Septic tanks and tertiary treatment plants

The definitive guide to septic tanks and related treatment plants is the current EPA Code of Practice

The EPA Code of Practice is an extremely concise, thorough and well laid out document which although somewhat lengthy is well within the scope of most people to understand with little specialist knowledge.

For the benefit of those not wishing to wade through 70 pages of detailed analysis we have explained a few simple key points here with illustrations taken from the CoP ;


This shows a typical septic tank

The key things about the tank itself are

  • Size;

    this is the primary factor controlling the efficiency of a septic system. If a tank is too small the effluent may not be retained long enough for settlement of solids or proper microbial action to take effect.

    The minimum size recommended by the CoP is 2600 litres and the formula for obtaining the correct size is

    Required capacity in litres = 150 x The Max poulation of the dwelling + 2000

  • Internal structure;

    The way in which incoming effluent is guided into the tank and baffles to ensure it cannot simply pass across without being treated is critical. The length of time the effluent spends in the tank determines how well it will be treated and this is controlled by these internal structures.

  • Outlet screens;

    These are one of the most important considerations in the whole system. A properly constructed tank will have a screen at the outlet which prevents scum and solids from entering the next stage of treatment, failure of this filter can cause a total failure if the system. Well designed units should make servicing this filter simple, it should be checked regularly but as with all parts of the treatment system proper precautions should be taken with regard to health and safety before touching anything.


    Tertiary treatment systems come in several guises and are designed to fit between the septic tank and final discharge point. A few are outlined here;

  • Sand filters

    these can be intermittent or continuous

  • An example of an intermittent sand filter

    In this example built onto the ground and covered over with grass, a pump regularly doses (delivers a measured quantity of effluent) the layers of sand through a network of infiltration pipes so that an even distribution is achieved. Here it is shown built above a porous substrate which allows the filtered wastewater to return to the water table from directly beneath the filter.

  • An example of an above ground sand filter

    Here the filter is constructed as a tank which may either be entirely above ground or sunken into it but completely waterproofed to prevent leakage or contamination. Like the last example a pump would periodically dose the effluent evenly over the top layer of sand but the filtered effluent would be collected from a pipe at the base and fed to a final polishing filter or percolation area.

  • Constructed wetland or reed bed

    A horizontal subsurface flow reed bed In this example the effluent is allowed to enter through an area of coarse gravel, it then flows slowly through the fine pea shingle where the root mass of the plants extract most of the nutrients and organic debris contained in the waste until it is collected from another area of coarse gravel at the other end. A weir arrangement keeps the level at an optimum from which the final effluent can travel to a polishing filter or percolation area.


    These are the final steps in the domestic waste treatment system and are designed to allow the infiltration of the treated effluent back into the environment.

    All such filters must have at least 900mm of free draining and unsaturated soil between the level of infiltration and the bedrock or water table below. The way in which this functions is by developing a biomat of active bacteria on the particles within the infiltration area. This acts on the already treated effluent to remove almost all of the remaining biological and chemical contaminants. The resultant high quality waste can then soak away without polluting nearby watercourses.

    A typical septic tank system

    for more detailed information go to EPA Code of Practice


    email- for any unresolved queries.

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