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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 (seelinks listed at Related Topics .) When the septic tank is pumped, measurements of the scum layer and sludge layer tell you the condition of the system.
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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.
At SEWAGE LEVELS in SEPTIC TANKS we explain how to interpret the meaning of high or low sewage levels in the septic tank as well as thick or thin scum or sludge levels.
Readers of this document should be sure to review our table of septic tank pumping frequency. This article is part of our series: 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.
A lot of septic pumping contractors and some other "experts" give a fixed rule of thumb that serves their own interest, such as "pump your septic tank every year" or "pump your septic tank every two years" without first having actually considered any information about the septic system capacity, level of usage, age, or other conditions. There are two septic tank pumping mistakes: pumping too infrequently, leading to an early drainfield failure and costly repairs, and pumping too frequently, wasting money (though you're wasting a lot less money than the cost of a new drainfield).
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.
WHEN TO PUMP the SEPTIC TANK - 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:
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 - 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,
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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HOW TO MEASURE the SCUM LAYER THICKNESS in the SEPTIC TANK- How to measure the thickness of the floating scum layer in a septic tank
Details of how to measure the septic tank floating scum layer are at HOW TO MEASURE SEPTIC SCUM LAYER. Excerpts are just below.
HOW TO MEASURE the BOTTOM SLUDGE LAYER THICKNESS in the SEPTIC TANK - How to measure the thickness of the settled sludge layer on the bottom of a septic tank
Details of how to measure the septic tank bottom sludge layer thickness are at HOW TO MEASURE SEPTIC SLUDGE LAYER. Excerpts are just below.
Details about how to make and use this septic tank scum and sludge measuring tool are at ELECTRIC MONITOR for SCUM & SLUDGE. Excerpts are below.
Details about how to buy and use this electronic septic tank scum and sludge measuring tool or grease trap monitoring device are at ELECTRIC MONITOR for SCUM & SLUDGE. Excerpts are below.
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