Septic tank floating scum layer thickness measurement:

This document explains the tool and the procedure for measuring the thickness of the floating scum layer in a septic tank as an aid in determining septic tank condition and whether or not the septic tank needs to be pumped and cleaned. We include descriptions of how to make septic sludge and scum measuring tools.

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

How to Measure the Scum layer thickness in a septic tank: How to measure the thickness of the floating scum layer in a septic tank

When the septic tank is pumped, measurements of the scum layer and sludge layer tell you the condition of the system. This procedure is modeled after the procedure described in classes for obtaining a Massachusetts Title 5 Septic Inspectors License.

However other states and authorities describe a similar procedure. Some septic contractors have built and bring to the pumpout job their versions of the equipment described below.

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 grease and oil being pushed out of the tank.

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

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

The septic tank scum layer probe shown above (courtesy USDA and US FPS) is also illustrated in a photograph just below.

Page top septic tank sketch courtesy of Carson Dunlop Associates, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

Septic tank top or floating scum layer thickness measurements are discussed on this page.

Separately, septic tank bottom sludge layer thickness measurements, discussed separately

at HOW TO MEASURE SEPTIC SLUDGE LAYER, 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
2. Whether or not there is evidence that the septic fields have been damaged by having not pumped the tank soon enough

Watch Out: Safety concerns when measuring septic tank scum and sludge levels: The procedure described below involves opening a septic tank and probing to determine the thickness of settled sludge in a septic tank and to measure the thickness of the floating scum layer in the tank.

This procedure is dangerous, involving possible exposure to methane gas and possible risk of falling into a septic tank. The procedure should be performed by a qualified septic contractor. No one should open, inspect, or examine a septic tank while working alone.

Sanitary concerns when measuring septic tank scum and sludge levels:

Wear gloves when handling the septic probe and watch where you're swinging the pole around so as not to splash septage on your assistant (never work alone on septic systems). The equipment should be washed after use if it is to be retained for future use. Some septic contractors carry their poles in a long plastic-lined box which goes back on the truck.

How to Make the septic tank probe for measuring scum & sludge layer thickness

The septic tank scum and sludge measurement tool shown here is one used by a septic contractor to probe the thickness of the scum and sludge layers.

A floating feeler board board hinged is to a second index board that is in turn attached at the end of a (roughly) 8 ft. pole.

The floating feeler board is to feel the under-side of of the scum layer - the contractor says more floating board length makes that easier as the board floats better.

The handle-attached fixed index board allows the contractor to see where the top of the floating scum layer has left a line on the board - marking the thickness.

We recommend using 2" poly piping for this purpose as it's easy to wash off. A simple door hinge is bolted to the end of the pipe and to the flapper board.

While some pumpers let the board float to feel the points described below, I've seen others who used a board which was fixed at a 90-deg. angle to the pole. (See illustration above).

Also see TUBE for MEASURING SCUM & SLUDGE for a tool that can make both scum and sludge thickness measurements with a single device.

• Open the septic tank access port.
  
  If the tank is a single compartment septic tank (USDA sketch at left) ,this inspection is preferably at the tank outlet end as that's the point where risk of discharge into the absorption system is critical.
  
  However some tanks provide a ready access port only at the inlet side - less desirable but you can look there, too.
  
  Watch out: if your tank is a two compartment type, the solids, floating scum and settled sludge are accumulating at the inlet portion of the tank. Inspecting at the final septic tank outlet end will not discover sludge and scum early enough to prevent septic system damage.
  
  Such tanks may have a center inspection port which admits tank access at the outlet of the sludge/scum containing compartment. That's where to test in two-compartment septic tanks.
• Insert the septic tank measuring tool:
  
  At the opened septic tank access port, probe with the flapper pole through the scum layer until you feel outlet baffle or sanitary tee. This means that the pole is turned so that the board will extend under and be able to feel the bottom of the baffle.
• Mark the pole
  
  (chalk or pencil will work) to indicate that point. We line up the mark with something easy to reach such as the top of the access port which serves as a uniform reference point for the later measurements.
• Now lift up the flapper pole
  
   to feel the resistance of the bottom of the floating scum layer.
• Mark the pole again
  
  using the same reference point as described above.
• Scum distance to baffle bottom:
  
  The distance between the two marks gives the distance from the bottom of the scum layer to the (lower) bottom of the outlet baffle. If the bottom of the scum layer is within 3 inches of the bottom of the baffle the tank needs to be pumped.
• Scum distance to baffle top:
  
  By visual inspection, if the top of the floating scum layer in the septic tank is within 1 inch of the top of the outlet baffle, the septic tank also needs to be pumped.

A similar sludge & scum measuring procedure is provided in Septic Tank/Soil-Absorption Systems: How to Operate & Maintain [ copy on file as /septic/Septic_Operation_USDA.pdf ] - , Equipment Tips, U.S. Department of Agriculture.

WHEN TO PUMP - 3 Rules on How thick can the septic tank sludge and scum layer be before septic tank cleaning is needed?

The following is an excerpt from WHAT SCUM SLUDGE THICKNESS = PUMP NEEDED.

"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%.   

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

• 1/3 of tank depth: Pump the septic tank when the total depth of scum plus sludge layers equals one-third of the depth of the tank

or

• Less than 3" between scum bottom & outlet baffle: 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

• Less than 6" from sludge top to outlet baffle bottom: 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

Watch out: 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.

Details are at EFFLUENT RETENTION TIME

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.

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.

Also see TUBE for MEASURING SCUM & SLUDGE for a tool that can make both scum and sludge thickness measurements with a single device.

For deep or hard to access or commercial septic tanks that may need close monitoring,

see ELECTRIC MONITOR for SCUM & SLUDGE and also take a look

at OTHER MEASURES of SCUM / SLUDGE.

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

At SEPTIC TANK LEVELS of SEWAGE 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.

...

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

Or see these

Recommended Articles

• 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

Suggested citation for this web page

HOW TO MEASURE SEPTIC SCUM LAYER 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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