Odor of end products.

[gkellett]

Fantastic site!

I'm thinking about incorporating a biogas system for my soon to be youth hostel and was wondering what the end product sludge is like. Can it be directly applied as fertilizer, or should it be composted? Does it smell?

Cheers

Gregory

[Bob]

I'm thinking about incorporating a biogas system for my soon to be youth hostel and was wondering what the end product sludge is like.

From my experimental 2-stage system, there are two end products besides the gas: a liquid effluent that is discharged at a rate equal to the feed rate--about 1/12 of the total digester volume/day; and (what I refer to as) the sludge, which only needs to be removed once every ?? years, depending on a number of factors (e.g. digestibility of feedstock, non-biodegradable fraction, grit, etc). With a feedstock of only human waste and toilet paper, I haven't pumped it out yet in about 4 years of operation.

The liquid effluent is like muddy water. I don't know the solids concentration, but I'd guess 1-2%.

Can it be directly applied as fertilizer, or should it be composted?

The simple answer is that the nutrient (NPK) value of the fresh effluent should be essentially the same as that of the of the original feedstock. The digestion process removes none of it. This assumes that it is used immediately, however. I have seen sources which indicate that aerobic composting results in about a 25% loss of N. The more complex--and probably more accurate--answer is that there is a lot of research yet to be done in this area. For example, though the total N in the anaerobic effluent is higher, the aerobic compost may have more of it in a "slow release" form which is not so easily lost to the atmosphere. And, of course, NPK is not the whole story either.

Does it smell?

Yes, if I stick my nose over it. But its not awful. I've smelled a lot worse. There are also a variety of ways to minimize it -- e.g. subsurface irrigation. In my current setup, I'm not using the effluent for fertilizer (though I should). Instead, it overflows the digester to a 55 gallon drum where it is aerated (6 watt air pump). Both the drum and the digester tank are located inside a plywood enclosure vented to a roof vent. The exhaust fan keeps the enclosure negative relative to the surrounding space, so there is no odor inside.