadequate Air exchange: Underground separators without proper vent develop dangerous gas buildup.

What Are the Best Practices for?

1. Employee Training: Kitchen staff should scrape plates, avoid pouring grease down drains, and use dry cleanup methods.

2. Strainers: Install sink strainers to catch food solids (reduces sludge accumulation).

3. Regular Inspection: Monthly thickness checks prevent overfilling and violations.

4. Documentation: Maintain cleaning logs for compliance inspections.

5. Bacteria Additives: Consider enzyme/bacteria treatments to reduce grease accumulation (supplemental, NOT replacement for pumping).

Our hydraulic calculations are based on established engineering principles.

Our hydraulic calculations are based on established engineering principles.

Our team developed this approach based on internal research and field validation.

Conclusion

Proper grease separator sizing ensures code compliance, prevents sewer blockages, and minimizes operating costs. Always size based on peak flow rate conditions, follow EN 1825 standards, plan for regular maintenance access, and train kitchen staff on FOG management.

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Key Takeaways

1. Nominal Size Calculation

Calculate nominal size using the EN 1825 formula:

Where:

• $NS$ = Nominal size (L/s)
• $Q_{\text{peak}}$ = Peak flow rate (L/s)
• $f_d$ = Density factor (typically 1.0)

Critical points:

• Nominal size determines separator capacity and must meet peak flow rate requirements
• Size for peak demand, not average flow rate
• Class I separators: $NS \geq 50$ L/s (large commercial kitchens)
• Class II separators: $NS < 50$ L/s (smaller establishments)

2. Peak Flow Rate Estimation

Estimate peak flow rate from multiple methods:

From meals served:

From fixture capacity:

Critical points:

• Peak flow rate determines nominal size selection and separator volume requirements
• Use actual fixture data when available (more accurate than meal estimates)
• Account for simultaneous fixture use with diversity factors
• Typical peak flows: 15-40 L/min for commercial kitchens

3. Retention Time Requirements

Provide minimum retention time for effective separation:

Where $V$ = separator volume (L), $Q$ = flow rate (L/s)

Requirements per EN 1825:

• Minimum: 3 minutes for Class I separators
• Recommended: 5-10 minutes for better performance
• Many jurisdictions mandate 5-10 minutes minimum

Critical points:

• Retention time ensures adequate separation of grease from wastewater
• Insufficient retention time allows FOG bypass, causing sewer blockages
• Calculate actual retention: $t = V / Q$ and verify it meets minimum requirements

4. Grease Storage Capacity

Size grease storage for accumulation period:

Where:

• $G_{\text{daily}}$ = Daily grease load (kg/day)
• $t_{\text{accumulation}}$ = Accumulation period (days)

Typical accumulation periods:

• 30-60 days: Standard for most commercial kitchens
• 15-30 days: High-volume operations requiring frequent cleaning
• 60-90 days: Low-volume establishments with infrequent service

Critical points:

• Grease storage volume determines cleaning frequency and maintenance requirements
• Insufficient storage requires more frequent (expensive) pump-out service
• Typical grease loads: 0.01-0.05 kg per meal served

5. Separator Type Selection

Select separator type based on application requirements:

Conventional Separators:

• Applications: Large commercial kitchens, full-service restaurants, hotels
• Capacity: NS 2-50 L/s, volumes 1,000-50,000 L
• Advantages: High capacity, reliable, cost-effective
• Disadvantages: Large size, requires outdoor installation, manual cleaning

Automatic (Mechanical) Separators:

• Applications: Restaurants with limited space, high-end establishments
• Capacity: NS 1-10 L/s, volumes 200-5,000 L
• Advantages: Compact design, reduced cleaning frequency, indoor installation possible
• Disadvantages: Higher initial cost, requires electrical power, mechanical maintenance

Compact (Under-Sink) Separators:

• Applications: Small cafes, coffee shops, bakeries, bar sinks
• Capacity: NS 0.1-1 L/s, volumes 40-200 L
• Advantages: Very compact, easy installation, lower cost
• Disadvantages: Limited capacity, frequent cleaning required (weekly)

6. Maintenance and Cleaning Requirements

Plan for regular maintenance to maintain effectiveness:

Cleaning frequency calculation:

Where separator should be cleaned when grease occupies 25% of storage volume.

Maintenance requirements:

• Pump-out service: Remove accumulated grease and solids
• Cleaning frequency: Typically 30-90 days depending on grease load
• Inspection: Monthly visual inspection for proper operation
• Documentation: Maintain service records for code compliance

Critical points:

• Grease separators require periodic pumping and cleaning to maintain effectiveness
• Neglected maintenance leads to reduced separation efficiency and code violations
• Plan for easy access and adequate space for service vehicles

Further Learning

• Water Tank Guide - Water storage tank sizing
• Hydropneumatic System Guide - Pressure tank sizing
• Water Pressure Loss Guide - Pipe friction calculations
• Grease Separator Calculator - Interactive calculator for grease separator sizing

References & Standards

Primary Standards

EN 1825 Grease separators. Provides standardized sizing methods, nominal size calculations, retention time requirements, and performance specifications for grease separators in commercial kitchens.

Supporting Standards & Guidelines

International Plumbing Code (IPC) Comprehensive plumbing code for safety and sanitation. Specifies grease separator requirements for commercial kitchens.

Uniform Plumbing Code (UPC) Model plumbing code developed by IAPMO. Provides grease interceptor sizing and installation requirements.

Further Reading

• ASPE Plumbing Engineering Design Handbook - American Society of Plumbing Engineers design standards
• EN 806 - Specifications for installations inside buildings conveying water for human consumption

Note: Standards and codes are regularly updated. Always verify you're using the current adopted edition applicable to your project's location. Consult with local authorities having jurisdiction (AHJ) for specific requirements.

Disclaimer: This guide provides general technical information based on international plumbing standards. Always verify calculations with applicable local codes and consult licensed professionals for actual installations. Plumbing system design should only be performed by qualified professionals. Component ratings and specifications may vary by manufacturer.