Rainwater harvesting offers a sustainable solution
to water shortages in these hosepipe-banning times.
Peter Mayer of Building LifePlans looks at the
options and analyses the whole-life costs.
There are many types of rainwater recycling systems, from the most simple water butt for garden use to a system that offers complete self-sufficiency without the need for utility mains water or sewage connections.
A typical system, somewhere in the middle of this range, will supply untreated rainwater for toilet, outside use and laundry. Water is collected from the roof, fed through a filter into a storage tank and the
Whole life performance issues
Water distribution pipes
Plastics or stainless steel pipes are commonly specified such as thermoplastics to BS 7291 - which include polyethylene, polybutylene and PVC - and stainless steel to German and US standards. Pipework is expected to have a service life of more than 50 years. Copper pipe should be to EN 1057. To prevent the risk of copper pipes corroding because of organic material and the acidity of rainwater, organic material should be filtered out and the system be flushed with mains water. This should be done before using rainwater, to allow formation of a protective patina lining to the pipes.
Water storage tanks
Typically tanks are made from plastics such as high–density polyethylene or glass reinforced plastics to BS 4994 or concrete (precast or in situ) and may be located below ground or above. Galvanised steel tanks lined with butyl rubber are used above ground in industrial or agricultural applications. Underground tanks would have a service life in excess of 60 years. Above ground tanks require space but have much reduced installation costs - however, plastics, exposed to weathering and solar radiation would have a considerably reduced service life of 20 – 30 years.
Filters, pumps and controls
Rainwater collection devices filter out leaves, dirt and excess water. Stainless steel, copper or zinc types would be expected to last over 60 years, as would underground filters made of plastics.
Submersible pumps located in the tank are sealed, and if there is a fault they have to be fully replaced. Life expectancy is related to running hours - typically 5 to 10 years. Alternatively pumps may be located outside the tank and connected to suck water out of the storage tank. This is a higher capital cost option but pumps can be serviced and failed parts replaced, which saves having to replace the whole pump, thereby reducing whole-life costs. Electronic controls have expected service lives of 10 to 15 years.
Rainwater recycling systems incur periodic inspection, servicing, cleaning and component replacement costs. Maintenance costs can be minimised by using self cleaning pump filters. An allowance should be made for unplanned maintenance.
Whole life cost factors
In contrast to a conventional water supply and drainage system, a rainwater recycling system comprises additional components, which result in higher capital costs. A whole-life cost analysis should determine whether or not the future water cost savings can offset the higher capital costs. Factors which influence the best value calculations include:
• Rainfall Can you collect enough rainwater? Mean annual rainfall in England varies by a factor of over 10 with low values of 500mm in the Thames Estuary compared with 5000mm in the Lake District. The average for England is about 900mm. The mean annual rainfall for Scotland nearly half as much again, at about 1500mm.
• Pattern of water usage Domestic water usage varies greatly; daily usage of 40 litres per person is possible although the UK average is between 100 and 160 litres a day per person. The less water households use, the more likely recycled rainwater can meet the demand.
• Building use The potential for cost-efficient recycling rainwater is excellent for commercial and institutional buildings such as schools or offices where there are large roof areas and high non-drinking water use. Furthermore, businesses may claim Enhanced Capital Allowances for rainwater harvesting systems.
• Scale of development On large projects the costs of rainwater recycling facilities can be offset against the savings from not having to construct a traditional stormwater drainage system with a treatment plant.
• Water provider The lower the cost of water and standing charges, the longer the payback period for a rainwater recycling system. Water costs are likely to increase, making rainwater recycling not only a sustainable option but a more cost-effective option.
Detailed advice and guidance on rainwater harvesting has been published by research organisations such as CIRIA, CIBSE, BSRIA, the BRE and the Environment Agency.
Net present value for 60 years
Conventional system; Utility’s supply for all water use and stormwater drains for disposal.
ainwater harvesting system; underground tank, partial mains water use, soakaway for stormwater drainage
• Whole life cost what-if scenarios are modelled on 100mÏ area for rainwater collection, a four person household, water consumption of 100 litres a person each day, rainfall of 750mm, rainwater is used for toilets, laundry and external use.
• Rainwater recycling system based on 3000-litre plastics tank, stainless steel submersible pump, plastics pipework. Conventional stormwater system based on 10m pipework and inspection chamber.
• Cost data relates to capital costs, component replacement and maintenance, water supply and disposal costs. Service lives are as indicated in the text.
The analysis is very sensitive to the costs of excavations, pipe–laying, soakaway construction and maintenance - a whole-life cost analysis based on project specific information is essential for a realistic best value appraisal.