I recently gave a presentation to a local business club on the broader merits of water re-cycling including rainwater harvesting.  This is a synopsis of what was covered:

The collection and use of rainwater and greywater is widespread around the world.

Rainwater harvesting is common in less developed countries, where it can provide an important and low-cost primary water use.

The conservation of water is becoming increasingly important - nearly 40 per cent of water companies believe that demand for water will outstrip supply by 2030

The conservation of water is becoming increasingly important – nearly 40 per cent of water companies believe that demand for water will outstrip supply by 2030

In the UK it is currently being used to supplement or reduce the use of mains water.

It can vary from a simple attachment of a water butt connected to a downpipe, to a larger system with a centralized collection and treatment system serving a number of people.

Having said that, the demand for water has dipped over the last couple of years, the recession is also having an impact, but demand is projected to increase on this resource in the future, whilst the supply may be adversely affected by global factors such as climate change.

This results in an increasing need to use water more wisely.  Alongside this is the desire of water consumers to reduce costs. Together, these factors mean that conservation of water is becoming increasingly important.  Recent research has found that almost 40 per cent of water companies believe that demand for water will outstrip supply by 2030.

One of the claimed benefits of rainwater, and greywater systems is that they reduce the cost of mains water use.  The payback periods vary, depending upon several factors

The cost benefit analysis is particularly sensitive to assumptions regarding component and overall system lifetimes.

The main barrier to widespread adoption of domestic rainwater systems in the UK is the cost.

Because of the number of variables, it is difficult to suggest costs and paybacks for a typical domestic system, but it seems likely to be in excess of 20 years even in an optimum situation with no major equipment failures (which seems unlikely).  Therefore, at the present time, small plumbed rainwater systems are unlikely to be economically attractive if the financial value of water saved is the only consideration.  The best one may consider in a domestic situation is to break even, but again, it is dependent upon a number of factors which will vary with locations.

There is however a demand for systems from environmentally conscious consumers and those who may value rainwater more highly than mains water.

Other drivers for using rainwater systems may be related to development and planning issues.  Some local authorities may insist that developers consider rainwater systems, whether to help balance local water resource demands, or to help balance local runoff.  This is normally linked to sustainability issues and Local Agenda.

Rainwater use should always be considered a part of a water conservation strategy.  Few buildings have exhausted the possibilities for conventional low cost measures such as taps that reduce flow, low-volume-flush WCs and waterless urinals.  Reduced water consumption, by using water efficient appliances, will allow more end-use demand to be met.  A fixed volume of rainwater can either serve the same end use more times, or serve different end uses as well.  Using water efficient appliances in combination with a rainwater system may make it more viable for some end users.

Having established an interest in rainwater and greywater use, it is vital to carry out an in-depth investigation on their viability, a comprehensive water audit.  It is usually most cost-effective for businesses to first implement water efficiency measures including better water management practices, such as repairing leaks, and not leaving taps dripping and fitting water-saving products. Some businesses may not find it cost-effective to go beyond this, but those with higher water usage may find that utilising water re-use in addition to implementing water efficiency can bring financial and other benefits.

Benefits of rainwater and greywater use include water savings for the end user and the potential to reduce their costs.  Reduced pressure on water resources and the supply infrastructure will only be realised with widespread uptake and this is unlikely at present.

Uncertainty over the volume of water saved, water quality, lack of design guidance, health risks and user perception, have all been identified as barriers that need to be addressed for rainwater and greywater systems.  The main barriers are though economic and allied to this system reliability.

At the present time, rainwater and greywater systems are economically viable where water consumption is above average, and there is sufficient rainfall.

In general, greywater systems are unlikely to be economically attractive where mains water is readily available but in specific circumstances could be economic, but this needs to be evaluated carefully case-by-case.  As systems evolve, develop and improve in the future, they will become more reliable and hence, economical.


About thewastewaterman

Chartered civil engineer with 30+ years' experience, specialising in the treatment and management of waste water. Expertise includes rainwater harvesting, water recycling, natural wastewater, reed beds, drainage, stormwater management (SuDS) and sludge/slurry

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