DIG-L: Post treatment of digestates

[CAVM@aol.com]

You raise several significant issues in your post.

Solid separation after digestion is more of a challenge than before. A very 
fine mesh filter may be the best solution, and some will work without 
flocculants.  Take a look at the Salsnesp filter, for example.  In the USA 
you can get info from Mark Thomas of sustainablenergy@qwest.net

Nitrogen is more readily available after digestion than before so it may 
become your biggest issue.  Wright-Pierce of Topsham, Maine, USA, is working 
on various methods to remove nitrogen post-digestion.  They are probably 
ahead of the pack in this work.  You might want to contact Mike Giggey, VP of 
Wright-Pierce, via
MDG@Wright-Pierce.com

Phosphorus is another nutrient of concern.  Alum and other simple chemical 
additions can help avoid P overloading.  Bacteria uptake and 
hyper-accumulator crops are also potential solutions.

We have been working for some time to evaluate the potential of using some of 
the post digestion solids for combustion fuel.  The heat provided cannot only 
be used to provide some of the warmth required by the digester bacteria but 
also to dry the separated solids post-digestion of pre-digestion.  By 
separating the solids before digestion you dramatically reduce the required 
sized of the digester vessel but only marginally reduce the gas produced from 
the remaining liquid.

Pre-digestion separation is much simpler than post-digestion separation.  We 
recommend the Maximizer separator by Accent Stainless of British Columbia, 
Canada.  Information can be obtained via 
Bcruz92312@aol.com

Pre-digestion separated solids are essentially odor free for several days.  
In the case of hog and dairy farms they can be composted into a very good 
soil amendment or dried to be a good combustion fuel.  For biosolids from 
municipal sewer systems the metal content dictates the options.  If a 
sufficient volume of solids is available electrical generation and steam 
production are options when combusted.

There are other means of treatment if necessary.

Cornelius A. Van Milligen
Kentucky Enrichment Inc.
byproduct processors
CAVM@AOL.com
============================================================
In Belgium there is a lot of chance that digestates will not be 
allowed to use on land because of the fact that they are considered 
by autorities as products rich in nutrients.  The reason is that also 
the excess of manure in Flanders, which is considered as 
nutrientrich, has to be exported out of Flanders.  In Flanders, 
normally only humusrich-nutrientpoor products wil be allowed to 
use.  Conclusion is that a digestate will have to be treated 
afterwards towards a compost or a pellet. (which is considered as 
humusrich, can be used in Flanders)

Does anyone have good data on the post-treatement of digestates? 
Things that would be of big value are:

Cost of solid-liquid separation, which separation equipment is 
used, which polymers and quantities are used for separation, does 
the type of inputmaterial (manure digestate or biowaste digestate 
for example) used for the biogas process influence the ease of 
separation, is digestate from slaughterhouse waste as easy to 
separate as digestate from biowaste,...

Is the solid fraction easy to compost, is a lot extra structure and N-
rich material needed to abtain good composting,...

what can be done with the liquid fraction, can it be easily stripped 
from nitrogen knowing that it will contain some floating material 
which has not been removed during separation,...

what is the opinion on techniques like inverse osmosis and 
ultrafiltration to treat the liquid fraction, what will the price be for
this kind of treatments,...

To resumate, every information (internetlink, report, cost 
analyse,...) that handels about post-treatement in general would be 
valuable for me.

Best regards, Jan 
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