The default, the four exceptions that permit the small unit, the 50 gpm ceiling and the four-fixture limit, and what each choice costs over ten years
The code has a default, and it is the big one
“Gravity or hydromechanical” reads like a design preference. In the code it is not. Section 1003.2.1 is written as a requirement with an exception attached, and the requirement is a vessel of adequate capacity for new construction, a change of occupancy classification, or a change of use. The small under-sink unit appears in the exception clause, available “at the discretion of the local jurisdictional code official” under four named conditions.
That framing settles most of the argument before it starts. If none of the four conditions apply to your job, the answer is a gravity interceptor and the discussion is about where to put it, not whether.
The two devices are not variants of each other
| Gravity grease interceptor | Hydromechanical interceptor | |
|---|---|---|
| Governing section | UPC 1014.3 | UPC 1014.2 |
| Sized in | Gallons | Gallons per minute |
| Sized from | Drainage fixture units | Sink compartment volume |
| Typical range | 500 – 15,000 gal | 20 – 100 gpm |
| Separation mechanism | Retention time, around 30 min | Baffles and air entrainment at rated flow |
| Usual location | Outside, buried | Inside, under or near the sink |
| Service | Pump truck | Hand cleaning, often by staff |
| Service interval | Weeks to months | Days to weeks |
| Needs a flow control? | No | Yes — vented, non-adjustable |
| Capacity rule | Table lookup | Fixtures ≤ 2.5 × rated gpm |
The retention-time row is the one that explains the rest. A gravity interceptor works by holding the flow still long enough for grease to rise — footnote 2 of Table 1014.3.6 states the tabulated volumes are built on about 30 minutes of retention. A hydromechanical unit has no room for 30 minutes of anything; it separates at its rated flow and only at its rated flow, which is why it needs a flow control and why exceeding its rating produces a straight pass-through rather than a graceful degradation.
The four conditions that permit the small unit
Section 1003.2.1’s exception lists them, and they are physical and site-specific rather than economic. A point-of-use hydromechanical interceptor, FOG disposal system or automatic grease removal device may be permissible where:
- A concrete slab would have to be broken at an existing building or facility to install a gravity interceptor properly
- An outside unpaved area exists, but it is more than 75 feet from the plumbing fixtures the interceptor would serve
- The installation is otherwise unfeasible — the section’s own example is a kitchen on an upper floor of a multistoried building
- The code official determines that minimal fats, oils and grease will be produced, based on the menu and mode of operation — the section names snowball stands and sandwich shops with low grease production using single-service tableware
Note the floor that survives the exception: “In no case shall a grease interceptor or automatic grease removal device be installed which has an approved rate of flow of less than 20 gallons per minute.” Granting the exception does not license an under-sink box of any size.
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The hardware either choice still depends on
A flow control on the small unit, a riser on the big one, and a way to measure what is inside either of them.
12 ft Sludge and Scum Sampler
- Reads the grease cap and the settled solids as depths rather than a guess
- Vessel volume only means something if someone knows how much is still liquid
- The same tool services a septic tank between pump-outs
24 in Tank Riser and Lid
- “Readily accessible to the equipment required for maintenance” is code, not comfort
- A buried interceptor with no riser gets serviced late, or never
- Brings the cover to grade so the cap can be measured without digging
DWV Pipe Hangers
- Table 1014.2.1 is computed at ¼ in per foot — a bellied line is not that
- Grease cools and sets in a sag long before it reaches the interceptor
- The published flow rates assume a pipe running at its design slope
Reusable DWV Test Cap
- The grease waste branch is tested with the rest of the drainage system
- Caps a stub for a water or air test without solvent-welding a knockout
- Comes back off, so the same cap tests the next branch
14 in Pipe Wrench
- Vented flow controls, cleanout plugs and threaded inlet fittings
- Section 1014.2 wants a vented flow control with no removable parts
- The flow control is what keeps a hydromechanical unit inside its rating
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The 50 gpm ceiling, and what a “trap” actually is
IPC-family municipal codes draw a hard line by flow rate. A grease trap is defined as a passive interceptor with a rated flow of 50 gpm or less located inside the building. The installation rules that come with that definition are worth knowing even when the code you are on words them differently:
- Maximum rated flow through each trap: 50 gpm. Minimum: 20 gpm
- No more than four fixtures connected to or discharging into any one trap
- Each connected fixture needs its own flow control or restricting device in the drain outlet, readily accessible
- Flow control devices shall not have adjustable or removable parts
- Total capacity in gallons of the connected fixtures shall not exceed 2½ times the trap’s gpm rating
- No food waste disposal unit connected to or discharging into it
- Water-jacketed traps are not approved
- Cleaned at least every 30 days, meaning emptied completely with sides and bottom cleaned
The four-fixture limit is the one that quietly decides the question. A kitchen with a three-compartment pot sink, a prep sink, a prerinse sink, a mop basin and three floor drains is seven fixtures before anyone argues. One trap cannot take them, and a second trap means a second flow control, a second access location and a second cleaning schedule.
