Terms for Anaerobic Digesters
This is the beginning of a document to guide sharing of information about Anaerobic Digesters.

The discussion can be found in the REPP Archives at http://listserv.repp.org/pipermail/digestion_listserv.repp.org/ starting in late December 2009 in the thread "Biogas from urban waste", later changing to "Biogas terms and tests" in January 2010

On December 31, 2009, Alexander Eaton posted a message in response to some eariler discussion in which he said:-
"is anyone aware at an attempt at defining terms surrounding biodigester use? I see this at invaluable, especially in forums such as this when those of us who are working in the field (often with small systems and crude measurement) are communicating with people in industrial or laboratory settings. Both ends of the spectrum can learn from each other, but better with a common language. Even just a general list of terms and agree upon definitions would be useful, and might open up some interesting debate in the process.

This is something that we are willing to take on (and translate), but I want to make sure we are not duplicating efforts.

Please advise.

Best in 2010,


On 1/1/2010 10:53 AM, David House (in reply to another message from Alexander Eaton) wrote (expanding on his own earlier message):

But, more productively and specifically:

  • For my part, I would suggest that any of us who are interested in
  • reporting our results should characterize substrates with at least
  • the following minimum parameters:
> * %TS
Per European standard SFS-EN 12880 (SFS-EN, 2000b), the sample must be
dried overnight in an oven at 105°C to a constant mass.

  • * %VS
Per SFS-EN 13137 (SFS-EN, 2001), the sample must be burned in an oven at
500-550°C to a constant mass.

  • * %grit
S and and similar materials are allowed to settle out of a sample, then
they are dried and weighed. Substrate must be collected as it would be
under conditions of use. This parameter can be of considerable
importance in the design or planning for a digester.

  • * Particle size profile
Sieve the normal (undried) substrate using appropriately sized screens,
then (drying if desired and) weighing each sieved portion. If desired,
volume may be estimated or measured as well. Knowing this profile may
have an impact on digester design.

  • * C/N
Can be done by an amateur chemist with suitable equipment, but more
usually requires analysis done in a lab.

  • * %H_2 O as digested
One wt/wt basis, with previously measured TS weight as the dry basis.

  • * Consistency at dilution
For low-tech situations, I recommend using a correlation based on common
substances such as water, half and half (50% cream/milk), butter milk,
pancake batter, cocktail dip or pickle relish (as shown in The Biogas
Handbook, p. 47). In any case, this parameter is most useful where
pumping the slurry is being considered. (See Hart, Moore and Hale, 1966,
"Pumping Manure Slurries".)

  • And likewise that digester parameters and biogas production be
  • characterized with at least the following minimum parameters:
> * Loading rate reported as VS/time/vol digester
Calculated based on known/calculated volume of digester and %VS as
previously determined.

  • * HRT/SRT
Hydraulic Retention Time is easier to calculate, based simply on daily
volume of substrate/slurry in, divided into the volume available in
digester. Solids Retention Time, for most simple digesters, will be
approximately the same, because solids and liquids will not be separated
either by design or at the outlet. (If solids simply settle and do not
move through the digester, the process will of course eventually fail.)
However, the best source of innoculum for the digester will be the
digestate, and methanogens prefer to adhere to solids. Thus separation
of solids at the outlet ("dewatering"), with some return thereof (which,
if memory serves, is done with some versions of the Nisargruna
technology), has certain advantages.

  • * periodic temperature of digestion
  • * periodic pH
  • * periodic VFAs/buffering
Finding pH and temperature are so simple as to not (in my opinion) need
any description.

Determining VFAs and buffering capacity in the digester, as I mentioned
previously, are the most complex of the suggested tests, but for any
really useful characterization of the digestion process, such a
determination is essential. (pH is important, but it is a lagging not a
leading indicator of process health.) The method of choice for
VFA/buffer determination is titration with solutes of known strength,
and there are a number of methods of titration which can be used.
Forgive me, however, for not taking the time to offer further details at
this time.

  • * Gas production, reported as vol. gas/vol. digester, and
  • as wt gas/wt VS
  • * %CO_2 (and by assumption, %CH_4 )
Gas production can be measured in a large number of ways, but as Mr.
Bapat has indirectly pointed out, the relevant temperature and
barometric pressure must be recorded at the time of measurement. As
regards the latter parameter, Paul has provided a very useful method of
determining CO2
and has been previously mentioned, for most situations knowing the
amount of CO2 and the temperature allows for a reliable determination of
the %CH4 and those parameters that follow therefrom.

With further regard to terms and tests, I recommend considering
"Defining the biomethane potential (BMP) of solid organic wastes and
energy crops: a proposed protocol for batch assays
which was released in March of this year. In some cases it offers
recommendations which would not work in low tech situations, but overall
it not only gives a fairly comprehensive approach to substrate
characterization, although it does not offer information about tracking
VFA production or buffering, crucial parameters for good process
control. (Of course, the paper is not aimed at characterizing the
digestion process, but only substrates per se.)

David House.


Good guidance. It would be useful to see a table of these qualities for
different substrates and reactors. Paul's biogas Wiki would be a convenient
place to build a table or database of these values. (Biogas Wiki
http://biogas.wikispaces.com/ ) You probably have tables of comparative
value in your book.

In the recent discussion I see four very different substrates and reactors:
1) the food wastes specified by ARTI (Dr. Karve) for their domestic
digester; 2) urban and domestic solid wastes described by Mr. Bapat for the
Greenleaf/ARI digesters which appear to be directed to industrial or
commercial users; 3) manures; and 4) sewerage described by Mr. Weisman.
Perhaps Dr. Karve, Mr. Bapat and Les Gornal could collectively characterize
the first two while others could characterize the last two in a variety of

It would be useful to have a common taxonomy of substrates so similar terms
would be used to described wastes of hopefully similar composition.

A good project for the New Year.

Tom Miles


There is a large amount of literature on biomethane potential for many
feedstocks and mixtures (starting with Buswell and Neave in the 1920's),
biochemistry of reactions, as well as AD system designs. More difficult to
find is reliable third-party (or peer reviewed) reporting on system
performance of large scale AD systems for mixed or separated ofMSW.
Europe has many operating AD plants for industrial wastes and ofMSW. The
IEA Biogas Task (Task 37) has some information;

For several reasons, there is much interest among California regulators and
solid waste management jurisdictions in anaerobic digestion of the organic
fraction of municipal (or urban) solid waste (AD of ofMSW). We produced a
report for CalRecycle (formerly Integrated Waste Management Board) in 2008
on "Current AD Technologies Used for Treatment of Municipal Organic Solid
Waste" which can be accessed here;


It has some information on system performance, not necessarily all from
third-party or peer reviewed sources.

Rob Williams


There is some information on Biogas Production Data here
In February the ISO started seeking input about Biogas Standards

Compiled/Edited by Paul Harris – paul.harris@adelaide.edu.au

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