[Digestion] landfill vs. AD

Sun Apr 29 16:51:42 CDT 2007

Hi
Don't forget that cow manure is already digested.

Regarding the digestion efficiency one must not forget that this is also a matter of economy. Normally the turn over time in a digestior is around 20 days but then all the digestibel matter is not completely digested. I you want to get a higher efficiecy it takes a longer time and then the treatment capacity in tons per day will be less and one must remember that for a commercial plant digesting for instance MSW the main income is the gate fee in spite of the fact that the methane gas can be sold as a fuel for city buses or for electricity production. What the owner of a commercial digestion plant wants is not necessarily high efficiency but return on the invested capital. There is a similar effect if you landfill and suck out the gas. After some 15 - 25 years the methane production has gone down so much that it is not economical to suck it out any more, but the lanfill will continue to produce methane gas at a low rate for many many years after that.

One thing one must also think about is the method of digesion in cells. A Cell could be a big concrete box filled with waste and covered by a gastight membrane. The digestion process would take 1,5 up to a couple of years. When the gas production goes down you take away the membrane or exchange it for a permeable type and ventilate so that the process turns into composting. Finally you dig out. Still the main income is the gate fee - as always.

So there are many ways of doing it.

Regards 
Bjorn Dahlroth

-----Ursprungligt meddelande-----
Från: digestion-bounces at listserv.repp.org [mailto:digestion-bounces at listserv.repp.org] För Jason Perry
Skickat: Sunday, April 29, 2007 7:19 PM
Till: digestion at listserv.repp.org
Ämne: Re: [Digestion] landfill vs. AD

Thanks Duncan. This is purely academic at this stage, and I appreciate 
your input. My question is really more about comparing the efficacy of 
the two methods in terms of useful methane output per amount of matter 
input. I agree, the transport schemes won't be exactly the same.

However, in this particular town there is no landfill, just a transfer 
station -- the landfill is run by the county in a different town. Most 
residents drive their rubbish and separated recyclables to the transfer 
station every Saturday. There is some curbside collection (taken 
directly to the landfill) but I think it's the exception. So, if people 
were convinced to source separate their food waste in a different 
container, and a central AD were set up at/near/on the way to the 
transfer station, then the transport scheme would be about the same.

As it is there is no official separation of kitchen waste, and the 
county does not plan to encourage it because they are implementing a LFG 
energy recovery system this year. There is some voluntary home composting.

The reason I suggested co-digestion with cow manure is that the town has 
nearly as many dairy animals (about 4500) as it does people (about 
6000). Unless I am grossly mistaken the amount of food waste from 
households is dwarfed by the quantity of cow manure.

Cheers,
Jason

Duncan Martin wrote:
> Hi Jason
>
> But a similar transport scheme seems highly illogical. Have I read 
> your proposal aright?
>
> If the vast majority householders can be persuaded to segregate their 
> biowaste efficiently (challenging!) and you set and pay for a separate 
> collection system, then an AD plant is a good option. It would be daft 
> to go all that trouble then dump it in a landfill - with all the stuff 
> you've kept it apart from.
>
> Landfill with LFG collection and use might be preferable (under US law 
> - but not in EU) where the biowaste is left in mixed waste or in a 
> residual waste fraction.
>
> However, if you had in mind some kind of biowaste AD cell within a 
> landfill site, my own research suggests that it would biodegrade much 
> more slowly than in a mixed landfill, because rapid acidification 
> would effectively ensile it.
>
> 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: "Jason Perry" <perry.jason at gmail.com>
> To: <digestion at listserv.repp.org>
> Sent: Friday, April 27, 2007 4:54 PM
> Subject: [Digestion] landfill vs. AD
>
>
>> Dear AD Listers,
>>
>> I may have asked this question in some form before, but I thought I'd
>> rephrase it.
>>
>> Consider a rural town in the Northeastern US. If you had a choice
>> between 1) sending the town's domestic food waste to a landfill that
>> collects its gas and runs a generator, or 2) sending it to a central AD
>> that co-digests food waste and dairy manure and runs a generator, which
>> do you think is a better use of the food waste in terms of energy
>> production? Assume similar transport schemes between the two.
>>
>> Many thanks,
>> Jason Perry
>>
>> _______________________________________________
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>> Digestion at listserv.repp.org
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>>
>>
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>
>

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