[Gasification] How anaerobic digestion produces renewable energy

Bruce Wilson Contracting bwc at ptd.net
Wed Jun 1 09:54:34 CDT 2022


On 5/7/2022 10:29 PM, hugh at austrop.org.au wrote:
> Thanks Bruce
> but what you are referring to would be a commercial digester with a
> well insulated tank, an integral mixer and perhaps a heater.
> Unfortunately Home Biogas systems are far simpler - but are dependent on
> local conditions to work. Even here in the wet tropics (16•S) winter
> temperatures can go as low as 13C. Putting in a heater, would consume
> a significant amount of our power (we are solar). Had we really been
> aware - we could have installed an insulated jacket, but in this 
> environment
> it would have failed quickly - moisture and various organisms setting
> up home there (in fact they could have punctured the tank - chomp!). 
> So these are
> great in warm and dry environments - and as they are inherently smelly
> most would not want them too close to the dwelling.
> However homogenisation really does speed things up.
> There is obviously a market for a better design - but we have the issue
> of gas storage - which HomeBiogas solves quite elegantly with the top
> bladder (which takes no extra space). A gasometer style storage would 
> be fine
> except for the space issue. Compression brings on a whole collection 
> of other
> issues.
> There is no free lunch! (or cooking heat)
> Cheers
> Hugh
> On 07.05.2022 9:14 pm, Bruce Wilson Contracting wrote:
>> Just a quick primer on methane digesters, it is a two part process,
>> first is the acidification stage where bacteria turn the waste to
>> acids. The bacteria do not have teeth or mobility, so putting what you
>> put into the digester through a grinder helps break it down so the
>> bacteria can access the nutrients. These acid producing bacteria
>> produce CO2. After the acids are produced the methanogenic bacteria
>> digest the acids and produce methane. Gentle mixing is called for
>> because so that the bacteria can access the nutrients to be digested,
>> the bacteria that produce the methane stop producing methane when
>> agitated which is why gentle mixing is called for. There are two ideal
>> temperatures for digesters, 120 degrees F, or thermophylic and 100
>> degrees F mesophylic. A ten degree drop in temperature leads to a
>> twenty percent drop in gas output. Thermophylic produces more gas but
>> is more finicky, mesophylic is much more forgiving. Bacterial seeding
>> can help a digester get started.
>> On 5/4/2022 1:19 AM, hugh at austrop.org.au wrote:
>>> Hi
>>>  we have one of the original Israeli HomeBiogas systems. Living 
>>> where we do - we had problems
>>> getting fresh cow shit. It was a disaster - mostly CO2.  So we 
>>> emptied, flushed and sterilized
>>> it and a (not so local) dairy provided 60 litres of fresh cow shit. 
>>> Then Covid hit and our supply
>>> of local ex-restaurant waste stopped. (we didn't produce enough 
>>> ourselves). 2 years later (the system
>>> having been been totally ignored), it started to inflate. Evidently 
>>> the battle between the CO2
>>> producers and the methanogens had finished, and to our delight the 
>>> gas appears to be pure methane. So
>>> we have been using this now for months for a significant amount of 
>>> our cooking.
>>> However - we homogenise everything.that goes in - this speeds up the 
>>> digestion process greatly,
>>> and means that we are not getting a residue of indigestible material 
>>> in the digester tank
>>> (which was a major problem with the initial clean out).
>>> And yes - we should be using the copious liquid fertiliser that the 
>>> system produces - too much
>>> else going on and not enough hands available.
>>> Homogenising - we use a modified Kambrook Power-drive stick mixer - 
>>> with a modified end (cut the
>>> loops off). So long as there is little fibrous material, or any 
>>> really hard stuff (bottle tops etc)
>>> it is a quite amazing machine.
>>> (also HomeBiogas have developed a toilet system that feeds the 
>>> digester directly. Wish we had had one!)
>>> Worth the effort.
>>> Hugh
>>> www.austrop.org.au
>>> On 04.05.2022 2:22 pm, Anand Karve wrote:
>>>> Dear all,
>>>> I developed in 2003 the urban domestic biogas plant which works
>>>> without dung, but uses only domestic food waste as feedstock. The
>>>> effluent slurry of this biogas plant is watery. I applied it regularly
>>>> to an experimental plot of garlic. It was just a feeler trial to see
>>>> how the plants responded to the effluent. To my surprise, the plot
>>>> receiving the slurry gave higher yield of garlic bulbs than the plot
>>>> receiving the recommended dose of chemical fertilizers. The area of
>>>> both the plots was equal.
>>>> Yours Anand Karve
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Bruce Wilson LEED AP, Project Developer at GEMMasterPlanning www.gemmasterplanning.com
Bruce Wilson Contracting, bwc at ptd.net, 610-282-0822
Environmentally Sustainable Renovation, Restoration and New Construction.
Energy Efficiency Improvement Contracting and Sustainable Consulting
PA Registered Contractor PA#10930,

  Celebrating 45 years in Business.
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