Septic tank schematic showing scum and sludge layers (C) Carson Dunlop Associates Septic Tank Cleaning / Pumping Objective Data
Use actual inspection & measurements to schedule septic tank pumping & cleaning

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Part II of Septic tank pumpout SNAFUS:

Hw to use objective data (sludge and scum measurements) from septic tank sludge and scum levels + other details to decide when next to have the septic tank pumped out or cleaned.

This article series describes common mistakes and misunderstandings about cleaning or pumping the septic tank. We explain why pumping too infrequently (or never) is a bad idea but we add that pumping more often than necessary is more or less tossing money down the toilet.

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Objective Data: Septic Tank Pumping Frequency Guideline, based on Measurements Rather Than Guessing

Septic Tank Scum Probe - USDA DJF

The table of septic tank sizes and number of household occupants as a good rule of thumb guide to how often a septic tank should be cleaned out well for most people.

But because we wanted to give an objective, measurement-data based alternative to simple use of the septic tank cleanout frequency table above, here we explain how to use the actual scum and sludge thickness to decide that a septic tank needs to be cleaned more often or less often.

Complete details of this approach are found at MEASURE SCUM & SLUDGE.

The "True" Need for Septic Tank Pumping Depends on These Factors

  1. Scum layer thickness: the actual observed accumulated thickness of the floating scum layer in the septic tank - a key factor that determines the septic tank retention time
  2. Sludge layer thickness: the actual observed accumulated thickness of septic tank sludge on the bottom of the septic tank - a key factor that determines the retention time. The USDA offers this simple advice on using the scum or sludge layer to know when a tank really needs pumping:

    • Pump the septic tank when the total depth of scum plus sludge layers equals one-third of the depth of the tank


    • Pump the septic tank when the bottom of the septic tank outlet baffle has less than three inches of clearance from the bottom of the scum layer (this may vary depending on the length of your outlet baffle or tee)


    • Pump the septic tank when the bottom of the outlet baffle is less than 6 inches from the top of the sludge layer found on the septic tank bottom
  3. Capacity of the septic tank - for the same level of septic system usage, a larger tank will need to be pumped less often as it will have a larger net free area and thus a EFFLUENT RETENTION TIME
  4. Volume of wastewater (related to size of household) being placed into the tank daily - daily wastewater flow determines the load on the drain field or soakaway system, and the solids in the waste water affect the rate of accumulation of solids in the tank
  5. Amount of solids in wastewater (e.g. garbage disposals produce more solids) - not all wastewater places the same load on the septic system. Chemicals in waste water can also affect solid accumulation in the septic tank.
  6. Septic tank retention time: the effective septic tank effluent retention time, given the above parameters. Retention time is the time provided for solids to separate from the wastewater and thus to be retained in the septic tank. Inadequate retention time results in a higher level of suspended solids in the septic wastewater being sent to the drainfield or soakaway system. Sending solids to the drainfield shortens its life.

Why You Should Have your Contractor Inspect the Septic Tank Before, During & After Pumping

For a better understanding of the condition of the septic system, when the septic tank is pumped, it should also be inspected by the pumper - in a series of steps

The answers to those more detailed septic tank condition inspection questions (and more listed at SEPTIC TANK INSPECTION PROCEDURE) give important information about the condition of the septic tank as well as the drainfield or soakaway bed, and can suggest repairs that can extend the life and safety of the system.

For the most objective approach to a very accurate septic tank pumping or cleanout frequency guide do this:

Septic tank schematic showing scum and sludge layers (C) Carson Dunlop AssociatesEither immediately if your septic tank is past due for cleaning, or at the next scheduled septic tank cleanout otherwise, ask the septic contractor to actually measure the thickness of the settled sludge and floating scum layer in the septic tank.

To understand just how these measurements are made, see our description of the whole process beginning at MEASURE SCUM & SLUDGE. That article series includes complete details such as:

Illustration courtesy Carson Dunlop Associates

Septic tank monitor

Below at References we also describe an electronic septic tank monitor or grease trap monitor from Worldstone. These devices can track sludge, scum, or grease levels in order to best schedule septic tank pumping or grease trap cleaning. This product is suitable for commercial installations and possibly for some residential septic tank systems.

According to the company, "Data from monitors can help establish appropriate service intervals, and document maintenance for regulatory compliance. Alarm features can help detect abnormal conditions and prevent costly backups."The company also produces an oil tank level monitor.

Thanks to reader Robert Shirley for this tip.

These Factors Increase the Recommended Septic Tank Pumping Frequency



Continue read at SEWAGE LEVELS in SEPTIC TANKS or select a topic from the More Reading links or topic ARTICLE INDEX shown below.


At TANK INSPECTION PROCEDURE we describe how to inspect the septic tank before, during, and after tank cleaning operations.

Suggested citation for this web page

SEPTIC TANK PUMPING MISTAKES at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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