markh at sonic.net writes:
>I'm looking at either composting (aerobically) the digester sludge and then using it for agricultural crops,
>applying the sludge directly to the fields, or using it as a food stock/nutrient source for fish, algae or
>pigs. Is their likely to remain a pathogen risk for any of these activities. Of course i consider the former
>to be the safest and the later as more risky, but I'd like some professional feedback.
Composting is an effective way to wipe out human pathogens. It also can
effectively reduce the carbon content of your sludge by a great deal. It
may also cause some loss of N through ammoniafication, and some loss of
S through the reduction of this atom to sulfide, which is volatile. If
I remember correctly, the loss of C causes sludge to become enriched
in N ans S.
>Another big issue with the biogas which is evolved is the loss of carbon in the form of CO2 and
>Methane. Of course the gas can be used as a fuel and I'm aware that most sewage treatment plants
>use it to operated their process turbines. But if it was possible to biologically scrub the CO2 from the
>gas by carbon fixing bacteria, perhaps by circulating the gas through a honeycombed chamber or cell
>containing Carbon fixing bacteria, then this carbon could be recaptured to maintain soil fertility and this
>would also boost the caloric value of the gas. Using such a scenario would the carbon fixing bacteria
>also assimilate or break down the methane gas?
I am confused. What do you want to keep C in your system? If I understand,
"soil fertility" refers to the ability of the soil to support plant growth.
Plants do not use organic carbon in the soil, instead they use inorganic
carbon in the air. Adding lots of organic C to your soil will probably
cause you unwanted problems, such as massive fungal growth and anaerobic
I can understand the desire to purify the methane by removing CO2. You
are correct that, under aerobic conditions, a very wide variety of
bacteria will oxidize methane. A biological solution to your problem
may prove more complex than a chemical one. For example, if you were
use the gas to sparge a tank of mild alkalai, this would cause the
CO2 to become dissolved in the liquid phase, leaving the CH4 in the
However, I think that sewage gas is high enough in CH4 that it can
be burned without further treatment.
One major problem with you have not mentioned is the presence of
heavy metals in sewage. Often this renders sewage sludge unusable
for agricultural purposes. You will need nice clean sewage, or you
will need to invent a cost-effective way to eliminate metals from
the sludge. By the way, if you accomplish the latter, you will
be a millionaire.
>Could you add an additional cell which would biologically scrub the H2S from the gas??
Plenty of bacteria will oxidize H2S to SO4 (sulfate) when O2 is present.
This bugs are present in soil everywhere, and thus you will not need
to add them. They will be in your system in any case.
If you run an anaerobic digestor, you may also have problems with
ammonia. Ammonia is volatile, but people do not like it when
ammonia fumes blow into their homes from compost heaps and digestors.
Ammonia can serve as an energy source for autotrophic bacteria when
O2 is present. The end product is nitrate.
By the way, you mentioned using sludge to fertilize a pond, and then
harvesting the pond biomass to use as fertilizer for fields. Because
sludge is full of N, I doubt that nitrogen-fixing organisms will
be needed for such a pond.
In fact, adding organic carbon to a pond will not encourage plant
growth. It will encourage the growth of heterotrophic bacteria.
These bacteria will devour all of the oxygen around, killing any
animals in the pond, and some of the rooted plants.
Adding nutrients like P and N will encourage plant and algal growth.
If you do not manage algal growth carefully, the algae will die from
the effects of overpopulation, and the bacteria will feed on their
remains. Again, you will have an anaerobic pond inhabited solely
My advice is to try and minimize the organic carbon going into any
of your systems. It is only desriably if you want to create conditions
which volatilize S or fix N by free-living bacteria (as opposed to
those living in plants.)
DEpt. Microbiology and Biochemistry