Can a facility switch from a hydromechanical interceptor to a gravity interceptor later if load increases?

Retrofitting from hydromechanical to gravity interceptor after initial construction is possible but generally involves significant additional cost and construction disruption, since gravity interceptors require substantial excavation and outdoor installation — this is exactly why initial sizing should account for reasonably anticipated future load growth, not just current day-one requirements.

Do hydromechanical interceptors require less maintenance than gravity interceptors?

Generally the opposite — hydromechanical interceptors, due to their much smaller volume, commonly require more frequent cleaning (weekly to bi-weekly in high-volume operations) compared to gravity interceptors, which can often go 4 to 8 weeks or longer between pump-outs due to their much larger grease storage capacity, as covered in the companion pumping frequency article.

Is one interceptor type generally less expensive than the other?

Hydromechanical units are generally less expensive to purchase and install for smaller applications, given their compact size and indoor installation without major excavation — but for larger loads where a hydromechanical unit would need to be oversized or multiple units used in parallel, a properly sized gravity interceptor can become the more practical and often more cost-effective solution despite its higher installation complexity.

Engineering·7 min read·August 14, 2026

🛢️ Hydromechanical vs. Gravity Grease Interceptors: Which One Applies

How hydromechanical and gravity grease interceptors differ in sizing method, installation location, and application scale, and which factors determine which type a given commercial kitchen needs.

By EngineersUniverse Editorial Team

Engineering Editorial & Technical Review Team

Published August 14, 2026

Two Fundamentally Different Devices for the Same Underlying Purpose

Hydromechanical grease interceptors (HGI) and gravity grease interceptors (GGI) both serve the same core purpose — separating fats, oils, and grease from wastewater before it reaches the sanitary sewer — but they achieve this through genuinely different mechanisms, are sized using entirely different methods, and suit different scales and types of food service operations. Choosing between them is a real design decision informed by the specific facility, not simply a matter of picking whichever unit happens to be available.

How Hydromechanical Interceptors Work and Where They Fit

Hydromechanical interceptors are compact devices, typically installed indoors near or beneath the fixtures they serve, that use internal baffles and a flow-control fitting to slow incoming wastewater enough for grease to float to the surface and solids to settle, while allowing the clarified water to pass through to the sewer. Because of their compact size, they are rated and sized by flow rate (gallons per minute, GPM) rather than by total liquid volume — the sizing question is fundamentally about how much peak flow the unit can process effectively, not how much total volume it can hold, as covered in more depth in the companion PDI G-101 sizing article.

How Gravity Interceptors Work and Where They Fit

Gravity grease interceptors are large precast concrete or fiberglass tanks, typically installed outdoors and below grade, that separate grease through extended retention time rather than flow-control baffling — wastewater sits in the tank long enough for grease to rise and solids to settle before the clarified effluent discharges to the sewer. Because their separation mechanism depends on retention time and total volume rather than instantaneous flow control, they are sized by liquid volume (gallons of tank capacity) using a formula based on meals served, not by a flow rate rating, as covered in more depth in the companion EPA/IPC gravity sizing article.

Why Facility Scale Is the Primary Deciding Factor

Hydromechanical interceptors are generally appropriate for smaller to medium food service operations with limited kitchen footprint and moderate fixture loads, where their compact indoor installation is a practical fit within the available space. Gravity interceptors are generally required for larger operations — high-volume restaurants, institutional food service, and facilities where the calculated hydromechanical sizing would exceed the largest practical standard HGI rating (commonly 100 GPM), a threshold this site's Grease Interceptor Sizing Simulator explicitly flags when hydromechanical sizing results exceed that largest standard rating.

Why Available Space Also Drives the Decision

Beyond load-based sizing thresholds, available physical space is a genuine practical constraint — a facility with a compact kitchen and no adjacent outdoor area suitable for a large buried tank may need to work within a hydromechanical solution even if load calculations suggest gravity interceptor territory, potentially requiring multiple hydromechanical units in parallel or other design accommodations. Conversely, a facility with ample exterior space and moderate fixture loads might still choose a gravity interceptor if local code specifically requires it for that facility's occupancy classification, regardless of what the hydromechanical calculation alone would suggest.

Why Local Code Requirements Often Override a Pure Engineering Calculation

Many jurisdictions specify which interceptor type is required for particular facility types or sizes directly in the local plumbing code or a municipal FOG (fats, oils, and grease) control ordinance, sometimes independent of what a pure sizing calculation alone would suggest — some jurisdictions mandate gravity interceptors for all new commercial food service construction above a certain size threshold, regardless of calculated hydromechanical adequacy. This is why confirming the local authority having jurisdiction's specific requirements is a necessary step before finalizing interceptor type selection, not an optional check performed only if the calculated sizing result seems ambiguous.

Why Terminology Confusion Is Common Between the Two Types

In everyday usage, "grease trap" often informally refers to a small hydromechanical device while "grease interceptor" refers to a large gravity unit, but code language does not always maintain this distinction consistently — some codes use "grease interceptor" as the umbrella term covering both types, distinguishing them instead as "hydromechanical grease interceptors" and "gravity grease interceptors" specifically. Being aware of this terminology variation matters when reading a specific local code or ordinance, since assuming a term means one specific device type based on common informal usage can lead to misreading an actual code requirement.

Topics covered

hydromechanical vs gravity grease interceptorHGI vs GGIgrease trap vs interceptorcommercial kitchen grease interceptor selection