Handout 2: Rainwater harvesting and grey waterrecyclingWhat is rainwater harvesting?Rainwater harvesting is a way of saving the rainwater which wouldnormally flow off a roof and down the drain, and using it as piped waterto flush toilets and for the garden watering, drive wash down,vehicle and car washing, instead of treated drinking (potable) water. Itis not a new idea: the collection and re-use of rainwater from roofs canbe traced back thousands of years in hot, dry countries around theMediterranean. In continental Europe, some 100,000 are installedannually; Germany has been using and refining the technology sincethe early 80s. Rainwater harvesting can also be used in schools,hospitals, offices, commercial premises; rainwater can typically beused for toilets, vehicle washing, yard wash down and watering plantpots/gardens.
Handout 2: Rainwater harvesting and grey waterrecyclingDepending on your normal usage, rainwater harvesting can save 30 to50% of the treated potable drinking water from the mains in housesand up to 80% of the treated drinking potable water in a business orcommercial building. This in turn can save up to 30 to 50% for thedomestic user and 80% for the commercial user on water bills. Havingmetered water is the best way of appreciating the savings that arebeing made. A rainwater harvesting system can collect on average upto 100,000 litres per household, depending on the area and angle ofyour roof, and your rainfall and even greater quantities for larger roofedcommercial buildings.
Handout 2: Rainwater harvesting and grey waterrecyclingNormally a builder would install a rainwater harvesting system. The skills are the same asinstalling a septic tank, but a plumber would be needed to complete the internalpipework. There are no specific regulations yet that govern rainwater harvesting as such,although Building Regulations Part H affect siting of the tank and pipe runs, while Part Grefers to internal plumbing regulations. In a properly designed system, the pipework isentirely separate from the mains water pipework and should be identified as non-potable.The system can be installed in existing buildings, but will cost more because of the extraplumbing required. In addition, bungalows and commercial premises are also verysuitable, the only limitation is the area of the roof to capture rain, compared to thenumber of users; this puts a limitation on flats and apartments. The tank should be buriedunder a car or vehicle park, landscaped area, garden, patio or drive, with space left forthe round access cover. Most systems are designed so that they can accept cars drivingover them if suitably installed. Domestic systems can cost from about £2500 up to over£4000 including installation costs, depending on the size of tank. Commercial systemscan cost a lot more depending on the size and requirement, but usually have a muchquicker pay back period due to the size of roof and high usage.
Handout 2: Rainwater harvesting and grey waterrecyclingA storage tank is fitted to your rainwater drain from your roof, and falling rain enters thetank through a filter which removes leaves and other matter. The storage tank is usuallyburied under car or vehicle parks, a garden or under the entrance access or drive, andcontains a pump which pumps the rainwater to the building where it is piped to the toilets,and to the outside taps. The filtered, untreated rainwater should only be used for non-drinking or bathing purposes: toilet flushing, gardens, vehicle and drive wash downs. Therainwater is filtered as it enters the storage tank, to remove particles and other matter. Itis kept in the dark and kept oxygenated to discourage algal growth, and properlydesigned systems are designed with calming inlets, which ensure that any sediment atthe bottom of the tank does not get stirred up. The water is not drinking water fit forhumans. The internal filter should be washed about once a quarter, otherwisemaintenance is not needed. The only moving components - pump and float switch havean extremely long life. The system should be designed to overflow a few times a year, toremove floating matter.
Handout 2: Rainwater harvesting and grey waterrecyclingWhen there is a prolonged spellwithout rain, the water level in thestorage tank will fall to a minimumlevel. At this level, a float switch willopen a valve from the normal mainssupply and keep the storage tanktopped up, until it is filled by rainagain. Normal system design shouldallow up to a week without rain, givennormal usage.245136A typical rainwater harvesting system1. Control centre ‘The Brain’2. Filtered inlet3. Pump4. Calmed inlet5. Underground tank6. Diverter valve
Handout 2: Rainwater harvesting and grey waterrecyclingThe water is collected in a butt or tank installed above or below groundlevel. However, for below ground installations the installation engineerhas to ensure adequate access for filter removal and cleaning as wellas disinfection purposes. Typical prices for grey water systems rangefrom £200 for the smaller system to £1000 for larger installations plusassociated plumbing costs.Grey water recyclingGrey water is the waste water that we drain away after we take abath, shower or from our hand basins. This water in much thesame way can be collected and reused for filling our toilet cisternsfor flushing, washing our vehicles, washing our drives and wateringour garden.
Handout 2: Rainwater harvesting and grey waterrecyclingWhen the water is collected from the baths, showers and hand basinsit has to be cleaned and filtered via a cleansing unit to remove allmicrobiological and other impurities to ensure that it is safe to reuse.When it has been adequately cleaned, the water can be piped to thetoilet cistern for flushing purposes, reused for washing our clothes,cleaning our vehicles and drives as well as watering our gardens.Maintenance of the cleansing unit involves the removal of a filter withinthe unit and rinsing in warm water as well as the replacement of adisinfection tablet at regular specified intervals.
Handout 2: Rainwater harvesting and grey waterrecyclingDepending on your normal usage, grey water recycling cansave up to 30 to 40% of the treated potable drinking waterfrom the mains in houses and up to 60% of the treateddrinking potable water in a business or commercialbuilding. This in turn can save up to 30 to 40% for thedomestic user and 60% for the commercial user on waterbills.The grey water recycling and rain water harvestingsystems can be combined and installed together forincreased long term savings.
Handout 2: Rainwater harvesting and grey waterrecyclingA typical grey water recycling system