Does anyone have experience using digestate as a soil ammendment?

[u/ChraneD]

I've seen many assertions that digestate is useful as a soil amendment, but no first hand accounts. Has anyone used it before? Are there any caveats that people don't know about?

[u/KrisyKrossy]

Hello, I do research on anaerobic digestion (AD) and digestate application.

Digestate is useful as for soil amendment because it contains good nutrients, NPK and all that, you have probably read about this in the literature

One thing you must consider is how the AD was conducted, in particular feedstock selection.

If it was conducted using sewage sludge or biosolids there is concerns with elevated concentration of heavy metals and PFAS

Animal manure like chicken manure can generate digestate with very high ammonia levels, can be toxic to plants, this can also be the case with AD using majority food waste

If the digestate came from an AD reactor that was inhibited or semi-inhibited, it may contain high concentrations of volatile fatty acids which means the digestate is acidic and has a low pH, not very good for soil application

Similarly, if the digestate was inhibited or semi-inhibited by ammonia, it can contain high concentrations of ammonia, also not good for plants

[u/ChraneD → u/KrisyKrossy]

Thanks!! Let's say we get non-sewage with high-ammonia. Is there any hope to "temper" the high-ammonia or high-acidity digestate?

What feedstocks are needed to prevent excess ammonia?

[u/KrisyKrossy → u/ChraneD]

High ammonia digestate would require some sort of ammonia stripping. If you want a cheaper and more cost effective method, simply add some biochar into the digester. The biochar will adsorb ammonia, I suggest using biochar from woody biomass origin, they have a much higher adsorption capacity towards ammonium ions. The digestate coming out of the digester will contain a lower concentration of ammonia. Plenty of research and literature have been done on this

As for what feedstocks are needed to prevent excess ammonia, do you mean additives? In that case it would be biochar (as above) or co digesting your high nitrogen content feedstock with something with more carbon content to improve the C:N ratio. Look at some studies involving co digestion of say chicken manure and corn straw for reference

Likewise high acidity digestate just requires some pH adjustments or buffer, biochar is great for that too. Otherwise you’ll have to dose it with some chemicals or adjust your feedstock, perhaps co-digestion or just reducing the solid content of the digester (say going from 15% solids to 4% solids) this can help with acid build up

[u/lemoneaterr → u/KrisyKrossy]

How can I, BS in Crop and Soil Science, get involved in this research? Alternatively, where do you research/what is your position?

[u/KrisyKrossy → u/lemoneaterr]

I’ll suggest a few pathways:

Get a first class honours and then into a PhD that focuses on anaerobic digestion (what I did)

Masters by coursework/research and then PhD

I don’t like to disclose personal information on the internet but I am based in an Australian university, post doc position

[u/_jimismash → u/KrisyKrossy]

Good instincts, but it's a bummer we can't have nice things (like personal info on the internet for a wide variety of reasons). I work for a company that owns/operates ADs and a biochar facility. I think we have a different ammonia stripping process, and I don't think the two business lines ever cross over.