Short definition
A garbage disposal — called a disposer in trade and food waste grinder in code — is an under-sink electric grinder that pulverizes soft food waste into a slurry small enough to flush through the kitchen drain. In Washington, disposers are legal on septic systems but actively discouraged by every county health department: they push more solids into the tank and shorten pump-out intervals.
What it is
A disposer mounts to the kitchen sink in place of a basket strainer. A 3-bolt sink flange (mounting ring, split snap ring, sink sleeve, fiber gasket) clamps the unit to the bowl from below; the motor and grinding chamber hang under the sink. The discharge tube exits the side and slip-joints into a continuous waste tee or directly into the trap.
Two operating types exist. Continuous-feed disposers turn on with a wall switch or counter-mounted air switch and run while you push food in. Batch-feed disposers only run when the lid is twisted into place — slower, but safer in a household with kids. Most US residential units are continuous-feed.
Residential motor sizes range from ⅓ to 1 horsepower; ¾ hp is the common balance of grind power and energy use. A side knock-out port on the body accepts a dishwasher discharge hose, so the dishwasher can drain through the disposer instead of a separate tailpiece nipple.
A standard kitchen drain stays at 1½-inch with or without a disposer under the Uniform Plumbing Code that Washington adopts.
Why it matters to a homeowner
Three practical concerns drive most homeowner decisions about disposers.
Septic load. If your home is on a septic system — common in rural Snohomish, Mason, Jefferson, Kitsap, and east-of-the-Cascades counties — a disposer increases the organic solids load on the tank by roughly 30 to 50 percent and shortens pump-out intervals by about 30 percent. Some private community sewer districts also prohibit them outright in their connection agreements.
Electrical. Most disposers want a dedicated 15-amp circuit. Older WA homes often share a kitchen branch with the dishwasher and other counter outlets, and adding a disposer means an electrician installs a new circuit before the install — typically $200 to $400 on top of the disposer itself.
Failure modes. The disposer is the most-Googled kitchen plumbing fixture for a reason. Humming, no-power, and bottom-seal leaks cover most calls. Two of those are DIY in five minutes if you know about the reset button on the underside and the ¼-inch hex slot for clearing jams.
Common failure modes
- Humming with no spin (jam): the motor is energized but the rotor is blocked. Power off at the wall switch or breaker first. Insert a ¼-inch Allen wrench into the bottom hex slot and rotate back-and-forth until the rotor turns freely. Remove the obstruction from above with tongs — never your hand. Press the red reset button on the underside, restore power, run cold water, and test.
- Dead unit (no sound): thermal overload tripped. Press the red reset button. If it won’t latch, wait 10 minutes for the motor to cool and try again. If it still won’t latch, the motor is shorted — replace.
- Leak from the bottom of the unit: internal seal failure. Disposers aren’t field-serviceable at this depth — replace the whole unit.
- Leak at the sink flange: putty failure or loose mounting ring. Drop the unit, reseat the flange with fresh putty, retorque the snap ring.
- Leak at the dishwasher port: the factory knock-out plug was never removed before the dishwasher hose was attached, or the hose clamp is loose. Pop the plug out from inside the disposer with a screwdriver, reclamp.
- Slow drain through the disposer: trap clog or grease in the trap arm. Clear from below. Never run chemical drain cleaner through a disposer — it corrodes the impeller and the seals.
Common variants and what a disposer is not
- Continuous-feed vs. batch-feed. Continuous = wall or air switch, run-and-pour. Batch = drop in food, twist lid to start; safer but slower.
- Standard vs. “septic-assist” disposer. Septic-assist models (InSinkErator Septic Assist, for example) inject an enzyme each cycle. The marketing implies septic compatibility, but the additional solids load is the same — the enzyme doesn’t reverse it.
- Disposer vs. dishwasher tailpiece. Without a disposer, a dishwasher hose connects to a special tailpiece with a side nipple beneath the basket strainer. The disposer’s body provides that connection point instead.
Washington note
Disposers are not categorically banned in Washington under WAC 246-272A, the state’s on-site sewage rule. But every county health department reviewed treats them as a known stress on septic systems and recommends against them. The practical guidance from Snohomish County Health and King County Public Health is consistent:
- Solids load increases roughly 30–50 percent with regular disposer use, and pump-out intervals shorten by about 30 percent.
- Size up the tank if you keep the disposer. The county sizing guidance is one tier above the bedroom-based minimum: a 3-bedroom home goes to 1,250 gallons instead of 1,000; a 4-bedroom goes to 1,500 instead of 1,250.
- Some community sewer districts prohibit disposers in their service agreements because their package treatment plants can’t absorb the extra load. Check before installing if you’re on a private community system rather than a municipal main.
- An “upcoming WAC revision” has been flagged by Snohomish Health that may add or strengthen food-waste-grinder language. Verify with your county before a new install on septic.
City sewer systems (Seattle, Tacoma, Bellevue, Spokane) accept disposers without restriction. The Washington Department of Health publishes the on-site sewage rules that the counties enforce; if you’re on septic, ask your local health department before adding or replacing a disposer.