14.1 Grease Interceptors

14.1 Grease Interceptors

Quick Answer: Size a Massachusetts grease interceptor from flow (GPM) and grease retention (pounds), not from drainage fixture units. 248 CMR 10.09(1)(b)5 requires not less than two pounds of grease for each GPM. Table 1 converts fixture volume to GPM (75 percent drainage load, one- or two-minute empty time). Table 2 converts that GPM to pounds (a 20 GPM unit retains 40 lb; above 100 GPM, double the GPM). Hydromechanical units and grease-removal devices need a flow-control between fixture and interceptor. Gravity units are the large outdoor tanks that hold FOG for 30 minutes or more. Put the interceptor within 25 feet of the most remote fixture. 10.15 Table 1 Note 1 does not let you pick an interceptor by adding pot-sink DFU.

The PSI Candidate Information Bulletin dated October 3, 2025 gives this cluster five written items: Grease Interceptors; Holding Tanks; Gasoline, Oil, and Sand Separators. This section is the grease third. Independent OpenExamPrep teaching follows 248 CMR 10.09 as posted on Cornell LII and the Mass.gov 12/8/2023 Uniform State Plumbing Code PDF. It is not a Board publication and it does not claim official approval.

248 CMR 10.03 defines an interceptor as a device that separates and retains deleterious, hazardous, or undesirable matter and lets normal sewage or liquid waste leave by gravity. Separator means interceptor. A grease interceptor (gravity) is the large unit, usually installed outside underground, because it needs 30 minutes or more for fats, oils, and grease (FOG) to rise by specific gravity. A grease interceptor (hydro-mechanical) is the compact unit, normally inside, that separates FOG continuously by hydraulic flow, air entrainment, and specific gravity together. 10.09(1)(b)1.c allows only gravity interceptors, hydromechanical, and engineered systems tested to the listed standards. A homemade drum is not a third legal type.

Food establishments, dwellings, and the fixture list

248 CMR 10.09(1) requires interceptors, separators, and holding tanks so oil, gasoline, grease, sand, and similar matter never reach building drains, public or private sewers, DEP-governed systems, treatment plants, or other environmentally sensitive areas. No wastes other than those requiring treatment or separation enter those devices. 10.09(1)(b)1.b then names grease: interceptors shall receive all fixtures and equipment that may produce grease-laden waste wherever food is prepared, and in other establishments where grease may enter sanitary drainage.

10.09(1)(b)1.d and 10.09(1)(b)8 keep grease removal off dwellings and off fixtures that convey human waste, except residential buildings that incorporate commercial cooking accommodations. A Worcester three-family with ordinary kitchens does not get a hydromechanical unit under the bathroom group. A basement catering kitchen in that same building does.

Unsewered sites send large grease quantities to 310 CMR 15.00 Title 5 as well as 10.09 (10.09(1)(b)1.e). Title 5 does not erase the plumbing interceptor; it adds the on-site-system overlay.

Individual fixtures to be protected (10.09(1)(b)2.c):

Interceptors are not traps unless they meet 248 CMR 10.08(2)(e) (10.09(1)(b)1.a). Keep the fixture trap, and put a vent downstream of all interceptors (10.09(1)(b)2.b).

Location (10.09(1)(b)2.f). Not more than 25 feet from the most remote fixture served. Exception: an exterior grease interceptor or trap within 25 feet of that fixture means an inside interceptor is not required. A Boston dish room 40 feet from a basement hydromechanical unit fails the 25-foot rule even if the unit is “big enough.”

10.09(1)(b)2.e. Individual or multiple floor drains, floor sinks, and commercial dishwashers that may encounter grease may run directly to an outside grease trap or interceptor. That run is a dedicated system. 10.09(1)(b)12 then says that when another authority requires the outdoor unit, the work within the property line shall comply with 248 CMR 10.03 Dedicated Systems. That authority’s design may specify the tank, but it does not supersede 248 CMR. The outdoor chamber vent is piped to the inside under 10.16, not less than four inches, and may join the building sanitary venting system.

Sizing is GPM and pounds — DFU is the branch footnote

10.09(1)(b)4. Size, test, and design interceptors to PDI-G101, PDI-G102, ASME A112.14.3, ASME A112.14.4, ASME A112.14.6, ANSI Z1001, or CSA B481. The Board may authorize an alternate design under 248 CMR 3.04. 10.09(1)(b)5 is the capacity floor: not less than two pounds of grease per GPM.

