Septic System Maintenance - Why Does a Septic Tank Need Cleaning?
     


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Why pump the septic tank? Here we explain why a septic tank needs to be pumped regularly, how septic tanks are cleaned, how much it costs to pump a septic tank, how to inspect and clean septic tanks safely, what is septic tank effluent retention time, how to measure septic tank scum and sludge levels, and other septic tank maintenance advice for homeowners. How often should septic tanks be pumped? Then we discuss reasons to pump out or clean a septic tank "off schedule" such as before a big party or for diagnostic reasons.

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WHY DOES A TANK NEED PUMPING? A Basic Explanation of What Happens in a Septic Tank

Photograph of  a Failed Septic Tank: fully impacted with solids and roots, this tank was ignored until it failed Why should septic tanks be pumped out? What causes septic field failures? Understanding the proper procedures for septic tank care, septic tank cleaning frequency, and other septic tank maintenance chores, will permit the owner of a home with an onsite septic system to maximize the life of the system and to assure that it is working properly.

See SEPTIC TANK PUMPING PROCEDURE for details of what should be done during a septic tank cleanout. See WHEN NOT TO PUMP A SEPTIC TANK for some warnings about when a septic tank should not be pumped out. Also see Septic Tank Pumping Procedure Detailed step by step photo-illustrated guide to find, pump and clean a septic tank. © 2008-1978 Daniel Friedman, All Rights Reserved. Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical reviewers are invited and are listed at "References."

The most common domestic wastewater treatment system used in rural areas is the septic tank-soil absorption system. The septic tank removes settleable and floatable solids from the waste water. The soil absorption field then filters and treats the clarified septic tank effluent and distributes it through the soil.

Removing the solids from the wastewater protects the soil absorption system from clogging and failure. In addition to removing solids, the septic tank also promotes biological digestion of a portion of the solids and stores the remaining undigested portion.

The first stage of the treatment system, the septic tank, removes solids by holding wastewater in the tank. This allows the heavier solids to settle as sludge and the lighter particles to form scum at the top.

To accomplish this, wastewater should be held in the tank for at least 24 hours. Up to 50 percent of the solids retained in the tank decompose; the remainder accumulate in the tank. Biological and chemical additives are not needed to aid or accelerate decomposition. The small, collapsing, and totally impacted septic tank shown here was ignored by the property owner for years, leading to total failure of the system.

Septic tank skectch (C) D FriedmanThis figure shows a tank in cross-section. You will see that the liquid septic effluent volume has as available space the area between the underside of the floating scum layer at the top of the septic tank and the upper side of the settled sludge layer on the tank bottom.

As a septic system is used, sludge continues to accumulate in the bottom of the septic tank. Properly designed tanks have enough space for up to three years safe accumulation of sludge, but the actual time you can allow between septic tank pumpouts depends on tank size and level of usage - see TANK PUMPING SCHEDULE for a table relating septic tank size and usage to pumping frequency.

When the sludge level increases beyond the maximum design point, sewage has less time to settle out as scum or sludge before leaving the tank (we explain this further at EFFLUENT RETENTION TIME) and more suspended sewage solids will flow out of the septic tank into the absorption area or drainfield.

If too much sludge accumulates in a septic tank, no settling occurs before the sewage flows to the soil absorption field. Infiltration of sludge into the soil absorption field will shorten the drainfield life and thus cause system failure by clogging the absorbing soils around the field. To prevent this, the septic tank must be pumped on the proper schedule. The material pumped out of the septic tank is known as septage, raw sewage, or blackwater.

Other reasons to pump out or clean septic tanks

Photo of septic tank sludge and scum layer being broken up prior to septic tank cleanout.Unscheduled septic tank pumpout advice: this document explains additional reasons for pumping the septic tank at times other than those provided by septic tank pumping tables.

We describe a list of circumstances that recommend an "extra" or unscheduled cleanout and inspection of the septic tank such as a marginal septic system that is about to be subjected to high usage during a party, or in response to a sewage backup into the building, following area flooding, and as part of diagnosing a failure of the septic system or any of its components.

Readers should see Septic Tank Pumping Frequency Guide for a table explaining when to schedule a septic cleanout. Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical reviewers are welcome and are listed at REFERENCES. This article is a supplement to Inspecting, Testing, & Maintaining Residential Septic Systems an online book on septic systems.

At WHEN to CLEAN THE SEPTIC TANK and in the table shown at Septic Tank Pumping Frequency Guide we've discussed the basic guidelines and tables used to determine when a septic tank should be pumped. But There are other reasons besides time or sludge/scum layer thickness that lead people to pump septic tanks out. Here's a list of other reasons to pump the septic tank along with links to more details:

  • Before the party: you can reduce the chances of a septic system backup as well as avoiding overloading the drainfield if you have the septic tank pumped right before any anticipated heavy usage such as having many visitors or having a large party with many guests in your home. We discuss this suggestion in detail at Septic Backup Prevention.

  • Septic Pumping in Response to Sewage backups: pumping a septic tank because drains are backing up in the building is a "band aid" that gives very short relief since the tank will fill again quickly, perhaps in just a day or so of use. The reason for the septic backup needs to be determined. If the septic system is backing up because the tank is totally blocked by solid waste the tank needs to be pumped but the owner needs to be warned that the drain field has been damaged and has less remaining life. If the septic system is backing up because the drain field itself is failing and refuses to accept effluent, the drain field needs to be replaced. See Sewage Backup Diagnosis & Hazards in buildings

  • If the septic system drains are clogged the clog needs to be found and cleared. (See CLOGGED DRAIN DIAGNOSIS & REPAIR)

  • Houses being sold: often an owner or realtor will arrange for a septic tank to be pumped just before a home inspection or before a septic system inspection or septic loading and dye test. Such a "favor" performed for the buyer is in fact preventing a valid septic inspection and test since an empty tank means the drain field can't be tested. This is a fraudulent act and should not be tolerated. (See When Not to Pump a Septic Tank)

  • Septic tanks exposed to flood conditions need to be pumped and inspected as the tank may have been filled with backing up silt or mud. (See SEPTIC SYSTEM FLOOD DAMAGE REPAIR)

  • Filled cesspools are often "repaired" by pumping and perhaps by other means such as agitating the cesspool bottom or sides. If a cesspool is found filled to within 6-12" of the top of the unit it is at end of life and needs replacement. Pumping some cesspools risks dangerous cave-ins or collapse. (See Cesspool Safety Warnings)

  • Evidence of Septic Failure such as effluent on the property surface in the drain field area or odors leads some people to pump the septic tank. As cited in our first example above, "Sewage backups", this is a futile effort and other diagnosis and repair are needed. See
  • Sewage odors sometimes lead people to pump septic tanks. Pumping is not likely to correct a sewer gas problem though tank inspection may provide some information about system condition. (See Diagnosing & Curing Sewer Gas Smells and Septic Tank Odors.)

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