[Stoves] [biochar] First report from Phnom Penh

[Crispin Pemberton-Pigott]

Dear AD

I am skipping Kevin's reply which is a suggested experiment (reasonable one)
and picking up on the death of the methanogenic bacteria. Very interesting
indeed.

The focus might be on the last month where perhaps something was taking
place that was not monitored. By that I mean just as the gas production rate
started to drop, one could expect to find that the growing conditions were
changing beyond some tolerable limit.

Suppose a 'poison' was accumulating at the higher feed rate - something to
do with not being able to breathe it into the atmosphere at the usual rate
per unit of gas produced. The system was working fine and producing 3 times
as much gas for whatever reason. Lots of bacteria digesting lots of
feedstock.

When the poison level became intolerable the bacteria population collapsed,
chased into extinction by the poison level. The bacteria would have evolved
in the meantime trying to adapt but the rate of change was too fast for them
to cope, at some point (a concentration analysis problem).

The char is obviously hosting or helping to host more bacteria. So what
changed over the 3 months that led to the switch flipping into extinction?
It may not have been a tipping point, it may have reached a critical level,
of the change in concentration may have been exponential. We don't know yet.

So I am thinking that adding something to deal with the poison or helping it
to leave at three times the rate would allow the system to continue working
indefinitely. It might be something as simple as having three times the area
between the rising gas accumulator and the digester tank which would give it
three times the breathing room. I recall you saying the starch/sugar system
had a low CO2 level and that it may have been outgassing. Perhaps there is
an accumulation of CO2 (and carbonic acid) in the mash which was the cause
of the problem. As Pritya commented once, the bacteria are not standard
digester flora and they have a (perhaps) unknown preferred pH tolerance
range.

Regards
Crispin in sunny Beijing

> Dear Crispin,
> the biogas plant filled with charcoal accepted three times the normal
> quantity of feedstock and also produced daily three times as much
> biogas. It did this for about three months, after which it suddenly
> stopped producing any biogas. We tried giving it a rest for a view
> weeks, but it did not recover. We reduced the feedstock quantity back
> to 1g per litre, but it did not produce any biogas. Three g per litre
> of digester capacity represent overfeeding under normal circumstances.
> As long as the charcoal in the biogas plant was active, the plant
> could accept the high dose of the feedstock. But at some point of
> time, the charcoal became inactive and without realising it, we kept
> feeding the system with three times the allowable quantity of
> feedstock. I think that the pH of the substrate became acidic, causing
> the biogas production to stop.
> Yours
> A.D.Karve