Rainwater harvesting part 2

Recent trends

The code for sustainable homes and its equivalent for commercial buildings, BREEAM has pushed the profile of rainwater harvesting from an enthusiastic technology amongst environmentally aware self-builders to become a construction material.  It is some way short of becoming a standard building material, found in every good builders merchant, or even something that every contractor knows enough about.  But rainwater harvesting is certainly on the right track.  The British standard, BS8515:2009 published last year set out new standards and really defined the different types of systems available, how to size storage and what to look out for.  The rainwater harvesting market in the UK is worth in the region of £12 million (at the time of writing) and is expected to increase 20% year on year for the foreseeable future.

Who’s buying these systems

Until say the last five year, the main market for rainwater harvesting or recycling systems was the environmentally aware self-builder, wanting to install a system purely to either reduce his/her environmental impact of the new dwelling or perhaps in some cases reduce the impact of the future cost of water.  Either way, they usually installed the systems themselves.  During this time a few developers were adding systems to the odd house within a larger development as a “trial”, expecting the regulations/customer demand to increase.  Commercial buildings were also early adopters of the technology, often because of the “business case” in other words a financial payback.  Generally domestic system have a very long payback and systems are not generally purchased on this basis.

Hosepipe bans & water restrictions

Whilst in the UK we have had fairly wet summers in recent years, in 2006 we did have water restrictions and that led to an increase in rainwater harvesting systems.  Tradition water butts have a capacity of say 200 litres each and as you can imagine this supply wouldn’t last year long during a dry period and this led to customers seeking a better solution.  Freerain and other supply 2-6,500 litres for garden only systems.  Retro-fitting a rainwater harvesting system is difficult, due to altering pipe-work and drainage runs, but a garden only system, generally doesn’t need for internal pipe-work and therefore, it is more attractive to retro-fitters.  See Freerain’s garden rainwater harvesting systems. A rainwater recycling harvesting system can be used during water restrictions, providing the water company enforcing the ban, hasn’t supplied the water.

What do these systems actually do?

In domestic terms, providing there is enough water from the roof, a system can provide around 50% of the total water demand.  This is the WC, washing machines and outside taps.  For garden systems, it is quite difficult to give percentages, but there is no reason why a large garden system shouldn’t be able to satisfy the vast majority of the demand.

Commercially it is very different and largely depends on the the type of commercial building.  An office for example might use 80% of its water for WC flushing, therefore, if the roof can supply enough water, the office could displace 80% of its mains water usage.  More likely, is the roof would/could only supply a proportion of that.  A warehouse with a massive roof, might only have 10 people actually working and therefore, would supply all of the non-potable water required.  Or the same warehouse might be washing 50 trucks a week and require even more.

What about off-grid/full potable upgrading

Whilst it is possible to completely upgrade rainwater for potable standard (that this drinking and bathing), generally it is not recommended as it requires fine filtration and a form of sterilisation.  This is usually carbon filter(s) down to 5 microns and then ultra-violet sterilisation to upgrade the water.  Whilst this might sound a good idea, there are two main issues to consider.  Firstly, would the roof supply enough water and secondly, the upgraded kit requires extra power and consumable parts.   I believe it is only viable if the property is not connected to mains water.

How rainwater harvesting meets the Code for Sustainable Homes

The datum for water consumption is 150 litre per person per day or 54,750 litres per year.  In order to achieve the highest levels of the code for sustainable homes this needs to be reduce to 80 litres or 29,200 for new dwellings.  With water saving measures such as aerated taps, shower heads and low-flush WC it is possible to reduce the 150 to say maybe 120 and then a rainwater harvesting system should be able to satisfy the remaining reduction.  The problems occur for higher occupied smaller dwellings.  It is currently not clear the best approach in this circumstance, but it can be tackled by communal systems and/or grey-water recycling systems.  I won’t be going into any detail on this, but hope to in future weeks.


Rainwater harvesting is becoming a main stream building/contractors product and as regulations become tighter it will become an effective tool, as part of tool box of techniques used by specifiers, developers and end-users alike.