Harvesting rainwater for domestic uses: an information guide. Reference number/code GEHO0108BNPN-E-E

Transcription

1 Harvesting rainwater for domestic uses: an information guide Reference number/code GEHO0108BNPN-E-E

2 We are the Environment Agency. It's our job to look after your environment and make it a better place - for you, and for future generations. Your environment is the air you breathe, the water you drink and the ground you walk on. Working with business, Government and society as a whole, we are making your environment cleaner and healthier. The Environment Agency. Out there, making your environment a better place. Published by: Environment Agency Rio House Waterside Drive, Aztec West Almondsbury, Bristol BS32 4UD Tel: enquiries@environment-agency.gov.uk Environment Agency January 2008 All rights reserved. This document may be reproduced with prior permission of the Environment Agency. Environment Agency Harvesting rainwater for domestic uses 1

3 Contents 1 Introduction 3 2 Design considerations 4 3 Installation and long-term considerations 9 4 Examples of small-scale rainwater systems 16 5 Conclusions 20 Suggested further reading 21 Useful contacts and other example sites 22 Glossary of terms 23 Annex 1 24 Annex Environment Agency Harvesting rainwater for domestic uses

4 1 Introduction This publication is about rainwater harvesting systems for non-potable domestic uses in houses, gardens, industrial and commercial premises. Intended for homeowners, house builders, planners, plumbers, architects and building managers, it contains information about the benefits of these systems, their design, installation and maintenance requirements and their cost. It also contains examples of housing schemes where these systems are already in use. The recycling of bath and shower water (greywater) is not included in this document. Why choose a rainwater harvesting system? Despite the fact that England and Wales appear to have plenty of rain, our growing population and the changing climate mean that our water resources are under pressure. The large number of new houses to be built over the next few years will increase the competition for available water between the environment and people, especially in the south-east of England which has been designated an area of water stress. Reducing demand for mains water can help to reconcile these competing needs. One way of reducing demand is to use a rainwater harvesting system to provide water for domestic uses that do not require water treated to drinking water quality. Rainwater harvesting systems can be installed in both new and existing buildings, and the resulting water used for all purposes except drinking (unless treatment to a potable quality is provided). There are no agreed water quality standards for rainwater use in England and Wales. If properly collected, stored and only used for non-potable purposes such as toilet flushing, harvested rainwater need not undergo any additional treatment such as chemical disinfection. Washing machines can also be fed by rainwater without disinfection, but occasionally colour and odour may cause a problem if the quality of the collected water is poor. When rainwater is used to supply a garden tap or rain water butt, care needs to be taken to ensure that the water cannot be accidentally drunk. The potential savings that can be made from rainwater harvesting depend on both the demand for non-potable water and the amount of rainwater that can be supplied, which depends on the roof area available for collection and the amount of local rainfall. Savings achieved by rainwater harvesting systems will be greater in larger buildings, such as industrial/commercial buildings and schools, due to their larger roof areas and potentially greater demand for non-potable water. In addition to the pressures on water resources, there are concerns over rainwater drainage from urban areas. Adverse impacts on flood risk and water quality mean that our existing approach to rainwater drainage systems will have to change. In England, Planning Policy Statement 25 (PPS25) on development and flood risk requires that planning authorities consider the effects of surface water drainage. This will result in increasingly strict controls on runoff from sites. The Environment Agency is promoting the use of SUstainable Drainage Systems (SUDS), including rainwater harvesting, to retain and control surface water. The need to reduce stormwater discharge rates may therefore be an incentive for choosing rainwater harvesting systems. Rainwater harvesting systems are not yet common in England and Wales, for two main reasons: The high cost of the systems compared to the low cost of water. Concern that the quality of the water may pose a health risk. Additionally, only metered customers (all industrial and commercial customers and around 30% of domestic properties) will benefit financially by using these systems. The majority of domestic customers, who don t pay for water by volume, have no immediate financial incentive to install rainwater harvesting systems. Environment Agency Harvesting rainwater for domestic uses 3

5 2 Design considerations Rainwater can be harvested from roofs and areas of hard standing, such as driveways. The most common method is to collect rainwater exclusively from roofs, as this avoids potential water quality issues from collecting water from more contaminated surfaces such as driveways, which can introduce oil and additional faecal material into the system. Rainwater may be collected and distributed in several different ways, and suppliers can advise on the best product for a particular situation. Figure 1 shows a diagram of one example of a system. System Design 1 Filter 2 Smoothing inlet 3 Suction filter 4 Pump 5 Control unit with intermediate storage 6 Water level monitor 7 Automatic change over 8 Type AA air gap 9 Overflow trap 10 Permeable pavement 11 Oil trap Figure 1. Schematic of a typical rainwater harvesting system Rainwater is collected from the roof area or hard standing by downpipes. A filter (1) stops leaves and other large solids from getting into the holding tank. The water enters the tank through a smoothing inlet (2), which stops sediment at the bottom of the tank from being disturbed by rainwater entering the tank. A suction filter (3) prevents the uptake of floating matter when the water is drawn up for use. As the water is non-potable, it travels through a separate set of pipes, as specified in the Water Supply (Water Fittings) Regulations A pump (4) pressurises the water. In this example the pump is submersible, although other systems may use suction pumps, located outside of the tank. The control unit (5) monitors the water level in the tank (6) and displays this information to the user. If the water level in the tank drops too low, the control unit will trigger an automatic change over to mains water supply (7). The system must have a type AA air gap (8) installed (see glossary) in order to prevent back flow of rainwater into the mains. 1 Water Supply (Water Fittings) Regulations 1999, Statutory Instruments No. 1148, No. 1506, Water Industry, England and Wales, HMSO 4 Environment Agency Harvesting rainwater for domestic uses

