These systems channel all rainwater from guttering into a centralised storage tank. Water is initially filtered in the guttering system to remove the largest debris. The water is then passed over a filter and enters the storage tank.
Filters have various designs this example the 3P Zisternen Filter ZF rainwater filter is taken from the Rainman™ Elite system.
The filtered fast flowing water becomes oxygen rich, it is directed down through the calmed inlet to the bottom of the tank where it is deflected upwards oxygenating existing tank water, ensuring a natural biological process takes place.
On demand the in-tank submersible pump delivers the clean rainwater to toilets, washing machines and outside taps. The system also has an automatic mains water tank top up facility, ensuring a constant water supply even when the tank rainwater levels are low.
The in-tank filter does not obstruct access into the tank and captures almost 100% of the incoming rainwater. The polyethylene holding tank is manufactured specifically for this purpose, it is of robust construction and ready fitted out with calmed inlet, filter and siphon overflow. The tank has push fit connections and telescopic entry access for adjustment to finished levels.
Systems can either feed a header tank or water from the main storage can be used directly. A variety of pumps and control units are available and there are numerous options for water storage.
Above ground storage systems are generally developed for garden watering purposes, however most firms that install rainwater-harvesting systems have options for fully direct systems that provide water for toilets and washing machines using above ground storage tanks. This is advantageous where there is little possibility for a below ground option such as in urban developments, terraces etc.
Domestic Potential
The size of the water tank used in a system is generally a function of the potential catchment of the roof and guttering system. For every inch of water that falls on a surface of 1000 square feet it is possible to collect approximately 600 gallons of water.
Annual rainfall for your area (mm)** x Roof area (m2) x Evaporation Factor* x Filter Loss Factor*
* assumed as 0.9
** example rainfall (838.0mm Mean England (Met office 2001))
Typical 3 bedroom house (4 people), the yield would be;
838** x 120m2 x 0.9*x 0.9* = 81,454 litres
A typical system would have a tank of approximately 5% of this volume i.e. 4073 litres.
If 45% of freshwater used in the home can be replaced by rainwater in washing machines, watering plants and flushing toilets etc
This amounts to a potential of ,550L in a house occupied by 4 people.
150*** x 4 x 365 x 0.45 = 98,550 litres
***(150L used per person each day)
Stormwater Protection
The urbanisation of our landscape has lead to a greatly increased demand on the handling capacity of waterways, streams and rivers. Rainwater falling onto man made surfaces enters rivers and steams at a much greater rate than it otherwise would by perculation through the conventional hydrological pathways that have existed for millions of years. This leads to events such as flash flooding as heavy downpours result in huge influxes of storm water, upsetting human communities and causing widespread ecological damage.
The use of rainwater harvesting systems has the additional benefit of control of storm water runoff.
Rain from roofs enters the storage tanks, filling them. If the system becomes full the majority of harvesting systems have an in built storm water Soakaways, these allow storm waters to be released slowly into the surrounding soils.
Building regulations do not apply directly to rainwater systems, although they may be linked with the Planning Permission for the storm-water management of the site. Building Regulations do cover the installation itself, tank sitting; pipe runs.
Domestic Example
Freerain™ are designers, manufacturers and national suppliers of rainwater harvesting systems and are one of the UK's leading brands in the field. Millennium Green has been studied by many academics. It is one of the first commercially built sites of its type and includes a number of environmentally friendly technologies and techniques.
One of which is a rainwater harvesting system fitted to each of the 24 plots. The site was completed in 2001 by Freerain ‘s construction company Gusto Construction.
A typical system such as lot 7 is a 4 bedroom detached house with a footprint of 153 M2. Resulting an annual expected collection of 92M3 of water per year. A 3,500L direct pressure system was installed. The system produced rainwater throughout the study year providing just under 50% of the total water supply of the household.
Capital cost of the full system £1990 + VAT
Estimated installation cost £350
Total cost 3500L system (2000) £2,340 + VAT (Freerain)
This costing is based on 2000 prices and the installation cost is based on multiple installations in a new-build site that has been developed/ designed with such a system in mind. This in principle is equivalent with an AROYH build as the factoring in of a rainwater harvesting system would be contiguous with the design. It typically takes 1.5- 2.0 kWh to pump 1 cubic meter of water (1000 litres). For a typical house using rainwater for WCs, washing machine and the garden, pumping costs are between 5-10p per week. There also exist options for solar water pumps that could be incorporated into a modified system.
Potential Development
Commercial systems also employ similar principles with larger and more sophisticated filters and pumps that have improved efficiency when compared to their domestic counterparts. The use of the technology in commercial/industrial applications has the additional benefits as 80% of water needs in certain businesses/industries is for non-potable water, and could therefore be provided by rainwater sources. Large commercial/industrial buildings with large roof areas have the potential for higher rainwater ‘yields’.
Theoretically there exists potential for the pooling of the rainwater resource at a community or neighbourhood level, using large storage tanks with a centralised filtration and pumping system. This would save on the energy cost of distribution and improve filtering efficiencies and maintenance procedure. However there currently exits no system of this kind in the UK and adjustments to existing models would be required, although this concept is useful for future consideration. Numerous rainwater harvesting systems are currently on the UK market, whilst the technology has reached maturity in the last 5-10 years it is currently employed in general at the individual self build level with around 400 units sold each year in the UK. However on the European continent sales are in excess of 100,000 units per year.
Community installation will reduce costs and is likely to improve catchment efficiency. The use of water recycling or rainwater harvesting in combination with other water use reduction technologies such as low flush toilets etc must form an integral part of any sustainable housing development.
Grey Water Recycling
The principle behind grey water recycling is that much of the application of potable water in the home can be replaced by a clean non-potable substitute, derived from the waste from showers baths and hand basins. This water can be used in flushing toilets, washing machines, watering plants and washing cars etc.
