Costs: Rainwater harvesting

There are many types of rainwater recycling systems, from the most simple water butt intended 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 then pumped to a gravity supply tank or directly to the end application.

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 PVCu - 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 insitu) and may be located below ground or above. Galvanized 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 to 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 more than 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 five 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.
• Maintenance 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 requires 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 that influence the best-value calculations include:

• Rainfall Can you collect enough rainwater? Mean annual rainfall in England varies dramatically, with values of 500 mm in the Thames Estuary compared with 5000 mm in the Lake District. The average for England is about 900 mm. The mean annual rainfall for Scotland is nearly half as much again, at about 1500 mm.
• 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, BRE and the Environment Agency.