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SAVING WATER
/ AT HOME
source:
http://www.environment-agency.gov.uk
/subjects/waterres/287169/
1370495/?lang=_e
http://www.environment-agency.gov.uk
/subjects/waterres/287169/
287245/
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Using rainwater for garden watering, toilet flushing and washing machines can save up to 50% of household water use…- correctly collected and stored, rainwater can meet all these requirements without further treatment. In practice most domestic roof areas are too small to satisfy all this potential demand regardless of storage cistern size, so it important to evaluate the potential savings before investing in an expensive installation; (1)
- in existing housing, it is generally more economic to employ water-saving measures than to reuse rainwater or greywater. However, with new developments, rainwater reuse becomes a more attractive option. Ultimately, rainwater systems installed with filtration and disinfection could potentially supply all domestic water requirements, although this is not currently recommended in at least many of Western countries;
- as it is ‘soft’, rainwater is ideal for clothes washing and toilet flushing. Trials in Germany have shown extended life for washing machines fed with rain rather than hard water.
Greywater, the waste water from baths, showers and washbasins, can be collected in a household-scale reuse system and treated to a standard suitable for WC flushing…- the amount of wastewater generated by any household will vary greatly according to the dynamics of the household and is influenced by factors such as the number of occupants, the age distribution of the occupants, their lifestyle characteristics and water usage patterns. Some water usage surveys carried out in capital cities have identified an average wastewater flow of 586 litres per day per household. Greywater represents about 68% of the total wastewater stream;
- according to the British Environment Agency, using greywater can save up to 18,000 litres of water a year for each person. This represents 33% of daily household water use in UK. Typically, about 1/3 of household water is used for flushing the WC;
- although personal bathing habits will influence the actual water savings, the tab below shows that volumes of greywater produced approximately equal the water use of the WC, giving potential savings of around 1/3 of daily household water demand. A trial by the UK Agency's National Water Demand Management Centre showed a range of water savings from about 5% to 36% for different trialists;
Components of household water demand (1997/ 98)* | % | | clothes washing | 14.0 | | personal washing | 33.3 | | dish washing | 7.7 | | car washing | 0.7 | | garden watering | 6.1 | | direct heating system | 0.1 | | toilets | 25.0 | | miscellaneous | 13.1 |
(*) source: British Environment Agency. Data refer to UK, but similar percentages can be found in almost all industrialised countries (with some slight variations on garden watering, due to different climate conditions).
Household greywater is usually clean enough for flushing the toilet with only basic disinfectant or microbiological treatment… - problems can arise, however, when the warm, nutrient-rich greywater is stored, since it quickly deteriorates as bacteria multiply. One solution is to remove any gross matter such as hair and skin debris by passing the greywater through a suitable coarse filter prior to disinfecting it. This prevents biological activity for long enough to allow the treated water to be stored until needed. Other components of the system include a pump to get the greywater to the WC (usually via a header tank) and a method of providing mains-water backup for when supply does not meet demand;
- another approach, better suited to larger-scale systems, is to treat the greywater in much the same way as sewage is treated. This has been done using traditional biological methods as well as newer membrane filtration technology. At this level of treatment it is possible to treat the combined sewage flow without separating out the greywater;
- untreated greywater (excluding any foul waste water from, for example, WCs) can be used for garden watering if used immediately after it is produced.
- a number of manufacturers claim to have products in development that will make greywater reuse practical at the single household scale, although economies of scale are likely to tip the balance in favour of shared systems. Some countries have already realised the benefits of recycling greywater. In Tokyo it is mandatory for buildings with over 30,000 m2 of floor space or with a potential water reuse of more than 100m3 per day.
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