Thursday, July 29, 2010

The energy cost of water

I have long been pondering the carbon footprint of the tap water we so take for granted.  Water is heavy, viscous stuff, and it takes a lot of energy to collect, deliver, and process.

Recently I came across a figure: running a tap for 5 minutes represents the same power consumption as running a 60 watt bulb for 14 hours. Unfortunately I have lost the source for that.

But today I get a figure from the Energy Saving Trust. "The energy needed to treat and pump mains water to our homes, and to collect and treat waste water from the sewage network, is responsible for nearly 1% of the UK's annual greenhouse gas emissions."

OK. The UK Carbon Footprint Project says: The UK's carbon footprint is over 500* million tonnes of CO2 per year. 1% of 500 is 5, so the embodied CO2 in our annual water is 5,000,000 tonnes, right? or 5 trillion grammes? (Correct me if I'm wrong, I am poor at sums).

Now, Water UK says "Each day the UK water industry collects, treats and then supplies more than 17 billion litres of high quality water to domestic and commercial customers and then collects and treats over 16 billion litres of the resulting wastewaters, returning it safely to the environment."

So 33,000,000,000 litres a day, or 12,012,000,000,000 litres a year. Divide CO2 in grammes  by volume of water in litres to get number of grammes of CO2 per litre, right?

= 0.42 grammes CO2 per litre.

In fact, 1 litre of tap water is about twice that, 0.8 grammes CO2, say, since once it's out of the tap, most of it has to be dealt with as sewage. Of course, sewage also  picks up some rain water runoff.

I should now go and check this against the thing of 5 minutes tap run = 14 hours of a 60watt bulb, but I need to go and have a little lie down after all this calculation.

Anyway, it all goes to prove that we shouldn't poo in our drinking water.


Jesse said...

Cannot fault your calculation at a quick glance! Seems like a small number to me.

Tiny compared to the 40% of annual emissions or so from electricity generation.

DocRichard said...

Of course, most of the power to shift water about comes through electricity.

DocRichard said...

Each person in the UK uses 150 litres a day. I make that 44 Kg CO2 a year per person through water usage.

Unknown said...

150 litres sounds quite conservative, it takes 4 litres to flush a loo. we're frugal with water as it always seems crazy that we flush our toilet with with drinking water. although we use shower water to flush ours and use our dish wash water to water the garden and we pump the washing machine water into a butt to water the garden too - it is still wasteful and we should do better.