Questions Regarding Digester Basics

Digester design and construction info

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brgionta
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Questions Regarding Digester Basics

Postby brgionta » Fri Mar 26, 2010 12:28 am

Hi Bob,

Your website is absolutely amazing, the best I have come across!! According to your two-stage design and corresponding literature you suggest that the acidogenesis stage and methanogenesis stage have a HRT of 3 days to 9-10 days respectively. After 3 days in the acidogenesis compartment, is the sludge then transferred to the methanogenesis compartment? And is this methanogenesis compartment initially empty? Is the sludge transferred from the acidogenesis tank to the methanogenesis tank via displacement following feeding the reactor after 3 days? I guess I am a little unclear of the flow and timing of organic/animal waste to the acidogenesis tank to the methanogenesis tank. My understanding is that pH is vital to the existence of each microbial stage. How or what would you recommend to control for the pH in each stage and suggest for "seeding" the reactor?

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Re: Questions Regarding Digester Basics

Postby Bob » Fri Mar 26, 2010 2:07 pm

After 3 days in the acidogenesis compartment, is the sludge then transferred to the methanogenesis compartment? And is this methanogenesis compartment initially empty? Is the sludge transferred from the acidogenesis tank to the methanogenesis tank via displacement following feeding the reactor after 3 days? I guess I am a little unclear of the flow and timing of organic/animal waste to the acidogenesis tank to the methanogenesis tank.

It's not quite accurate to say that sludge is transferred after 3 days from one compartment to the next. It would be more accurate to say that the contents of the acid tank are displaced by incoming new material to flow into the methane tank. And that, depending on daily flow rate, the average retention time in the first tank is 3 days. HRT (Hydraulic Retention time) is simply the volume of the tank divided by the daily flow through it.

For example, lets say you have a 100 gallon tank partitioned into two chambers of 25 gallons and 75 gallons respectively. To get an HRT of 3 days in the first, you would need to feed it 25/3 = 8.33 gallons of mixed slurry per day. And because the larger chamber is 3 times bigger than the first, the retention time in that one would be 3 times longer, or 9 days.

My understanding is that pH is vital to the existence of each microbial stage. How or what would you recommend to control for the pH in each stage and suggest for "seeding" the reactor?

That is a large subject. There has been a fair amount of discussion hereabouts about startup. Check out this topic, for example:


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