On Monday, June 16, 2003, at 06:06 pm, Jeff Kasten wrote:

> Les,
>
> Thank you for the practical tests for biogas!

My pleasure !

>
> Can you help explain the difference or equivalence of COD and VFA?
> My dilemma is simple, how does a VFA concentration of 250 mg/l equate
> to COD? Perhaps the community would like to weigh in on this,
> please?

Firstly Chemical Oxygen Demand - is a measure of the concentration of
mushy stuff, the pulp and soluble carbon, in a bucket of sludge. Grit
does not contribute much to COD. The one bulk characteristic that
corresponds most closely to COD is Volatile Solids which is sometimes
known as loss on ignition at 550 deg. C. Across a wide range of
sludges and manures I get the experimental relationship that 1g-VS =
1.2g-COD +/- 0.1

[except for chicken faeces which generally sits at the high end (1.3) -
but this may be due to the fact that cooking chicken droppings in a
small laboratory is a malodorous business and the threat of having
small parts of you body removed and served up on toast by the occupants
of the next work bench provides an incentive to bias the results
towards the quick burn, turn the fans on and run for lunch end of the
accuracy scale.]

OK so if COD is generally so closely tied to VS and VS can be
ascertained in two ovens and a couple of weighings, why use COD and all
that glass-wear and expensive lab time?

Qu. where are you getting that bucket of sludge from?

1. Sampled from the inlet of the digester - OK the COD is an easily
measured indication the amount of mushy stuff you are feeding the
digester as in 1200kg of COD per day which as we now know is equal to
1000kg of VS/day in a typical system. This is good to know and is
obtained from the simple formula sludge concentration mg/l - COD x
volume per day in litres. Divide this by the volume of the
digester and you get - well about 5 generally - the specific daily
loading rate of the digester in kg/m3-digester . You have a choice of
deciding which is preferable to measure kg-VS or kg-COD. I generally
choose VS because this also gives me Total Dry solids fed to the
digester and the % of the Total Solids which is Volatile - a measure of
freshness of the sludge. Fresh is good for gas. There is no point in
blaming the digester if the sludge coming in has degassed in a tank 20
miles away down a hot road before it gets to the digester.

2. Sampled from the inside the digester - this should tell us what is
going on inside the digester today right? Well no - the sludge in the
digester has been there on average for say 20 days and the large
overload you gave it last night when the contacts of the pump relay
welded together still only represents about 5 or 10% of what you are
about to measure. The signal you are looking for which is the state of
the digester relative to recent events is lost in all that ancient
noise.

3. Sampled after the digester - The output flow stream is a continuous
stream of samples of the inside of the digester. This means that the
efficiency of the digester can be ascertained by checking the VS before
and after the digester. Divide the output VS by the input VS. This
ratio is about a half. Half the VS comes out as goes in - the
difference is the mass of biogas generated by the digester. What comes
out is devoid of mushy stuff (now gas) - all that is left is salts (low
COD) and easily separated fibre.

OK so where is all the VFA (volatile fatty acids - vinegar and such
like)? These are a process intermediary for fresh sludges. This means
that they build up inside the digester when the digester is fed and at
the same time they are consumed by the bacteria inside the digester.
Think of it as two boxes inside the digester - one contains the VFA-
making bacteria - young sporty types with fast cars, these guys
reproduce rapidly, and will eat the mushy stuff you feed them faster
than you can say butyrate and produce clouds of CO2 in the process.
In the other box is a population of ancient beings, slow, bearded
probably, - wrinkly old ancients that reproduce very very slowly and
eat with great precision and infinite care. These are the most ancient
creatures on the planet, archeabacteria. They sup their VFA slowly and
......... well, ' make methane' and a bit of CO2 from their food.

What we measure in the VFA is the concentration of the daily shipment
of VFA from the sporty acetogenic bacteria to the Methanogens. In a
stable system there is a large population of methanogens and the
concentration of VFA will not exceed about 250mg/l VFA while in
transit between the two bacterial groups. If the VFA builds up slowly
due to increased feed, the methanogen population will grow and reduce
the level to the optimum.

