Septic tank scum & sludge layers:

This article explains the necessity of measuring the thickness of the floating scum layer in a septic tank and of measuring the thickness of the settled scum layer in a septic tank as well. We include descriptions of how to make septic sludge and scum measuring tools and how to use them.

Whether your system is a conventional septic tank and drain field, an above ground septic system, or even a holding tank, pumping and inspecting the septic tank is a critical septic tank maintenance and septic system maintenance chore.

Septic tank pumping frequency guidance is provided in a table at our website (see links listed at the ARTICLE INDEX the bottom of this article .)

When the septic tank is pumped, measurements of the scum layer and sludge layer tell you the condition of the system.

How and Why to Measure Septic Tank Scum & Sludge Thickness

Septic tank sludge & scum thickness measurements, made or at least roughly estimated by the septic pumping contractor at the time that a septic tank is pumped, provide key information that permits the home owner to know

1) whether or not the septic tank is being pumped often enough, and

2) whether or not there is evidence that the septic fields have been damaged by having not pumped the tank soon enough.

Article contents

• MEASURE SEPTIC TANK SCUM & SLUDGE - CONTENTS: When to Pump the Septic Tank - based on scum & sludge layer thickness - how and why to measure septic tank scum & sludge levels
• FREQUENCY of TANK PUMPING DEPENDS on scum & sludge layer thicknesses
• FLOATING SCUMLAYER DEFINED at what floating scum layer thickness does the septic tank need pumping?
• SETTLED SLUDGE LAYER DEFINED at what bottom sludge layer thickness does the septic tank need pumping?
• OTHER MEASURES SCUM / SLUDGE
• PUMP AT WHAT SCUM SLUDGE THICKNESS
• NET FREE AREA - Septic Working Volume

At SEPTIC TANK PUMPING SCHEDULE we discuss three septic tank pumping frequency mistakes that can be avoided by either using a septic tank cleaning table based on actual wastewater production (number of occupants etc) or by making objective measurements which we discuss in this article. First the mistakes:

1. Failure to pump the septic tank frequently enough: leading to an early drainfield failure and costly repairs
2. Pumping or cleaning the septic tank too frequently, wasting money (though you're wasting a lot less money than the cost of a new drainfield.
3. Pumping the septic tank with the fantasy that doing so will "fix" a clogged or failed drainfield. All you really gain is a few days of toilet flushing before the tank has re-filled.

Actually inspecting the septic system, diagnosing any problems or failures, and inspecting conditions inside the septic tank will tell us whether the tank is being pumped at the correct frequency.

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
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
   
   an 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.

What is The floating scum layer in a septic tank

The floating scum layer in a septic tank includes oil and grease which, if pushed into the leach field, will clog that component of the septic system.

Oil and grease are particularly harmful to the aerobic portion of septic effluent treatment in the soil absorption system.

Therefore we want to clean the septic tank if the scum layer has thickened to the point of risking pushing grease and oil out of the tank.

The septic tank needs to be pumped when the floating scum layer has accumulated to reach 3 inches of the bottom of the outlet baffle or tee.

Septic tank schematic courtesy of Carson Dunlop Associates, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

See SEPTIC TANK LEVELS of SEWAGE for details on normal and abnormal levels and what they mean.

What is the settled sludge layer in a septic tank

The sludge layer on the bottom of a septic tank includes various solids which are not dissolved in the septic effluent and which are dense enough to fall to the bottom of the tank.

The septic tank bottom sludge is comprised of "settleable solids" and that portion of "suspended solids" which will, given enough time, also settle out. These accumulate at the bottom of the septic tank until they are removed by a septic tank cleanout procedure.

Normally a septic tank should be pumped when the bottom layer of sludge is within 18 inches of the tank outlet.

How thick can the septic tank sludge and scum layer be before septic tank cleaning is needed?

Thickness of the Floating Scum Layer & Bottom Sludge Layer in the Septic Tank Tell When to Pump Out the Septic Tank

This discussion explains how to decide when to pump & clean the septic tank based on an actual inspection of the septic tank sludge & scum layer thicknesses.

An alternative that you can use if your septic tank is difficult to access or you don't have the ability to make these measurements (or to ask your septic contractor to make them) is to use the septic tank pumping schedule

at SEPTIC TANK PUMPING SCHEDULE.

That schedule considers the septic tank size and level of usage.

