What are Septic Tanks?

Septic tanks (ST) are used to deal with sewage water from lavatories, kitchens, bath, showers, and wash basins. They are not meant to take rainwater as large volumes of rain water can wash all the carefully stored solids out into the soakaway!

Septic Tank Lifespan

The life of such a soakaway varies depending on ground conditions. Typically, a soakaway will last 10 to 20 years, but 60 years is possible too! While I have described the effect of not de-sludging on the bottom solids, the top crust, or floating solids can be an even bigger menace.

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Floating solids are fat, oil and grease, or FOG. It is essential that floating solids do not enter the soakaway. Grease will live in the soil for years and bacteria there do not readily break it down. To prevent this happening, a Tee pipe should be installed on the outlet pipe. This way, floating solids will not simply float out! However, if there is no Tee pipe or it has been broken off, the Fog goes into the soakaway, quickly blocking it.

If the ST has a tee pipe but is not de-sludged each year, then the floating solids layer gets thicker, and so goes over the top of the tee. So again, the soakaway is under attack. Instead of a decent 20 year life, it can have less than say five years.

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How Brick Build Septic Tanks Are Designed

The design of these tanks has often been modified to suit ground conditions at the time of construction. Their depths often not as planned due to  hard rock or wet running sand or too much water ingress creating excavation problems. Once circumstances affect design the purpose of the ST is compromised. Ideally, a ST should have two chambers, the first twice as big as the second. Often, they are constructed with one chamber only. Another compromise based on cost!

Consequences

The consequence, generally not understood by all, is that the amount of solids carried out into the soakaway is far more than it should have been. The ST is designed to have about 600mmm (2ft) of solids in the bottom. More than this and the tank is full! Once it’s full of solids it should be emptied immediately.

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The height of the inlet pipe should be 1.4m above the tank bottom. This allows for a settlement zone, 800mm deep and with a volume of water which allows the incoming effluent to gently sink to the bottom storage zone. The speed of water through the tank, if allowed to increase, increases the amount of solids the water can carry.

The solids that do accumulate on the bottom increase in depth and so the speed of water increases, as the settlement zone volume decreases, carrying most of the solids, including fine solids scoured from the bottom into the soakaway or drainage field.

Read: septic tank full symptoms

Rule of Thumb

• Each household is different, but it is generally accepted that raw sewage has 300 ppm, solids to water.
• The effluent from a good ST is considered to be about 200ppm.
• Treated water, using a small packaged treatment plant is 20ppm
• Roof water is about 5ppm with tap water zero of course!!

So, only a third of solids are retained, two thirds goes into the soakaway.

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