NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/sci/space/19923

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Subject: Biosphere 2 Oxygen and other Questions
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Biosphere 2 Oxygen and other Questions
January 27, 1993

Sixteen months down and eight to go.

Sorry to be so long in posting answers to the many
questions, I simply have not been able to make enough time
in here.

We don't know where all the oxygen has gone. This is one
of the biggest mysteries so far. While the CO2 removal
system can account for some of the O2 loss, as oxidation
of organic material producing CO2, which was then
removed from the air by the system. The amount of CO2
removed from the air only accounts for about one fifth of
the total oxygen deficit. We have essentially two leading
theories, first is oxidation of reduced compounds that may
have been present at closure, like possibly a reduced iron.
The second is oxidation of organic material in the soil and
subsequent precipitation of the CO2 as a calcium
carbonate, the parent materials for this are abundant but
the necessary conditions are in question. We are also in the
process of doing small chamber experiments with soil
similar to that in Biosphere 2. Carbon and Oxygen isotopes
are also being extensively studied for clews to what is
going on. The soils are the main point of interest because it
is the only place in the biosphere big enough to hide 12,000
Kg of O2.

The CO2 is now at an average concentration of 3500 ppm,
with a 400 to 500 ppm diurnal swing. The sunny weather
we are having now is bringing the average CO2
concentration down at a rate of 80 ppm per diurnal cycle,
without any use of the CO2 removal system which is now
off.  This means that the draw down during the day is 80
ppm greater than the rise at night. To put 3,500 ppm in
perspective, a room with people in it and normal ventilation
can easily get this high. Also CO2 concentrations between
5,000 and 8,000 ppm CO2 is not uncommon in the space
shuttle and submarines. Human health difficulties seem to
begin at about 9,000 to 11,000 ppm. Increased CO2 does
result in faster growth rates with many plants.

There was no bias against the Sabatier or Bosch systems,
they were simply not considered necessary at the time. We
had no idea that the oxygen phenomena would occur. What
we did know was that during the winter we would have an
excess of CO2 and in the summer quite possibly a deficit.
We are able to put the CO2 back in the air during the
summer and have it made into plant material, thus
completing an annual cycle of CO2 storage and release.
The summer release of CO2 should stimulate plant growth
and replenish some of the oxygen. A basic problem with
the Sabatier or Bosch systems and our situation is that we
do not have enough CO2 in the air to significantly change
the O2 using a system that would liberate the O2 form the
CO2. Even in the summer with low CO2 in the air, the O2
loss is at about 0.26% O2 per month, while the average
CO2 concentration is fairly constant at only 0.12%. Even
converting all the CO2 in the air to C + O2 would have
little effect on the O2 situation. Using such a system to
drive the CO2 even lower would begin to reduce food crop
production rates, because the plants would become limited
by the availability of CO2. So there is no need to transform
the CO2, just temporarily store it in the winter. Note that if
we had constant high levels of light, like in space for
instance, this CO2 storage scenario would be mute
because we would have no need to try and survive a low
light season!

The CO2 removal system I designed uses sodium
hydroxide as a scrubbing fluid, removing a fraction of all
acid forming gasses in the air. This is done using a large
reaction column with a high air flow rate, even so we can
only remove 100 to 200 ppm per day with the system. On a
low sunlight day the CO2 can jump up 300 to 400 ppm, so
the system can only moderate trends. The sodium
carbonates formed from the CO2 and sodium hydroxide
are reacted with calcium hydroxide to form CaCO3
(limestone) which regenerates the scrubbing fluid back to
sodium hydroxide. The CO2 is thus stored as CaCO3 until
time to release the CO2, which is done by heating the
CaCO3 making CaCO and CO2. The CaCO can then be
re-hydrated with the water in the scrubbing fluid and all the
chemicals are restored and ready for the next winter. This
may not be the best system, but it works. It is too heavy for
a space application but may possibly be made from local
materials.

Also a basic problem with any system is the need to
process a large volume of air. If the CO2 was at 1000
ppm, then for every liter of CO2 removed, one needs to
process 2000 liters of air with a 50% efficient system. This
can be done either by pre-concentrating, which is difficult
with a volume as big as Biosphere 2, or processing the
whole air which is also hard. Ambient temperature
scrubbing of air in a reaction column, using a hydroxide,
can handle large volumes of air quite easily. A similar
system is used in space craft with solid lithium hydroxide.
Pressure swing systems are being considered for space
station using materials that adsorb CO2 at cabin pressure
and release the CO2 to space vacuum and/or with heating.

The smell is usually great. I have a pure air generator that
makes very clean air by pressure swing adsorption, air with
no smell. By breathing this air for a period of about 30
minutes, it tends to "zero" my sense of smell so for a short
time after, 4 to 10 minutes, I can smell the air in the
Biosphere. It smells like rich farm soil (not potting mix), a
sweet fresh smell. There are some exceptions to this, like
when the sewage system has a problem such as
inadvertent overflowing, but after cleanup the smell only
persists for one to three of hours. Also if a container of
some wet organic material is not tended, it can smell rather
bad.

Methane buildups have not occurred, this is probably due to
methanotrophic bacteria increasing to consume the
methane thereby keeping it in check. We have thus-far had
no problem with any trace compounds building up in
the atmosphere or water supply. We haven't even needed
to use the soil bed reactor system to clean the air. The soil
bed reactor is designed to force air through the soil in the
agriculture biome as a means of cleaning the air.

We are now at the two thirds mark, I am sorry to say that I
can not be more specific than to say that group dynamics
are quite exciting. Some studies I have seen say that in a
situation with a confined crew, the third quarter is usually
the most difficult time. Oxygen additions are continuing, we
are now at about 17.5%. The low light conditions that we
have had over the last two months may delay harvest times
with some crops as much as a month. The effect of this on
our total food production will not be fully seen until mid
March, at which time we will be able to evaluate the
situation decisively.


Taber MacCallum
Biosphere 2 Crew