6 When the water in the tank reaches a certain level, an overflow trap (9) allows floating material to be skimmed off into the storm drain. A non-return valve should be fitted to prevent contamination of the tank by backflow, together with a rodent barrier (see glossary). Water soaking through a permeable pavement (10) can also be collected, although as well as a filter, an oil trap (11) should also be fitted. Collecting water from this source increases the potential for oil and animal faeces contamination of the rainwater stored in the tank. To overcome this, additional filtration and disinfection may be necessary. The choice of collection surfaces should be considered on a site by site basis. It is usually simplest to collect water exclusively from the roof of a property but collecting from additional hard surfaces increases the yield and may be necessary in some cases. Variations on this system The design shown in Figure 1 and described above shows a system with a submerged pump, which directly feeds water using appliances via a small intermediate tank within the control unit to allow mains back up when rainwater is not available. Although rainwater harvesting systems share common principles and components, there are key areas where the approach differs. Three of these areas are discussed below: The method of distributing rainwater (direct feed or header tank) The location of the mains backup supply The type of pump used Direct feed or header tank Systems can either supply water using appliances directly with rainwater (direct feed systems). Or supply rainwater to a header tank in the loft (header tank systems). Where a header tank is used, the mains back up will feed directly into the header tank. In direct systems the approach to mains backup differs. A comparison of these approaches is given in Table 1. Table 1. A comparison of direct feed and header tank rainwater harvesting systems Direct feed systems Header tank systems Benefits Issues Benefits Issues Rainwater is supplied under pressure, which allows effective use in the garden or for vehicle washing. When rainwater is unavailable, mains water must be pumped, using unnecessary energy. Mains backup is direct to header tank, which allows mains water to be supplied to appliances during Pressure may be too low for modern washing machines and garden irrigation. WC s may fill slowly. Does not require a loft area. Tried and tested equipment available from European market where rainwater harvesting is more established. If the pump fails, no water will be available for appliances normally fed by rainwater. Mains top-up and controls may be more complicated. pump failure. Low cost pump and simple controls possible. Energy efficient as pump runs at full flow and mains water need never be pumped. Loft-Space is required for a tank. Limited range of systems commercially available. Mains backup Direct feed systems can either add mains water as required into the large storage tank which is usually underground or into a smaller, intermediate tank or cistern as shown in Figure 1. Some dual feed toilet cisterns are also available which makes a mains top up within the rainwater harvesting system itself unnecessary. Dual feed toilets are not common, but are a useful Environment Agency Harvesting rainwater for domestic uses 5

7 technology, because they allow WC cisterns to be supplied by harvested rainwater when available and by mains water (without additional pumping) when rainwater is not available. Locating the mains backup in the underground storage tank is often cheaper than using an intermediate storage tank, but using an intermediate storage tank allows mains water top-up closer to where it is required and therefore reduces the distance it has to be pumped. An intermediate cistern can also be smaller, allowing a smaller volume of mains back up each time it is topped up. Location of main pump In both direct feed and header tank systems, the main rainwater supply pump can either be a submersible unit in the main storage tank, or a suction pump outside the main storage tank. Suction pumps are generally located within the control unit and must be positioned relatively close to the tank, in frost-free conditions. Locating the pump in the tank (submersible pump) generally means that the pump will not be audible in the building. A submersible pump can also be more powerful than a suction pump. However, pumps located within the control unit are generally easier to inspect during servicing and maintenance and avoid the need for an electricity supply to run to the underground tank. Rainwater harvesting systems consist of components, such as tanks and filters, that are common to all variations of this technology. These components are discussed below. Tank Tanks vary in size from a small water butt to large underground tanks that contain many thousands of litres of water. A wide range of water butts are now available, made from a wide range of materials, from re-used, wooden wine barrels to imitation boulders and of course the familiar green plastic model. In the UK, larger tanks tend to be constructed from Glass Reinforced Plastic, Polyethylene or Concrete. Different tank materials suit different installations, and advice should be sought from a reputable rainwater harvesting supplier, see UKRHA for contact details 1. A tank should be located in a place that will moderate the water temperature, reducing bacterial growth in summer and frost damage in winter. The tank should also be shielded from direct sunlight, to avoid overheating and the development of algae. Usually the best solution is to house the tank underground. As the tank is often the most expensive part of a rainwater system, costs can be reduced by carefully considering how large it needs to be. The size of a rainwater holding tank must match the demand for water with its availability as closely as possible. The tank size chosen should be a balance between cost, storage capacity, and the need to enable an overflow at least twice a year, to flush out floating debris. A rule of thumb for household water use is to size the tank at 5% of the annual rainwater supply, or of the annual demand, using the lowest figure of the two. Industrial / commercial premises may need to make more detailed calculations. The tank size is calculated from the collection area, drainage coefficient, filter efficiency and rainfall. Tank size (litres) = effective collection area x drainage coefficient x filter efficiency x annual rainfall x 0.05 Details of the variables in the above equation are set out in Box 1. In order to optimise the tank size for a larger system, a more rigorous analysis is necessary, which can be provided by the supplier of the system, some large systems will use storage of less than 5%, but it is unusual to size the tank at more than 5% of available supply or demand, as this quickly becomes uneconomic Environment Agency Harvesting rainwater for domestic uses

