Grey water; see our section on Greywater recycling
Black water; also known as brown water and sewage.
Effluent from toilets, kitchen and food preparation sinks and dishwashers all contain a cocktail of harmful and generally undesirable substances. The most obvious of these are the potentially harmful pathogenic organisms and parasites which thrive in human excrement. The combination of relative warmth, water and abundant food in household effluent gives rise to very large populations of these microscopic pests and as a result this sewage should be regarded as a hazardous substance and contact should be avoided.
Effluent also contains significant quantities of chemical pollutants, some of these may have been deliberately introduced in the form of detergents, antiseptic cleaners and bleach. These may have served a useful purpose within the home but once in the discharged effluent they pose real problems. The corrosive and toxic nature of some of these substances is a problem in its own right but more seriously they impede the natural order of biological degradation which would normally break down much of this waste.
Methods of treatment;
many people connected to a public sewer will never know or see the processes which their waste undergoes. Once effluent has passed through all the pipework to bring it to a treatment plant it has to be screened. The raw effluent is passed through metal gratings where non organic materials like plastics and metal are removed. An astonishing amount of plastic applicators, cotton bud stalks, condoms, sanitary wear and even syringes find their way here and have to be removed and dealt with. Large items like dead rats, pieces of wood and plastic bags all have to be extracted and due to the level of contamination these items are usually incinerated. The bulk of these unwanted items should never be allowed to enter a sewer and the environmental and financial cost of this carelessness is significant.
Once screened the effluent will usually be allowed to settle in large ponds where heavy silts and sands, small metallic objects and waste which has not broken down will create sediment sludge. This sludge will to some extent break down biologically to a state where it poses little threat to human health and is generally removed periodically and allowed to decompose further on land before disposal.
The remaining liquid effluent passes onto a stage of aeration, by stirring the liquid in large tanks or ponds aerobic bacteria are able to digest much of the solids and further reduce the harmful contaminants it then passes to a filter. These are usually circular tanks filled with a biologically activated carbon material. The carbon granules have enormous surface area at a microscopic level and bacteria which eat the sewage thrive on this material. A spray boom rotates and doses the liquid effluent over the surface at a steady rate and the water which emerges is comparatively clean
Next a final filtration through what is known as a polishing medium is carried out, this essentially involves pouring the liquid through enormous beds of sand until what emerges is fit to be returned to the water cycle. The bulk of commercially treated sewage in Ireland appears to be finally discharged into rivers which dilute any remaining bacterial populations to a harmless level.
This is all well and good in theory but it is likely that much sewage is inadequately treated before finding its way back into water courses and the sea with the result that drinking water can become contaminated and bathing water quality suffers on beaches.
for most people in Ireland this means a septic tank. From the latest census it appears there are at least 440,000 septic tanks in Ireland and in reality it is likely that there are many more. A good many of these are entirely inadequate for purpose and legislation is being enacted to address this.
The subject of septic tanks and their related treatment systems is too big to cover here but see our guide to Septic tanks and tertiary treatment plants
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