Code Stories · why the rule exists
Grease Interceptors (FOG)
The rule in Massachusetts today
Not in facts.json. Verbatim, 10.09(1)(b)2.f: "Interceptors shall be located not more than 25 feet of the most remote fixture being served." 5: "Interceptors shall have a capacity of not less than two pounds of grease for each gallon-per-minute (GPM) of flow." 2.b: "A vent shall be installed downstream of all interceptors". 3.b: "The waste discharge from a commercial food waste disposer shall not discharge into the sanitary drainage system through an interceptor."
Why it exists
Grease goes down the drain as a hot liquid, cools in the pipe, and solidifies, sticking to the walls and building up until the sewer backs up or overflows. EPA's report to Congress found grease was the most common cause (47%) of reported sewer blockages. An interceptor slows the flow and gives it time to cool, so grease, which is lighter than water, floats and is held back while the clearer water leaves. Federal pretreatment rules also bar discharges of 'solid or viscous pollutants in amounts which will cause obstruction' in the public sewer.
How it came to be
- 1884 — Nathaniel T. Whiting of San Francisco patents a grease trap (US 306,981, Oct. 21, 1884). patents.google.com
- 1940 — BMS 66 leaves grease interceptors to local authority, and allows an interceptor plus a fixture trap if a vent intervenes. archive.org
- 2004 — EPA Report to Congress on CSOs and SSOs (EPA-833-R-04-001), as quoted in EPA's 2012 food-service fact sheet. hampton.gov
- 2012 — EPA fact sheet (EPA-833-F-12-003) citing the pretreatment prohibition at 40 CFR 403.5(b)(3). hampton.gov
Make it stick
Mnemonic: 'Hot down the drain, hard in the main.' Grease is liquid in the sink and solid in the sewer, so the interceptor has to catch it in between.
On the exam
Expect MA specifics (248 CMR 10.09(1)(b)): which fixtures must connect (pot and scullery sinks, floor drains and floor sinks, dishwashers regardless of temperature, pre-rinse sinks, soup kettles, wok stations, hood washers); which must not (commercial food waste disposers; fixtures conveying human waste); the residential exemption; the 25 ft limit to the most remote fixture; capacity of 2 lb per gpm (double the gpm rating above 100 gpm); flow control devices and their venting; the vent downstream of every interceptor; the ban on water-cooled interceptors and on emulsifying chemicals; and maintenance by the owner.
What people get wrong
- Hot water and dish soap flush grease safely away. They only move it downstream, where it cools and solidifies. EPA's point is that grease 'solidifies, reduces conveyance capacity, and blocks flow.'
- An interceptor is a second fixture trap, so it's prohibited double trapping. In 1940 BMS 66 allowed an interceptor in addition to a fixture trap if a vent was between them. MA today says interceptors 'shall not be considered traps unless they meet' the 2-to-4-inch seal rule (10.09(1)(b)1.a), and 'a vent shall be installed downstream of all interceptors' (10.09(1)(b)2.b).
Try it: 3 real practice questions
Q1. A grease interceptor (grease trap) is required for:
Fats, oils and grease flow as liquids when hot but congeal as they cool in the building drain and public sewer, forming blockages. A grease interceptor slows and cools the flow so grease separates and is held back before it leaves the building; commercial kitchens produce that load continuously, while laundries need lint interceptors, not grease interceptors.
248 CMR 10.09Q2. A grease interceptor is rated at 25 GPM flow-through. What minimum grease retention capacity does 248 CMR require?
Interceptors must hold not less than 2 pounds of grease for each GPM of flow: 25 GPM x 2 lb/GPM = 50 lb. Table 2 in 10.09 lists the same 25 GPM = 50 lb pairing.
248 CMR 10.09(1)(b)5Q3. An inside grease interceptor must be located not more than how far from the most remote fixture it serves?
248 CMR requires interceptors within 25 feet of the most remote fixture served, so grease does not congeal in a long branch before reaching it. If an exterior interceptor is within 25 feet of the most remote fixture, no inside interceptor is needed.
248 CMR 10.09(1)(b)2.fThese come from the PressureTest bank. Start 7 days free for the full bank, timed exam simulations and drawn isometric problems.