8 Box 1. Variables in the tank sizing formula Effective Collection area The roof area is the length in metres (L) multiplied by the width in metres (W). Figure 2 shows the measurements needed to calculate the roof collection area, in metres squared (m 2 ). The effective collection area is equal to the area of roof or hardstanding that can be reasonably used to collect rainfall (in plan). This may not equal the entire roof area, as the arrangement of down pipes or location of different roof areas may not allow the entire roof to be used. The total rainwater collection area of a property can include both roof water run-off and hard standing infiltration into SUstainable Drainage System (SUDS), where demand is greater than the roof area alone can supply. The quality of run-off water should be good enough not to need extra treatment other than an oil trap (if a driveway is used). An area likely to be heavily contaminated with animal faeces is therefore not suitable. Figure 2. Roof catchment area Even if rainwater is not collected for non-potable use, PPS25 advocates that SUDS should be considered. SUDS can improve the quality of run-off water and reduce peak storm flows. This allows rainwater to be returned to the environment at an appropriate quality and flow rate for replenishing ground and surface water supplies. Drainage coefficient It is impossible to collect every drop of rain that falls on the collection area. Light rainfall will only wet the surface and then evaporate and heavy rainfall can overflow from the gutters and therefore not be captured. A drainage coefficient is used to adjust the tank size calculation to allow for this. Table 2, below shows which drainage coefficient (also known as the run-off factor ) to use for different roof types. Table 2. Drainage coefficient Filter efficiency Roof type Drainage coefficient Pitched roof tiles Flat roof smooth tiles 0.5 Flat roof with gravel layer The amount of water captured also depends on the efficiency of the filter. Not all the water in gutters will reach the holding tank. Most manufacturers recommend that 90% of the potential input can be used. To adjust for this a coefficient of 0.9 should be included in the calculation. Annual rainfall Annual rainfall can vary significantly over a small area, so a reading (in millimetres) within 10 miles of the site is recommended. Your local Environment Agency office or the Meteorological Office should be able to supply rainfall data. Using rainfall data in millimetres gives the tank size in litres, using rainfall in metres gives the tank size in cubic metres. Environment Agency Harvesting rainwater for domestic uses 7

9 How to use the tank sizing formula Consider a property with an effective collection area of 100 m 2. The roof area used in this example is a pitched roof with tiles, so a drainage coefficient of 0.9 is used. The filter efficiency is assumed to be 90%, so a filter efficiency coefficient of 0.9 is used. Annual rainfall is assumed as 905 mm/yr, the 1961 to 1990 long term average from met office data for England and Wales 1 (in a real example local rainfall must be used, as rainfall varies significantly across England and Wales). Table 3. Summary of parameters for tank sizing calculation Effective collection area (m 2 ) 100 Drainage coefficient 0.9 Filter efficiency coefficient 0.9 Average rainfall (mm/yr) 905 Tank size =100 x 0.9 x 0.9 x 905 x 0.05 = 3665 litres or 3665/1000 = 3.7 cubic metres (m 3 ) Treatment The first line of treatment is the filter. This should automatically prevent leaves and other solid debris from entering the holding tank. No further treatment is usually necessary for rainwater harvesting systems that collect water for toilet flushing and garden watering. Any taps fed by untreated rainwater should be clearly labelled, and where appropriate have their handles removed to prevent children from drinking the water. Other aspects of a rainwater harvesting system that cannot strictly be classified as treatment are also critical to maintain good water quality. The calmed inlet which stops sediment at the bottom of the tank from being disturbed and contaminating the stored rainwater is a simple but effective method of maintaining a consistent quality of water in the tank. The floating extraction with additional filter, gives additional filtration and ensures that the rainwater is taken from the tank just below the surface level where the water is cleanest. The overflow siphon allows floating material to be removed and must form an effective rodent barrier. To ensure good water quality the tank must be installed correctly in an appropriate location, preferably underground to avoid sunlight, give a constant temperature and avoid damage from accidental collisions. The use of rainwater for drinking is not recommended due to the potential for bacterial contamination from collection surfaces and difficulties of monitoring the water to ensure its safety. Pump Most commercially available systems use automatic pressure and flow-activated pumps. When a toilet is flushed the pump switches on and refills the cistern. Alternatively, rainwater can be pumped to a header tank from where it is fed to the WC. The differences between suction pumps and submersible pumps are discussed on page 6. A suitable, cool, frost-free location is required for the header tank. Dry-run protection should also be considered to protect pumps in dry spells. Control unit During dry periods, there may be insufficient rainwater to meet demand. A display on the control unit should indicate when tank water levels are low, so users are aware that they are receiving mains water Environment Agency Harvesting rainwater for domestic uses

