What do you do with the biogas waste - is it toxic to soil?

Hi,

I wondering what you do with your biogas waste?  I have read bits that it is not that good for the soil, but can't find a lot on it.

I did find this article, and read the more understandable parts, and its conclusion is the need for more research.  
https://www.intechopen.com/books/biofuel-s-engineering-process-technology/utilisation-of-waste-from-digesters-for-biogas-production

Quite heavy reading, and it seems that it is staying that it gives an immediate flush of nitrogen small green growth, but long term does not support bacteria life required for the breakdown of organic waste in to mineral nutrients available for plants.

It did say in one small section that when fermentation was 18-20C rather than 40C or more (and so different bacteria), then it was less of a problem.  Do most home biogas systems sit closer to this 18-20C range?

I would think that depends on what you already have for soil conditions.

My soil is already at the optimum level for organic matter due to years of spreading manure upon my land, so I am in pretty good shape. In fact, nitrogen without organic matter would be really good for my land at this point as my levels are so elevated (yes, you can have too much organic matter).

But all biogas is, is a ruminant animal like a cow or sheep.

I mean consider this, most biogas generating plants use raw food waste for fuel because it gives them 75% more production. One by me would have had only 250 KW production if it has just gone with their cows manure waste, but instead they included food waste, and upped their power generation to 1,000.000 KW. The reason that could do that was, their cows process their feed pretty darn well. By using raw food from grocery store waste, it did not go through a cow or a human, so they processed the biogas out of it instead.

But manure still has some great attributes, and I have used it for years to fertilize my fields just fine. My NPK was increased, along with my organic matter. Yeah my sheep and cows burped and farted out 75% of the biogas, but it still has value, just as a biogas system waste would.

I used composted biogas slurry one season when I lived in Nepal, and it worked like a very nice composted manure. I have read that the slurry from biogas is very high in nitrogen and it's best to add some carbonaceous material and let it compost aerobically before using it in fields or gardens.

I was talking with a guy in Maine who has a pretty big biogas generator (1 megawatt) and he said in just a few years they think they will be able to separate the NPK separately from the waste side, so that if a field needs just potash, they ca apply just the right amount of that, instead of trying to apply all three NPK as they have to now. That will be really cool to do.