4.4 Interceptors, Catch Basins & Chemical Waste Treatment (SPS 382.34)
Key Takeaways
Grease interceptors must be maintained on a cycle not exceeding 90 days or per the manufacturer's instructions (SPS 382.34 (3) (e)).
Exterior grease interceptors have 42 to 72 inches liquid depth, at least two compartments, and a minimum capacity of 750 gallons to a municipal sewer or 1,000 gallons to a POWTS.
Exterior interceptors and catch basins must be at least 5 feet from a building, 10 feet from a water service, 2 feet from a lot line and 10 feet from a cistern or lake high-water mark.
Garage catch basins for public buildings need at least a 36-inch inside diameter, 24-inch depth below the outlet trap, a 4-inch outlet with a 6-inch seal, and a 23-inch cover.
Chemical wastes below pH 5.5 or above pH 10.0 must be treated to pH 5.5 to 10.0 in a dilution or neutralizing basin, or held for disposal.
The general rule (SPS 382.34 (3))
Any deleterious waste discharged into a plumbing system must go to a wastewater treatment or holding device that separates, dilutes or neutralizes it. Deleterious waste means waste, other than from dwelling units, that can congeal in drains, interfere with treatment, pass through treatment and pollute, create explosive or toxic mixtures, or damage pipes and structures.
| General requirement | Rule |
|---|---|
| Maintenance | Clean as needed; grease interceptors on a cycle of no more than 90 days or per the manufacturer |
| Reassembly | Replace baffles, weirs, orifice plates, vents and covers after service |
| Access | Never buried or covered; top no more than 6 feet above the surrounding floor; 18 inches of clear space above |
| Exterior location | At least 5 ft from a building or pool, 10 ft from a water service, 2 ft from a lot line, 10 ft from a clearwater cistern and 10 ft from a lake or stream high-water mark |
| Saturated soil | Anchor against flotation |
| Retained material | Not introduced into a drain, sewer or water body without approval |
Grease and oil treatment (SPS 382.34 (5))
Occupancies other than dwelling units that put cooking grease into the drain need grease treatment. New, altered or remodeled systems discharging to a public sewer need one or more grease interceptors.
- With exterior interceptors, all and only kitchen wastes go to the exterior interceptor.
- With interior interceptors, a food waste grinder and the sanitizing or rinse compartments of a sink may bypass. The wash compartment of a scullery sink and a pre-wash compartment that doesn't drain through a disposer must go through the interceptor.
- For a POWTS, interceptors must be sized so effluent meets SPS 383.44 (2). Only kitchen and food wastes go to the exterior interceptor, except in hard-to-separate remodels.
- Grease or fat emulsifiers may not be introduced into an interceptor.
Exterior grease interceptors
| Feature | Requirement |
|---|---|
| Liquid depth | 42 inches minimum, average 72 inches maximum |
| Rectangular tank | At least 36 in. wide and 72 in. long, long side parallel to flow |
| Horizontal cylinder | At least 52 in. diameter and 72 in. long |
| Vertical cylinder | At least 72 in. diameter |
| Compartments | At least two for any new or replacement tank |
| Inlet baffle or tee | Extends to 1/3 of liquid depth |
| Outlet baffle or tee | Extends to 2/3 of liquid depth |
| Baffles above liquid | At least 6 in., with 2 in. clear above |
| Manholes | At least 23 in., at or above grade, locking cover; no wall more than 4 ft from a manhole edge; label "GREASE INTERCEPTOR" at least 4 x 6 in. |
| Minimum capacity | 750 gallons to a municipal sewer; 1,000 gallons to a POWTS |
Capacity is calculated with the SPS 382.34 (5) (c) 2. formulas. For restaurants with seating, the inputs are seats (a drive-in space counts as 3 seats and a drive-up window as 60), serving hours (6 to 12) and an appliance factor (0.75 with no dishwasher or grinder, 1.0 with one, 1.25 with both). For dining halls, hospitals, schools and carryout kitchens, the inputs are meals per day at 3 gallons per meal, serving hours and meal periods. All-paper service may halve the result, but never below the 750 or 1,000-gallon floors. Exterior tanks sit on at least 3 inches of bedding that passes a 3/4-inch sieve.