10 3 Installation and long-term considerations The correct installation and maintenance of a rainwater harvesting system is key to ensuring that water is saved. A badly installed or maintained system may result in the owner losing enthusiasm for the system, which in turn may lead to it being ineffective or even removed. Plumbing legislation If a rainwater harvesting system is inadequately installed, it could become a public health hazard. Of specific concern is accidental contamination of mains water with rainwater. The Water Supply (Water Fittings) Regulations are designed to prevent this. The main safeguard that the legislation requires is that a type AA air gap must be used at the point where the mains top-up enters a rainwater harvesting system. A type AA air gap ensures that there is a physical separation between the two types of water, ensuring that no rainwater can be drawn back into the mains water supply. Advice on the Water Supply (Water Fittings) Regulations 1999 is given by the Water Regulations Advisory Scheme (WRAS) 2. Their Information and Guidance Note number dictates that all pipework must be marked. Pipes carrying non-potable water must be clearly distinguishable from those carrying mains water, to ensure that there can be no accidental cross-connection. Marking tape and warning labels are available from rainwater harvesting system suppliers. The WRAS Information and Guidance Note number , Issue 1, aims to support water conservation and to prevent reclaimed water systems from contaminating potable mains water supplies. The note offers information about installing, modifying and maintaining reclaimed water systems (including rainwater harvesting systems), and encourages a standard approach to their design and fitting. The note also provides a method for assessing health risks, depending on the end use of a rainwater harvesting system. It recommends: hazard assessments for each project; prevention of cross-contamination by clearly labelling pipework; screened inlets and outlets for rainwater storage tanks to prevent rodents and faecal matter entering the tank; insulation to minimise heat gain and frost damage to the system. Many companies in England and Wales now offer advice on the planning and fitting of rainwater harvesting systems. The UK Rainwater Harvesting Association (UKRHA) 3 is an organisation, comprising of suppliers and installers of rainwater harvesting equipment that is dedicated to safeguarding standards in this relatively young industry. The UKRHA website contains a list of members, which is a good starting point for anybody looking to purchase a system. For a nominal fee the Centre for Alternative Technology 4 also provide a list of suppliers. The water technology list is an online database of products that qualify for the Enhanced Capital Allowance scheme (ECA) and therefore another good resource for consumers. The ECA scheme enables businesses to claim 100% first year capital allowances on investments in technologies and products that encourage sustainable water use. The water technology list is therefore an excellent place for businesses to look for water using technologies, but also a useful resource for domestic customers who want to find out what products are available. 1 Water Supply (Water Fittings) Regulations 1999, Statutory Instruments No. 1148, No. 1506, Water Industry, England and Wales, HMSO Water Supply and Treatment Resource Guide, Centre for Alternative Technology Environment Agency Harvesting rainwater for domestic uses 9

11 Demand On average, a person in England and Wales uses around 150 litres of water per day, but this varies according to many factors. Metered water bills will show individual household consumption more accurately. Harvested rainwater is typically used to flush toilets and can also be used to supply the washing machine and water the garden. Figures 3 and 4 show where water is used in households 1. These are estimates based on modelling carried out by WRc and are used here as examples to demonstrate trends in changing water consumption. Figure 3. Micro-component water use in existing households Microcomponent household water use in existing build WC Basin Shower Bath Kitchen sink Washing Machine Dish Washer Outdoor Figure 3 shows that the largest single component of water use in existing households is WC flushing. Substituting mains water for rainwater here could clearly make significant savings. Figure 4. Micro-component water use in new households. Microcomponent household water use in new build WC Basin Shower Bath Kitchen sink Washing Machine Dish Washer Outdoor 1 Assessing the cost of compliance with the code for sustainable homes, Environment Agency (2007) 10 Environment Agency Harvesting rainwater for domestic uses

