Septic Tanks & Leach Drains

In some Perth Metro area’s main sewer isn’t available or hasn’t been brought to the area yet and in those instances a septic tank and leach drain system is required to be installed to treat and dispose of effluent. There are quite a few options on the market, from plastic, fiberglass and concrete tanks to the more advanced ATU’s which allow effluent to be drip fed into garden beds or even used in above ground sprinkler fields. A septic tank is simply two chambers (Sometimes located inside one tank) where anaerobic and aerobic bacteria break down the solid and liquid waste, effectively “treating” it, allowing the treated water to be leached / evaporated into the soil through leach drains, french drains, soak wells or french drains. From time to time, septic tanks will have to be pumped and cleaned, and blocked or broken leach drains replaced – Eventually the soil around the leach drains will become saturated and no longer absorb the treated water like it used to, in this case fresh soil needs to be installed in the drainage area to allow it to work correctly. The Health department does a good job of explaining the workings of a septic tank Health Department – Septic Tanks.

Septic tanks and leach drains need to be approved by the department of Health. A list of approved tank manufacturers, sizes and contact information can be found Here.

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For every Septic Tanks system installed – there must be means of draining away the treated effluent – depending on the age and location of your house, you will either have a single tank with a baffle, then two or four 1200×1200 concrete soak wells, or leach drains constructed out of special leach drain brick, with concrete pavers ontop. After time, the soil around the leach drain area (infiltration field) will become clogged and be less efficient at soaking away – this is the usually the reason why septic tanks stop working properly – as there is a backup of treated water. The way to fix this is to install a new leach drain field, which will allow the treated water to drain away.  Modern day leach drains are manufactured from different types of material – Concrete leach drains, Plastic leach drains, Polyethylene leach drains, Fiberglass leach drains. Depending on the type of treatment you can also dip feed the effluent into your garden or field, or use above ground sprinklers in an effluent absorption field.

There are three different types of leach drain installations:

  1. Below ground – it means the leach drains are buried below ground, and the inverts / levels match up with the inlet and outlet of the septic tanks, allowing gravity to drain from the septic tanks into the diverter and then into the leach drains
  2. Semi inverted – it means the leach drains are not completely buried, however they stick out of the ground slightly – they will need to be covered with the correct soil and the correct amount for an effective leach infiltration area. This is used when there is a high water table, or the ground isn’t able to be excavated properly.
  3. Fully inverted – This means the leach drains sit ontop of a bed of soil, depending on the water table in the area. For example, if you were living on a flood plane or drainage line, and the water table came to ground level, you would required a 1.2m sand pad to be constructed before you installed your leach drains on top, then covered them with additional sand. With this method you would also require a pump pit with electric pumps inside to move the waste to a higher level.

Modern leach drains are installed in pairs, and the size of them depending on a multiple of factors:

  1. The physical size length of the module
  2. The Infiltrative area (m2/m)
  3. Amount of daily waste (Hydraulic loading per day)
  4. The soil (Sand, Gravel, Clay)

For example, if you had a large commercial premises that was using 10,000L per day, in sandy soil, using a galvins concrete leach drain (10,000 / 30 / 1.58) = 211~m of leach drain required.

The Health Department of WA has a list of approved manufactures and information, it can be found here Health Department – Leach Drains.


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