[Digestion] Mixing in digesters?

[Katahdin Energy Works]

Dear Frank, and Biogas people,

Just a few questions if you don't mind.

Katahdin Energy Works wrote:

>Depends on the design.
>
>We do tanks, and depending on the % of solids may include a bubbler in the
>bottom of the tank. 
>  
>
What percent solids value do you to make this decision and what do you 
consider the optimum % solids?---Sort of depends on where the manure is
coming from  and how much we emulsify it. Beef cattle have up to 23% solids,
while diary is looser, we like 11-12% ...don't forget the role of bedding in
all this!

And is the bubbler recirculating the gas that is produced in the tank?...no,
liquid separated from the spent manure slurry. Gas goes through scrubbers,
why get it wet again? 

>Other tanks include a heating core and agitator to have a uniform
>temperature.
>
>Temps are a concern; as is exposure to the two key microbe
>groups--Acetogenic and methanogenic. We use imbedded colonies to maximize
>exposure. 
>  
>
What is the optimum temperature you try to maintain and will the microbe 
activity be able to produce the needed heat without the need for outside 
heat source?-----Summer use is 135 F(high temp, fast cycle); winter use is
95-100 F.  Tanks are insulated and hold heat from the preheater.

>There is a natural bubbling that occurs as solids sink to the bottom and
get
>acting upon.
>
>CHD lagoons have a patented circulation system w/pumps and baffles.
>
>I haven't check evaluations of agitator systems; remember injection is from
>the bottom and that provides mixing; and digestion is from the bottom to
the
>top.  
>
>In two tank designs; the acidification process is often a 'fast' one with a
>lot of agitation; while the methane one sees solids somewhat settling and
>methane given off bubbling to the top. 
>  
>
What is the optimum pH value for the system to work at optimum
conditions?...about 7.5, remember in Maine we have sawdust bedding and that
lowers the pH.

Can the pH be used to control the CO2 given off?....depends on the microbe
strain, I think?

Higher pH to keep the CO2 in solution , perhaps adjusting at the end?  to
reduce the CO2 in 
the out gas. Good question; really haven't adjusted the pH levels to account
for CO2; we are mostly seeking to shorten the digestion cycle so we can
process more manure with a smaller tank faster; and to increase gas yields.
We are trying to sell the CO2 to a greenhouse or it is just scrubbed out. ?
Are you going for REC's?


>Hope this helps.
>  
>
It helps a lot as I am on a steep learning curve.
And the top of Mt Katahdin is my favorite place on Earth.

It is nice; and the trail has been improved. Really haven't be up it, in
years. Thanks for reminding me!

Thanks
Frank



>Frank J. Heller, MPA
>KATAHDIN ENERGY WORKS
>12 Belmont St.
>Brunswick, ME 04011-3004
>207.729.6090
>http://mysite.verizon.net/fjheller/
>
>
>-----Original Message-----
>From: digestion-bounces at listserv.repp.org
>[mailto:digestion-bounces at listserv.repp.org] On Behalf Of Harmon Seaver
>Sent: Wednesday, September 12, 2007 2:16 PM
>To: Digestion at listserv.repp.org
>Subject: [Digestion] Mixing in digesters?
>
>   Someone just posted that  "well-stirred digesters with mixed contents
>will always out-perform single feedstock digesters by volume". Sometime
>back, when I asked about having a mobile digester (to provide fuel for a
> motorhome and process waste), I was told that it would not work because
>disturbing the digester would cause it to stop working.
>    So is mixing bad or good?
>
>  
>

-- 
Frank Shields
Soil Control Lab
42 Hangar way
Watsonville, CA  95076
(831) 724-5422 tel
(831) 724-3188 fax
frank at compostlab.com
www.compostlab.com




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