When it rains, water from most urban buildings and hard surfaces is directed through downpipes, drains and gullies into the drainage system. Some downpipes from the roofs of domestic properties are directed into a soak-away and so put to ground but in urban areas, or from large buildings, this is not generally the case. There are huge consequences of this water management system:
- Drain systems can't cope in times of peak rainfall, causing flooding
- New drainage systems cost more to build, to cope with increased peak loads
- Groundwater levels drop as water doesn't permeate through and replenish the watertable
- Water management, and our bills, become more expensive.
- Low groundwater levels mean water shortages!
- A HUGE resource is wasted, both on a national and individual level.
Earthships are energy and water independent
When it comes to the last point, we finally have something that we can work with, as individuals. The average house roof in the UK sheds some 45,000 litres of water per year, and a household of three consumes about 175,000 litres of tap water per year. Nearly 25% of the water we currently use could be harvested from our roofs! Furthermore, by efficient use and reuse of water this could be cut to well under half. In fact in the few examples of UK houses that are designed to run off-grid, they are not connected to any mains services at all. “Earthships” are one example of housing designed to run off-grid (right).
So, what can we do to capture some of this rainwater, and why should we bother when we can turn on a tap. In fact, if you live in the SE region of the UK, you might not be able to take that for granted for much longer. Already this year (Feb 2006) the authorities are talking about water shortages and hosepipe bans, after two very dry winters. We can expect this pattern to become more severe. And self interest immediately kicks in with the prospects of using standpipes! Those on water meters (and we soon all will be) also have an obvious reason to save water. Other environmental reasons are more philanthropic in their rewards, but just as important.
The simplest way to capture rainwater is to use diverters on your downpipes and feed this into a rain barrel or barrels linked in series. You can buy manufactured systems and water butts or set up your own reusing large plastic containers, such as those sold off from companies that purchase (non-toxic) water-based liquid or food supplies. Dustbins can be used, although they are strictly not meant to store water and may not be strong enough, nor have well sealed lids.
All this can look a little unsightly, to say the least. If the optimum figure to store is 5% of annual roof capacity (industry standard calculation), then for an average house that's around 2,200 litres, or over 20 dustbins! It is also fiddly to effectively use this water as the rain-barrels are unlikely to all be in one place.
A better, but more costly, system is storage in an underground tank, from which the water is pumped on demand. This greatly increases the usability of the system as the water can then be used to flush toilets, feed the washing machine (perfectly safe and soft, ie no calcium content) and pressurise your garden taps. In fact, as only 5% of the water we use needs to be potable (drinking water), the uses are many. It is worth noting that the water from a green (living) roof may be less suitable for household reuse as it can be discoloured from passing through the roof substrate. There will also be much less of it, as green roofs retain most moisture, which is evaporated back to the atmosphere.
There is an expense involved in setting this system up and there are other considerations, such as mains connectivity in case of no rain for extended periods, filters, overflows to mains and electrical consumption for the pump (this could be solar powered, of course). There is currently limited financial incentive for such systems and the payback, in hard cash may never fully materialise. A part of the problem may be that manufacturers tend to over specify and overcomplicate simple solutions, pushing up production and installation costs. There are always cheaper alternatives, especially for the DIYer. However, as energy decline and global warming become rapidly more serious over the next decade, water harvesting will become more viable and indeed, a necessity.
If environmental improvement is the main concern, then rainwater could be fed into a landscape of dry swales, ditches and ponds and allowed to gradually permeate into the earth, enriching the groundwater table. This can create a vibrant and changeable garden ecology. Water could still be cleaned with bio-filters and used from open ponds, but only for garden watering due to the biological life inherent in pond water. This could be combined with the storage tank system to both reuse water and create wetland-style landscapes, especially when combined with greywater (everything but the toilet) cleansing. This may not be a viable option on heavy clay soils with low permeability.
Rainwater collection from driveways is another viable source and may capture as much again as the house roof does. However, because this is subject to pollutants, such as oil, this will need filtration and cannot be reused inside the house. However this is an ideal source for irrigation, perhaps leaving the roof catchments for internal reuse.
Urban planners and engineers are beginning to use this approach to counter the drainage problems of our towns and cities. Called SUDS (sustainable urban drainage systems), authorities, engineers and town planners are finally waking up to the realities and limitations of fragmented, over-engineered solutions.
Like a lot of sustainable design, these systems can often work best when implemented on a local community scale. A housing cluster around a village pond can use this as a stormwater catchment, greywater recycling (or blackwater too in a reed bed system), water reservoir for garden irrigation and of course, recreation and ecological diversity.
Whilst we might want to keep a reedbed sewerage system away from a central feature (they still smell a bit), it makes a lot of sense to clean and recycle greywater as this is a continuous source of water if it comes from the mains. Obviously, rainwater only appears, well, when it rains! First flush rainwater can be stored in a larger, community tank and pumped to a number of nearby houses for loo-flushing etc. Larger systems are cheaper to install than individual ones, which all do the same thing. however, more pipework is needed and stronger pumping facilities, plus a centralised management arrangement.
Such systems are still in their infancy and there is a lot of experimentation required to know just what is viable, and how exactly to do it. At the recent EcoBuild exhibition at Earl's Court, London (Feb 06) rainwater and greywater systems were noticeable by their absence. This situation will change, however - watch this space!