Water Usage And The Green Roof Water Recycling System

The Naked Scientists spoke to Professor David Butler, University of Exeter
05 March 2006

Interview with 

Professor David Butler, University of Exeter


Chris - Tell us about your work.

David - I'm very interested in water and reusing water because at the moment we use a lot of water and we waste it in my opinion. We've been looking at ways to reuse water where we can.

Chris - Have you got any figures up your sleeve as to roughly how much water we do use on the average day in the average household here in the UK?

David - The average person uses about 150 litres per day. Now if you imagine 150 one litre bottles sat on your table, that is an awful lot of water. Of that 150, we only drink maybe a litre or so.

Chris - To put that in perspective, if I was someone in Africa, how much water would I use in a day, assuming that I don't live in on eof the big cities?

David - 10 litres maybe.

Chris - So therefore we're actually 1500 per cent over what we actually need to get by.

David - That's absolutely right.

Chris - So we have a terrifically wasteful culture. What's your solution?

David - My solution is to try and reuse water that we've already used once before. We can take water from our showers or baths and try and get it back to a quality that's suitable for, let's say, flushing a toilet. Why do we need to flush our toilets with drinking water?

Chris - It's always surprised me. Is it just because it would cost the environment so dear in laying down a second set of pipes to deliver slightly less pure water to every house?

David - Yes and it's very expensive. Some places in the world where they are very short of water do that. But we thought it might be worthwhile looking at whether we could recycle water at the smaller scale rather than the larger scale.

Chris - So what is your solution?

David - It's to try and treat this grey water using a low-tech solution. You plant vegetation, plants and flowers into a gravel bed, maybe on top of a large building on a flat roof and trickle this grey, once-used water through it. We then use these natural processes to get it back to a suitable quality.

Chris - Ok, that's the theory behind how you set it up. But what's happening if you zoom in to the small scale? What are these plants doing and what is the gravel doing to clean the water? David - In the gravel you're getting a build up of micro-organisms that will remove some of the polluting material. The roots of the flowers will also open up the pores and draw in oxygen as well.

Kat - What sorts of polluting materials are these?

David - They're oxygen-demanding material. If you put this water straight into the environment, it would suck the oxygen out of rivers and so on. We don't really want to be flushing our toilets with that sort of water.

Chris - So in other words it would suck the oxygen out the environment because it would promote the growth of bacteria due to the phosphates and things in it. It's also bacterially laden too.

David - yes, and we're actually asking our bacteria to grow at a specific point and we're making them a nice environment in which to do that.

Kat - My mum does things like putting the washing up water out in the garden and I've often wondered whether you could put a plastic tub in the shower and catch some of the water that you don't wash with. Is that not so good to put on the garden?

David - Well I wouldn't really recommend putting it on your tomatoes for example.

Chris - In case someone's peed in the shower! But how does it actually work?

David - There's no problem at all with water with pee in it and unless you've got an infection, it doesn't have bacteria in it. And anyway, the water that we're dealing with would be very low in urine concentration.

Chris - So what you would then do would be collect the water that comes from a certain set of appliances such as a washing machine and pipe that off separately to the flower bed.

David - Yes, to the flowerbed on the roof and trickle it through there.

Chris - So you have pump it up?

David - Yes, and that's going to consume energy, which is the negative side of this.

Chris - Why is it on the roof?

David - To save space and then we can use gravity to get it back down into the building.

Chris - So it trickles down through the plants. How much water can this flowerbed process?

David - This can more or less process all the water we need for flushing our toilets and that is about a third of the total amount of water we use individually.

Kat - Does it collect rainwater as well?

David - Yes, that's another option. We can certainly collect rainwater and rainwater is of better quality than this grey water. We've certainly been looking at techniques for recycling that as well. You do need a big tank to collect rainwater if you want a good secure supply.

Chris - Is the water that comes out safe?

David - It's safe for flushing the toilet and I think it's safe for doing garden irrigation, but I wouldn't water my tomatoes with it.

Chris - Do you need a special type of plant that you need to do this or will any old plant do?

David - Well this is exactly what we're trying. We're trying local species of plants to see how well they do and that's why this system is under development. We want to see which plants will do what.

Chris - And what size flowerbed do you need to process the output from a standard family?

David - Well more or less you need a couple of square metres for each individual.

Chris - That's not much.

David - No, that's not a lot and that's why we think that this is a promising technique.

Chris - What about in winter time though when the whole thing freezes?

David - That's a good question. It shouldn't freeze because grey water will be warm from your showers, and we also collect the water in a tank in the house and store it. But we are a little bit concerned that some of our plants will die off in the winter so we're looking for hardy varieties that will grow throughout the season.

Chris - Interestingly, in China they're trying to prepare air quality for the Olympics. China has air quality that's so bad that on over 150 days in the year Beijing has air unfit for humans to breathe. They're trying desperately to try and improve it, so they're implementing a series of roof top gardens. They have a particularly hardy type of grass which they're planting on all their high rise buildings in an attempt to use it like the lungs of the city.

David - I think these green roofs are very important and promising for the reason you mention and we can also use it to treat our grey water, our rainwater, and they are insulating as well.


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