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Gas yield in anaerobic digestion - mass balance doesn't add up if calculating from COD

Gas yield in anaerobic digestion - mass balance doesn't add up if calculating from COD

Gas yield in anaerobic digestion - mass balance doesn't add up if calculating from COD

(OP)
I'm designing biogas plants.
Our usual precedure to calculate gas yield is to look at the volatile solids (VS) content of the substrate and use a literature value of Nm³/kgVS (Biogas produced) to get the gas yield. The results make sense and are true for substrates encountered in waste to energy and renewable energy plants (total solids (TS) between 5% and 20% or even more)
In the wastewater field, the collegues calculate with chemical oxygen demand (COD) and COD destruction, and biogas yield per mass unit of COD destruction.

Now, I looked at a gas yield calculation (done by a collegue) for a wastewater plant (TS < 1%), and if I calculate the way I learned the mass of biogas produced is higher then the VS input into the process - so the mass balance doesn't add up.

While I trust the person who did the calculation, I want to understand the discrepancy and deepen my understanding. Hence these questions:

Is it (in anaerobic wastewater digestion) possible that a signifiant amount of gas is generated from input that isn't accounted for as TS or VS? (like substances with a steam pressure smiliar to water, so they also evaporate in TS measurement)

The collegue mentioned water dissociation as one source for biogas mass. I found no reaction path from water to biogas, and my gut feeling is that this would be strongly endothermic - Why would any bacteria in its right mind do a metabolic pathway that costs energy? Is there any literature on this one of you can point me to?

RE: Gas yield in anaerobic digestion - mass balance doesn't add up if calculating from COD

Approximation is 2 cubic feet per person per day. But depends on freshness of the sludge. It's a good starting point though.

RE: Gas yield in anaerobic digestion - mass balance doesn't add up if calculating from COD

(OP)
It's wastewater from a food processing operation. The gas yield you give is basically the same on a per COD basis. But the question remains: how is it possible that the gas yield is more than the VS input?

RE: Gas yield in anaerobic digestion - mass balance doesn't add up if calculating from COD

You collegue probably used some parameter that based the gas volume on plant capaity rather than volatile solids (VS) content. There is a method to do this calculation in Metcalf and Eddy. You are correct, the gas yield can not be greater than the VS input.

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