How often to pump a septic tank in Michigan: the table, the agencies and your county

Four sources, four answers

Four sources give four different pumping schedules for a Michigan septic tank, and none of them is a rule. EPA recommends inspecting a tank every one to three years and pumping it every three to five years, with the timing set by tank size, household size, water habits and the solids that have built up. EGLE’s yearly checklist for owners says to have the tank pumped if it is more than one-third full of scum and sludge. Michigan’s loan program, which sets the standard for work it funds, says to pump when sludge and scum exceed 30 percent of tank volume or reach the baffles. Oregon State University Extension publishes a table that converts tank size and household size into years.

None of the four is a Michigan statute, and nothing we can cite shows a state mandate for any interval. The counties give advice, not requirements. In the places we can cite, pumping becomes mandatory only inside a sale: Shiawassee County’s ordinance requires servicing as part of the evaluation if the tank has not been pumped within two years, and the Wayne, Macomb and Saginaw evaluations include pumping.

The Oregon State table, and what it assumes

Oregon State University Extension, in publication EC 1343 (“Septic tank maintenance”, revised March 2022, reviewed 2025), gives estimated pumping frequencies in years for year-round residences. The figures assume at least 24 hours of wastewater retention and 50 percent digestion of the solids that stay in the tank. It is an Oregon publication and not Michigan guidance.

Tank (gal)	1	2	3	4	5	6	7	8	9	10
500	5.8	2.6	1.5	1.0	0.7	0.4	0.3	0.2	0.1	n/a
750	9.1	4.2	2.6	1.8	1.3	1.0	0.7	0.6	0.4	0.3
1,000	12.4	5.9	3.7	2.6	2.0	1.5	1.2	1.0	0.8	0.7
1,250	15.6	7.5	4.8	3.4	2.6	2.0	1.7	1.4	1.2	1.0
1,500	18.9	9.1	5.9	4.2	3.3	2.6	2.1	1.8	1.5	1.3
1,750	22.1	10.7	6.9	5.0	3.9	3.1	2.6	2.2	1.9	1.6
2,000	25.4	12.4	8.0	5.9	4.5	3.7	3.1	2.6	2.2	2.0
2,250	28.6	14.0	9.1	6.7	5.2	4.2	3.5	3.0	2.6	2.3

Source: Oregon State University Extension Service, EC 1343, Table 1 (columns are household size in people). We left out the 2,500-gallon row because its figures for five and six people do not follow the pattern of the other rows. The pumping interval calculator reads this table for you.

Reading the table for a Michigan house

Take a 1,000-gallon tank, which is the size the Michigan Criteria set as the minimum for new systems. The table gives 5.9 years for two people, 3.7 for three, 2.6 for four, 2.0 for five and 1.5 for six. A family of five on a 1,000-gallon tank would therefore pump about every two years, sooner than the three to five years EPA describes. A single occupant on the same tank gets 12.4 years from the table, and that is where the table and the agencies part company: Oregon State’s own summary says that for a properly designed system the tank should be inspected and pumped every one to five years, and EPA still asks for an inspection every one to three years. Treat a long table figure as the end of the range and not as a reason to skip the inspection.

A tank on an older system may be smaller than that, and the table shows what that costs you: a 750-gallon tank with four people comes out at 1.8 years. Counties size tanks under their own codes, so ask the county health department for the permit and as-built drawing, which EPA says the local permitting authority usually holds, to learn what is actually buried in your yard.

What shortens the interval

The table carries one general note: tanks need more frequent pumping if residents use a garbage disposal. EGLE agrees, saying a disposal increases the sludge and scum and how often a tank may need pumping. Neither gives a number. With a disposal, treat the table figure as a ceiling and measure.

Water softeners are a different problem. The table makes no adjustment for them, and EGLE says residential septic systems in Michigan are not designed for softener backwash, that most local sanitary codes prohibit connecting such discharges, and that a reverse osmosis unit can discharge four gallons per gallon it filters. The fix for a softener is to route the backwash elsewhere, not to pump more often. Additives are no shortcut either: EGLE does not recommend bacteria, enzymes or chemical products, because a tank already contains the microorganisms it needs.

Michigan county advice

Counties that say anything about frequency give advice, and the advice varies. Monroe County recommends pumping by a licensed pumper every one to two years, though its code says only to pump at such intervals as necessary. St. Clair County’s septic page gives two figures, every three to five years in one place and every three years depending on the number of residents in another. Barry-Eaton District Health Department tells Eaton County homeowners that pumping every two to three years is typical. These are recommendations, and the spread shows how little the counties agree.

What the pumping should measure

The interval is a guess until someone measures the tank. EPA gives the working test: pump when the bottom of the scum layer is within 6 inches of the bottom of the outlet, when the top of the sludge layer is within 12 inches of the outlet, or when sludge and scum are more than 25 percent of the liquid depth. EGLE’s one-third and the loan program’s 30 percent are looser versions of the same idea. Ask the pumper to record the depth when the tank is opened, and use the reading to set the next date. The receipt guide lists what else belongs on the paperwork.

Who may pump

Michigan licenses pumping as a business. Part 117 of the environmental code requires an EGLE septage waste servicing license and a vehicle license for anyone who cleans out, hauls or disposes of septage, and the responsible agent for the business needs 30 hours of continuing education to renew. Ask for the license number before the truck backs in. The septic pumping page covers licensing and what the published sources say about price.