CORE Off-Mains Drainage · Module 1 of 4
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What is a septic tank?
A septic tank is an underground chamber made of concrete, fiberglass or plastic through which domestic wastewater (sewage) flows for basic treatment. Settling and anaerobic processes reduce solids and organics, but the treatment efficiency is only moderate (referred to as "primary treatment"). Septic tank systems are a type of simple onsite sewage facility (OSSF). They can be used in areas that are not connected to a sewerage system, such as rural areas. The treated liquid effluent is commonly disposed in a septic drain field or Leach field which provides further treatment. However, groundwater pollution may occur and can be a problem.
The term "septic" refers to the anaerobic bacterial environment that develops in the tank which decomposes or mineralizes the waste discharged into the tank. Septic tanks can be coupled with other onsite wastewater treatment units such as biofilters or aerobic systems involving artificially forced aeration.]
A septic tank/ sewerage treatment plant are used where the property is unable to connect to a mains drainage or sewer system.
Tanks come ion a variety of shapes and sizes.
Generally made of plastic or fibreglass these days
Septic tanks come in many shapes and sizes
Septic tanks come in many shapes and sizes. Traditional systems were constructed of brick or concrete and could be comprised of single or multiple chamber systems. The inlet pipes and outlet pipes to the various chambers are known as “DIP PIPES” .
Doubled dip pipes connecting through an internal baffle wall are known as “H”pipes.
Septic tanks come in many shapes and sizes
Dip Pipes. T and H pipes
On traditional (older systems) the Dip and H Pipes are of vitreous clay which is unfortunately both heavy and brittle.
As can be seen in Figure above the pipe are simply suspended / fixed in place by the mortaring of the socket of the Dip / H pipe over the spigot of the pipe passing through the chamber wall. Invariably there are no pipe clips used to secure the vertical leg of the Dip/ H pipes to the chamber wall.
Consequently the pipework is vulnerable to damage from a variety of causes and one can often find the dip pipes to be broken off and laid at the base of the tank.
Dip Pipes & H Pipes In Septic Tanks
These are placed within the external and internal dividing walls of a chamber to isolate the solids and waste matter. They are designed to allow water to freely move through the system, they are no great mystery and are easily constructed out of standard bends and square (90 degree T) junctions. The base of the dip pipes and H pipes re extended down towards the base of the chamber so that the deeper sediment free waste water is able to pass up through the pipe and into the next stage of the system.
Septic Tank how they work
The inlet pipe discharges directly into section (a) where some solids float and others settle, clarified effluent passes up through the slotted baffle into area (b) where finer particles of scum and waste are again separated before the waste water passes into section (c) and into the outlet pipe work.
Modern tanks constructed from fibre glass and plastics work in much the same way as the older systems as the image to the right shows, the central baffles divide the tank into three sections whilst allowing waste water to move through the system.
The central ball rises to the top of section (a) forming a seal when the tank initially fills, when the tank is emptied the ball lowers allowing access to the sludge at the bottom of section (a)
Three parts of investigating a tank
The waste from the house runs out to the tank, Its then treated in the tank and then onto the soakaway.
3 parts of investigating a tank.
1 Drainage upstream of the tank to make sure no sw on the drian or the drain is damaged causing ground water to enbter the system which willoverload the system
2. The tank its self.
3. The Soakaway.
Why soakaways block
Sludge builds up in the primary settlement tank
Why soakways block
When the system isn’t emptied regularly Sludge builds up and enters the soakaway or if the T pipes or H pipes are damaged
Damage to the baffle can also occur if the tank is not emptied regularly as the sludge builds up and pushes on the baffle.
Why soakways block
Sludge enters the soakaway eventually completely blocking it.
Other issues can be damaged pipework from tree roots or ground movement as usual damage
Septic Tank and herring bone soakaway
Perforated soakaway pipe
Most Perforated pipe soakaways use a shingle bed, A shingle covering and a membrane. This allows the effluence to drain from the pipe and into the shingle and then into the ground strata,
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