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

  • Beginning before anything is touched (is the tank cover safe, is the tank damaged, leaky, are the baffles lost or damaged, has the tank been flooded),
  • During the septic tank pump-out (subjectively how thick were the scum and sludge layers, was there effluent backflow into the tank during pumpdown)
  • After completion of the septic tank cleanout (is the tank cracked, tipped, damaged, are the inlet and outlet baffles intact)

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

  • Use of a garbage disposer or food waste grinder increases septic tank pumpout frequency - we agree, but not all experts do. See GARBAGE DISPOSAL vs SEPTICS for details.
  • Use of a sewage ejector pump connected to the septic system may increase septic tank pumpout frequency by adding macerated sewage that increases the risk of pushing floating solids into the drainfield. See SEWAGE EJECTOR / GRINDER PUMPS
  • Use of a water softener or water conditioner such as an iron filter whose regeneration cycle water empties into the septic tank may increase the tank pumping frequency, though this is arguable in our opinion. Issues are excessive salt dosage that is going to reduce septic tank bacterial action but worse may seriously damage a drainfield; also the water volume itself can flood a marginal drainfield. See REDUCE IMPACT of SOFTENER on SEPTIC and also our discussion of the effects of SALT OR WATER INTO SEPTIC
  • Frequency of use of laundry facilities; similar questions and arguable facts as with water conditioners. However installing a septic filter to keep fabric filters out of a septic tank or drywell can give significant benefit as can avoiding excessive volumes of powdered detergents. And as with water conditioners, the effect of water volume on the drainfield may be more immediate and serious than the effects of laundry water on the septic tank.
  • The U.Minn. table adjusts the laundry impact on septic tanks and septic tank risk levels downwards if you
    • use a water-conserving top loading washer or a front-loading washing machine [presumably they mean that either unit should be water-conserving]
    • have installed low flow shower heads
    • have installed low water usage toilets
    • repair [water supply system] leaks [such as drippy faucets and running toilets] quickly
    • use mild cleaning products & detergents and limit use of anti-bacterial products - in our opinion arguable as the level of antibacterial effect of at least some popular brand dish soaps has been estimated as trivial; I would have instead increased the risk level for constant or frequent use of antibiotics by occupants of the building, as nursing homes, for example suffer septic tank bacterial action reduction from that cause
  • An in-home business that increases water usage (daycare, taxidermy, hair salon) increases septic tank damage risk - same arguments as we suggested above should lead one to be concerned about the drainfield when added waste water volume rather than added solids are present; some home businesses (photography or taxidermy) include use of chemicals that should not be flushed into a septic system. See TOILETS, DON'T FLUSH LIST.
  • Having 3 or more overnight guests at a time or large groups visiting the home add septic tank risk; the table provided at SEPTIC TANK PUMPING SCHEDULE does a poor job of distinguishing between 3 overnight guests once a year and regular extra sleepover guests; similarly, occupancies that place more than the usually-assumed 2 occupants per bedroom should cause a septic tank pumpout frequency increase
  • Septic tank size vs number of bedrooms: here we are in full agreement about the impact of high number of bedrooms and small septic tank sizes, but adding risk points for "don't know" is a bit vague.
  • Time since last time the septic tank was cleaned affects the risk level - we agree completely. The worksheet adds one risk point if it's been 3-5 years since last septic tank cleanout and 2 risk points if it's been more than 5 years. In our OPINION this is a ridiculously weak weight placed on this critical factor.

    We regularly hear from people who have lived in a home for fifteen or twenty years and don't recall ever pumping out their septic tank, nor where the tank is located, nor what size it is. Those are likely to be much higher risk situations than a 1500-gallon tank at a home with two people that was last pumped out six years ago.



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


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

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

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