The potential for grey water equates to around half the daily domestic usage of a household
The AQUA-Recycling Control ARC4-Standard
Grey water from sink, shower and bath sources is channelled in a separate waste system (to that of lavatory and washing machine waste) to the treatment process. This water is then pumped into a storage system that can be combined with rainwater storage.
The system employs biological treatment in a loop reactor with special aeration. Then the water is pressed through the filtration membrane plates by simple gravity. The pore size of the plates is 0.00005mm and guarantees complete retention of germs and bacteria (size of bacteria by comparison: 0.001 mm). The membrane is constantly cleaned due to air bubbles passing the plates.
The gravity-effect in combination with the air bubble cleaning ensures a continuous flow of filtrate and thus long service life with low maintenance. After the filtration the water is supplied to the applications. If there is a lack of treated Grey water the system will automatically switch to mains water supply and therefore ensure supply to the applications. All processes are monitored centrally by electronic controls. The intelligent control optimises the process with different patterns of consumption. Faults are displayed by an optical and acoustic alarm.
This process produces clean non-potable grey water that can be pumped into the home for use in flushing toilets, washing machines and car washing, gardening applications. This system has potential to save around 50% on home water consumption.
Ecoplay
The Ecoplay system won the sustainable building services awards best sustainable product 2007. The system is designed to reuse bathroom grey water to flush toilets
The maintenance-free Ecoplay unit can be installed in a standard size void, which can be concealed and decorated as normal. It collects bath and shower water, skims off light materials such as hairs from the surface and allows heavier materials to sink to the bottom. The remaining ‘clean’ water is then transferred to a storage tank ready to flush toilets, and with a storage capacity of 100 litres one Ecoplay unit can provide enough water for approximately 20 toilet flushes.
Ecoplay’s intelligent system manages the quality of the stored water and if the toilets are not flushed within 24 hours after a period of regular use, Ecoplay will purge the retained recycled water to waste in order to clean the system. It then imports a minimum of mains water so that toilets can be flushed. The Ecoplay unit requires little extra installation work and is equally suitable for all domestic projects whether single-storey or duplex, new-build or renovation.
This system is designed to be fitted in the void or studwork of a bathroom. Ecoplay does not produce water suitable for use in washing machines, gardening and car washing etc. This means that at maximum the system results in a 30% saving of potable water used for toilet flushing.
Pontos Aqua Cycle
The Aquacycle system appears to be the most established and widely used grey water treatment system. The AquaCycle 900® works by taking grey water from the shower, bath and sink, and recycles it for use in flushing the toilet, cleaning the house, washing clothes and watering the garden, without the use of chemicals. By mechanically and biologically cleaning water, it supplies clean, odourless water that conforms to the EU 76/160/EWG directive for recreational water. The AquaCycle 900® is suitable for the daily needs of a household of 4-6 people and, as well as being modular and self-cleaning, it's also silent and maintenance-free.
According to the manufacturer the average payback period for this size household is ten years. However when using the figure 0.18p per litre for UK water (Envirowise), a saving of 90,000 litres per year would equate to a £162 per anum saving based on these figures. With the retail price around £3,290.00 (incl VAT) (Hansgrohe) the payback time for the system appears to be more in the region of 20 years.
However there is significant variation in water prices across the UK. These variations are due to range of factors such water availability/infrastructure/logistics. This table shows various mean water bill prices per bedroom across the different water boards and the projected payback time for the Aquacycle 900® based on these figures.
The discrepancy between the Envirowise data and the mean water bill data can be explained by the fact that the mean regional water bills are not based on usage. The installation of a water meter is likely to contribute to a reduction in Water bills and incentivise a reduction in usage.
The next generation of ‘low impact’ homes will have low water usage toilets and appliances incorporated into their design and fitting out, these homes will have significantly reduced water usage. This means that the handling capacity of max 600l/d max of the 900 system may present a treatment surplus for one household. The system could be used to treat grey water from several new build dwellings such as the low impact developments of AROYH. The modular nature of the systems on offer means different combinations of systems could provide treatment solutions for multiple adjoining dwellings.
Code 5 and 6 houses must have a daily water usage of less than 80 litres per person. The use of a rainwater recycling or Grey water treatment system is likely to reduce mains water usage in a house by around a half, or 75 litres a day when 150 is taken as the current average figure. The application of one of these systems is a necessity if a house is to meet the higher code levels.
Combined systems
The use of rainwater or Grey water in the home is limited to around 50% of total usage .
Further treatment of water means there is potential for the use of rainwater to provide potable water within a household. However, if the rainwater will also be used to meet the potable water needs of a home's residents, it is important that the homeowner uses care in selecting materials and coatings which will come into contact with the water as it is collected, since some impurities can be picked up by the rainwater as it travels through the collection system.
Some materials, such as the asbestos roof materials used in older homes, should not be part of any system used to provide drinking water. In addition, products such as asphalt shingles can contribute particulate matter into the water, requiring additional filtration before the water reaches the storage tank or cistern. Lead materials in any form, such as lead flashing, could be problematic in a rainwater collection system.
There are a range of options for bringing water up to potable standards these include ceramic, carbon and Ultra Violet filters. Many of these are designed to filter mains water as it enters a household. The plumbing system of a household could be modified so that a portion of water is filtered to provide potable water for faucets and showers. The resulting grey water could then be recycled and used for the suitable applications of toilet flushing, washing machines and outdoor use. This hybrid system could result in a home that is almost fully self sufficient in its water usage although installing both Grey water treatment and rainwater harvesting with potable filtration would have high initial installation costs the long term environmental and economic benefits make the concept extremely attractive.
Hybrid system
© Donal Liam Kinnear Brown, June 2008
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