[Digestion] Crystalline deposits in digester

Sun Sep 16 14:23:52 EDT 2007

Frank, I am afraid that I do not recall the composition of the crystals. I 
did not work on this digester myself but heard the story from a colleague, 
so my knowledge of it is limited.

However, I do not think the crystals were either a valuable byproduct or a 
toxic hazard. I recall them as something readily explicable (in retrospect!) 
for the effluent being processed.  Bear in mind, this was not manure, it was 
a pharmaceutical effluent, so I do not think crust formation, in the usual 
sense, had anything to do with it.

By voids, I mean holes, passages, cavities, channels -- in other words 
spaces through which fluid could flow. The crystals did deposit on the walls 
and floor but I suspect that the mechanism had more to do with low shear 
rates near the walls than with temperature. As the deposits gradually 
thickened, the effective working capacity of the digester was gradually 
reduced.  Eventually, as I described, the working capacity was limited to a 
roughly spherical cavity around each of the three impellers, linked to each 
other and to the feed and discharge points by channels through the 
crystalline mass.

As I recall it, it took several years for this severe reduction in capacity 
to develop. So, yes, an annual clear-out might have helped -- however, that 
is a major undertaking, not something to be lightly considered. For an 
industrial effluent, the consequence might be a month or more interruption 
of production.

Emulsification of the feed might have helped but my purpose in relating this 
story was to emphasise the necessity of good mixing.

Duncan

----- Original Message ----- 
From: "Katahdin Energy Works" <KatahdinEnergyWorks at verizon.net>
To: "'Duncan Martin'" <duncanjmartin at eircom.net>; <frank at compostlab.com>
Cc: <digestion at listserv.repp.org>
Sent: Friday, September 14, 2007 12:38 PM
Subject: RE: [Digestion] Floating manure

>I give up...crystals of WHAT....valuable byproduct or a toxic hazard or
> what? How was the digester heated; did the crystals precipitate on the
> cooler walls or elsewhere..not sure what you mean by a "void".
>
> Crust formation is a problem I thought was solvable by proper 
> emulsification
> of manure beforehand, and a shorter digester cycle, as well as an overflow
> type of emptying of the digester.
>
> I've heard of the crust being allowed to build up and harden...yikes!
>
> Bubbler vs. agitation at the top of the digester?  Higher temp operation?
> Any solutions, besides cleaning it out yearly?
>
> 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 Duncan Martin
> Sent: Thursday, September 13, 2007 7:27 PM
> To: frank at compostlab.com
> Cc: digestion at listserv.repp.org
> Subject: Re: [Digestion] Floating manure
>
> Frank, that's a very good question - but I think based on a slight
> misunderstanding.
>
> I suspect the problem is not that the reaction stops but that the
> non-digestible fraction becomes concentrated in the floating layer, so is
> never discharged from the digester and accumulates..
>
> Any digestible material entrained with it probably digests quite well,
> though perhaps a little slower than in the well mixed zone. However, I am
> not aware that there have been any studies done on this.
>
> Essentially the same problem arises with solids that settle on the bottom 
> or
>
> crystallise on the walls. This is why it is so important to have a mixing
> system that mixes the whole digester.  As the digesting material is 
> usually
> a non -Newtonian fluid, it can adopt a stable mixing pattern in which the
> zone around the agitator is well mixed while the remainder is stagnant. 
> This
>
> can happen even when the feedstock is a relatively thin liquid.
>
> There was a case over 30 years ago in Yorkshire, England, involving a
> pharmaceutical effluent where the performance of the digester gradually
> deteriorated, over a period of years.  When they opened it up, they found 
> it
>
> was almost full of crystalline deposits, so the effective capacity of the
> digester was reduced to only a small fraction of its nominal size. There 
> was
>
> a void around each of the three side-entry agitators, linked to each other
> and to the feed and discharge pipes by "rat-holes"! And that's all - the
> rest was crystals!
>
> Such situations rarely arise at lab scale, which can lull you into a false
> confidence. At large scale, unconfined sequential gas injection is much 
> more
>
> effective at avoiding such situations than gas injection into a draft tube
> or agitator-based systems, because there is "no hiding place" for deposits
> of any kind.
>
> The mechanism of flotation probaly isn't primarily large bubbles of gas
> getting caught under lumps, as you describe it. It is more likely to be 
> the
> attachment of multiple microbubbles to small particles, which then cohere 
> to
>
> form a crust when concentrated near the surface. This principle is widely
> applied in a number of industries, including mineral processing and 
> effluent
>
> treatment, to separate fine particles from a dilute slurry.
>
> Best regards
>
> Duncan J Martin
>
> Chair
> Republic of Ireland Centre
> Chartered Institution of Wastes Management
>
> ================================
> CONTACT DETAILS
> Duncan J Martin, PhD, CEng, CSci, MIEI, MCIWM, MIChemE
> 24 Townsfield, Cloughjordan, N Tipperary, Ireland
> Mobile: +353 86 8377 906
> Home: +353 505 42087
> Email: duncanjmartin at eircom.net
> ================================
>
>
>
>
>
>
> ----- Original Message ----- 
> From: "frank" <frank at compostlab.com>
> Cc: <digestion at listserv.repp.org>
> Sent: Thursday, September 13, 2007 5:02 PM
> Subject: Re: [Digestion] Floating manure
>
>
>> Biogas people
>> I assume the floating manure is methane getting caught under it and it
>> floats to the top? But why does this stop the reaction, production, of
>> methane? It should still be wet and under anaerobic conditions.
>>
>> Thanks
>> Frank
>>
>>
>>
>> Jaime Marti Herrero wrote:
>>
>>>>I haven't worked with cattle manure much. It seems to> float and form a
>>>>heavy crust that interrupts gas flow.
>>>>
>>>>
>>>
>>>i rescue a mail i sent weeks ago about this subjet:
>>>
>>>Hi everybody.In the last moths im meeting people who is installing 
>>>tubular
>
>>>biodigesters and they are using 1:2 manure:water rate to feed the
>>>biodigesters. From five years ago i started using 1:4 rate (recomended by
>>>Lylian Rodriguez and Preston), but from two years ago im using 1:3 
>>>because
>
>>>the in some places the are not that much water for 1:4.the reason tu use
>>>1:3 or 1:4 is to avoid the formation of the foam that stop the production
>>>of biogas. But if really you can work wiyh a lower rate (1:2) without
>>>problems with the foam, this will be a great issue, because the volume
>>>required wil be lower and the cost os materials and biodigester as 
>>>well.So
>
>>>please, have you got good expiriences with low rate manuere:water for 
>>>long
>
>>>time with out foam?thanks everybodywe keep in contactjaime> Date: Wed, 12
>>>Sep 2007 19:52:41 -0700> From: weiswar at yahoo.com> To:
>>>Digestion at listserv.repp.org> Subject: [Digestion] Floating manure> > > > 
>>> >
>
>>>Do large digesters just add more water? > > My thinking w!
>> as to weight it down with a steel screen.> Thoughts? > > > >
>>
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>>
>> -- 
>> 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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