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Septic Tank Pumping Guide: When NOT Pump Out A Septic Tank SepticAPedia ©

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• When not to have the septic tank pumped?
• Septic tank pumping and testing when buying a home
• Septic tank pumping after area flooding

When is it a bad idea to pump out the septic tank? Under some conditions pumping the tank may be unsafe, or it can lead to damage to the septic system. Examples discussed here include pumping after a septic system has been flooded and pumping some systems which can lead to dangerous or even a fatal collapse. For a septic tank pumping table and explanation of when septic tanks (onsite sewage disposal system holding tanks) should indeed be pumped and related information readers should see PUMPING FREQUENCY. © 2008-1978 Daniel Friedman, All Rights Reserved. Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. No warranties are expressed or implied. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical review by industry experts has been performed and is ongoing - reviewers are listed at "References." This document is a chapter of Inspecting, Testing, & Maintaining Residential Septic Systems. Also see Diagnosing Clogged Drains Is it a blocked drain or the septic system? If you can't find your septic tank see How to Find The Septic Tank - tips on locating septic tanks for inspection and septic tank pumping. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

When is Pumping a Septic Tank Not Recommended

When the Septic System is Flooded Don't Pump It Out

If you property has been flooded by rising water such as from a storm, hurricane, or a river overflow, pumping out a septic tank when ground waters are still flooding the area of the septic tank can lead to some unexpected problems:

1. If the septic tank is plastic or fiberglass, and if ground water is still high around the septic tank, the tank may actually float up out of the ground, leading to damaged septic piping and more costly repairs. Wait until floodwaters and ground waters have receded around the septic tank.
2. Even if the septic tank is concrete, if ground water is over the septic tank or even over its inlet piping or drainfield piping, if you pump out the flooded septic tank, floodwaters, including silt and mud may simply flow into the tank, filling it with silt and debris that will need to be removed. Wait until floodwaters have receded and ground water is low enough that mud and silt and floodwater won't simply run back into the tank.

More about how to inspect, pump, and repair your septic system after flooding is at Flood Response what to do after a septic system has been exposed to flooding.

If a Septic Tank or Cesspool is Old, of Unknown or Fragile Construction Don't Pump Without Inspecting

An old, site-built septic system built of dry-laid stone or concrete block, and in particular an old home-made cesspool is at high risk of collapsing when it is pumped, or collapsing shortly thereafter. I assisted in a tragic case on Long Island, NY in which a cesspool was pumped leading to a collapse the next day when the owner walked over the cesspool and it caved in on him. If you don't know what the septic system is made of, be sure that the septic cleaning contractor proceeds with appropriate care.

In an "unknown construction" condition, you might not even know if what's installed is a conventional septic tank and drainfield or a simple cesspool. In such cases the septic contractor should be expected to proceed with caution, perhaps pumping from high in the "tank" only a portion of its contents, just sufficient to inspect (flashlight and mirror on a pole) the tank interior to see how it was constructed and what is its condition.

Before completing any septic pumpout or other septic repair work, be sure you have safe covers over the system(s) and any access openings. More about septic system hazards and safe procedures is at Septic System Safety: Septic System, Septic Tank, & Cesspool Safety Warnings for Septic Inspectors, Septic Pumpers, and Homeowners

If the septic system sludge level is very low and the floating scum layer thickness is minimal

If the septic system sludge level is very low, perhaps just an inch or two in a 5 foot deep septic tank, and the floating scum layer thickness is also minimal, perhaps just an inch or two, the only reason I can think of that you'd pump the tank anyway would be a need to inspect or repair it. It's quite possible to estimate the thickness of the sludge layer and scum layer using probes especially constructed for that purpose.

Measuring the septic tank scum and sludge thickness is not a normal homeowner job, and it can be unsafe (falling into tank, methane exposure, etc.) You should leave this job to a professional. How to measure septic tank sludge thickness and floating scum layer thickness are explained and illustrated at MEASURE SCUM & SLUDGE - show this information to your septic contractor if s/he says "it can't be done".

If you're stuck, the septic contractor has already come to and insists on pumping the septic tank, or is going to charge you the tank pumping fee just to do a "look-see" then go ahead and let the contractor pump out the tank and inspect it for damage.

But be certain to ask about and notice yourself just how much sludge there was on the tank bottom, and just how thick the floating scum layer was at the septic tank top. If these thicknesses were minimal, as I described above, then if the level of occupancy of the building is unchanged in the future, you can probably go a bit longer before the next septic tank pumpout is required.

See my table at Septic Tank Pumping and adjust that data to suit what you've just observed. In other words, if you had five building occupants and a 1000-gallon septic tank, the table told you to pump it after two years. If you did so but there was almost no sludge or scum, you can probably go out three years, and maybe longer depending on what you see after three years.

Do Not Pump the Septic Tank Right Before a Septic Inspection and Test

The photo at left shows red septic dye ponding at the entry to a basement at a home with a septic system in total failure. The dye would never have appeared after our 150 gallon test if the septic tank had been pumped right before we got to the site. Rather the dye would simply have sat in the empty 1000 gallon septic tank, hiding a problem until after the septic tank was re-filled with wastewater. Here is a totally-failed septic system which was not discovered until the morning after the new owners moved into their "new" home. When a house is being sold: often an owner or realtor will arrange for a septic tank to be pumped just before a home inspection or before a septic system inspection or septic loading and dye test. Such a "favor" performed for the buyer is in fact preventing a valid septic inspection and test since an empty tank means the drain field can't be tested. This is a fraudulent act and should not be tolerated. If you are purchasing a home with a septic system it would be smart to inspect and test that system before completing the purchase. Knowing the age, location, type of equipment, and condition of the septic system can reduce (not eliminate) the chances of an expensive surprise (like a septic system that does not work), and it can reduce the chances of a dangerous site condition (like an old cesspool or tank about to collapse).

