Is Anaerobic Sewage Treatment Exothermic?

[russ_watters]

And if so, how much heat is produced?

Ok, so here's the problem. We are working with a generator salesman who is trying to sell a bunch of generators to a sewage treatment plant to capture the waste methane and burn it to make electricity. Great idea. The complication is that currently *some* of the methane produced is used to fire boilers to heat the crap to its optimal temperature of 95F for digestion. And we don't know how much gas is currently used for that, so we don't know if we can completely eliminate it (we will recover some of the waste heat from the generators for that purpose, but the efficiency is low).

We're also considering insulating the digester tanks (outdoor concrete tanks 175 feet in diameter) to reduce the need for heating them, but if we insulate them and they produce heat, we'll kill the bacteria.

We have numbers for the methane that is currently being burned-off and for the gas being produced, but they don't match: they are often burning as waste (not in the boilers) more than they produce. This could be because the production numbers are methane only and the burning (flaring) is methane + carbon dioxide, but we don't know those proportions either.

I know aerobic digestion produces heat, but I don't know about anaerobic. Anyone have any insights...?

 

[lalbatros]

This reference might be useful:

http://www.cropgen.soton.ac.uk/publication/14_MC%20Process%20Calculations.pdf

Unfortunately, it doesn't answer you question, but it provided a simple heat balance.
My feeling however is that the heat balance is not subtantially influenced by the reactions taking place.

 

[russ_watters]

Well, the fact that it talks about heat loss and heat input without mentioning biochemical heat production implies that it isn't a significant factor...

Thanks for the link.

 

[turbo]

Russ, you need to see case-studies, as the amount of methane (and other combustible gases) will vary wildly with the quality of the feed effluent, the temperature at which it is digested and the type of feed-system and mixing and the configuration of the reaction vessel. The vendor has to pony up, here. No consultant should have to predict the efficiency of such a system based on the (probably vastly inflated) claims of a sales-person. Good luck getting good data, though. The vendors will not refer you to their failures, only to their successes. Been there.

 

[chemisttree]

This link might be helpful. It is a model of energy inputs for a plug flow anaerobic digester.

http://asae.frymulti.com/abstract.asp?aid=21157&t=2

 

[russ_watters]

Russ, you need to see case-studies, as the amount of methane (and other combustible gases) will vary wildly with the quality of the feed effluent, the temperature at which it is digested and the type of feed-system and mixing and the configuration of the reaction vessel. The vendor has to pony up, here. No consultant should have to predict the efficiency of such a system based on the (probably vastly inflated) claims of a sales-person. Good luck getting good data, though. The vendors will not refer you to their failures, only to their successes. Been there.

My issue isn't quite that bad. For starters, this is an existing water treatment plant and they know they are burning off a lot of gas that they could be using to generate electricity (they just don't exactly know how much), so there is a definite upside and a very short payback. The system we are designing will have all the controls necessary to run the existing boilers to keep the digesters warm if the turbines don't put out enough heat for that. So the primary risk is buying 12 turbines and having only 6 run at a time because the gas is needed to fire the boilers. That would piss off the client...

Insulating the tanks could be a secondary project that could be done after a year of datalogging.