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

[MartinLe]

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?

 

[ch81pc]

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

 

[MartinLe]

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?

 

[bimr]

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.