What not to put in a Michigan septic system: EGLE's list and what counties add

EGLE’s list, in its own order

Ask Michigan’s environmental regulator what to keep out of a septic system and the list is mostly about water and grease. EGLE’s septic FAQ tells owners to keep sump pumps, water softener discharges and gutter downspouts away from the system, to avoid putting excessive cooking grease or waste oil down the drain, to skip tank additives, and to understand that a garbage disposal raises how often the tank needs pumping. It also tells owners to fix leaking faucets and toilets. The rest of this guide takes those in turn and adds what county codes say, because a county code can be stricter than EGLE’s advice.

Water that does not belong in the system

EGLE says residential septic systems in Michigan are not designed for water softener backwash, and that most local sanitary codes prohibit connecting such discharges. A reverse osmosis unit is a related problem: EGLE says it can discharge four gallons for every gallon it filters, and that any such discharge should be located 50 feet from your well. A septic system is sized for the wastewater of the household, and an extra flow of backwash or reject water adds volume that was never in the design.

County codes spell the rule out. Genesee County says footing drains, roof drains, storm water and softener waste must not connect to a septic system, and notes that a permit gives no assurance of performance. Barry-Eaton District Health Department says tile that carries water to a drain, lake, stream or field tile is not a sewage system. Sump pump water is on EGLE’s list too. If a sump pump in your basement has been connected to the tank, the drain or the field, ask a septic company and the county health department how to reroute it.

Grease, food and the garbage disposal

Excessive cooking grease and waste oil should stay out of the drain, EGLE says, and grease does not break down in a septic tank. On disposals, EGLE says an in-sink garbage disposal increases the sludge and scum and how often the tank may need pumping. Oregon State University Extension, whose pumping table is on our interval guide, goes further and says garbage disposals are not recommended because they add a solids load, and its table notes that tanks need more frequent pumping when residents use one. Neither source gives a number for the effect. If you have a disposal, plan for a shorter interval and ask the pumper to measure the sludge.

Restaurants face a separate rule. Livingston County’s sanitary code requires food-service septic tanks and grease traps to be pumped annually, and Michigan treats grease trap waste as its own category of septage, as the grease trap page explains.

Additives, enzymes and starters

Additives get no support from EGLE, which does not recommend any kind, bacterial, enzyme or chemical, and says the microorganisms a tank needs already exist from normal use. Oregon State agrees that biological and chemical additives are not needed to speed settling or decomposition. If a product label promises to clean the tank, remember that what removes sludge is pumping. The same Oregon State publication says up to half of the solids that remain in a tank decompose naturally, and the rest accumulates until a pumper takes it out.

What goes on top of the system

Vehicles do not belong on any part of the system, EGLE says, and neither do heavy machinery or dumpsters. It says to keep trees such as walnut, maple, cottonwood, aspen and willow away from the system, along with large shrubs, because root infiltration is one of the leading causes of failure in Michigan. It also says to direct gutter downspouts away. Before a driveway, a shed, a patio or a swimming pool goes in, ask the county health department where the drain field and its reserve area lie. Livingston County requires a site review for additions, Jackson a change of use review for a new bedroom, and Monroe says additions, pools and outbuildings may need approval.

Wipes and what the label rule leaves open

We found no EGLE list of household chemicals, medications, cleaners or bleach to keep out of a septic system, and no EGLE statement on wipes, so we do not publish one. What Michigan did do is regulate labels. According to the Legislature’s bill listing, House Bill 4596 of 2023 became Public Act 43 of 2024, which sets labeling standards for disposable, non-flushable wipes. The signing date is not in the sources we cite, and a label rule is not a statement that any wipe is safe for a tank. If you are unsure, put it in the trash.

How the wrong inputs show up at pumping

The damage tends to appear in the same two places. EPA says solids that leave the tank clog the drain field, and a grease-heavy or disposal-heavy household sends more solids to the tank. The pumper sees it as a thick scum layer or a sludge depth that reaches EPA’s marks: scum within 6 inches of the bottom of the outlet, sludge within 12 inches of it, or more than 25 percent of the liquid depth. The second place is the field, where EGLE says wet spots or bright green grass over it are warning signs. If your receipt shows deep sludge a year after the last visit, check the inputs in this guide before you blame the tank.

Part-time use and rentals

Oregon State’s pumping table is written for year-round residences. The Michigan sources we can cite do not adjust an interval for a cabin, a rental or a weekend house, so the table figure is only a starting point for those properties. If guests come in numbers, a softener or a laundry room is running, or a rental changes hands often, have the sludge measured at the first pumping and ask the pumper what the numbers mean for your next date.

A short routine

Fix leaking fixtures and route softener backwash and sump discharge elsewhere. Keep the field clear: no vehicles, no new structures, no trees nearby, downspouts pointed away. Pump by the measurement, not by the calendar, and keep every receipt. For signs that something has gone wrong, read the failure signs guide.