From: Mark Walker (mark@permanentend.org)
Date: Sat Aug 23 2003 - 13:29:10 MDT
> I would like to see someone cite some Google references on the actual
> land area required to produce 2500 calories per day. (There seems
> to be a lot of hand waving going on here).
>
To coin a phrase: it depends. It just so happens that we were discussing
something tangentially related to this across the street at wta-talk. Here's
the relevant part:
Is it possible to "close the loop" within
the confines of a city. By this I mean grow all the food for the city
residents and recycle all human waste. Here's my back of the envelope
calculations to say
that this might be possible: Take central Toronto as an example of a densely
populated "modern" city.
Land area per person = 264 m2 (density of 3,788 per km2).
(http://www.demographia.com/db-toronto-ward.htm )
Food can be grown by using an aquaponic set-up. Aquaponics uses fish waste
to grow plant matter, and growing plants to clean the fish water. Based on
research (both theoretical and practical see www.permanentend.org ) it is
possible to grow enough food for a person in a space that is 6m2. So for a
single person you might have a 2000 liter pond filled with about 100kg of
carp or tilapia. Water from the pond is used to irrigate plants grown
hydroponically above the fish pond. The hydroponic beds are made of crushed
stone--no soil is used in the system. The energy for this sort of system is
quite minimal--the pumps use about 40/watt hr per person, which could easily
be supplied by solar or wind energy. This handles the inputs, our food, what
about the outputs? Based on calculations from
http://www.prismusa.homestead.com/files/Duckweed.htm (see for derivation
http://www.permanentend.org/home.html under resources) a 3m2 pond should be
sufficiently large to
process the waste of a single human. The idea would be to grow duckweed in
these ponds, dry
it, and then feed it to the carp closing the loop. Waste water that is
processed with duckweed is far cleaner than what comes out of modern sewage
plants. In fact the water is drinkable if you run it through a uv
sterilizer.
Bottom line then is that an optimistic figure of 9m2 per person to process
all food and waste. (Of course I'm not suggesting that everyone ought to
grow their own food--aquaponics lends itself to the division of labor and
automation even more so than traditional agriculture).
Of course this figure is optimistic in someways, e.g., even under glass (or
plastic) growth would be seasonal (without enormous external energy inputs)
in some cities like Toronto.
This could in part be ameliorated by increasing the size of food and waste
facilities and grow more in the warmer seasons. So even if the calculations
are optimistic by an order of magnitude it would still be possible for
Toronto to grow its own food and process its own waste in a closed loop
within the confines of its present geography. Obviously this would require
redesigning the city, e.g., houses might be built with roofs capable of
supporting an aquaponic facility. Roadways might be covered with a canopy to
allow sewage ponds on top to grow duckweed, etc.
As far as I can see, there is no principled biological or engineering reason
why such a city could not be built, really what we lack is the political
will.
Cheers,
Mark
Mark Walker, PhD
Research Associate, Philosophy, Trinity College
University of Toronto
Room 214 Gerald Larkin Building
15 Devonshire Place
Toronto
M5S 1H8
www.permanentend.org
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