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In many parts of the world, human waste is used as
fertilizer, at least for crops that will be cooked (wheat,
potatoes, etc.) or agricultural products not used as food
(jute, hemp, cotton, etc). But for fresh produce this is not
a good idea, since sewage carries diseases such as
Sewage can be pasteurized to kill any harmful bacteria, but
this requires a good bit of energy. However, any power
plant that uses a steam turbine or piston engine will
produce waste heat, usually in the form of steam that
must be condensed. By using sewage as coolant in a heat
exchanger, the steam will be condensed, and the sewage
will be pasteurized.
Furthermore, the anaerobic fermentation of sewage
produces methane gas, which, once filtered, can be burned
in a conventional natural gas power plant. This is also true
of landfills, and the waste from cattle yards and
Imagine a community that exists in a river valley or some
other sloped landscape. At the top of the slope (ie,
upstream), there is a city that produces sewage, and
consumes electricity and food. Downstream there is a
waste treatment plant, a landfill, various industrial plants,
and a power plant. And downstream of everything are a
series of farms.
Sewage from the city, along with other biodegradable
waste, is fed into an anaerobic digester to produce
methane gas. The methane, including some from a
landfill, is filtered and then burned in the power station.
The waste steam from the power plant is condensed in a
heat exchanger, while also heating the sewage and killing
the harmful bacteria within it. The pasteurized sewage
then flows to the farms, where it is used as fertilizer
without risk of spreading disease.
By using waste to produce electricity and fertilizer, less
fossil fuels will be used by this community, making it more
||If the admins feel this would fit in another category,
please feel free to move it (or I can move it myself).
I also thought of placing it in Food:Farming.
||I'm reasonably sure that if your anaerobic digester is
doing its job properly, the waste is already sterile (at
least with respect to pathogens) at that point. That
being said, this couldn't hurt.
||This has been done in simplified form for thousands of
years. In rural Vietnam, for instance, the rice paddies
double as latrines.
||How would you compare the fertilizing properties of
||a) untreated sewage
b) heat-treated sewage
c) anaerobically digested sewage
||[FlyingToaster], I would think that they would be
pretty similar chemically, although untreated sewage
might have microbes that would be more beneficial
to plants, but worse for humans. But I'm no
||Animal waste has long been used, safely, as a crop
fertilizer. Human waste is no more hazardous (at
least, the main bacterial hazards are common to
human and animal waste; and viruses won't survive
||Therefore, I'm pretty sure that human waste can
be used as effectively and as safely as animal
waste. The main objections are either
squeamishness (for some reason, we're happier
with animal manure on our crops than human
manure), or the fact that animal waste isn't
generally mixed with condoms and loo paper.
||what I meant was what's the difference between pasteurized sewage and sewage which has had a portion of C and H stripped out by methane production vis-a-vis fertilization.
||Sorry, [ft], I was replying to the idea rather than
to your annotation. None of the treatments is
likely to have much impact on its fertilization
powers. Plants mainly need simple compounds of
nitrogen and phosphorous, which come mostly
from the breakdown of complex organics (such as
proteins and nucleic acids) in sewage. Plants
don't need (and usually can't use) carbon,
hydrocarbons or carbohydrates in the soil.
||Stewing things first won't have much impact; nor
should methane production. If anything,
fermentation might assist the breakdown of some
complex molecules into simpler
nitrogen/phosphorous compounds, and thus make
the fertilizer faster-acting.
||To clarify my comment re:pathogens, human
pathogens tend to like a very limited temperature
range (~96F, 32C). The organisms that do anaerobic
digestion tend to prefer a significantly higher range
(140-160F, I believe), and most digesters operate in
that range, so the pathogens go away.
||I've seen prototype runs of human waste processed into
fertiliser. It works quite well... at least as well as similar
products made from chicken manure.
||The concept was never followed through because it had
trouble attracting funding.
||Generally speaking the issue with our effluent is that it is mixed with far to much water to be effective for anything but fertigation. Additionally human effluent is contaminated with cleaners, chemicals and a plethora of drugs, all of which are hard to separate from the essential fertilizing action. To make and effective portable fertilizer from sewage waste would mean changing the way in which we shit an piss. We would have to stop mixing it with so much potable water and cleaners and the shit of people taking certain extremely persistent compounds would have to be excluded. The problem of pasteurization/disease is a minor one, easily resolved by the aerobic biological digestion that will have to take place before we can use the fertilizer for direct soil application anyway. As for the shit that we can collect from domesticated animals in a non liquified, relatively pure form, it is already being used in exactly this way and some sewage plants already operate "sewage gas" power plants, either in turbine form, or in a fuel blend in piston engines with diesel.
||I'd forgotten about drug metabolites.