Pee-cycling
20 December 2006
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Graham Lawton
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You recycle your household waste. You buy locally grown food, fit low-energy
light bulbs and try not to use the car unnecessarily. Maybe you even
irrigate the garden with your bath water. But you've still got an
environmental monster in your house. Your toilet is wrecking the planet.
Before you point to the brick you've put in the cistern, it's not about the
water - well, not entirely. The big problem is pee. Your pee. Do you flush
it away without a second thought? Tsk, tsk. Lose the green halo.
At first sight urine looks like an unlikely environmental menace. What harm
could come from flushing away a fluid that is mostly water, plus a smidge of
proteins and salts? Surprisingly, the answer is "a lot".
The problem with urine is that it is the main source of some of the chemical
nutrients that have to be removed in sewage treatment plants if they are not
to wreck ecosystems downstream. Despite making up only 1 per cent of the
volume of waste water, urine contributes about 80 per cent of the nitrogen
and 45 per cent of all the phosphate. Peeing into the pan immediately
dilutes these chemicals with vast quantities of water, making the removal
process unnecessarily inefficient.
To be fair, if you use conventional western plumbing there's not an awful
lot you can do about your personal pee-print right now. A lucky few,
however, live or work in one of the buildings in continental Europe where
you can find a future must-have eco-accessory: the urine separation toilet.
These devices divert urine away from the main sewage stream, allowing the
nutrients to be recycled rather than treated as waste. They could solve all
the environmental problems associated with urine and even turn sewage plants
into net producers of green, clean energy.
"If you use conventional plumbing There's not a lot you can do about your
personal pee-print right now"
So how do standard sewage systems deal with urine? Known in the business as
"yellow water", urine enters the sewage system and mixes with solid waste
("black water"), "grey water" from household sinks and baths, and sometimes
rainwater. It eventually arrives at a treatment plant, where it must be
cleaned up enough to be discharged into a river.
The first step is to filter the sewage to remove large objects such as
condoms, tampons and a random assortment of dead goldfish and false teeth.
What remains flows into settlement tanks, to allow the faeces to sink to the
bottom. This solid sludge is separated off and stored in oxygen-free tanks,
which are gently warmed for about two weeks. Bacteria break it down,
generating methane gas that can be burned to produce electricity. The end
product is an inert solid that is usually burned or dumped in landfill.
Meanwhile, the liquid portion of the sewage flows into oxygenated "aeration
tanks". Here microbes guzzle the nutrient-rich organic material and multiply
like crazy, converting nutrients into biomass. This eventually sinks to the
bottom of settlement tanks as yet more sludge, while the liquid heads off
for a final, energy-intensive "polishing", which strips out any leftover
nitrogen and phosphate that the aeration stage couldn't get rid of.
Minority pursuit
The whole process is very good at converting yellow, black and grey water
into more or less clear water, but all that pumping, stirring, aeration and
heating uses a lot of power - about 11.5 watts per head of population.
That's only a tiny portion of your personal daily energy consumption, but it
mounts up. In the UK, population 65 million, it means waste water treatment
consumes 65,000 gigajoules a day - about a quarter of the output of the
country's largest coal-fired power station. In a world where energy
efficiency is ever more important, that's not to be dismissed.
According to civil engineer and urine-separation expert Jac Wilsenach, it's
highly inefficient. Wilsenach spent six years at Delft University of
Technology in the Netherlands working on the intricacies of sewage systems,
and according to his calculations, today's methods mean we are literally
flushing energy down the pan.
In a paper published last March, Wilsenach calculated that if we were to
separate out just half of our urine, the microbes in the aeration tanks
could eat up almost all the nitrogen and phosphate (Journal of Environmental
Engineering, vol 132, p 331). The energy-intensive polishing stage would
become completely unnecessary.
There's another, even bigger gain to be had from separating out the urine.
Lower starting levels of nitrogen and phosphate mean that the microbes in
the aeration tanks can do their job much more efficiently, taking just one
day compared with about 30, thus reducing the energy demand of the aeration
tanks. What's more, the resulting sludge is richer in organic matter and
generates more than three times as much methane. In fact, says Wilsenach,
separating out 50 to 60 per cent of the urine could turn sewage works from
net consumers to net producers of energy to the tune of about 2.5 watts per
person.
So far so good. But how do you stop pee from getting into sewage in the
first place? The answer is to install a special WC called a
"urine-separation toilet" or often just a NoMix, after one of the leading
brands.
On casual inspection a NoMix toilet looks pretty much like a normal one. But
peer into the bowl and you'll see that there are two waste pipes - a small
front one and a larger rear one. The front one collects urine and diverts it
into a storage tank (sometimes aided by a tiny trickle of water) to await
its fate. The rear works like a standard flush toilet.
