Seroney
Here's something else that you might find useful. It's an excerpt from another foreum.
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(Posted on AD listserver in response to a question about measuring digester
performance - CH4, CO2, H2S, pH and what is useful in day to day operation.)
If you want to keep it simple, forget pH - this will only tell you
what you did wrong last week and has no place in the operation of a
digester on a day to day basis.
The biogas composition and quality is the pulse of the digesters
condition.
golden rule No.1 If the boiler does not light - stop feeding the
digester until it does light. The understanding of this is that
overloading generates VFA (Volatile Fatty Acids) in the digester in
excess of the consumption by methanogenic bacteria. Since production
of VFA is accompanied by CO2 gas production, if there is a biological
overload caused by excess feed, this will cause an immediate increase
in CO2. High CO2 and relatively low methane gives a gas that will not
ignite in a boiler set up for burning normal gas. If you are using
Drager tubes for CO2 analysis, then if the CO2 increases to above 50%
slow down the feed and stop the feed at 55%CO2.
golden rule No. 2 Feed daily and never feed more than 30% VS today than
you did yesterday. (this is to do with the rate of growth of the
methanogenic populuation under ideal conditions.)
This assumes temperature control to better than 1 deg C per day (we
work to 0.1 in the control loop) in digesters with a minimum of 35 deg.
(we run in excess of 38 deg. C)
Thats it! - how to run a digester without a lab.
However,
If you really want to monitor the digester especially at start up then
the TVA [Total volatile acid] level in the digester is the best
indicator of inhibition in the methanogenic stage. Good for digesters
in the 1-4 volumes of biogas per day range. Use 1000mg/l VFA as the
peak level during start up and aim for 280mg/l in normal production,
350 mg/l max peak after shock loading. If the VFA level goes higher
than this level, shut off the digester feed pump for a couple of hours.
On very high rate digesters, you could also use chromatography to
measure the concentration of individual species of the VFA - these form
a series with heavyweights at one end (proprionate and butyrates) and
light VFA (acetate) at the other. By tracking these every other day,
you will be able to see the development of heavyweight VFA at times of
chemical inhibition or overloading and hence you can throttle the
digester feedpump or feed composition to maximise gas production
without overloading the biology. Useful if you are working at 5-7
volumes of biogas per volume of digester per day and have a white coat
management structure.
In case you are wondering what happens to pH - well during high VFA
concentrations of say 1000 or even 3000mg/l VFA, the pH stick at around
7.2 due to two buffering systems, the carbonate-bicarbonate system and
ammonia system from decomposition of proteins. When finally it
unsticks and the pH is seen to jump to 6.8 and falling it is all too
late, the buffers are used up and the low pH inhibits the consumption
of VFA by poisoned methanogens. The VFA build up accelerates making
the inhibition of methanogenesis worse and bingo you now have a tank
of vinegar! This is what the pH meter will tell you - but then you
will know this from the smell! :-/
Best Regards
Les. Gornall
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It's not me. Hope this helps.
LAsludge
