How to Measure Septic Tank Sludge Depth

How do you know when to pump your septic tank?

In a recent post, you have learned that there is no way to know for certain when to exactly pump your septic tank other than an actual measurement of accumulated sludge and scum. By design, the septic tank should be pumped when the sludge and scum layer displace 30% of the tank volume.

For example, if a septic tank has a liquid depth of 48”, the tank should be pumped when the sludge and scum layer combined measure 14 ½” (48” X 0.30).

People in the septic system industry measure scum and sludge layer thicknesses with a product called a sludge judge, which cost about $75. It is a long hollow plastic tube with a check valve in the bottom.

The proper way to use the sludge judge is as follows:

To measure the scum layer:

• Push the sludge judge through the scum layer until it just breaks through the layer.
• Make a visible note of the relationship between the top of the scum layer and the location on the tube.
• Pull the tube up and measure the length on the tube. Many times you will see some of the scum layer stuck to the tube to identify the location.

To measure the sludge layer:

• Slowly lower the tube into the septic tank until it touches the bottom of the tank.
• As the device is slowly pulled out of the water, the check valve closes capturing a liquid/solid profile of the septic tank water. The thickness of the sludge layer can be measured.

In this example, the accumulated sludge inside the tube measured approximately 8” – 9” and there was no scum layer. If there was, the scum layer thickness would simply be added to the 8’ – 9” measurement. In this case, the scum/sludge layer combined displace about 18% of the tank volume (8 ½”/ 48”).

Reviewing the pumping records indicate that this septic tank was last pumped 26 months ago. Based on this date, the septic tank should be pumped within 43 months of the last septic tank pump out (0.18 / 26 months X 0.30 = 43 months). Because the rate of scum/sludge accumulation can change over time, it is suggested that this measurement be made annually, especially if the number of people living in the home changes or a different family moves into the home.

If $75 to purchase a sludge judge is beyond your budget, you can build a device that will accomplish the same task. You need a long slender stick or pipe of approximately 8’ in length for most applications. If your septic tank is buried deep you will need a longer stick. Loosely wrap the bottom 2’ of the stick with cheesecloth and secure the cheesecloth to the stick at the bottom, middle and top with plastic wire ties, string or mechanics wire. The key is to NOT wrap it tightly around the stick.

To measure the scum layer, push the stick cheesecloth side down through the scum layer until it just breaks through the layer. Note of the relationship between the top of the scum layer and the location on the stick. Pull the stick up and measure the length on the stick. Many times you will see some of the scum layer stuck to the stick to identify the location.

To measure the sludge layer, slowly lower the stick with the cheesecloth downward into the septic tank until the stick rests on the bottom of the septic tank. Slowly and gently move the stick back and forth in a plus (+) pattern about 2” in each direction to allow the solids to flow into the cheesecloth. Slowly draw the stick out of the septic tank. Measure the witness line of solids that are imbedded into the cheesecloth. Measure the “wet” mark on the upper end of the stick. Calculate the % capacity as in the example above.