Septic System Questions

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    • #278513

      We live in Rhode Island and are in the process of purchasing 21 year old home with an original 1,000 gallon septic system. The home is in a rocky area with a high water table. The concern with the Septic System is that there was sludge at the top of the tank when inspected. The tank is 21 years old. Is there need for concern? How long do septic systems usually last?

      Thanks in advance.

    • #298665
      John Aldrich1

      ea_web, there are basically 3 layers in a functioning septic tank. The “sludge layer” is on the bottom, the next layer above the sludge layer is called the “sludge-free zone”nd the top layer is called the “Scum Layer”.
      If the septic tank was full, then you were looking at the scum layer.

      An increase in the depth of the sludge layer reduces the hydraulic detention time of the sewage, and therefor, reduces the treatment capacity of the tank. The quality of the septic tank effluent declines as the sludge layer accumulates. The poorly treated effluent is then applied to the leach field where it will cause an increase of the thickness of the clogging mat. The thicker clogging mat will reduce the percolation rate of the effluent into the soil. At some point in time, the “percolation rate” through the clogging mat will be “less” than the “application rate” of the septic tank effluent. When this occurs, the water either backs-up into the septic tank, or surfaces above the leach field, or both.

      I use the “1/3 rule” when determining the need for sludge removal from a septic tank. When the measured sludge depth is 1/3 the liquid depth in the tank, the sludge should be removed by a septic tank pumper.

      If you are still negotiating to purchase this septic system, require that the contents of septic tank be removed so that a thorough inspection can be performed. I also recommend that you employ a local expert in on-site system design to inspect the system to determine its condition, and its adequacy to serve your needs. Do not rely upon the opinion of the local regulatory agency technician.

      Properly designed, properly constructed, and regularly maintained septic systems have the potential to have an infinite service life. Design standards pertaining to septic tank/soil absorption systems are codified in most Individual Sewage Disposal System Regulations (ISDS). The product of these antiquated design standards are systems that are designed to fail. At best, the average service life of these inadequate systems is 15 to 20 years. Depending upon the many variables impacting the performance of the system, some systems fail almost immediately, while other systems have been performing successfully for over 100 years. If you need further direction send me an e-mail message at;

      John W. Aldrich
      Septic System Consultant

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