Paving the way for rainwater harvesting

Paving the way for rainwater harvesting

Robinson, Katherine

Each of us uses about 220 litres of water per day; this equates to filling and carrying around 23 buckets. Yet, from these 220 litres, less than 18 litres are actually used for life-sustaining purposes.

Although the vast majority of this water is used for nonpotable purposes, such as flushing the toilet, watering the garden, in washing machines and dishwashers, it is still being passed through expensive water purification processes and stored in massive land-consuming reservoirs. In industry, where the proportion of non-potable water usage is even higher, the potential environmental and financial savings to be made by water harvesting are considerably greater.

Rainwater harvesting systems

It is estimated that water bills account for more than 1% of company business turnover, and with water prices set to increase on average by 18% over the next few years, there is an ever-growing need to consider water conservation measures in both businesses and homes across the UK. Rainwater harvesting systems for secondary uses could account for up to half an organisation’s total water consumption.

One of the biggest challenges in specifying and implementing ‘recycling’ systems within a scheme or organisation has been the justification – a compelling return on investment that is measurable and delivers a payback in a relatively short timescale.

Formpave is known for its market-leading Aquaflow Permeable Paving and Sustainable Urban Drainage Systems (SUDS). The company’s patented Aquaflow paving system can deliver immediate environmental benefits and financial savings when compared with traditional drainage. One major saving is that the water harvested by the system costs nothing to an organisation and that they do not have to pay to dispose of large quantities of roof and car park runoff (see Figure 1). With over 600 sites now completed within the UK, a number of systems have been installed specifically to harvest rainwater, both from hard landscaping (see Figure 2) and roofs.

Case study

At Meadowside Primary School in Quedgeley, Gloucestershire, 2200m^sup 2^ of tanked system was constructed for the county council, with an increased depth of sub-base to provide a greater water storage volume (see Figures 3 and 4). The rain that falls on the playground and car park is harvested and reused to flush lavatories. Since the successful installation of this system, the school now uses three times as much water from the harvesting system as compared with mains water – which equates to a sizeable saving on the water bill.

Water conservation, recycling and reuse

The built environment provides two main catchment areas – the roof of a building and the associated hard landscaping, such as roads, car parks and paved areas. The incorporation of these hard landscaped areas around buildings has changed the course of natural drainage, often exacerbating the effects of flooding. This has resulted in changes to Part H3 of the Building Regulations with regard to Sustainable Urban Drainage Systems

The Building Regulations state ‘paved areas around a building shall be so constructed as to be adequately drained’… (providing)… ‘an adequate soak away or some other adequate infiltration system’… (where)… ‘methods of drainage, other than connection to a public surface water sewer, are encouraged where they are technically feasible.’ Incorporation of a water harvesting system to satisfy these requirements delivers both environmental and financial benefits.

Over a number of years, Formpave has worked closely with Coventry University to research and develop permeable pavements and sub-bases that retain and clean storm water runoff. Through this partnership the company developed its market-leading, patented Storm Water Source Control System.

The Formpave SUDS system allows rain to infiltrate through a permeable concrete block paved surface (see Figure 5) into a unique stone sub-base where it is cleaned, by filtration and microbial action, before being released in a controlled manner into sewers or watercourses, or infiltrated directly into the sub-grade. The water can also be harvested and used for non-potable purposes, such as flushing lavatories and horticultural uses.

Building a water harvesting system

It is vital that the specification of a water harvesting system considers the technical points that are outlined as follows.

The laying course and sub-base

The granular stone laying course and sub-base must be laid exactly to specification so that the crushed gravel and rock not only provide 30% voids for water attenuation but also act as a trickle filter to remove pollutants, such as heavy metals and organic matter. The 30% voids ratio also creates the right environment for the naturally occurring microbes, which degrade oil dropped by vehicles.

The inclusion ofgeotextiles and/or a membrane

Depending upon whether the site is a tanked or infiltration system, the sub-base is underlain by an impermeable membrane, or Inbitex geotextile. Tanked systems are suitable for use when harvesting water for reuse or discharging water in a controlled manner into watercourses or sewers. Infiltration systems are suitable for use when infiltrating the water directly into the sub-grade, for example to recharge groundwater base flows. Both systems incorporate a layer of Inbitex geotextile beneath the laying course. This is where silt and loam that enter the system are held and retained. Heavy metals that are washed into the system have an affinity to the silt and loam and adhere to them. Trials at Abertay University have shown that the system retains 85% of heavy metals that enter the system.

Concluding remarks

Suitable for pedestrian, trafficked and heavy-duty applications, Aquaflow blocks are manufactured to BS EN 1338(1) and can be used in the same way as normal paving blocks. Special vertical drainage channels allow water flows of 9000mm per hour (9000 litres per nr per hour) to infiltrate the sub-base below. In 2002 Formpave received the Queen’s Award for Enterprise, in the innovation category, for their patented Stormwater Source Control System and Aquaflow range of Permeable Paving. It has licencees for this system in six overseas countries.

Reference:

1. BRITISH STANDARDS INSTITUTION. BS EN 1338: Concrete paving blocks. Requirements and test methods. 2003.

KATHERINE ROBINSON, FORMPAVE

Copyright The Concrete Society Sep 2005

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