Carey’s Bio Digester

[Carey Ma]

Here you go folks. In collaboration with an old time friend, we have developed a portable, family sized methane production unit that harvest bio-gas (methane). This is the first stage in my three-stage compost system.

Features

• 20 year lifespan
• 2 man assembly (4 hours)
• Weighs only 15 to 20 kilos/ 30-40 pounds
• Operates at –40 to 60 deg C
• Daily output of  0.3 to 0.6 m3 per day
• Made or recycled rubber

 

All for a mere USD $400.00 FOB Tinjing, P.R.China

Whatchall think?

[Carey Ma]

A cutaway of my bio-digester.

Key code: Blue = air. White = soil/ pit base.

[Carey Ma]

@ Jon P. This is a straight forward stomach design (literally taken from the design of our own stomachs), no baffles or anything. Incoming waste is driven by gravity into the separation part where separation occurs. Some waste naturally sinks to the bottom until reaction occurs were it starts to float. It is the floating layer that is most active (which may need supplemental heating in winter to keep steady gas production).

 

The densest layer is the bottom sludge layer. The separation layer is a mixture of flush water and solid waste, the consistency of thin mud. Sludge is forced up (slowly) via built pressure from gas production and from incoming waste volume. So yes (kind of), 5 gal in 4 gal out.  Farmers can either add this sludge directly into their watering cans or preferably toss it onto their compost pile to dry and further process.

 

Daily input volume is expected to be one gallon human poop and two gallons animal poop mixed with two to four gallons of water.

 

Temperature and pH are the main factors to watch for.

I like that pic…poop powered rocket…who would ever guess…hahaha

 

@ Cris S: Good luck with your project. Please let me know how I can help.

 

Cheers

[Rick Welland]

Hi Carey..Here are two scale drawings of laSenda as it is developing..first one is overview of our buildings and growing areas with ponds..second one shows the trays descending the incline for greenhouse/shade cloth area..hope this helps.

In the second drawing the digestors are on the east/west ends of the main building- feeding into the constructed wetlands next to the large fish pond in front of the living units..what think ye?

[Jon Parr]

Thanks so much, Carey. More questions I haven't been able to find while researching. Do you have to monitor the pH inside the digester?  If so, what is the range you try to keep it at?  Do you keep it in that range by using a pH up like ashes or lime, or by altering the feedstock input?  I assume hi pH is never a problem?
I have seen differing opinions on biodigester websites as regards the pathogens in effluent. Some claim that the high methane, high pH environment kills all pathogens, and is safe to be applied directly to food crops. Others say that the pathogen count is merely reduced, and the effluent should be further treated before use on food crops. One design I chanced upon suggests heating the input to 70 C before adding, which renders the effluent pathogen free, warms the digester, and nets more methane from the same feedstock.

[Carey Ma]

No monitoring required. If you don't have gas to cook dinner, put some lime in the dang thing...hehe. The perfect range is neutral. Feedstock is expected to be continuous from the same sources.

As for pathogens, I consider this a low temp or mesophilic digester so don't have faith that temperatures alone will kill them. I think anytime for whatever reason the reactor malfunctions, a majority of microbes are lost. As I am not a micro biologist, I cannot tell you what microbes die at what level but I don't think 70deg C is high enough to kill Jack Diddly. In fact, though the national gov has approved the direct use of this sludge for food production, I would suggest that it gets introduced to a high temp (hot) aerobic compost and then mixed with garden compost to age/mature before using. But what do I know. One point four billion people eat sewage grown crops here without any apparent negative effects.

[Chris Smith]

Thanks for the diagram. I would like to scale it down a little and use plastic 55 gallon drums. With properly placed inlet and outlet ports do you think it will work in a drum? I can get then very cheep and would like to use them if possible.

[Carey Ma]

@Joe L: Thanks for the poop info. We experimented with several designs before coming to the conclusion that the height of our system does not need a separator because the slurry automatically separates. If our input was solids and liquids as in modern toilets we might need to separate, especially if toilet paper is present. However, Chinese have been taught to place used toilet paper in waste paper baskets to be composted or incinerated. Fecal mater (human & animal) is placed in buckets and mixed (stirred) with water into a wet mud. The only down side was that our system does not have the time required to maximize biological activity before it is expelled. This reduces extraction efficiency of methane production  but since we intend the byproduct to b used in fields, this is deemed acceptable.

[Carey Ma]

@ Chris S: Drums can be used, however the design needs consideration. I'll try to draw a diagram later and explain further but might have to wait till next weekend when I come back from an out of town trip.

Cheers

[Jon Parr]

And I thought used TP in the waste basket was just a Mexican tradition. I've been spending an unhealthy amount if time researching biodigesters lately, and many add newspaper or straw to balance the carbon/nitrogen ratio, actually improving gas production. I would think TP would benefit the digester, or at least not negatively impact it. And IMO, used TP in the waste basket is straight up disgusting, justified or not.

Carey, what the volume of your digester bag? If you added a second bag with the input connected to the output of the first, then you'd double your retention time and add a baffle, eh?

[Carey Ma]

Yes news paper and straw would be better in some systems. However, we are trying for simplicity. The negative impact would be that cellulose requires a bit more time to digest and TP by itself is mostly cellulose while NP and straw have more C value. Well it's a cultural thing as well as necessity. Older buildings have too small a pipe to accommodate TP wads, clogging the system, causing a mess for neighbors below.

Yes a second bag, bigger system or second stage digester would improve efficiency but at a cost...too expensive for peasants. Remember, this is for primitive use so we tried to keep it cheep and simple...Poop in: gas and ferts out.