12 Figure 4 shows that the proportion of water used for toilet flushing is decreasing and the proportion used for showering and bathing is increasing. The total water use per person represented in Figure 4 is also lower than Figure 3 with new households using around 150 l/p/d and older households using around 167 l/p/d. In a new household around 35% of the total water use could potentially be substituted by rainwater. In a house with four people, this amounts to a potential saving of about 207 litres per day, or 75 cubic metres (m 3 ) a year. However, for a system to deliver these savings it would require a large collection area and high rainfall. Using average rainfall in England and Wales (905 mm/yr 1 ) and assuming a system efficiency of 60%, a roof area of 138 m 2 would be required to meet the annual demand of 75 m 3. In areas that enjoy less than average rainfall this roof area will need to be bigger still to satisfy the demand for rainwater. This suggests that the benefits of rainwater harvesting will be small in modest homes in areas with low rainfall such as Eastern England and significant in larger homes in areas with abundant rainfall such as South West England. While water consumption for bathing is generally increasing, in other areas of the home water use is decreasing. Progressively less water is used for toilet flushing, due to changes in technology and legislation (maximum toilet flush has decreased from 7.5 litres to 6 litres and most toilets sold are now dual flush). Toilet flush volumes are set to decrease further and toilets are currently available that flush 4.5 litres. Savings can therefore change in relation not just to the rainwater supplied, but also to the demand for non-potable water. Savings could be greater where rainwater is used for clothes washing. However this could potentially cause clothes to become discoloured or smell, due to contamination from leaf litter. However, some rainwater harvesting companies recommend using rainwater for clothes washing, claiming that a properly specified and installed system should not cause these problems. Interestingly, the Millennium Green study mentioned on page 18 involved using harvested rainwater to supply washing machines for one year. The study found that the water quality was acceptable for use in washing machines and no discolouration was reported. It is not however certain whether this good water quality is sustainable as collection surfaces age. Rainwater is soft (less limescale / hardness content), so substituting rainwater for mains water in hard water areas can reduce the amount of detergent needed and the build-up of scale in washing machines, therefore potentially increasing their operational lifetime. The amount of water used for domestic purposes in industrial / commercial premises will vary depending on the number of fittings and appliances and their frequency of use. When looking at ways to reduce water consumption, first consider simple efficiency measures such as changing water using practices and habits and installing simple water saving technology such as low flow taps, aerated showers and low flush toilets. This is often more cost effective than rainwater harvesting and, by reducing your use of hot water, you could also significantly cut energy usage. For more information on water efficiency see our website 2. Water quality and standards for rainwater use Rainwater is the source of all our water, it fills rivers, aquifers and lakes, from where it is abstracted by water companies for the public water supply. Before mains water is distributed, it is treated to make it safe for human consumption. While stringent standards guard the potable water quality in the UK 3, there are no standards for the quality of non-potable water. Defra intend to produce appropriate standards for non-potable water to overcome the concerns about potential health hazards and bolster public confidence in using non-potable water. The cost and practicality of long-term water quality monitoring of rainwater systems is a problem. One possible solution is to ensure that system design and installation standards prevent problems from occurring, rather than undertaking continuous water quality monitoring. Best practice guidelines are currently in progress through a number of routes. The Building Services Research Water Supply (Water Quality) Regulations 2000, Statutory Instruments No. 2000/3184 Environment Agency Harvesting rainwater for domestic uses 11

13 and Information Association (BSRIA), through the Market Transformation Programme (MTP) have developed a guide for specifiers of rainwater harvesting technology 1. The UKRHA are currently developing a code of best practice and a BSI standards development committee is looking at developing a British Standard for rainwater harvesting systems. The Water Supply (Water Quality) Regulations specify the quality standards for drinking water, but these seem too strict to apply to non-potable harvested rainwater. The standards in the Bathing Water Directive (76/160/EEC) 3 are considered by some to be more appropriate for nonpotable use, as water that meets the bathing water quality criteria should be safe for total immersion and occasional ingestion and therefore should also be safe for flushing toilets and watering the garden. It has been suggested that these standards are used as the minimum requirement for non-potable water quality. The MTP has recently looked at the potential for water quality standards for non potable water (rainwater and greywater). This work was carried out by BSRIA and built on principles that were previously discussed in the Buildings that Save Water project BTSW 4. The report, Rainwater and Greywater: Review of water quality standards and recommendations for the UK 5 found that the main hazard from rainwater or grey water is exposure to pathogenic micro-organisms derived from faecal contamination. Ideally, water quality guidelines should be specific to the end use and be based on a risk-assessment approach. In practice, using a risk-assessment approach is impractical because of the uncertainties concerning the composition of rainwater and greywater, the extent of individual exposures and the infective dose of pathogenic organisms. The report recommends that the guidelines should be based on the Bathing Water Directives (1975 and 2006) and identifies different guidelines for different end uses (see Table 4). The report also identified a traffic light system for interpreting the guidelines, which is included in Table 4. 1 Rainwater and Grey Water: A Guide for Specifiers, MTP (2007) 2 Water Supply (Water Quality) Regulations 2000, Statutory Instruments No. 2000/ Bathing Water Directive (76/160/EEC) 4 Rainwater and Greywater Use in Buildings: Project Results From The Buildings That Save Water Project; Best Practice Guidance (C539), CIRIA, Rainwater and Grey Water: Review of water quality standards and recommendations for the UK, MTP (2007) 12 Environment Agency Harvesting rainwater for domestic uses

