How to install a septic tank and sewage treatment plant

The installation of a septic tank or a sewage treatment plant is seen by many to be a complicated process. This simple guide aims to give a step by step walk through on septic tank installation. Following this basic guide will make sure you do not make any mistakes and will save you money on your septic tank installation costs. All work should always be in accordance with the building regulations for your region.


Sewerage and wastewater from a development is collected in sewers pipes below the ground connected to toilets, showers, sinks, etc.  All of the wastewater generated from activities in the house or building should be discharged to the septic tank or sewage treatment plant. Anything from outside the walls of the house such as storm water and roof water is not permitted to enter the septic tank or sewage treatment plant.

septic tank installation

Contrary to widespread belief, all washing machine and dishwasher wastewater can enter the septic tank or sewage treatment plant. A certified tank will have no issue breaking down this type of wastewater. Sewers around the house or building are constructed from 4” or 110mm uPVC pipes. These pipes should be laid at a gradient of 1:60. If the gradient is too much or too little then it can result in solid and liquid waste separating, thereby causing a blockage.

Access junctions [or AJ’s] should be installed at bends and on long straight runs of sewer for access and maintenance. An AJ should be installed at the inlet and outlet to the tank for this reason. For larger developments with bigger flows of wastewater, larger sewer pipes should be used.

Septic Tank Installation

You will need to prepare an excavation for the tank. When purchasing a tank you should consider a shallow dig tank. When installing one of these tanks you only need a small digger so it saves on cost.

When digging out for your tank you will already have the sewer pipe running to the tank location. You will then need to take a measurement from the bottom of the inlet hole on the tank to the base of the tank plus an extra 50mm. This is the depth you then dig down below the sewer pipe running to the tank location. Typically, you will dig the hole 250mm wider on each side than the actual tank size.

septic tank installation

After your digging is complete you need to create a level base in the excavation by putting in 50mm of gravel or sand. This is why you dig an extra 50mm deep.

Next you place the tank in the excavation. This is an important part of your septic tank installation project. All Biocell domestic sewage treatment plants and septic tanks have lifting points on the tanks so they can be easily lifted into place by the digger.

With our reinforced septic tanks you do not need a concrete backfill. You can backfill around the tank with gravel or sand. It is important to use rounded gravel, not sharp angular material. Gravel types like clause 804 should be avoided as they can compact after installation causing settlement around the tank over time.

As you backfill around the tank with gravel or sand, it is important to then fill the tank with water incrementally. This keeps the tank in position and will counteract the load from the backfill material.

If you have installed a sewage treatment plant then you will need to bring a power supply to it. A low energy system is always recommended as it will save on running costs. Typically, the power cable being brought to the plant is a standard three core SWA cable.

Alternatively, if you have purchased a non-electric sewage treatment plant then you will not need a power supply to be brought to the tank.

System startup

If you have installed a septic tank then you can start using it immediately. If you have purchased a sewage treatment plant then you can also start using it immediately. With an electric sewage treatment plant, you will just need to make sure it is turned on. While some old fashioned and inefficient systems may require the use of chemicals or additives to the system, all Biocell systems are totally natural. They biologically treat the sewage via natural bacteria that grow in the tank. No need for any expensive enzymes, septic tank worms or septic tank bacteria.


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