[Digestion] Food-waste anaerobic digester at University of Colorado

[Björn Dahlroth]

Hi
>From energy point of view composting can never beat digestion. 

If you have a large waste incineration plant in the vicinity producing
district heating or using the energy for some sensible purpose composting
can't beat that either but it depends on the moisture content of the food
waste and on whether they extract heat from the moisture in the fluegas by
condensing. 

Composting "burns up" huge amounts of bio-energy and even if you consider
the fertilizing effect of compost and the energy saved in the manufacturing
of commercial fertilizer, composting is still loosing in comparison. In a
future when bio-energy is scarce larger scale composting might be less
common. You also have much less fertilizer loss in a digestion plant. 

However today composting can be cheaper depending on how you do it. Compost
can be a soil improver and may be the present nearby soil mixing market is
paying more. For composting pure food waste you also need to add structure
material like garden waste and similar and that adds to the soil improving
properties but entails an additional cost.
 
The digestion plant will of course also produce some sludge that can be
treated afterwards in a composting process.

Something that you might find interesting for small plants is to digest in
concrete cells with gas tight membranes. The process takes much longer time,
but is not so sensitive to impurities due to bad source separation. When the
gas production is going down you switch over to composting. Afterwards you
dig it out. This method is in use. Of course such a process has its fair
share of practical problems but I guess there are no digesting plants
anywhere of any kind where they don't have some problems. That is why you
employ people to run them. 

Regards 
Bjorn Dahlroth







-----Ursprungligt meddelande-----
Från: digestion-bounces at listserv.repp.org
[mailto:digestion-bounces at listserv.repp.org] För Jason Woods
Skickat: den 24 oktober 2007 19:43
Till: digestion at listserv.repp.org
Ämne: [Digestion] Food-waste anaerobic digester at University of Colorado


Hello,
 
I'm a mechanical engineering student at the University of Colorado in
Boulder and I'm currently working with a professor and four undergraduate
students on a feasibility study about using anaerobic digestion on campus.
The university produces around 1500 tonnes of food waste per year.  There
will also be some yard waste, but from what I've seen, it still seems the
waste stream is still too small to make the project viable.  Would
composting make more sense in this instance?
 
I was also trying to figure out how much waste water will be generated, and
it seemed pretty high.  If the waste stream is ~70% moisture content and the
process is run as a "high-solids" digester, and the digestate/compost is
dried to 50% moisture content, my mass balance showed lots and lots of waste
water.  Am I forgetting something?  Is there very much water lost to
evaporation?
 
Any other advice, suggestions, or comments are welcome.
 
Thanks much,
jason woods
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