What each choice really costs over ten years
The capital comparison is the one everybody runs, and it points at the small unit. The ten-year comparison usually does not.
| Cost driver | Gravity | Hydromechanical |
|---|---|---|
| Vessel and install | High — excavation, crane, risers, backfill | Low — a box and a flow control |
| Service event | Expensive, infrequent | Cheap, frequent |
| Who does it | Licensed hauler with a manifest | Often kitchen staff |
| Failure mode when skipped | Degrades gradually over weeks | Pass-through within days |
| Indoor nuisance | None — it is outside | Odour and spill risk in the kitchen |
| Survives a menu change | Usually — large margin | Often not — sized to a specific sink |
| Survives a tenant change | Usually | Frequently resized |
The row that matters most is the failure mode. A gravity interceptor that is pumped late loses working volume gradually — performance declines over weeks and there is time to notice. A hydromechanical unit that is not cleaned fills its small chamber in days and then passes grease straight through at full flow. The capital saving is real; so is the operational exposure, and it lands on the operator rather than the installer.
There is a related point about resilience. Table 1014.3.6’s bands are wide — everything from 9 to 21 DFU gets the same 750 gallon vessel — so a gravity interceptor usually has genuine headroom for a menu change or an added fixture. A hydromechanical unit sized to a specific sink at a specific drainage period has none, and the 2.5× capacity rule will catch you the moment a fixture is added.
Choosing, in order
| Question | If yes |
|---|---|
| Does the local pretreatment ordinance name a device or a minimum size? | That is the answer. Stop here. |
| Is this new construction, a change of occupancy or a change of use? | Gravity is the default; the exception has to be argued. |
| Does one of the four Section 1003.2.1 conditions genuinely apply? | Hydromechanical is available — with the code official’s approval in writing. |
| Are there more than four grease-bearing fixtures? | The four-fixture limit pushes you to gravity or to multiple units. |
| Does the required flow exceed 50 gpm? | Above 50 gpm you are not specifying a trap. |
| Do the fixtures exceed 2.5 × the rating you can fit? | The capacity rule rules the small unit out independently. |
| Will anyone actually clean it weekly? | If not, the hydromechanical option is a compliance risk whatever the paperwork says. |
Two genuine cases for the small unit survive all of that. The first is the sandwich shop the code itself names — low grease production, single-service tableware, one or two fixtures, where a gravity vessel would be ridiculous. The second is the upper-floor kitchen in a multistory building, where the code’s own “unfeasible” example fits exactly and there is no realistic alternative.
Everywhere else, the honest answer is that the gravity interceptor is what the code expects, and the conversation worth having with the client is about where it goes and how the pump truck reaches it — not whether it can be avoided.
If you do go hydromechanical, get these right
- Run both tests. Required flow from the compartment volumes, and fixtures ≤ 2.5 × certified rating. They fail independently.
- Default to the one-minute drainage period unless the authority has told you otherwise. A one-minute unit satisfies a two-minute requirement; the reverse never holds.
- Full inside depth, one 0.75 factor. Applying a reduced working depth as well undersizes by about a third.
- Vented flow control, no removable parts, readily accessible and visible, with no system vent between it and the interceptor inlet, and its air inlet to the vent system or through the roof — never open inside the building.
- Vent downstream of the interceptor, per Section 1014.2.2.
- No disposer upstream unless the local ordinance permits it, and size for it if there is one.
- Write the cleaning interval into the handover, with the sides-and-bottom definition rather than “skim it”.