Grease Interceptors: Protecting Drainage Infrastructure from Fats, Oils and Grease

What Do Grease Interceptors Do and Why Do They Require Maintenance?

Grease interceptors are critical wastewater treatment assets designed to separate fats, oils, grease (FOG), and food solids from wastewater before they enter the public sewer infrastructure.

When hot, liquid fat mixes with commercial dish-washing detergents, it creates a temporary emulsion. However, as this wastewater travels down the drainage line and cools below 38°C / 110°F), a destructive chemical reaction occurs:

1. Separation & Solidification: The grease separates from the water and hardens.
2. Saponification: The solidified fatty acids bind with calcium present in both the wastewater and concrete sewer walls.
3. Calcification: This chemical bond transforms the mixture into a rock-hard, soap-like solid.

Without a local grease interceptor to slow down, cool, and retain these fats at the source, this calcification occurs directly inside building and utility pipes. Over time, this can lead to blockages and back ups which can lead to structural damage and environmental pollution within local waterways.  

Case Study: The Whitechapel Fatberg (London, 2017)

The catastrophic potential of unmanaged FOG is best illustrated by the 130 tonne “Whitechapel Fatberg” discovered blocking a Victorian-era sewer under London in 2017. Forensic analysis revealed that nearly 90% of the obstruction consisted of solidified cooking oils and fats from commercial kitchens that had bound together with non-flushable household waste.

While often hidden from view, grease interceptors play an important role in protecting building and town drainage systems, maintaining hygiene standards, reducing environmental impacts, and ensuring compliance with wastewater authority requirements. Without regular inspection and maintenance, these systems rapidly lose efficiency, increasing the risk of blockages, odours, drainage failures, and regulatory non-compliance.

How do Grease Interceptors / Traps Work?

Grease interceptors slow waste water flow and operate on the simple principle of gravity.  Fats, grease and oil are less dense than water which allow them to float to the top of the interceptor tank / separation chamber while heavier solids sink to the bottom. The cleaner, clarified water in the middle is then allowed to safely discharge into the main sewer line.

Although commonly referred to as a single asset, grease interceptor systems typically consist of multiple interconnected components, including:

• Interceptor tanks or separation chambers
• Inlet and outlet pipework
• Baffles and flow control devices
• Access covers and inspection openings
• Ventilation systems
• Pump systems
• Monitoring equipment and alarms
• Associated drainage infrastructure

Each component contributes to overall system performance and should be considered within maintenance strategies.

What Regulations and Compliance Requirements Apply to Grease Interceptors?

Grease interceptor systems are required under Building Regulations internationally with the same purpose of treating FOG waste at the source, avoiding blockages / disruption downstream. This puts the onus on the building designer, owner / operator to ensure that they comply with the requirements which are enforced by local authorities.

United Kingdom

• Building Regulations: Part H (Drainage and Waste Disposal), Section 2.21 dictates that any drainage serving a commercial hot food kitchen must be fitted with a grease separator. 
• Under Section 111 of the Water Industry Act 1991, it is a criminal offence to discharge any matter into the public sewer system that interferes with the free flow of wastewater.
• The Environment Protection Act 1990 requires you to keep a record of collections in case you need to prove the contractor is licensed and registered.

Australia

• Trade Waste Agreements are required by local water retailers (such as Sydney Water, or SA Water), with mandates to entirely pump out waste by a licensed liquid waste contractor on a rigid schedule (typically every 1 to 3 months).
• AS/NZS 3500 Plumbing and Drainage Series: Provides the minimum requirements for sanitary plumbing and drainage systems, including design principles relevant to grease management systems and discharge control, ensuring compliance with National Construction Code (NCC).

Canada

• National Plumbing Code of Canada (NPC): Establishes baseline requirements for plumbing systems, including drainage design principles relevant to grease interception in commercial applications.
• Provincial Plumbing Codes (e.g. Ontario Building Code, British Columbia Plumbing Code): Most provinces adopt or adapt the NPC, often adding local amendments requiring grease interceptors for food service establishments.
• Municipal Sewer Use By-Laws: Municipalities such as Toronto, Vancouver, and Calgary enforce strict sewer use bylaws that typically:
  •         Mandate grease interceptors for commercial kitchens
  •         Define sizing and installation requirements
  •         Set discharge limits for FOG concentrations
  •         Require routine cleaning and maintenance logs
  •         Permit inspections and enforcement actions

Failure to maintain these systems is typically treated as a compliance breach, resulting in fines and possible imprisonment, due to the direct impact on public sewer infrastructure and environmental performance. 