14 Table 4. MTP proposed guidelines for the water quality of non-potable water from rainwater and greywater Potential sources of contamination Usually, harvested rainwater for non-potable use requires no more treatment than basic filtration to remove organic debris. Bird and animal faeces as well as decomposing leaf litter on roofs and in Environment Agency Harvesting rainwater for domestic uses 13

15 guttering could pose a health risk, if washed into a rainwater harvesting system. For this reason manufacturers of systems recommend that gutters are cleared regularly. As driveways may be contaminated with oil and more faecal material than roofs, collecting rainwater from these surfaces increases the potential risk. Using oil traps removes some of the oil, but odours may still be a problem. For these reasons some suppliers advise against using driveways to collect rainwater. The use of a suitable permeable pavement with appropriate substrate below can provide a reasonable level of treatment. Some types of roof cover are less appropriate for rainwater harvesting than others: asbestos-cement roofs can cause filters to block, and collected water may pose a health risk; metal roofs (except stainless steel) can release small amounts of leachates, which can stain water fixtures, for example green from copper; bitumen felt or coated roofs can lead to discoloration and odour problems; grass (and other vegetation) covered roofs reduce the amount and speed of run-off water, which might need extra treatment as the water is likely to be discoloured by soil. Maintenance This is not yet fit and forget technology. If the user is not relatively enthusiastic about the rainwater harvesting system, routine maintenance might not be carried out and benefits will be minimal. The following maintenance is recommended to keep the system running properly and avoid contamination: filters need cleaning around three times a year, depending on tree cover over the collection area; gutters need to be kept free of debris that could block the system; a visual inspection of the tank is recommended at least once a year. Excessive silt should be removed. the mains water top-up should be checked once a year, to ensure that it is functioning. Determining who will be responsible for maintenance and repairs of communal rainwater harvesting systems can be a significant barrier to the spread of rainwater harvesting systems. To counter this the Construction Industry Research & Information Association (CIRIA) have produced a guide which provides basic advice on the use and development of model operation and maintenance agreements for rainwater and greywater use systems together with simple guidance on their incorporation into developments 1. This guide is available from the CIRIA website (see further reading). For more information on the technical aspects of rainwater harvesting systems see the MTP reports, Rainwater and Grey Water: A guide for specifiers 2 and Rainwater and Grey Water; Technical and economic feasibility 3. Benefits Rainwater harvesting systems can reduce demand for mains water, making the available supplies stretch further, this also reduces metered customers water bills. Harvesting rainwater reduces the volume of surface water discharged to drainage and may contribute to reducing flood risk and the load on combined sewer overflows as well as improved river water quality. The main environmental benefit of these systems is that they reduce the volume of water that must be abstracted from lakes, rivers and aquifers. This water can therefore remain a benefit to ecosystems and help sustain environments. 1 Model agreements for sustainable water management systems. Model agreement for rainwater and greywater use systems (C626), CIRIA, Rainwater and Grey Water: A Guide for Specifiers, MTP (2007) 3 Rainwater and Grey Water: Technical and Economic Feasibility, MTP (2007) 14 Environment Agency Harvesting rainwater for domestic uses

