What should you not flush with a septic system?

Only two things belong in a septic-connected toilet: human waste and toilet paper. That is the whole list. Toilet paper is designed to break apart in water so the solids settle in the tank and the rest moves through. Human waste is what the tank's bacteria are built to digest.

Your septic system is a small, living treatment plant buried in your yard. As the EPA explains, "all of the water from your house, except for water from the toilet, flows into your drainfield" only after the tank does its job, so anything that does not break down throws off that balance. The tank separates solids from liquid, and bacteria slowly digest the organic matter. Items that do not digest pile up as sludge, fill the tank faster, and shorten the time between pump-outs.

A good habit is to picture the tank every time you reach for the flush handle. If the item is not body waste or toilet paper, it goes in the trash, not the bowl. This one rule prevents most avoidable septic repairs.

Keep this list out of any toilet tied to a septic system. None of these break down, and several actively harm the tank.

• "Flushable" wipes (the label is misleading; they do not break apart like toilet paper)
• Paper towels and facial tissue
• Feminine hygiene products
• Diapers
• Cat litter, even types marked flushable
• Dental floss
• Cigarette butts
• Coffee grounds
• Pharmaceuticals and unused pills

The biggest repeat offender is the flushable wipe. The name suggests it is safe, but these wipes stay intact in water and tangle into ropes that block pipes and clog the tank. Plumbers find them snagged at every turn in a drain line. Dental floss and cigarette butts are small but do not degrade, so they collect over time. Cat litter turns to a cement-like clump that no bacteria can break apart. Coffee grounds sink and add to the sludge layer that you eventually pay to pump out. Paper towels and diapers are built to stay strong when wet, which is exactly the opposite of what a septic tank needs.

Pharmaceuticals deserve special mention. Flushing pills sends drug compounds into your soil and toward groundwater, and some can disrupt the bacteria doing the work in your tank. Take old medication to a drug take-back site instead. When in doubt, the safe choice is the trash can.

The toilet is not the only entry point. Your kitchen and utility sinks feed the same tank, so what you pour down a drain matters just as much. Keep these out:

• Grease and cooking oil (they congeal, coat the tank, and clog the drainfield)
• Harsh chemicals and bleach in volume
• Paint and paint thinner
• Solvents
• Pesticides and other lawn or pool chemicals
• Chemical drain cleaners

Grease is the quiet killer. Hot oil pours like a liquid, then cools and hardens inside your pipes and tank. Grease is one of the most common causes of sewer and drain blockages nationally, and on a septic system it floats as scum and can escape to the drainfield, where it clogs the soil and pipes that filter your wastewater.

Strong chemicals are the other danger. A septic tank works because it is full of living bacteria. Pouring bleach in volume, drain cleaner, pesticides, or solvents down the drain kills those bacteria, and a tank without bacteria stops treating waste. Paint and paint thinner are just as bad; they coat the tank and carry chemicals into the soil. Normal household cleaning in everyday amounts is fine; the problem is dumping concentrated chemicals or large volumes at once. For occasional drain clogs, a plunger, a snake, or an enzyme-based cleaner is far safer than a caustic product. Save leftover paint, solvents, and pesticides for a household hazardous-waste drop-off instead of the drain.

A city sewer carries your wastewater to a large municipal treatment plant with industrial-scale equipment. A septic system treats everything on your own property, with nothing more than a tank, gravity, soil, and bacteria. As the EPA describes it, a septic system "treats wastewater close to the source," which is why it has far less tolerance for the wrong materials than a public sewer does.

That difference is the whole reason for the stricter rules. On city sewer, a flushed wipe becomes the utility's problem downstream. On septic, it stays in your tank until you pay to pump it out, or it migrates to the drainfield and causes a repair that can cost far more than a pump-out. The bacteria that do the digesting cannot be replaced with a quick additive; University of Arizona Cooperative Extension and other land-grant studies have found that septic additives provide little benefit because the system already grows the bacteria it needs. Protecting the bacteria you have is cheaper and more reliable than trying to add more.

There is also a hard limit on what the tank can hold. Anything that does not break down accumulates as sludge and scum, and overloading the tank pushes solids into the drainfield, where they clog the soil. The University of Arizona Extension recommends pumping a septic tank roughly every 3 to 5 years, and the cleaner you keep what goes in, the longer you can stretch between pumpings.

Good habits keep a septic system healthy for decades. The core move is controlling what enters the system, then easing the load on the tank.

Use your garbage disposal sparingly, or not at all. A disposal grinds food into fine particles that the tank cannot fully digest, so they settle as sludge and you pay to pump more often. Scrape plates into the trash or a compost bin first, and treat the disposal as a backup, not a routine tool.

Spread out water use, too. Doing every load of laundry on one day floods the tank and can push solids into the drainfield before they have settled. Stagger laundry across the week, fix running toilets and dripping faucets quickly, and install water-efficient fixtures to lower the daily volume your system has to treat.

Finally, watch for warning signs so a small issue does not become a failure. The EPA lists slow drains, gurgling plumbing, sewage odors, soggy or standing water over the tank or drainfield, and a strip of unusually green, spongy grass over the drainfield as signals that something is wrong. If you notice these, stop adding water and have the system checked. To learn the maintenance schedule, see how often to pump a septic tank, review the signs your septic system is failing, and check what not to put down the drain for the full kitchen-sink list.