When is a Septic Tank Considered Full? (a mathematical approach)

Posted by Tank Techs on December 31, 2019


Note from the inspector: This information is provided for those individuals who love the precise details. For those less inclined, watch the TankTechsRx video “How Often Does my Tank Need to be Pumped?” It will get to the conclusion very quickly.

The Home septic system is designed to work when the tank is full of water. Measuring by time between pump outs is really just guess work. According to the NAWT the septic tank only needs pumping when the sludge level and the scum level reaches between 25% and 33% sludge and scum level accumulation. Some sources recommend as high as 50% sludge can be acceptable. The variation in depth of sludge is dependent on the thickness of the sludge. A watery light sludge could wait to 33% before pumping. A very heavy gooey sludge should be pumped at 25%. To be effective the tank needs a 50% or more clear level of water to accelerate the break down process. If the sludge level gets to high or the scum level floating on the top gets to thick the sludge/scum can pe pushed into the drain field and clog the field. None of the current products on the market except TankTechsRx for Septic Systems has the ability to consume sludge in the leach filed and the tank because it is the only tank treatment that uses a living and multiplying bacteria blend that will consume solids, leaving basically wastewater, CO2 and antioxidants.

To establish the percentage of sludge /scum in the tank make a sock tool described in FAQ: How to make a tool to measure sludge and scum levels in a septic tank.

Expose the first opening in the tank were the lid is. If you do not know where it is See FAQ: How to find a septic tank. In newer tanks this is where the filter is and has a manhole cover over it. In older tanks the lid can be anywhere from inches to feet underground. Dig down to expose the tank. Open the lid and slowly lower the pipe, sock first into the tank. You will probably feel resistance when you hit the scum level floating on the top of the water. MARK 1 THE PIPE at this point at the opening to the tank. You will use this tank reference point for all marking of the pipe. This will be the top of your water level. Note about how thick it feels after you break through. MARK 2 THE PIPE again. This will show you the thickness of the scum level floating on the top of the tank. This level could be anything from a light film to solid scum several inches thick. You may have to force the pipe through this scum.

Continue lowering the pipe slowly into the tank noting the next resistance you feel. Stop lowering when you feel resistance. This will be the top of the sludge level in the tank. MARK 3 THE PIPE.

Lower the pipe to the bottom of the tank. Loose sludge will let the pipe slip easily to the bottom of the tank. Thick sludge may require you to push the pipe down to the bottom of the tank. When you hit the bottom MARK 4 THE pipe one last time using the same tank reference spot you used before. The distance between the 3rd mark and the 4th mark will indicate the thickness of the sludge level.

Slowly remove the pipe from the tank and examine the sock. Note the condition of the sludge accumulation on the sock. Is it heavy and gooey or light and watery?

Do the math:

The 1st mark shows the water level. This should be equal to the line leaving the tank for the leach field. Record this level. If this drops more than 3 inches under normal usage this likely indicates a leak in the tank that must be repaired. Call a professional.

The distance between the 1st and 2nd mark is the scum level in the tank. Record this measurement. It will be added to the sludge level to ascertain the sludge/scum level in the tank.

The distance between the 2nd and 3rd mark is the water level in the tank. This must be no more than 50% of the total tank height for a proper functioning septic tank.

The distance between the 3rd and 4th marks is the sludge level in the tank. Record this level to be added to the scum level.

The distance between the 1st and 4th mark is the overall water/scum level of the tank.


Scum Level 2” Plus sludge level of 23” = 25”

Overall water level of tank is 47”

25” sludge divided by 47” level equals 53% sludge level in the tank. Time to pump IF the tank has heavy, gooey sludge.

If the sock is just off color and the sludge is light and under 50%, you can add a liter of TankTechsRX for Septic Systems and check it again in 6 months to a year, depending on the condition of the tank. Be sure to take a picture of the sock and pipe lines you drew for reference for the next checkup.

Remember to take pictures of everything so you don’t forget.

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