16 Unlike greywater (see glossary), for most non-potable uses, rainwater does not require chemical or biological treatment before use. This makes the maintenance of rainwater harvesting systems easier and cheaper than greywater systems. Rainwater harvesting systems also use less energy than greywater systems. Costs In addition to the cost of components, there is also the cost of having the system installed. Setting the holding tank into the ground requires labour and excavation equipment. Both collection and distribution pipework needs to be fitted by a trained plumber. It costs less to install a system during construction, than to retro-fit a system to an existing building. The cost of the equipment needed for a basic household rainwater system starts at around 2,000. Plumbing and fitting costs can exceed 1,000, depending on factors such as soil type and size of system, and whether excavation equipment is on site. This cost will increase as the size of tank and the installation of a system become more complex. The financial benefits from a small-scale rainwater harvesting system are limited by the low cost of mains water in England and Wales compared with other European countries. However, this cost varies significantly across water company boundaries. Rainfall patterns and maintenance requirements are among the long-term variables that will also influence the savings made. Water bills include both mains water supply and wastewater treatment charges. Wastewater charges to a property on a water meter are based on a percentage of the amount of mains water supplied to the property. Reusing rainwater reduces the potable water volume, but the amount of wastewater returned to sewers remains the same. Therefore, theoretically the savings from reusing rainwater should only be based on the potable water use reduction and a slight reduction in surface water drainage charges. In practice, however, most water companies do not make the adjustment to accommodate the larger volume of wastewater compared to the potable use, so the wastewater charges also reduce with rainwater harvesting. This tariff discrepancy could become an issue if the uptake of rainwater harvesting systems increases in the future. German companies often measure or estimate the amount of rainwater in wastewater discharge, and charge customers accordingly. Water companies in England and Wales may consider using this method in the future. Suppliers of rainwater harvesting systems currently claim that systems can save around 50% of the water used in an average household. The volume that is actually saved depends on the supply of and demand for non-potable water. The amount of money saved also depends on the maintenance requirements and the lifetime of parts before they need replacing. Maintenance costs will depend on the user s ability to carry out simple maintenance tasks themselves, for example regularly cleaning guttering. Larger-scale housing developments with shared maintenance and infrastructure are more likely to make the systems financially attractive because of economies of scale and coordination of maintenance programmes. These systems also tend to be installed during construction, which makes them more cost effective than retro-fitted systems. In agricultural and industrial schemes, rainwater harvesting could be more cost effective because of the relatively high water use and large harvesting potential of roof and hard-standing areas. However, livestock farms should not use water draining from hard standing which has been in contact with livestock, since this run-off would have a high bacterial content. For most homes with gardens, the most cost effective rainwater system is a simple water butt. These hold around 200 litres and cost about 40. For a larger storage capacity, multiple water butts can be linked together, where there is sufficient space. Although they do not offer year-round savings, they can help reduce the peaks of summer water demand, by reducing a gardener s need for mains water. Environment Agency Harvesting rainwater for domestic uses 15

17 4 Examples of small-scale rainwater systems Around 2000 rainwater harvesting systems were installed in the UK in 2006/2007. This figure has increased from around 500 systems in 2003/2004 (data from UKRHA 1 ). Most of these schemes are domestic, although there are an increasing number of large commercial schemes - notably, some supermarket chains are now installing systems. The following schemes have published detailed reports of the experience gained from installing and monitoring rainwater systems. Housing Co-operative Hockerton Housing Scheme In 1998 the Hockerton Housing Co-operative in Nottinghamshire built five homes designed to minimise water and energy use. The site has been designed to allow the occupants to be independent of the mains water system. Water from the road, earth covered roofs and surrounding fields is collected via a series of dykes and channelled to a sump, from where it is pumped to a reservoir of 150m 3 capacity. This water is used for everything apart from drinking and cooking. It is passed through a sand filter, which removes particles and has some bacterial action. In addition to this, the housing scheme collects rainwater falling on the conservatory roofs, which is filtered and treated for drinking water. The 330 m 2 conservatory roof space can collect 200m 3 per year. The costs were calculated and reported in Saving water on the right track 2 2. Table 5 summarises the rainwater harvesting section of this report: Table 5. Initial outlay costs for the Hockerton Housing Scheme in 1998 Costs scheme-wide Unit cost ( ) Total capital cost ( ) Labour installation cost ( ) Fresh water collection and pumps (inc. reservoir) - 9,000 2,231 Conservatory drainpipes and gutters 1, Foul and rainwater drainage - 1,776 1,017 Total 1,737 11,276 3,248 Table 6. Annual maintenance costs Annual maintenance Time taken (man days) Cost ( ) Management of reed beds Emptying of septic tanks Change of filters 1 75 Maintenance of sand filters New filters - 40 Total Saving Water on the Right Track 2, Environment Agency (1999) 16 Environment Agency Harvesting rainwater for domestic uses