Interior grease interceptors
- Rated for at least 15 gpm, and not less than the manufacturer's rating.
- Three-fourths of the total holding capacity of all connected fixtures may not exceed the rated flow.
- Grease holding capacity in pounds must be at least double the rated flow in gpm.
- Install flow controls if the manufacturer requires them, accessible and vented if orifice type.
- No water-cooled interceptors, and none where the temperature can fall below 40 degrees F. No more than 12 inches of horizontal inlet pipe may be submerged.
Worked example. A three-compartment pot sink holds 30 gallons per compartment, or 90 gallons. Three-fourths of 90 is 67.5, so the interceptor must be rated for at least 67.5 gpm (a 75 gpm unit, for example). It then needs a grease capacity of at least 2 x 75 = 150 pounds.
Garage floor wastewater (SPS 382.34 (4))
At public buildings, service areas drain through a garage catch basin or oil interceptor. Areas where vehicles are only driven or stored may use a floor drain with a solid-bottom sediment bucket.
| Garage catch basin (public) | Requirement |
|---|---|
| Inside diameter | 36 inches minimum |
| Depth below lowest part of outlet trap | 24 inches minimum |
| Outlet | 4 inches minimum, trapped with a 6-inch seal (external trap within 36 inches) |
| Water line | At least 2 inches below incoming horizontal drains |
| Cover | At least 23 inches |
| Untrapped inlet drains | At least 4 inches, own 6-inch seal, length no more than 24 x diameter |
One- and 2-family garage floor drains need a removable solid-bottom sediment basket. Their catch basins need an 18-inch minimum inside diameter.
Other interceptors
| Device | Key requirements |
|---|---|
| Automatic car wash interceptor | Outlet seal at least 15 inches; bottom of seal 30 inches above the floor of the interceptor; capacity 5 x max gpm above and 15 x max gpm below the seal; 2 or more hand wands need one |
| Commercial laundry interceptor | Basket stopping solids 1/2 inch or larger; in-line unit at least 24 inches, airtight cover, vented (at least 2 inches), 4-inch outlet with 12-inch seal |
| Oil and flammable separator | 3-inch individual vent to at least 12 ft above grade; 3-inch fresh air inlet within 2 ft of the inlet, at least 1 ft above grade and 6 ft below the vent outlet |
| Sand interceptor | Outlet seal at least 12 inches |
| Plaster trap | Installed as the fixture trap, within 36 inches of the sink |
| Bottling, dairy, meat processing | Interceptor required |
Chemical waste (SPS 382.34 (14))
Chemical wastes with a pH below 5.5 or above 10.0 go to a holding tank or through an approved dilution or neutralizing basin that brings them to pH 5.5 to 10.0 before the building sewer. Without enough dilution water, fill the basin to the outlet with marble or limestone chips 1 to 3 inches in diameter. The inlet or outlet is submerged to form at least a 4-inch seal. A basin with a submerged inlet has a sanitary vent on the basin and a chemical waste vent on the inlet pipe; with a submerged outlet, the arrangement is reversed.
What is the maximum maintenance cycle for a grease interceptor under SPS 382.34 (3) (e), unless the manufacturer's instructions say otherwise?
30 days
60 days
180 days
90 days
What minimum liquid capacity applies to an exterior grease interceptor that discharges to a municipal sewer?
500 gallons
750 gallons
1,000 gallons
1,500 gallons
A laboratory discharges waste with a pH of 3.0. What does SPS 382.34 (14) require before it reaches the building sewer?
Neutralizing to pH 5.5 to 10.0 in a basin, or holding it for disposal
Dilution with an equal volume of cold water upstream of the fixture trap
Passage through a grease interceptor sized for at least 15 gpm of flow
No treatment, because the public sewer will dilute the waste adequately
Sections you finish are checked off in the contents.