We only measure VFA to see if the methanogens are thriving and not over
fed. The digester is controlled by monitoring VS or COD fed to the
digester per day in order to KEEP IT CONSTANT.

>
> As you can imagine I do not have the means to determine VFA in my
> (hypothetical) prototype digester.

It is not a difficult test if you have about two meters of bench space
and an electrically heated pad, a pH meter and a titration pipette or
two.

> Not all is lost though if I feed
> controlled concentrations in a controlled fashion it should be
> possible to optimize [biogas] production without measure of VFA, I
> hope.

yes! Build the feed rate up slowly, no more than 30% today than you
fed yesterday - start very slowly at a rate which would give you a
retention time of about 120 days - you will be surprised how far you
can go. I have seen very successful digesters operated at thermophilic
temperatures and 6 days retention time.

>
> I read your VFA to gas volumes (1-4 and 5-7 volumes) as an important
> estimate of digester volume or rather digester potential in my case.

I think I was not clear enough here. I was not relating VFA
concentration to Specific Gas Volume production (although there can be
an indirect relationship). The point is the rate a digester is running
determines how the digester is managed.

A few examples:

It is possible to classify digesters by looking at the volume of
biogas produced by a cubic meter of digester volume.

Digester space is expensive to build but the smaller the digester for a
given feedrate, the harder the digester works and the less time you
have to react to foaming incidents after overloading. In a really
heavily loaded digester [producing lots of money] the operation is
managed like a nuclear power station [- as in the Simpsons.?] - 24
hour round the clock observation, 15 minute call out and a check sheet
and shut down sequence for every failure mode.

Some numbers:

A lagoon type digester with little temperature control might produce
0.25m3/day of biogas for each cubic meter of hydraulic digester volume
- something goes wrong with the loading and as long as you fix it this
month there should be no problem long term.
Heat the digester a little better, control it and a plug flow digester
might produce 1 cubic meter of biogas per cubic meter of digester per
day.
Heat and stir, and up the influent concentration to 7% you can get 1 or
2 m3/m3-digester
Heat, stir, prepare the waste for regular feeding and leave out
rainwater, keep the feed concentration higher than 10%TS and you get
2-3m3/m3 digester.
Add fat to the input and this becomes 4-7 volumes of biogas per volume
of digester per day.

Note the golden rule of digester design - it is the feedstock that
produces the biogas NOT THE DIGESTER.

To reiterate - the faster the digester is working, the less time you
have to react when something goes wrong. This is important when
considering the design of a digester for a specific location. There is
no point in asking a farmer to respond in 15 minutes to an overload
problem during harvest!

VFA analysis gives EARLY WARNING of potential foaming and overloading
problems to come in the next few hours or days.

For low rate digesters daily VFA analysis is not required at all.

For medium rate digesters I would use VFA analysis during start up and
weekly until I was comfortable with the operating regime with a check
every time the operating regime was changed.

For High rate digesters I would check those VFAs up to twice a day
during start up if there was a need and every other day during
operation and on the fastest digester I would definitely need to know
just what the archaebacteria are being fed today and ask for a
chromatograph of the various types of VFA.

> Nearly all AD material I've discovered focuses on the waste
> (influent), usually the specifics of the waste and how to digest it.
> The common denominator as I see it is either COD or VFA hence my
> dilemma...

fear not- the common denominator is COD or Volatile Solids - as above.
VFA levels in the inlet are only important with technically difficult
wastes like pickling factory effluent, silage effluent from
agricultural silage clamps etc. These have to be carefully handled and
fed slowly as they react and form methane in about 15 seconds.
>
> Another (unrelated) question, what was the title of your thesis?

'Interaction between anaerobic and aerobic processing of farm effluent '

>
> Thank you.

At your service.

Les. Gornall

Return to Beginners Tour of Biogas