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With water shortages reported recently in Australia, in many of the Mediterranean countries and in a string of other countries across the globe, we are increasingly becoming aware of the fragility of this planet's future and our place within it. We have huge quantities of water on the blue planet, yet only 2% of it fresh water. We have a growing population, set to hit nine billion by 2042 - that's 2.7 billion more than now - so not an inconsiderable rise in numbers. Not unreasonably, they will all want drinking water, bathing water, better food and a better quality of life - all of which demand more high quality water.
For some, the answer to all potentially catastrophic situations is to rely heavily on currently non-existent technology emerging from somewhere in time to be the white knight that saves us from the horrors of the moment.
So first of all let's ask the fundamental question - why do we want to harvest rainwater? The answer is simple. Because it's free and with the changing weather patterns we are experiencing, it often arrives in huge quantities that sees much of it wasted as it either makes its way into the river network or worse, if the rivers can't cope with the run-off it causes - results in floods that can damage crops and cause soil erosion. Neither of which are helpful to a world that needs more food and more land to grow it on. Unless we harvest it, the rainwater has a tendency to flood us out of our homes and then disappear eventually into streams and rivers before it heads for the sea.
We have to stop seeing rain as an inconvenience and begin to see it as a valuable resource.
Whilst it comes from the sky, you never know exactly in what quantity or exactly when it will come. So instead, we currently participate in a major waste of expensive, cleaned water. For years, here in the UK we have happily cleaned the water supply before flushing it down the toilet. Actually and metaphorically.
Do we need to use potable, drinkable water to flush our toilets? To wash our cars? To water our gardens? Clearly the answer is no.
A clean source
Water that falls from the sky is in the main clean and an excellent source of water for those tasks that don't involve drinking, cooking and personal hygiene. What would make huge sense is if all homes and businesses had two sources of water - one for the aforesaid drinking, cooking and personal hygiene and another for flushing toilets and washing cars and arguably our clothes too. That might happen in time but probably not without legislation, but in the meantime, many people are beginning to look at the option of voluntarily harvesting the rainwater that falls on their roofs, that then makes its way through a mesh filter, into a holding tank, usually underground, before pumping it from that tank to be used in a number of non-potable applications. This can clearly reduce the demand on the potable water supply provided by the water companies and it can reduce water costs - particularly if you have a metered water supply. It is of course imperative that you have the facility of ‘topping up' your water needs from the mains should the supply in your harvesting tank fall short for any reason. But it's only a necessary ‘belt and braces' solution.
There are of course practical limitations on rainwater harvesting. Any aerial photograph of a large town or city will highlight immediately a wide selection of potential rainwater gathering surfaces. Even in rural areas, a number of potential rainwater gathering sites will stand out like a sore thumb. Large flat areas - usually roof areas - will collect the largest quantities of water and that water, channelled into drainage systems where the clean rainwater can be stored underground in a large tank for future usage, can be used for a range of uses from irrigation on farms to toilet flushing in office buildings.
Harvesting water
It is possible to harvest rainwater from surfaces other than roof areas but with this comes additional problems. Road surfaces for example might seem like a good place to collect rainwater from - we've all seen huge quantities of rainwater surging down the gutters before heading down the drains to be disposed of - usually through the sewage system. With gathering rainwater from such surfaces comes the very real problem of gathering sediment and whilst you will get some sediment in roof gathered rainwater, it is nothing like as much. Built-in filtration systems will remove the levels of sediment comfortably from roof gathered rainwater - much more robust filtration would be required for rainwater gathered from other surfaces and at this moment in time, it makes sense to harvest from the most obviously simple and most cost effective sources. The need to harvest from more complex surfaces may become necessary in the decades ahead of course.
When deciding on whether your specific roof is right for rainwater harvesting, you have to look at the roof area and its collecting ability in conjunction with the number of people in the property where you are considering using a system and incorporating rainwater harvesting and utilisation. A tall, narrow office block with a relatively small roof area and huge numbers of people working in it may not be ideally suited for a rainwater harvesting project. Typically in offices, around 60-65% of all water usage is to flush toilets and urinals. However, it might be ideal for a factory with a large roof area relative to the number of employees, or even on a smaller scale, a new home for example, where there's a large roof area relative to the number of people living in the property. So each opportunity is different and you have to look carefully at each case to see if it's a viable option.
It's clear that rainwater harvesting is a technology that has a lot to offer and its one that is of growing potential and real importance as water becomes rarer in many areas of the world and more expensive in real terms, pretty much everywhere. It's also a technology that is being considered much earlier in the construction process. There's no real problem adding rainwater harvesting technology to an existing building - it can be done, but if it's planned into a new building as part of the construction process, it clearly makes storage of rainwater and distribution of that rainwater to the required outlets in the building far simpler. And this is the trend that is being seen across the world. Forward thinking that incorporates in some cases the facility for vast amounts of water to be stored underground for both on-going and later use.
Providing enough potable water to meet the demand here in the UK is an on-going battle for the water companies. Taking full advantage of this wonderful free resource seems to make a huge amount of sense, whether on a micro or macro basis.
