How to desludge a septic tank

Written by tom raley
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How to desludge a septic tank
(note pad image by Christopher Hall from

The bacteria in a septic tank breaks down the solid waste, and the remaining liquid is dispersed through the field lines and absorbed into the soil. While the majority of the waste products entering the tank are broken down, there are certain remnants and items which the bacteria will not break down. These items remain in the septic tank in the form of sludge and slowly gather at the bottom of the tank in the form of sludge. This sludge must periodically be cleaned out.

Skill level:

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Things you need

  • Long stick or rod
  • Tape measure
  • Pad and pencil

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  1. 1

    Locate the inspection/cleaning access port on your septic tank. The access port will allow you to inspect and clean out the septic tank without doing any actual digging. If your system does not have an inspection port it will be necessary for you to dig down to the tank, usually buried no more than 24 inches below the surface, and expose the access hatch in the top of the tank. The process for checking the tank and pumping out the sludge remains the same regardless if you have an easy access port or have to dig.

  2. 2

    Push the stick/rod into the tank opening until it touches the bottom of the tank. Remove the stick and measure the distance from the end of the stick to the highest point where the water touched the stick. This should be the overall height of your septic tank. Dry the rod and put it into the tank again, this time pushing it down very slowly. As you do so, attempt to feel when you come in contact with the sludge lining the bottom of the tank. As soon as you feel the sludge, stop and remove the rod. Measure from the end of the rod to the water line. This should be the open distance above the sludge.

  3. 3

    Calculate the amount of room the sludge is occupying in the tank. Subtract the first number from the second number to determine how many inches of sludge are in the tank. Divide that number by the overall height of your tank. Example: The first number was 60 inches, and the second measurement was 35 inches. 60 - 35 = 25 , and 25 divided by 60 = 41.6 per cent. If 35 per cent or more of the tank is filled with sludge, it is time to have it pumped out. Allowing the tank to fill above 35 per cent can cause solid waste to enter the field lines or to clog the inlet lines. This can result in costly repairs and down time.

  4. 4

    Hire a professional who will use a high-powered vacuum truck to remove the sludge from the septic tank. A large hose will be inserted into the tank and attached to a pumping truck. The sludge and other solid waste are pumped out of the septic tank and captured in a large holding tank on the truck. Another reason you need to hire a professional is that they have the proper permits and locations to dispose of the sludge once it is pumped out. On average, it will cost approximately £260 to have your septic tank pumped out. The average tank should be pumped out every three to five years.

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