On Monday, November 28, 2016 at 2:24:42 AM UTC+13, Alain Fournier wrote:
On Nov/27/2016 at 1:17 AM, Fred J. McCall wrote :
JF Mezei wrote:


100pax over 3-4 months will consume large amounts of food. That is a lot
of mass that you have to lift and accelerate out of earth's orbit
towards mars most of which will become waste. Not doing anything with it
means wasting that mass which you spent much fuel accelerating.


I know it's hard for you, but think about it. Most of the mass of
food (and feces) is water. You're going to get the water back for
recycling on the back side of the process. That means each person
will generate 1-2 ounces of solid waste per day once the water has
been recovered (and you'll get 3-6 ounces of water out of the same
waste stream). Let's use the larger number as more 'favorable' to
your case; 100 people (not sure what 'pax' are when they're up and
dressed) will generate around 12.5 pounds of solid waste per day. That
waste is a mix of dead bacteria, indigestible food elements like
cellulose, minerals, and indigestible fats. You're not going to turn
it into methane without giving up a lot of the recovered water and
even then most of it isn't going to 'convert'. Recovering the water
is more valuable, since you can make things like breathing air out of
that stuff. So you're going to accumulate a little over half a ton of
such cruft during the course of the trip.

[You should use normal units instead of those ounces and pounds, it
would make your post easier to read for normal people, and lessen the
risk of errors from unit conversions.]

It's not one or the other. You can very well recover the methane and the
water and grow food. Plants don't need the methane from human waste to
grow. So after extracting methane, the waste isn't any less fertile than
it was before extraction.

If you want to recuperate the water that was lost in the fermentation,
you can burn the methane and make electricity, water and CO2. The CO2
will be taken by the plants you want to grow. Of course, if you do so,
that means you can't use Mr Mezei's idea of burning the methane as
rocket propellant. Which probably isn't worth the trouble anyway.

Even growing food on the spaceship probably isn't worth the trouble. The
trip is not long enough to do serious farming. I think that the best use
of human waste on a spaceship bringing colonists to Mars is to store it.
Land it on Mars. And then, once on Mars compost it and use it to grow
food. You will want to have lots of fertilizer handy for your colony on
Mars.


Alain Fournier

Depends entirely on the size of the ship. A 100 passenger ship, it makes sense to recycle air and water certainly, and reduce all waste to carbon and phosphates. A 10,000 person colony ship - now you get to the size where growing stuff makes sense. But even this is a function of technology. The Stanford Torus studies of the 1970s involved growing conventional crops and livestock. Today, with the ability to grow meat in a test tube, and assemble it using 3D bio-printers, it is not beyond the realm of near-term possibility that we will be able to have self sustaining systems for as few as 100 people or less.

11% conversion efficiency of sunlight to biomass - using advanced algal strains - a square meter of solar collector exposed to sunlight in space - is capable of capturing sufficient energy to produce 0.55 kg of dry matter per day. We consume 0.55 kg of dry matter per day.

Since chlorophyll uses only specific colours we can even sort through the spectrum to efficiently drive photovoltaics and use dyes to shift colour intensities, to improve efficiencies - and collect sufficient power to run a self contained system.

Conceptually Algae produces agar. Agar grows cells in culture. Cultured cells are assembled with 3D printers into food items. Waste is processed into feed for algae. A litre of contained fluid is required to produce about 5 grams per day under ideal conditions. So, each square meter of sunlight at 1 AU translates to 110 litres. Ideal intensity, at the colours required are around 310 Watts per square meter. The algae reactor is 110 kg per person. Cell cultures and other hardware add another 40 kg per person. A total of 150 kg per person. So, 272 days at 0.55 kg per day - we have break-even.

Now changing up the atmosphere getting rid of O2 - or lowering it substantially - permits lowering the amount of water - in a sort of aeroponic setup -

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC426129/

Which could vastly reduce the amount of water needed to perhaps 10 litres - insteads of 110 litres - reducing the break even flight time to less than 30 days.