Food Waste and Biogas, Part 3

| 9/19/2014 10:26:00 AM

How much biogas from how much food waste? In general, that’s the question we left unanswered in that last blog, part 2 in this series. (Here… And part one is here, in case you missed it.)

Well, the rule of thumb is that a biogas digester kept at the proper temperature — body heat, which is 105 degrees Fahrenheit … or at least it is for a cow — will produce its own volume in biogas every day. According to this rule, if your digester is a cubic meter, and you keep it properly warm, you will get a cubic meter of biogas from it, every day. (That’s about 35 cubic feet, and comfortably more than most families will need to cook their lunch and dinner, but not a lot more.)

But that rule of thumb comes from experience with manure-fed digesters. That is, if you have a digester and you’re just putting manure in it, then the rule of thumb applies. But the fact is that different substrates produce different amounts of biogas. Remember when we said that food waste makes great biogas? (No? Well, we did….) You can see the difference by looking at the following chart which I produced, using data published by the Bavarian Association for the Promotion of Solar Energy:

substrates chart

Click on the graph to see a larger version. Source data derived from Solarenergiefoerderverein Bayern e. V., “Biogas– Strom und Wärme aus der Natur”, pg. 9 (here)

Freshly cut grass clippings can ultimately produce better than 1½ cubic feet of biogas per dried pound. By contrast, the same dry weight of cow manure, under the same conditions, will produce less than a quarter of that. If you’re lucky enough to have enough of what the Bavarians call “residual fats,” then the comparable pound will produce 24 times the amount of biogas as the cow manure. So like I said: different substrates produce different amounts of biogas. In spades.

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