What Maintenance Tasks Are Required for Grease Interceptors?

Maintenance requirements vary depending on usage patterns, wastewater characteristics, system size, and authority requirements; however, structured preventative maintenance programs can be categorised as follows:

1. Routine Inspection & The “25% Rule”

• • Frequency: Weekly to Fortnightly.
  • Procedure: Facilities or building managers must perform a visual inspection of the interceptor’s internal layers using a core-sampling tool (often called a “Sludge Judge”). This clear tube measures the precise depth of the top scum layer (floating FOG) and the bottom sludge layer (settled food solids).
  • The 25% Rule Threshold: Internationally recognised asset standards mandate that a grease interceptor must be professionally serviced before the combined volume of the top FOG layer and bottom solids layer exceeds 25% of the total liquid capacity of the tank. Beyond 25%, the hydraulic retention time (the time wastewater needs to sit and cool) is severely compromised, allowing grease to escape into the public sewer.

2. Comprehensive Pump-Outs & Hydro-Jet Cleaning

• • Frequency: Monthly to Quarterly (1 to 3 months, or strictly as dictated by the Local Water Authority Trade Waste Agreement).
  • Execution: This task must be performed by a fully licensed liquid waste removal contractor to completely remove all liquid, floating grease, and settled solids. Merely skimming the top grease layer is unacceptable.
  • Scraping and Hydro-Jetting: Once emptied, the technician must use high-pressure water jets or scrapers to clean the internal concrete or fiberglass walls, corbels, and structural corners. This removes hardened, calcified grease buildup and eliminates anaerobic bacteria that cause highly corrosive hydrogen sulphide (H2S) gas and foul odours.

3. Internal Component & Baffle Inspection

• • Frequency: Concurrent with every pump-out service (when the tank is fully empty).
  • Procedure: A structural and mechanical assessment of the tank’s internal components must be conducted immediately after evacuation. Inspectors must check that the inlet and outlet T-pipes (sanitary tees) are intact, unclogged, and properly submerged. The internal baffle walls must be checked for cracks, shifting, or chemical erosion (concrete degradation caused by fatty acids). If a baffle wall fails or a T-pipe breaks off, wastewater will “short-circuit” directly from the inlet to the outlet without separating, rendering the interceptor useless.
  • Gasket & Lid Seal Check: Inspect the heavy-duty access lids and gas-tight neoprene seals. Worn gaskets must be replaced immediately to prevent dangerous, foul-smelling sewer gases from escaping into commercial spaces or public walkways.

4. Mandatory Regulatory Record Keeping & Auditing

• • Frequency: Continuous / Post-Service immediately.
  • Procedure: Maintaining a digital or physical manifest logbook is a legal requirement under regional Trade Waste Agreements, including:
    • The date and exact time of the pump-out.
    • The total volume of liquid waste removed.
    • The licensed waste hauler’s corporate details and vehicle registration.
    • The official waste tracking manifest number (e.g., EPA waste tracking certificates).
    • Notes on any structural defects or repairs required.
  • Auditing Compliance: These records must be kept on-site for a minimum of 2 to 5 years (depending on local jurisdiction). Water authority inspectors perform random, unannounced audits; failure to produce these manifests on demand can result in immediate, heavy statutory fines or the revocation of the facility’s commercial discharge permit.

Why Do Grease Interceptors Require Ongoing Maintenance?

Grease interceptors are often overlooked because they operate out of sight; however, their failure can have significant operational, environmental, and financial consequences. Excessive grease build-up, poor maintenance practices, or inadequate inspection regimes can contribute to drainage failures, odour issues, environmental incidents, and expensive emergency rectification works.

A structured, risk-based maintenance approach ensures grease interceptor systems continue operating effectively, protects downstream infrastructure, supports regulatory compliance, and reduces operational disruption. Ultimately, effective maintenance is not simply about preventing blockages, it is about protecting critical building infrastructure, maintaining operational continuity, and ensuring wastewater systems continue to perform safely and reliably throughout their service life.