12/23/2023 0 Comments Modern septic tank design![]() Grease will live in the soil for years and bacteria there do not readily break it down. It is essential that floating solids do not enter the soakaway. 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.įloating solids are fat, oil and grease, or FOG. The life of such a soakaway varies depending on ground conditions. So, only a third of solids are retained, two thirds goes into the soakaway. Roof water is about 5ppm with tap water zero of course!!.Treated water, using a small packaged treatment plant is 20ppm.The effluent from a good ST is considered to be about 200ppm.Each household is different, but it is generally accepted that raw sewage has 300 ppm, solids to water.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. The speed of water through the tank, if allowed to increase, increases the amount of solids the water can carry. 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 height of the inlet pipe should be 1.4m above the tank bottom. More than this and the tank is full! Once it’s full of solids it should be emptied immediately. The ST is designed to have about 600mmm (2ft) of solids in the bottom. ![]() 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. Often, they are constructed with one chamber only. Ideally, a ST should have two chambers, the first twice as big as the second. Once circumstances affect design the purpose of the ST is compromised. Their depths often not as planned due to hard rock or wet running sand or too much water ingress creating excavation problems. ![]() The design of these tanks has often been modified to suit ground conditions at the time of construction. How Brick Build Septic Tanks Are Designed 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 tanks (ST) are used to deal with sewage water from lavatories, kitchens, bath, showers, and wash basins. ![]()
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