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Avatar photoJohn Aldrich1

    Sally Beard, the event that triggers the need for pumping the sludge (indigestible solids) from the septic tank is when there is a reduction of hydraulic detention time of the sewage in the tank. When a large percentage of the available digestion volume is filled with indigestible solids, the velocity of the sewage through the tank is increased. The increase in velocity results in less time available for microbial digestion of the organic matter in the sewage. The quality of the effluent being applied to the leach field then declines causing organic overloading of the field. Organic overloading will cause leach field failure. The reduction of available digestion volume also results in fewer microbial populations to digest the applied organic matter.

    So, that said, it seems to me that the most efficient approach to determining that point in time when the sludge must be removed is to implement the 1/3 Rule.

    THE 1/3 RULE

    “When the measured sludge depth in the primary digestion chamber of the septic tank is 1/3 the depth of the liquid level of the tank, it is time to have the sludge removed.”

    Typically, septic tanks have a 4 foot liquid level, so when the sludge depth is measured to be 16 inches, it is time to call a Licensed Professional Septic Tank Pumper.

    So, how does one measure the sludge depth in a septic tank? I recommend that a riser to the GROUND SURFACE be placed over the access hole of the primary compartment of the tank. This will allow easy access to perform the annual sludge measurement. Then purchase a device known as “THE SLUDGE JUDGE.” This is a rigid, clear plasic tube with a foot valve mounted on the bottom. It allows for taking a core sample of the contents of the tank. The Sludge Judge costs about $80.00 US. Check out this website.

    Alternatively, a measuring stick could be fabricated with an 8 foot 1″X1″ stick with about 2 feet of terry cloth stapled to the bottom. Insert the stick to the bottom of the tank and let it stand for a couple of hours. The very black sludge will adhere to the rough fibers of the terry cloth, and the depth of the sludge can be readily determined.

    So Sally, what event will remind you of your annual responsibility to maintain your septic system. If you are an American, then I suggest the 4th of July, SEWAGE INDEPENDENCE DAY. Celebrate your independence of the SEWER GRID, and remember that with this independence comes the responsibility of a septic system operator. If you are a citizen of another great country, then select a national holiday that will act as a trigger for your memory.

    This is the whole slimey story of sludge removal from septic tanks. Good Luck!

    John Aldrich (Septic Tank Yank)
    Septic System Consultant
    Advanced Professional Engineering, Inc.
    Fort Collins, Colorado USA

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