[Digestion] Floating manure

Thu Sep 13 16:27:29 EDT 2007

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
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Email: duncanjmartin at eircom.net
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----- 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
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