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Sewage as Fertilizer, Pasteurized by Power Plant

Use poop as fertilizer, but kill the bacteria by heating it in a power plant's steam condenser
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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 salmonella and cholera.

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 slaughterhouses.

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 self-sufficient.

discontinuuity, Feb 12 2013


       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.
discontinuuity, Feb 12 2013

       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.
MechE, Feb 12 2013

       This has been done in simplified form for thousands of years. In rural Vietnam, for instance, the rice paddies double as latrines.
Alterother, Feb 12 2013

       Ima +   

       How would you compare the fertilizing properties of   

       a) untreated sewage
b) heat-treated sewage
c) anaerobically digested sewage
FlyingToaster, Feb 12 2013

       [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 biochemist.
discontinuuity, Feb 12 2013

       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 intact anyway).   

       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.
MaxwellBuchanan, Feb 12 2013

       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.
FlyingToaster, Feb 12 2013

       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.
MaxwellBuchanan, Feb 12 2013

       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.
MechE, Feb 12 2013

       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.
UnaBubba, Feb 14 2013

       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.
WcW, Feb 14 2013

       I'd forgotten about drug metabolites.
UnaBubba, Feb 14 2013


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