digester for suburban community

Digester design and construction info

Moderator: Bob

dwroblewski
Posts: 2
Joined: Tue Dec 09, 2008 11:55 am

digester for suburban community

Postby dwroblewski » Wed Dec 10, 2008 8:55 am

I am currently working with a friend to build a digester for her home. If we can build one that works efficiently, our end goal would be to market the idea to her small suburban community, to see if they would let us build a larger model that the community could collevtively use to turn their waste into energy. Our research on the subject has aroused some questions.

1. Since she lives in a small suburban community, we would like to make a digestor that could run on grass clippings in the summer, tree leaves in the fall, and dog waste / food scraps year-round. Would this be possible? I've played with the calculator a little bit but haven't been able to figure it out.
2. Given this type of waste, would a continuous or batch system be preferred?
3. What is the best way to macerate the waste and heat it up to 95 degrees F? Keep in mind this is in a suburban community and we don't have access to farming equipment / heavy machinery.

Help on any of these questions would be greatly appreciated. Thanks Bob! Love the site!

Drew

User avatar
Bob
Posts: 631
Joined: Tue Jul 03, 2001 11:01 pm
Location: Willow, Alaska USA
Contact:

Re: digester for suburban community

Postby Bob » Fri Dec 12, 2008 10:35 am

Hi Drew,

I've played with the calculator a little bit but haven't been able to figure it out.

It is based on a continuous feed design. The inputs (on the left side) are what would be added on a daily basis. Enter the estimated daily quantity of each feedstock, adjust the mixture till you get an approximately 30:1 C/N ratio, then add enough water to get an approximately 6-8% slurry. The outputs (on the right) tell you about how big a tank you will need, and approx how much gas you can get per day. (Use it as you would EPA mileage stickers, bearing in mind that "your mileage may vary". There are many variables that will affect actual output.)

2. Given this type of waste, (grass clippings in the summer, tree leaves in the fall, and dog waste / food scraps) would a continuous or batch system be preferred?

A short answer is that fibrous material is difficult to pump. Continuous feed systems are better for finely macerated material and thinner slurries that won't clog the pumps & piping. You might consider a so-called 'hybrid' system, consisting of a number of 'acid' modules, each feeding a 'methane' module. The acid modules are batch feed -- throw the leaves (or grass clippings, whatever) into it, add water and let it ferment. Then pump the liquid from it (food for the methanogens) into the methane module. An advantage is that only the methane module may need to be heated.

3. What is the best way to macerate the waste and heat it up to 95 degrees F?.

That will depend on the type of design, ambient temperature, amount of insulation, and what heat sources are available. (Are you in the sunny south? Frigid north? Will it be in a heated space or outdoors?) The online calculator assumes using some of the methane generated, to heat the digester. Re maceration, a batch feed (or 'hybrid') design doesn't need it.

dwroblewski
Posts: 2
Joined: Tue Dec 09, 2008 11:55 am

Re: digester for suburban community

Postby dwroblewski » Sun Dec 14, 2008 8:01 pm

Thanks for the post. To answer your questions, we are located in Pittsburgh, PA, and the digester will be stored outdoors. Working indoors will create too many problems. Your comments raised some other questions.

1) Once we have obtained the gas, how do we purify it to determine how much is methane and how much is CO2?
2) What is the projected total cost of materials for the digester you show on your site?

User avatar
Bob
Posts: 631
Joined: Tue Jul 03, 2001 11:01 pm
Location: Willow, Alaska USA
Contact:

Re: digester for suburban community

Postby Bob » Mon Dec 15, 2008 1:18 pm

1) Once we have obtained the gas, how do we purify it to determine how much is methane and how much is CO2?

You can scrub the CO2 from the biogas by bubbling it through limewater. H2S can be removed by filtering it through steel wool.

That said, I think most people don't bother -- at least not for simple small scale systems where the gas is to be used only for cooking or heating water. For larger scale systems and/or where you need to compress and store the gas, it may be worth the extra cost and complexity to remove the 30-35% CO2. The primary benefit is efficiency -- reducing the volume of gas to be compressed and stored. The reason to remove H2S is that it is corrosive. But unless you are feeding it into an engine, that may not be worth the bother either. You can minimize the amount of H2S generated by careful selection of feedstock (eliminate fats, proteins), and/or pretty much avoid the need to scrub it at all, if all you're going to do is pipe it to a burner in non-corroding tubing.

2) What is the projected total cost of materials for the digester you show on your site?

That's a complicated question, Drew. What I have shown is a 90 gallon digester only. Not shown is an input method (e.g. mixing tank, pump, piping, valves, etc.), gas-holding & usage (e.g. gas bag, compressor, floating cover collector, etc.) or effluent handling. I don't have the total cost at my fingertips, and it was built a few years ago, but I'm going to guess that it was about $1,000 at the time, maybe $3-400 for the digester itself.

A more useful question (or answer) would be that I think a design which improves in significant ways on the one shown, can be built for less than $1,000, installed (including input, output & gas-handling).


Return to “Anaerobic Digester”

Who is online

Users browsing this forum: No registered users and 1 guest