Water Saving

Rainwater Harvesting

The concept of capturing rainwater and storing it for later use is well documented from pre-Roman times and on all the major continents, although in industrialised countries, until recently, the practice had largely died away with the introduction of reliable mains-supplied water. With the ever-growing demand for water (and subsequent increases in cost), and the known adverse impacts this can have on local environments, the Irish market demand for rainwater recycling systems is likely to increase - mirroring what has already taken place in other industrialised countries.

The strange cycle of droughts and floods serve to highlight the importance of rainfall on people's lives.  Recent water-shortages tend to point up the fact that both time and water are running out – and we have to act now.  A proven, cost-effective way of addressing this particular problem is through the use of rainwater-harvesting and grey water harvesting systems. Rainwater systems were once widespread but  fell out of fashion with the introduction of mains water supplies. It is likely that they will experience major growth over the next decade as the effects of climate change are more noticeable.

A rainwater harvesting system, collects water that falls onto the roof of a property for subsequent use in non-potable applications, such as toilet flushing, clothes washing machines, car washing and garden watering. Typically, independent trials have shown that a domestic rainwater harvesting system reduces mains-water consumption by around 50%.  Similarly Grey Water from domestic use can be captured treated recycled and reused.

When light rain falls, much of the water is re-absorbed into the atmosphere, rather than finding its way into the water table or reservoir system. A rainwater harvesting system, however, intercepts this water conserving it for subsequent use. Conversely, during heavy downpours, rainwater harvesting systems help to alleviate flood-risks by easing flows into the storm-water management grid, minimising waste and disruption. Use of harvested rainwater for non-potable applications also saves the energy that would otherwise be wasted in bringing water un-necessarily up to mains, drinking water standard. The following facts illustrate where grey water recycling and rainwater harvest can play a part.  A typical domestic rainwater harvesting system provides around 50% of a household’s total consumption. The roof on a typical 4-bed family home captures more than 100,000 litres of water each year.  A typical family uses 70,000 litres each year on toilet-flushing, clothes washing & outside us.

Development pressure for housing will increase the emphasis on the use of grey water and harvested rainwater to reduce demand on mains water, to help manage development storm-water management, and to save on the energy used in producing potable water.

Areas of application

Today rainwater is still often considered a problem because it can cause flooding. The perception is that rainwater needs to be drained and disposed of as quickly as possible.

Rainwater harvesting aims to change this perception by retaining the water for non-potable use, turning it into an asset rather than a problem. Rainwater is free of charge and can be used for many commercial applications in:

Factories, Offices, Hotels, Plant Growing Nurseries etc.

Rainwater can be used in many areas of application to substitute mains water:

Reasons for rainwater harvesting

Potential reasons for installing rainwater harvesting systems include:

Financial reasons:

·        Reduction of water costs

·        Possible avoidance of surface run-off charges

·        Reduced capital & installation cost for attenuation (by substituting harvesting tanks for storage tanks)

Promotional reasons:

·        Enhanced image by showing investment in environmental technology

·        May assist in planning applications

Technical reasons:

e.g. rainwater is soft and is therefore well suited for cooling systems, for cultivating plants and for many process uses.

Ecological reasons:

• Reduced consumption of potable water
• Reduction of stormwater discharge

General concept of grey water and rainwater harvesting systems

There are a variety of systems on the market but the general concept is

always the same:

1. The water is first filtered and then stored.
2. Water is then pumped either directly to the points of use or to an internal break tank.
3. If the system runs low on rainwater a mains water back up unit will guarantee continuous supply feeding mains water into the system.
4.  When it rains again, the system changes automatically back to the rainwater supply.

Typical Integrated Domestic System utilising both Grey Water and Harvested Rainwater

Typical system components are:

·        Filtration – depending on contamination of the water and on requirements, different methods of treatment are used, but always a pre-storage filter employed.

·        Storage – to balance water yield and consumption. Different material are used; plastic, GRP, concrete

·        Technical components – pumps, controls, mains back up

·        Overflow – storage has an attenuation function but secure drainage is needed to soak-away or storm drain.

Note: At the point of use separate pipe work is needed in order to avoid potable water and non-potable water coming into contact.

Rainwater yield, water consumption and quality aspects

When calculating the size of a system one must first look at the volume of water that can be collected (rainwater yield) and then compare it to the amount of water needed (water consumption).  Using the collection area, the local precipitation, filter efficiency and run-off factor the rainwater yield can be calculated. Additional water from some internal processes or other systems, as mentioned before, could be included. The water consumption is calculated by simply adding up the consumption of the different application areas that can be supplied by rainwater. Sometimes it is useful to consider using different water cycles due to certain water quality requirements. The consumption of different applications is considered as well as the required water quality.

Water charges & economics

The economy of a system depends on various factors: Charges for water supply and sewage services can vary greatly and these charges can significantly affect the potential savings of any system. There may also be storm water charges to be considered. The capital cost of the installed system is affected by several factors, including the equipment used and the ground conditions on site. Ongoing costs that should also be taken into consideration are the energy cost of running pumps etc and cleaning/maintenance costs.

There may also be interest on the capital investment to consider.

Sustainable Drainage

(SUDS)

Sustainable Drainage is an environmentally-friendly way of dealing with surface water runoff to avoid problems associated with conventional drainage

practice. These problems include exacerbating flooding. This new approach to drainage is often termed “SUDS” (or sustainable urban drainage systems), but applies equally to rural and urban sites.

The basic requirements of sustainable drainage are ;

• Water run-off from an area following development should be no greater than it was before development.
• Following development, there should be no deterioration in downstream waterways or habitat
• Water resource management be integrated into the design of a development from the outset

A wide variety of off-the-shelf sustainable drainage solutions are readily available commercially, which quite simply are designed to avoid, or delay (giving drainage infrastructure more time to cope), more rainwater leaving a site post-development than before.

Examples of how this can be achieved are:

• Soakaways - Where ground conditions permit, modern soakaways built to modern void-efficiency ratios can continue to play an important part in achieving sustainable drainage provided the dangers of infiltration-based contamination are considered and avoided.
• Rainwater Harvesting - Collecting rainwater for re-use, makes excellent commercial sense. Typically, with domestic installations a rainwater harvesting system will reduce the demand for treated mains water by up to 50%, while in commercial applications even greater savings can be achieved. Such systems therefore provide a double environmental benefit – firstly by contributing to sustainable drainage, and secondly by reducing the demands on water treatment plants.
• Grey Water recycling – Collecting and treating grey water from domestic and commercial/industrial premises allows reuse of a resource which will otherwise be wasted
• Attenuation - Where the other on-site sustainable drainage technologies are insufficient to contain run-off rates at the required level, under-ground storage containers – usually in the form of tanks, large diameter pipes, or plastic “crates” can be used to store large volumes of water which can then be safely released over time at a controlled rate. This can make a very important contribution to flood avoidance where existing infrastructure is already operating close to its limit predevelopment.
• Landscaping Solutions - In a properly integrated design, sustainable drainage can play an important part in adding to the visual impact of a development. Depending upon the type and style of a development, some or all of the following can be considered with, by employing suitable associated vegetation, potentially stunning results:

·        Permeable pavement areas – which can be achieved using a range of materials and technologies

·        Swales (shallow ditches) and basins can be used to hold water for gradual water dissipation into soakaways or held in a “balancing” pond, or wetland.

PIW August 2007

"With thanks to various sources including The UK Rainwater Harvesting Association and Rainwater Harvesting Ltd"

The UK office is best positioned to provide this advise but please do not hesitate to contact either office.