[Digestion] Biogas COD/CH4 yield

[Gasan Osojnik]

Dear all,

I think the discussed  parameters regarding the COD/CH4 yield have to be
further clarified.

The reference value 0.35 m3 methane / kg of COD is a theoretical value
everyone can calculate, and is used for engineering purposes in the
designing & optimization stage of the biogas plant. As Marcus already
explained, it is calculated on the basis of COD *of* methane, therefore:

CH4 + 2xO2 -> CO2 + 2xH2O

m(CH4) = 16 g
m (2xO2) = 64 g
COD(CH4) = 16 g CH4 / 64 g O2 = 4 g COD

As 1 mole of gas = 22,4 dm3 @ stp,

ρ(CH4) = 16g  / 22,4 dm3 =  0,71 g/dm3

And 1 g of CH4 occupies @stp

V(CH4) = 22,4 / 16g = 1,4 dm3 CH4

To summarize, 1 g of CH4 is 4 g of COD and occupies 1,4 dm3 CH4 @stp,

therefore,  per 1 g of COD

Y= 1,4 dm3 CH4/4g COD = 0,35 m3 CH4 / kg COD

Anaerobic degradation of 1 kg COD of substrate (which is "transfered" to the
COD of methane) therefore theoretically yields 0,35 m3 of methane.
As expected, this values are strongly under matched in real operating
conditions.

The value mentioned by Mel, derived from experimental conditions, is most
likely coincidental, meaning that the operating conditions in that
particular case (0,35 m3 CH4 / 4 kg COD) meaning, that the Yi = (0,35/1) /
(0,35/4) = 25% of theoretical yield, or can be derived from inappropriate
interpretation of the theoretical values, as 1000 mol of CH4 do indeed
correspond to 4 kg of COD (4 g COD/mole of CH4).

BR from SI, Gasan




> > Thanks Marcus for your response to my query:
> >
> >
> > > All,
> > >
> > > Do you know of (have) a way to "translate" BOD and/or COD to methane
> > > potential? It would of course have to be in terms of BOD/COD removed
> > > giving rise to so much methane.
> > >
> > > Mel
> >
> >
> > Hi Mel,
> > >
> > > if you consider the removed COD is completely transferred to CH4 in the
> > > Biogas (CO2=0 COD) you can calculate:
> > >
> > > 1g COD removed ->  0,25g CH4
> > > Or 1g COD removed -> 0,35 L CH4 (273K, 1013 mbar).
> > >
> > > If you want to estimate more exactly you may consider that some fo the
> > > removed COD flows into H2 and microbial biomass.
> > >
> > > Markus
> >
> > Your calculation, however, is at odds with the statement in Grady et al.
> > 1999, page 600 in "Biological Wastewater Treatment 2nd edition" Marcel
> > Dekker. The statement is that 4 kg COD removed generates 0.35 m3 of
> > methane (0.35 L CH4/4 g COD removed). Also that figure or nearabouts is
> > found in other documents. Apparently it was derived from theoretical
> > calculation (not understood by me) and also from serious observation of
> > real systems. If repetition = validity, thhan th figure is usable.
> >
> > Perhaps others can weigh in on the theoretical end, or report other
> > "sightings of the translation.
> >
> > Mel
>
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