Rainwater harvesting

The Watercycle at the Milennium Dome 2000

1 Introduction
2 Policy
3 Types of System
4 Green roofs
5 Calculations
6 Cost-Benefits
7 Conclusion
8 Related articles on Designing Buildings
9 External references

[edit] Introduction

Rainwater harvesting (RWH) is a process of collecting and storing rainwater that falls on a catchment surface (typically a roof, although almost any external surface could be suitable) for use, independent from, or supplemental to the mains water supply. This reduces demand on the mains supply, offers some resilience from local supply problems and reduces the amount of energy used for water treatment and transportation. Collection and diversion of surface run-off can also mitigate flood risk and control drainage as part of a sustainable drainage system (SuDS).

Rainwater is a relatively clean water source, needing only minimal treatment (generally UV filtration). Collected water can be used for non-potable purposes such as flushing toilets and urinals, supplying washing machines, irrigation systems, vehicle washing, sprinkler systems and so on. It is claimed that up to 50% of domestic and 85% of non-domestic mains water supply can be replaced in this way (ref Kingspan Water).

Rainwater harvesting systems range from the humble water butt used to water domestic gardens, to schemes such as the Millennium Dome in London in 2000, where rainwater was collected from the 100,000 sqm roof and filtered through reed beds in the landscape before being returned to the Dome and used to flush the 700 toilets. In this case 'greywater' from handbasins was also recycled, and the water supply was supplemented by groundwater from under the Dome site.

Rainwater harvesting at the Millennium Dome.

[edit] Policy

Rainwater harvesting is increasingly important in urban water management strategies in the UK. It is considered to have an important part to play in 'Future Water', the government's water strategy, and in October 2012, Schedule 3 of the Flood and Water Management Act 2010 was enacted, requiring sustainable drainage of surface water to be included in developments that require planning approval or have drainage implications (see sustainable urban drainage systems).

The Code for Sustainable Homes has provisions to restrict surface water run-off for new build developments and encourages fitting underground tanks to new-build homes to collect rainwater. In addition, rainwater harvesting scores highly in the BREEAM and LEED rating systems.

BS 8515:2009 'Rainwater Harvesting Systems, Code of Practice', establishes standards for the installation, testing and maintenance of rainwater harvesting systems for non-potable applications. It includes standards for filtration, for the manufacture and installation of storage tanks and a series of approaches for calculating the sizes of tanks. Annex C also sets out colour standards for pipework to distinguish it from the mains supply.

[edit] Types of System

There are three main types of rainwater harvesting system:

Water collected in storage tank(s) and pumped directly to points of use.
Water collected in storage tank(s) and fed by gravity to points of use.
Water collected in storage tank(s), pumped to an elevated cistern and then fed by gravity to the points of use.

The first option is viable in domestic buildings as the water does not have to be pumped a large distance. For larger commercial buildings the second or third options are likely to be more efficient. More than one storage tank may be used in large buildings.

Innovative tank design means that the collected water can be stored in large-volume bladders, in plastic matrix tanks, in-slab tanks as part of the building structure, as well as more conventional ferrocement or glass reinforced plastic below-ground tanks. Often there are benefits to modularity, where several tanks are linked together to enhance final water quality (through additional settlement), increase the volume stored, or simply spread the cost (the storage component is typically the most expensive part of a rainwater harvesting system). Inconsistent rainfall patterns are buffered by scaling the storage tank appropriately.

Rainwater harvesting systems can be found worldwide, and whist tending to be suited to the tropics or 'rainy' areas, can be applicable in arid or semi-arid areas.

[edit] Green roofs

Rainwater can be collected from green roofs, although the volume will be much reduced (small rain events may produce no run-off), and some substrates may be more RWH-friendly than others - specifically those with a high mineral content.

Leached nutrients, vegetative matter, sediments and organic load (including bacteria) mean that the quality is lower than water harvested from a 'normal' roof. Fertilizers that may cause contamination should be avoided. The main concern however is the brownish colour that rainwater collected from a green roof tends to acquire. For this reason it is recommended that the collected water is used outside buildings, ie for irrigation, where discolouration is not such an issue.

[edit] Calculations

The Environment Agency suggests a shorthand method to determine the storage tank size for domestic buildings:

Size of storage tank (litres) = Annual rainfall (mm) x effective collection area (m2) x drainage coefficient (%) x filter efficiency (%) x 0.05

Where:

Annual rainfall (mm) is the average yearly rainfall.
Effective collection area is the area of the roof.
Drainage coefficient depends on the roof type, (see below).
Filter efficiency is specified by the manufacturer, commonly 90%.

Dividing the answer by 1,000 will give the size of the storage tank in cubic metres.

Drainage Coefficients:

Pitched roof - 0.9
Pitched roof with riles - 0.8
Flat roof with gravel layer - 0.8

Guidance for non-domestic buildings is available in the British Standard Code of Practice 8515:2009 – Rainwater Harvesting Systems. This gives a more detailed approach based on more accurate demand data where demand is variable throughout the year, as well as in-depth rainfall data, such as the daily rainfall in the area for the past 5 years. These values are then used in a computer model which predicts the systems likely behaviour over time.

UK rainwater harvesting system suppliers commonly use short-hand methods when designing storage tanks for their systems (ref 10). There are, however, more detailed models better suited to the design of RWH systems and their performance and these are researched in 'Rainwater Harvesting: Model based design-evaluation' (Ward, et al., 2010).

A comprehensive assessment of modelling methods has been produced by Roebuck (ref 5). Roebuck shows that previous reports showing the cost-benefits of rainwater harvesting systems are unreliable and have not been extensively investigated. To properly determine the feasibility of a rainwater harvesting system all possible costs must all be incorporated into the financial analysis. In addition, mains water prices have been increasing and should be predicted to continue for the foreseeable future.

[edit] Cost-Benefits

Potential savings can be assessed based on:

The demand for non-potable water.
The amount of rainwater it is possible to collect.
Whether or not the property is on a water meter.
How much the system will cost to maintain.
The energy cost associated with pumping harvested rainwater. A method has been developed by Ward, et al. (ref 4) to estimate energy costs associated with rainwater pumping. It assumes the system used has a header tank and assesses volumes of harvested rainwater against pump parameters to estimate the energy used by the pumps.
Investment capital for the full system.
Pump replacement costs.
Maintenance costs.
Mains water supply charge.

Savings are likely to be higher in large commercial buildings as they tend to have a larger roof catchment area, as well as a greater demand for the non-potable water than private property.

[edit] Conclusion

Pros:

Planning friendly (and for Code for Sustainable Homes / BREEAM / LEED rating systems).
Reduces runoff from site.
Takes pressure off mains supply and stormwater systems.
Adds value to development.

Cons:

Perceived health risk. It is important to ensure that all pipework containing non-potable water is clearly labelled.
Payback is often more than 5 years.
Systems need regular maintenance; cleaning filters every few months, cleaning roofs annually and replacing UV lamps.
Space requirement for storage tanks.
May reduce flowrates in cold water systems below the norm which can cause other issues.
The results in (Roebuck, 2008b ref 5) suggest that rainwater harvesting systems are not always cost-effective, as the long-term savings tend to be less than the initial capital invested. It may still be worthwhile if the funds for installation are not met by the client but rather are provided through government investment. However, the report only takes into account systems on a single-building scale. On a larger scale with a number of buildings the overall savings may be sufficient to return the investment.
The Environment Agency suggest that reducing water consumption is generally more energy efficient than rainwater harvesting (ref Environment Agency: Re-using and harvesting water).