Houseworks: A septic system failure that isn’t really a failure 

The tee fitting at the inlet into some septic tanks is prone to clogging that looks catastrophic but is really an easy fix.  Photo by Stephen Hutchings 

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A septic system can sometimes behave as though something has gone terribly wrong, when the fix is surprisingly simple. 

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I was reminded of this in the spring when I was called to a house where no amount of plunging would get the toilet flushing again. Raw sewage was also backing up into the shower stall. It looked like two problems at once but was actually caused by one thing happening at the septic tank. The problem wasn’t a failed septic system – it was a blockage right where the sewer pipe entered the septic tank. 

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This can happen more easily than you might think, because of the way septic tank inlets are usually designed. I’ve seen it be the root cause of what seemed like septic failure in other houses. The horizontal sewer pipe from the house typically enters a vertical tee fitting with open ends inside the tank. Sewage enters through the middle, while the vertical section directs incoming material downward. 

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It’s not a straight shot into the tank, and there’s a good reason for this. A septic tank works partly by allowing heavier solids to settle to the bottom while lighter material floats to the top. The inlet fitting helps reduce turbulence that might otherwise stir things up and send solids farther through the system. 

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The downside is that this inlet arrangement can become a collection point for toilet paper and other solids. If enough material accumulates, the inlet can plug completely.  

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When this happens, every drain upstream of the blockage is affected. From inside the house, the symptoms look alarming enough to make you think the entire septic system has failed. 

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Fortunately, an inlet blockage is remarkably easy to correct once the problem has been found. With the appropriate tank access cover safely opened, the obstruction at the inlet can easily be broken up from above with a stick. 

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Having your septic tank pumped every two or three years is important maintenance, but it won’t necessarily prevent a blockage at the tank inlet. Photo by Steve Maxwell 

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There’s an important safety point here. Septic tanks can contain dangerous gases, and an open tank presents a serious fall hazard. Never enter a septic tank, and don’t put your head or upper body down inside one. On dual-compartment tanks, the inlet is right near the front access hatch, so there’s no need to kneel down and search. 

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There’s another relatively simple blockage worth knowing about, too. Many modern septic tanks have an effluent filter at the outlet. Its job is to prevent suspended solids from leaving the tank and entering the leaching bed. That’s important because once solids begin plugging the perforated pipes and surrounding soil in a leaching bed, repairs become extremely expensive. 

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The filter protects against that, but filters themselves need cleaning. Whenever your septic tank is pumped, the technician usually cleans this filter, but if it doesn’t happen sewage can’t leave the tank properly. Once again, you may see slow drains or backups and assume something catastrophic has happened when it hasn’t. 

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So when a septic system suddenly stops accepting sewage properly, two basic questions are worth answering before assuming the worst: Is sewage getting into the tank? Is it getting out?