Grey water recycling

Grey water is the waste water produced from domestic activities such as washing clothes and bathing.

These include;

  • Water from hand basins
  • Water from showers
  • Water from baths
  • Water that has been used to wash cars etc
  • Washing machine waste (sometimes better left to go unused as the levels of soap and contaminants is too great for some systems to deal with)
  • Q: Why not dishwasher waste and kitchen sink waste?

    A: These both contain extremely high levels of organic solids which lead to unhealthy populations of pathogens and excessive nutrients both of which pose a risk to health.

    Grey water treatment and uses

    Grey water can fulfil the same basic functions as rainwater if treated sufficiently.

    These include;

  • Flushing of toilets
  • Water supply to washing machines
  • irrigation of plants but not recommended for those intended for consumption
  • outside tap for general cleaning
  • supply to water features and ponds
  • Unlike rainwater greywater must be treated and filtered to remove biological and chemical contaminants.

    Treatment methods;

  • Settlement;

    most greywater contains suspended solids, mostly dirt and skin particles these need to be allowed to sediment out before they clog up any further treatment. Most systems will have a primary settling tank with a calmed inlet to allow this to take place.

    If practical the water entering the house and intended for topping up the hot water tank can be taken through this settling tank via a heat exchanger thus recovering some of the heat lost when our hot water is going down the sink. Alternatively the recycled water heading towards being used for toilet flushing can pass through this tank via a less efficient heat exchanger. This will slightly raise the temperature of the incoming water before it reaches the toilet cisterns and prevent condensation forming on them.

  • Mechanical filters;

    These range from large tubes filled with layers of carbon granules and sand( which currently require yearly filter changes @ around €140 each!) to large gravel and sand beds onto which the greywater is fed evenly and allowed to trickle through. The solid particulates, grease, oil and detergents are trapped on the surface of the gravel and sand where microbial action helps break them down. The resulting water is cleaner but still carries a higher biological and chemical load than is desirable. The efficiency of this filter medium decreases as the surface covering builds up and eventually becomes blocked leading to significant maintenance issues.

    A typical stratified sand filter

  • Biological filters;

    these are usually large shallow tanks or ponds filled with either soil or gravel and planted with suitable aquatic or moisture loving plants.

    These are most commonly known as reed bed filters because the principal plant of choice is Phragmites Australis or common reed. Other plants such as flag iris, brook lime and water cress are often used but the choice of plant can be critical as some are more dominant and others more efficient at removing undesirable elements from the grey water.

  • A section through a typical reed bed

    The quality of water that has been through a reed bed is generally higher than that which has simply been filtered because the plants work at a microscopic level to change the chemical and biological balance of the water.

    We have attempted to keep this brief and simple, if you have any queries that you cannot find answers for on this site please email-

    examples of reed beds can be seen in our Gallery

    Diagrams are courtesy of the EPA from their Code of Practice for single dwellings

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