The following example was provided to us by Effluent Services, Ltd., an experienced septic service company in New Zealand. In New Zealand septic tank sizes range from 2000-6000 liters (500 gallons to 1,500 gallons U.S.) with the average being 3100 liters (about 800 U.S. gallons and below the minimum septic tank size permitted in most U.S. jurisdictions).

"Generally at a two year interval for septic tank pumping service the average septic tank in these size ranges will have a 400 mm scum layer with about a 200 mm sludge layer.  With an average depth of 1600 mm, the solids content is about 600 mm thereby reducing the settling time by nearly 40%.   

Therefore one may be right to suggest that the increase in total suspended solids exiting the tank will rise exponentially.

This may be a simplistic approach to calculating the frequencies between cleanings, however the evidence of it’s success is in the reductions of replacement leach fields."

This is very sound reasoned. Homeowners should use the scum and sludge layers found when their septic tanks are pumped as a way to determine whether or not the tank is being pumped often enough.

In areas where most septic tanks are on the comparatively "small' side, a two year regular pumping schedule is reasonable for light-usage. In our experience, a 500-gallon septic tank in a home with a family of four is going to need very frequent pumping if the drainfield, leach beds, or soakaway system is to be protected.

US Department of Agriculture Guidance on What Scum or Sludge Levels Require Septic Tank Pumping

Similar rules of thumb offered by USDA on how much sludge or scum mean the septic tank needs cleaning include:

• Tank-Depth One-Third-Rule: Pump the septic tank when the total depth of scum plus sludge layers equals one-third of the depth of the tank

or

• Outlet Baffle Three Inch Rule: 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)

or

• Outlet Baffle Six Inch Rule: 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

Septic tank effluent net free area or effective septic tank working volume

Don't wait too long before removing septic scum and sludge People who wait until their septic system stops working due to a clogged or over-filled (with sludge and scum) septic tank have waited too long. As the bottom sludge layer increases in thickness, and as the top septic scum layer increases as well, the remaining "net free area" or "effective septic tank volume" of effluent in the tank is reduced in volume.

When a septic tank is operating with a low volume of "net free area" of septic effluent, the system lacks adequate volume to provide adequate settlement time (EFFLUENT RETENTION TIME) - time needed for sludge to settle to the tank bottom and scum to coagulate at the tank top.

In this circumstance, although drains in the building seem to be just fine, the septic tank effluent is remaining in a constant state of stirred-agitation. In turn, that means that the system is pushing floating debris into the leach field or other absorption system.

Simply put and quoting the USDA,

The smaller the [net free area] space, the greater amount of solids that leave the tank via the outlet. That is why septic tanks should be pumped out long before the sludge and scum layers build up to the point where they completely fill the [septic] tank and block the flow of wastewater.

... [ in contrast] The unpreventable failure of the soil-absorption system eventually occurs when growth of the organic material in the wastewater [the biomat (SEPTIC BIOMATS) that forms under and along the sides of a drainfield trench] becomes so large [thick] that they plug up the soil.

Watch out: Pushing grease, scum, and small solid debris out of the septic tank and into the leach field reduces the future life of that expensive component of a septic system.

In fact if a septic tank has become blocked or even nearly-blocked by solids, the system has already had a history of pushing solids into the drainfield and even if the system appears to still be working properly, the future life of the drainfield has been substantially reduced.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-04-25 - by (mod) -

Jim

Your second question was helpful too - thanks.

It's best to direct surface runoff away from the drainfield since saturating the drainfield means that during that period it's not likely to work - it's not sanitary.

If you are seeing wet areas only at the end of leach lines and the soil is otherwise uniform in slope then I suspect the lines are failing.

On 2020-04-25 - by (mod) -

Thanks for the helpful question, Jim.

In my opinion, and based on the authoritative articles I've reviewed, pumping a septic tank every 5 years is arbitrary and in a sense "capricious".

I'm sorry to not be more charitable but the right answer to "how often should I pump my septic tank" is .... it depends -

- on the septic tank size

- on the septic tank usage expressed either in daily wastewater flow volume or in number of building occupants

So a single person living in a property with a modern 1,200 gallon septic tank may not need to pump the tank on more than a fifteen-year interval

while

A family of four living in the same home with the same septic tank need to be pumping it every 3-4 years.

Waiting too long to pump the tank destroys the drainfield. Pumping more-often than is really needed won't hurt the tank, it helps keep the septic pumping companies in business, but that's about it.

Please see details at SEPTIC TANK PUMPING SCHEDULE https://inspectapedia.com/septic/Septic_Tank_Pumping_Schedule.php

On 2020-04-25 by Jim

I have a lot of water draining over my septic field when it rains and I notice that at the ends of the lines sometimes are leaching, is that normal. Is it okay to place extra dirt over the lines where you see depressions in the ground along the line. I think that is where the water goes and runs to the end of the line.

