Why doesn't pumping always stop a backup?
The EPA describes a conventional septic system as a tank and a drainfield. All water leaves the house through one main drainage pipe into the tank, solids settle and scum floats, and the liquid flows out into the drainfield, where the soil accepts and treats it. Pumping deals with only one part of that chain: the solids stored in the tank.
A tank is also never meant to sit empty. Washington Sea Grant's guide to pumping, from the University of Washington, says the normal liquid level in a septic tank sits at the bottom of the outlet pipe. So the useful question is not whether the tank is full of liquid, but whether water can get into the tank from the house and out of the tank into the field. A backup that returns after pumping means one of those two paths is still blocked.
Penn State Extension puts the drain field side plainly: by the time sewage has backed up into a home from a tank that went too long, the soil absorption area is often clogged with a nearly impermeable biomat, and pumping the septic tank will not remove the biomat.
Is the problem between the house and the tank?
If the line from the house to the tank is blocked, an empty tank does not help, because the wastewater never reaches it. One clue is a pumper's report of a normal liquid level in the tank while drains in the house still back up.
The EPA lists items that should never go into a septic system, including non-flushable wipes, paper towels, feminine hygiene products, diapers, dental floss, cat litter, coffee grounds and cooking grease. Any of them can catch in a drain line or at the tank inlet. The EPA also advises against chemical drain openers on a septic system, and Maricopa County warns that corrosive products such as peroxide and sulfuric acid destroy the bacterial action the system relies on.
A clog in the building drain is a plumbing repair, not a septic repair. Our septic repair page explains how the two are told apart.
Could a baffle or the effluent filter be the cause?
Washington Sea Grant explains that baffles direct flow through the tank and help keep sludge and scum from traveling into the drainfield. A two-compartment tank has an inlet, a center and an outlet baffle; a single-compartment tank has no center baffle. The pumper should check that they are intact and working.
Many tanks also have an effluent filter at the outlet. Maricopa County calls it the last protection for the disposal field and recommends cleaning it at least once a year by hosing the solids back into the tank and reinstalling it. Washington Sea Grant says the filter should be pulled and cleaned at the time of pumping. If you are not sure that happened, ask your pumper; a filter left packed with solids is worth ruling out before assuming the field has failed.
What if the drain field is saturated?
The EPA says that if a drainfield is overloaded with too much liquid, it can flood, sending sewage to the ground surface or causing backups in toilets and sinks. When the field cannot take more effluent, the tank level rises again with normal use no matter how recently it was pumped.
Washington Sea Grant notes that after pumping, the pumper will listen or look for water running back into the tank from the drainfield. Backflow can mean the drainfield is malfunctioning, the tank has settled, the outlet grade is wrong, a line between the outlet and the field is broken, or a pump is not working. A high liquid level before pumping can likewise point to a saturated drainfield, leaky seals or a cracked tank.
Too much water is a common way to overload a field. Maricopa County's maintenance recommendations list several:
- Leaking faucets and running toilets. The EPA says a single leaky or running toilet can add as much as 200 gallons of water per day.
- Laundry bunched into one day. The county suggests 1-2 loads a day instead of 12 loads in one day.
- Rain water, roof runoff or flood irrigation ponding over the disposal field. Divert it away from the tank and field.
- Draining a swimming pool into the system or over the field, which the county says can drown the drain field.
What if the system has a pump or float?
Some systems, including many alternative systems, use a pump to move effluent from a pump chamber to the field. Washington Sea Grant says a high liquid level in the pump tank might be caused by a tank, float or pump problem, and that floats can become disconnected. On these systems, pumping out the tank only buys time: the level climbs again until the pump, float or control is repaired.
The EPA recommends that alternative systems with electrical float switches, pumps or mechanical components be inspected more often, generally once a year.
What should you do next?
Start with what the pumper saw. Washington Sea Grant says the pumping receipt should record effluent levels, tank condition, baffle condition and anything abnormal, such as water flowing into the tank while all household water is shut off. Those notes often point straight at the cause.
- Cut water use to the minimum until the cause is found.
- Note which drains back up, whether the yard is wet over the field, and whether an alarm is sounding.
- Keep people, pets and vehicles off any wet area, and never open the tank or lean in. Penn State Extension warns that septic tanks contain gases that can kill.
- Ask for an inspection. The EPA says an inspection can include checking the inlet and outlet pipes, the effluent filter, pumps and controls, and possibly a camera look at pipes for blockages or collapsed piping.
- If the cause turns out to be a failed tank or disposal field, Maricopa County requires an Alteration permit to replace it.
Common questions
Is it normal for the tank to look full again a few days after pumping?
A tank full of liquid up to the outlet pipe is normal; Washington Sea Grant says that is where the liquid level belongs. A level above the outlet, or drains backing up, is not normal and points to a blockage, the field or a pump.
Can I pump the drain field the way I pumped the tank?
Penn State Extension says pumping the septic tank does not remove a drain field biomat. Its fix involves pumping the ponded wastewater from the absorption area itself and letting it aerate, which is a job for a septic professional.
Will a septic additive clear the backup?
No. The EPA does not recommend septic additives, and Maricopa County says not to use a starter because the bacteria already in household wastewater digest the solids naturally.
Should I use a chemical drain opener while I wait?
No. The EPA advises avoiding chemical drain openers on a septic system, and Maricopa County warns that corrosive products such as peroxide and sulfuric acid dissolve material in the tank and let it flow into the disposal field.
Sources
- U.S. EPA: How Septic Systems Work
- U.S. EPA SepticSmart: How to Care for Your Septic System
- U.S. EPA: Frequent Questions on Septic Systems
- Maricopa County Environmental Services: Operation and Maintenance Recommendations for an Onsite Wastewater Treatment Facility
- Maricopa County: Onsite Wastewater (Septic) Systems FAQs
- Penn State Extension: Septic Tank Pumping
- Washington Sea Grant, University of Washington: Pumping Your Septic Tank (EPA-hosted copy)