You don't even have to do anything special to make this separation happen -
apart from one thing. "The toilet is constructed in a way that if a man or
woman sits on the toilet most of the urine is collected," says Bjartur Swart
of engineering firm Grontmij in Drachten, the Netherlands, which is
conducting urine separation trials across the country. Yep, that's right. In
the urine-separating future, men will sit down to pee.
Although small-scale urine separation has been practised for centuries (pee
has been used in industries ranging from textile dyeing to blacksmithing,
for example), it is something of a minority pursuit today. Modern
experiments started in Sweden in 1994 with the founding of two "ecovillages"
- Understenshöjden in Stockholm and Björsbyn in the far north - whose houses
and apartments were fitted with urine-separation toilets. There, the urine
is stored for collection by local farmers who use it as a fertiliser.
Other villages have followed suit and Sweden is now the urine-separation
centre of the western world, with around 3000 NoMix toilets in use. Denmark
has also set up urine-separation projects including one at the Svanholm Gods
farming collective near Skibby, the largest producer of organic vegetables
in the country (bear that in mind if you ever buy organic vegetables from
Denmark).
The urine from all these projects ends up being sprayed directly onto
fields, which works fine as long as only a few people are contributing. It
doesn't take much, though, for supply to start outstripping demand.
"Recycling urine directly is not feasible in cities, but that is where the
focus should be because that's where the biggest amount of waste comes
from," says Wilsenach, now at South Africa's national research institute
CSIR in Stellenbosch.
So what to do with the urine? The answer is, recycle it indirectly - in
other words, extract the nutrients and turn them into fertiliser. In the
Netherlands, Grontmij trucks the stored urine to a special treatment plant
where the phosphate is precipitated out as a mineral called struvite
(ammonium magnesium phosphate). This is a useful fertiliser and can help
reduce demand for mined phosphate, which can only be a good thing: phosphate
rocks are often contaminated with heavy metals, and mining and refining them
generates waste and uses lots of energy. Some estimates suggest the world's
phosphate mines will be exhausted in 100 years. Yet at the moment we
literally pour tonnes and tonnes of perfectly good phosphate down the drain.
Green halo
The other nutrients in urine can also be turned into fertiliser. Novaquatis,
a branch of the Swiss Federal Institute of Aquatic Science and Technology
(EAWAG) on the outskirts of Zürich, is experimenting with extracting
nitrogen and potassium in forms that can be sprayed directly onto crops.
Once the urine is treated it is clean enough to go directly into a river.
Crucially, these methods of extracting nutrients directly from urine consume
much less energy than dealing with its vastly diluted form in general waste
water. There's an extra energy cost from trucking the urine in, but
Wilsenach says it's minuscule compared with the savings.
If all these benefits weren't enough, using a NoMix toilet saves water too.
According to research done by EAWAG, it reduces your use of flush water by
80 per cent, cutting the average household's overall water use by about 25
per cent. Bear in mind that the water that fills up the toilet cistern is
clean enough to drink: "We use good quality drinking water to flush away
urine," says Wilsenach.
So where next for urine separation? Grontmij and Novaquatis have set up
pilot projects across the Netherlands and Switzerland, and a handful of
places are now doing it for real, including the public library in Liestal,
Switzerland. Swiss citizens can even buy their very own NoMix toilets and
storage tanks if they want, even though the sewage system is not yet ready
and the urine ends up flowing into the waste water stream as normal. Urine
separation is also taking root in Austria and Germany.
In surveys, people say they would be happy to use NoMix toilets and buy
vegetables fertilised with processed urine. There is even a way round what
could seem the biggest obstacle to widespread acceptance. "If a man doesn't
like sitting, he can urinate just in a normal way and use the [front] hole
as a target," says Swart.
One day we may look back at our habit of flushing pee away with drinking
water as staggeringly wasteful. "Water and waste are two of the greatest
challenges the world faces at the moment," says Jacob Tompkins, director of
Waterwise, a London-based water efficiency campaigning group. "Anything that
looks at our low-efficiency way of dealing with the waste stream is
extremely important."
Of course it would take time and money to convert existing sewage systems.
But even if urine separation isn't coming to your area any time soon, that's
not an excuse for inaction. Keeping urine out of the waste stream any way
you can pays dividends. So what are you waiting for? Next time you need to
take a leak, give the bathroom a miss and head straight for the flower beds.
Then you can replace your green halo.
>From issue 2583 of New Scientist magazine, 20 December 2006, page 45-47
http://environment.newscientist.com/article/mg19225831.600-peecycling.html