18 The benefits of this project were significant. Savings on water supply and sewerage charges for the five households were 1,000 in the first year alone. There are no external labour costs for the development, as occupants carry out their own maintenance The Hub Water and energy efficient community centre The Hub 1 is an innovative, multi-functional community centre, which has been designed to minimise its impact on the environment. It incorporates a rainwater harvesting system, which collects around 900,000 litres of water a year. This water is used for garden watering and for flushing toilets. As well as using harvested rainwater, the Hub also uses low flush toilets, showers with flow restrictors and automatic (infra-red) taps with spray fittings. A display board is used in the reception area to communicate to the public the amount of rainwater in storage and the amount of water being used. The water saving equipment is expected to save 1280 per year. This gives a calculated payback period of 11 years. Earthship Brighton The Earthship 1 is a community building and a model three bedroom house that is entirely independent of mains services. It relies directly on the sun, the wind and the rain to provide heat, power and water. The Earthship collects up to 73,000 litres of water a year from its roof. Once rainwater is collected it flows through two filters and is stored in four 5000 litre underground tanks. The water is then gravity fed to a series of filters in the Water Organising Module this treats the rainwater to potable standard ready for use in washbasins and showers. Greywater from showers and washbasins is also collected, treated and used to flush toilets. In addition to collecting rainwater and reusing greywater, the Earthship also uses water efficient kit. This includes a dual flush toilet that uses 2 litres for a reduced flush and 4.5 litres for a full flush. The Earthship approach to saving water is an excellent example, as it uses rainwater harvesting and greywater reuse alongside (not instead of) water efficiency measures. By combining alternative supplies with careful water usage, the Earthship has been able to operate without a connection to the mains water supply. The Earthship is an ambitious project which takes rainwater harvesting to another level, by using rainwater for purposes where mains water is usually required. This necessitates additional treatment and inevitably does not provide the same level of water quality assurance as mains water. Large savings can be made while still using mains water for certain purposes, but the Earthship takes this further, aiming to demonstrate that by combining a rainwater harvesting system with water efficient appliances it is possible to operate off the grid. Frog Firle Grade 2 listed Youth Hostel Frog Firle 1, historic youth hostel worked with South East Water to make use of a large, concrete underground tank built by a previous owner and originally intended for garden irrigation. This existing tank was adapted to act as a rainwater storage tank for toilet flushing in the youth hostel. Rainwater was collected from the roof area and transported via down pipes into the large underground storage tank. A mains backup system was also installed in the storage tank to ensure water was always available. From here the rainwater was pumped, using a submersible pump, to three GRP cisterns that were installed in the roof. Rainwater was then fed to toilets in the youth hostel. Due to the listed status of the building the installation was relatively expensive for a project that excluded a main storage tank. The total cost of the installation was 11,400. Special care was taken to ensure that the installation was in-keeping with the style of the building. This included constructing a replica down pipe to carry both the electrical wiring for the system and the pipe work from the underground storage cistern to the tanks in the roof. This project has reduced the water usage of the youth hostel by around 30 per cent. Frog Firle is an interesting example that was complicated by the historic status of the building. It highlights the difficulties of retrofitting, but also shows that although retro-fitting a rainwater harvesting system can be expensive, it is possible even in difficult circumstances. This rainwater 1 Water Efficiency Awards, Environment Agency (2007) Environment Agency Harvesting rainwater for domestic uses 17

19 harvesting system is also interesting because it makes use of existing resources on site, which keeps costs down and also avoids the environmental impact of manufacturing, transporting and installing a large rainwater harvesting tank. Gusto Homes Millennium Green Millennium Green 1 is a development of resource efficient housing, built by Gusto Homes. The houses each incorporate the individual Freerain rainwater harvesting system, as well as many other environmental features. The rainwater harvesting systems collect water from all available roof areas, filter it and store it underground in individual 3,300 litre tanks. The harvested rainwater is then used to flush toilets, feed washing machines and supply outside taps. Severn Trent Water and the Environment Agency took the opportunity to monitor the Freerain systems to see how effective they were at reducing demand for water. The performance of the Freerain systems was monitored in two properties for approximately one year. One property was a four bedroom detached house occupied by a middle aged couple and the second was a larger six bedroom detached house which was occupied by a family of five. This project showed that during the trial period, the smaller property was able to meet approximately 43% of its demand from rainwater and the larger property was able to meet about 37% of its total demand from rainwater. The year the systems were monitored was exceptionally wet, receiving 862 mm of rainfall. The average rainfall from 1995 to 2000 was 581 mm. This led to larger water savings than would be expected in a typical year. Both of the properties monitored were also significantly larger than average and therefore had a larger than average roof area (the effect of doubling the roof area is effectively the same as doubling the rainfall). The combination of the large roof areas and high rainfall in this trial mean the savings are not directly transferable to other schemes. Gusto homes calculated that the payback period for the systems, excluding energy usage and maintenance was around 18 years. This is likely to be significantly longer in a house with a more typical roof area in a year with average rainfall. The actual payback period will also be increased further by including operating and maintenance costs. Users perceptions of the systems were positive throughout the trial. One especially interesting aspect of this was that the users were happy to keep using harvested rainwater for clothes washing, which suggests that the quality of rainwater was sufficient for this purpose. The Millennium green is an example of the sort of savings that can be achieved with a large roof area and high rainfall and equipment installed during construction. This scenario shows significant water savings but does not offer an attractive payback period, especially since maintenance and operating costs are excluded. Additional examples of systems with quick payback periods Gusto Homes have provided some summaries of non-household installations that demonstrate quicker payback periods than the above example. These are summarised in Table 7 2. Table 7. Examples of two rainwater harvesting schemes with short payback periods Office in Manchester Community Centre in Kent Roof area (m 2 ) 3, Rainfall (mm) Tank Size (m 3 ) Collecting (m 3 ) 2, Supplying WC s, 550 employees WC / Clothes Washing Capital Cost ( ) 12,000 6,500 Annual water savings ( ) 4,000 2,200 Payback* (years) The Comprehensive Monitoring of a Rainwater Harvesting System and its Potential Impact on Future Water Supply Demand, Malcolm Day, Severn Trent Water 2 Presentation by Stephen Wright, Chief Executive, Gusto Group Limited (2007) 18 Environment Agency Harvesting rainwater for domestic uses