248 CMR 10.09 Table 1 — Recommended procedure for sizing interceptors inside buildings (single compartment):

Favorable policy in the same paragraph: rated capacity is never less than 40 percent of the individual fixture capacity in gallons. For 59.8 gal, 40 percent is 23.9 gal. When you buy on grease-retention pounds only, multiply the fixture’s gallon flow-rate capacity by two.

248 CMR 10.09 Table 2 — Capacity of interceptors:

For flow more than 100 GPM, double the flow-through rating to get pounds. A 110 GPM load is 220 lb, not a Table 1 DFU number.

Worked example — Springfield two-compartment pot sink. Each bay is 24 in × 24 in × 12 in = 6,912 cu in = 29.9 gal. Drainage load per bay = 0.75 × 29.9 = 22.4 gal. One-minute period = 22.4 GPM per bay. Two bays dumped together are 44.8 GPM. Table 2 has no 44.8 row; select the larger listed size, so 50 GPM / 100 lb. The 40-percent check is 0.40 × (29.9 + 29.9) = 23.9 gal of rated capacity — the 50 GPM unit is larger than that floor. Do not call this sink “8 DFU because Table 1 says 4 per section” and then shop a 20 GPM box.

10.15 Table 1 still lists commercial pot, scullery, etc. (each section) = 4 DFU with Note 1: See 248 CMR 10.15(1)(b)2.d for sizing using grease interceptors. Cornell prints that pointer under Table 1. The matching installation sentence in 10.09(1)(b)2.d is: waste branches for interceptor-protected fixtures may be sized equal to the inlet connection of the properly sized interceptor. That is how you pick the pipe into the interceptor. It is not how you pick the interceptor. After the interceptor, remaining sanitary load still uses Table 1 / Table 2 for building drains (Chapter 11). Do not substitute interceptor gallons for those DFU columns.

Hydromechanical flow control (10.09(1)(b)6). Hydromechanical units and grease-removal devices shall have a flow-control device, vented (air intake) or integral non-vented. Place it between the fixture and the interceptor per the manufacturer. An external flow-control vent connects to the building sanitary venting system and is not a fixture vent. Gravity outdoor tanks are not this PDI flow-control paragraph.

Pre-rinse and disposers (10.09(1)(b)3). A dishwasher pre-rinse without a food-waste disposer that wastes to a dishwasher drain shall discharge through an interceptor, minimum two-inch, and the horizontal developed length from the pre-rinse outlet to the weir of the dishwasher trap shall not exceed eight feet (Example 4). Commercial food-waste disposer waste shall not discharge through an interceptor (Example 5). Pre-rinse sinks with a disposer follow 248 CMR 10.10(4)(b).

Water-cooled interceptors are prohibited (10.09(1)(b)7). Emulsifying chemicals that move grease through the interceptor into the drain are prohibited unless the Board accepts them (10.09(1)(b)9).

Maintenance (10.09(1)(b)10). The owner or agent removes grease and solids and disposes of them under federal, state, and local health rules. Local boards of health may add monitoring or registration. That is cleaning frequency as the authority writes it — 10.09 no longer prints the old one-inch “IMPORTANT” stencil as a sizing rule.

Product-acceptance and design policy. Do not install a grease product that needs 248 CMR 3.04 Product-acceptance until the Board has voted. Alternate designs also travel 3.04. The Mass.gov policy Grease & Vacuum Drainage System Design (enacted 11-18-2009, posted 12/07/2017) states that a Massachusetts registered professional engineer, regardless of discipline, may design grease and vacuum drainage systems. Current 10.06 / 10.23 vacuum-drainage text still wants a Massachusetts professional engineer; the policy is what lets that PE be civil, mechanical, or another registered discipline for grease and vacuum design. It does not let an unlicensed kitchen manager pick a tank from a catalog and skip 10.09 Tables 1 and 2.

Official resources

• 248 CMR 10.09 Interceptors, Separators and Holding Tanks (Cornell LII) — 12/8/2023 grease, Tables 1–2, 25-foot rule, disposers
• 248 CMR 10.03 Definitions (Cornell LII) — gravity versus hydromechanical grease interceptors
• 248 CMR 10.15 Table 1 Note 1 (Cornell LII) — DFU footnote to interceptor-branch sizing
• Grease & Vacuum Drainage System Design (Mass.gov policy) — PE of any discipline, enacted 11-18-2009
• 248 CMR 10.00 (Mass.gov, effective 12/8/2023) — official PDF