Often a building owner will, as a "favor" to the buyer, have the septic tank pumped. This is a generous and nice thing to do if the owner agrees to take this step after any onsite septic system testing and inspection have been completed. But otherwise it may be a dirty trick.

The step taken by a building seller of pumping a septic tank right before a septic system inspection and test to be performed before purchase of a home may sound like a nice favor but in fact there is a serious risk that the septic tank cleanout will hide a problem:

• Pumping a septic tank shortly before testing prevents an actual effective test of the septic system since it produces an empty septic tank and thus subverts the system loading portion of a septic loading and dye test (water and dye are run into the septic system to look for evidence of a backup or breakout of effluent on the yard surface).
• Therefore risks hiding an imminent or already existing septic system failure
• Therefore risks presenting the new buyer with a costly surprise that could have been discovered before the purchase

If a building is not in active use, pumping the tank before a septic inspection and loading and dye test will prevent a valid test as the septic tank will remain empty. Pumping the septic tank at a building occupied by 3 or 4 people should not be done within two weeks of a septic inspection and test. This (conservative) advice helps assure that the septic tank is full before the loading and dye test are begun.

If there is a readily accessible and safe septic tank access port to permit a view of the septic tank interior (one that is not buried and not too heavy for the inspector to remove) then opening this cover can permit the inspector to confirm that the septic tank is at a normal level and thus that the test to be performed is reasonable.

More Reading For Buyers of a Home With a Septic System

• Buyer's Guide Home Buyer's Guide to Septic Systems Inspection & Testing - What to Do, Step by Step.
• Cesspools Home Buyer's or Owner's Advice
• Cesspool Age Estimates help evaluate cesspool condition and need for repair or replacement
• Drywells Home Buyer's or Owner's Advice
• Dye Amounts, Water Volume: how much septic dye and how much water to use to perform a septic dye test
• Dye Tests: how to perform a Septic Loading and Dye Test - the complete procedure for septic loading & dye testing, a septic function test
• Failure Causes - Septic Systems for Septic Systems, septic tanks, septic drainfields, cesspools, drywells, distribution piping
• Failure Causes - Drainfields: how to inspect septic leach fields & what causes septic field failure
• Failure Spots - Where septic system trouble is likely to show up regardless of septic dye testing
• Inspecting & Testing Septic Systems: Online Book, Inspection, Test, Diagnosis, Repair, & Maintenance: Our Online Septic Book: Septic Testing, Loading & Dye Tests, Septic Tank Pumping, Clearances, details of onsite waste disposal system inspection, testing, repair procedures.
• Inspecting Onsite Waste Disposal Systems class, Explaining Septic System Operation to Clients; Visual Inspection, Testing, Invasive Inspection , Evaluation, Reporting; Future Trends, Legislation
• Outside Inspection - - Simple Visual Site Inspection Gives Key Information About Septic System Condition
• Septic Tank Inspection Procedure - How to Inspect Septic Tanks and evaluate the septic tank condition, baffles, sludge levels, damage, evidence of septic failure, etc.
• Septic Tank Location - How to Find the Septic Tank, how deep will the cover be, how to document its location
• Spotting Breakouts - Spotting Dyed Septic Effluent Breakout Outside - Where Septic Dye is Likely to Show Up During a Septic Dye

More Information on Septic System Diagnosis, Maintenance, & Repair

• The Septic System Information Website home page for this topic
• Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
• The Home Buyer's Guide to Septic Systems
• Septic Tank and Septic System Maintenance - online articles giving details of many septic system maintenance and repair topics.
• Septic Tank Pumping Guide: When, Why, How to pump the septic tank
• Tank Location - How to Find the Septic Tank, how deep will the cover be, how to document its location
• Drainfield Inspection Procedure Septic Leach Fields - how to inspect and diagnose septic drainfield failures
• Drainfield Layout: septic drainfield or leaching bed shape and placement considerations
• Drainfield Layout: septic drainfield or leaching bed shape and placement considerations
• Drainfield Location: how to find the septic drain field or leaching bed


• Home & Outdoor Living Water Requirements
• Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
• Septic System Design Basics: Choosing Septic Tank Size, Absorption System Size
• How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
• Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
• Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.


Pennsylvania State Fact Sheets relating to domestic wastewater treatment systems include

• Pennsylvania State Wastewater Treatment Fact Sheet SW-161, Septic System Failure: Diagnosis and Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-162, The Soil Media and the Percolation Test
• Pennsylvania State Wastewater Treatment Fact Sheet SW-l64, Mound Systems for Wastewater Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-165, Septic Tank-Soil Absorption Systems
• Document Sources used for this web page include but are not limited to: Agricultural Fact Sheet #SW-161 "Septic Tank Pumping," by Paul D. Robillard and Kelli S. Martin. Penn State College of Agriculture - Cooperative Extension, edited and annotated by Dan Friedman (Thanks: to Bob Mackey for proofreading the original source material.)

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