The county requires you to pump the tank every 5 years and send them the a copy of the ticket. Is that soon enough.

On 2019-12-24 by Jason

The septic company I use comes out to do quarterly checks of sludge. The last 3 checks for sludge levels were 8 then three months later it was a 5 and now 3 months longer it is a 10.

Can these levels of sludge go up and down like this or is my septic company giving me false reports. How can sludge be at a 8 and then drop down to 5 3 months later?

On 2019-04-22 - by (mod) -

Hoyle

Well yes - maybe.

Check that the baffle or tee at the septic tank outlet end is installed an intact. IF it is in place then yes you're fine.

But if the tee or baffle is missing we may be sending floating grease and oil out into the drainfield and clogging it up.

If that's the case let me know and I'll recommend our repair article on septic tees and baffles.

On 2019-04-22 by hoylep

Is it ok to have 0 scum but have 12" of sludge in the septic?

On 2018-04-16 by Glen

I put a new 750 Gallon (1st chamber) and 350 Gallon 2nd chamber about 2 yaers ago. I thought I would check the Scumb and slude levels today. I have about 3" of sludge on the bottom but no scumb level on top. I's that good? Thank You Glen

On 2017-10-18 - by (mod) -

Thanks for asking, Annika.

The floating scum layer and settled sludge layer accumulate in the septic tank until the tank is pumped / emptied by the septic pumping contractor. In turn, the septic pumping company then hauls the septage to an approved disposal site, most-often to a waste treatment plant. There the pumper pays a fee to dispose of the septage. That disposal fee is a substantial portion of what the septic pumper charges you for pumping your septic tank, and it explains why septic tank pumping doesn't make rich people out of septic pumping truck drivers.

On 2017-09-08 by Annika Farson

You never said where the sludge and scum goes.

Comment:

(Jan 27, 2015) Jane Fairfax said:
This was very informative, especially since I knew next to nothing about septic systems before. I like learning about these kind of topics, though, since doing so helps me take better care of my home. After reading this, it would appear we need to start looking into sludge pumps.

Question: 3" of scum on top of the inlet side of the septic tank?

(Mar 29, 2015) Steve D said:
What does it mean when when the inlet side of the septic tank has appx 3" of scum on top, but the outlet side of the tank has no scum, and the effluent is not draining to the leach field? Should I try adding emergency enzymes?

Reply:

Steve there is no independent, unbiased expert source that recommends septic tank additives and they are prohibited in some jurisdictions.

It sounds as if a normal pillow of scum is sitting inside and floating up inside the inlet baffle - something that is removed when you have the tank pumped. As long as inflow to the tank is not being blocked you're ok. If the inlet to the tank is the blockage problem then of course you want that pillow of crud removed - pump the tank.

If however water is flowing into the tank but not out into the drainfield the problem is elsewhere: a blocked line, D-box, or a failed field. You need to have the outlet end of the tank and the D-box inspected for flow.

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Continue reading  at HOW TO MEASURE SEPTIC SCUM LAYER or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• SEPTIC TANK PUMPING PROCEDURE
  • CLEANING SEPTIC TANKS
  • WHEN to CLEAN THE SEPTIC TANK
  • WHEN NOT TO PUMP A SEPTIC TANK
  • FIND the SEPTIC TANK, HOW TO
  • HOW TO OPEN a SEPTIC TANK
  • INSPECT the SEPTIC TANK BEFORE PUMPING
  • SEPTIC TANK INSPECTION PROCEDURE
  • SEPTIC TANK LEVELS of SEWAGE
    • EFFLUENT RETENTION TIME
    • MEASURE SEPTIC TANK SCUM & SLUDGE
    • HOW TO MEASURE SEPTIC SCUM LAYER
    • HOW TO MEASURE SEPTIC SLUDGE LAYER
    • SEPTIC TANK BACK FLOODING
    • TOOLS for MEASURING SCUM & SLUDGE
  • PUMPER TRUCK OPERATION PROCEDURE
  • PUMPING the SEPTIC TANK
  • RECORD SEPTIC TANK LOCATION
  • SEPTIC PUMPING CONCLUSIONS
• SEPTIC TANK SAFETY

Suggested citation for this web page

MEASURE SEPTIC TANK SCUM & SLUDGE at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to SEPTIC SYSTEMS

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