How to build a Culhane/Solar CITIES HDPE Bio-Digestor

(Design by T.H. Culhane, photos courtesy of Brandon Shaw)

History:
T.H. Culhane designed and innovated this HDPE Plastic IDB Tote 3-tank water-displacement pressurized biogas system (henceforth known as "the Solar CITIES biodigestor system") in Cairo, Egypt and built the first one working with Solar CITIES associates Hanna Fathy and Mike Rimoin and students from the SEKEM Environmental Science Center in October 2009 after many experiments on his own porch in Essen Germany. After building many ARTI India style telescoping biogas digestors in Cairo Egypt throughout the previous year Culhane realized several improvements were necessary for spreading the technology out of the East and Middle East to colder climates. 1st, the necessary round water tanks used by such digestors in tropical and subtropical regions are not normally available in his home countries (Germany and America). He also realized after building an ARTI style digestor on his porch in Germany that the heat losses from a typical floating tank telescoping digestor would make them very difficult to adapt to cold climates. Also, the slight amount of methane that escapes from the open floating tank digestors made them unsuitable for indoor use. Finally, pressurizing the gas for delivery to a distant stove or to an electric motor at a constant pressure that could be regulated was difficult using the floating tank system because of the need for placing weights on top of the tank. For these reasons, Culhane designed a new system using the ubiquitous pallette delivered IDB Tote tanks made of High Density Polyethylene that he and his wife Sybille had already used for their greywater system.

 Here is a 3D animation of version 1:

With the success of the Culhane biogas digestor design in Egypt, Germany, California and now Alaska, Solar CITIES is publishing the plans and materials list here for the first time so that you can join our teams around the world in bringing low cost, climate-friendly clean fuel-from-kitchen-garbage solutions to your home and community.

Material to be bought (per complete system):

First, you will need tank fittings:



You will need 5 1/2" Poly Tank Flanges (part TF050) (1 for the digestor gas output, 1 for the gas collector side input, 2 for the two top gas collector gas outlets and 1 (optional) for the water pressure tank top water input :

You will need 1 1" Poly Tank Flange Assembly (part TF100): (for the digestor effluent output)

And you will need 1 2" Poly Ethyl Tank Flange Assembly (part TF220) (for the digestor feed tube that goes through the HDPE tank cover)

(yes, Virginia, they all look alike; only the size and costs differ. These are all internally threaded for the dimensions specified. You will need larger hole saws to cut the hole to fit them in, i.e. a 2" hole saw will NOT drill the hole for the 2 inch fitting, you will need a 3.25 inch hole saw).



Can't find tank fittings? Home Depot rarely has them (we've sometimes had to resort to using electrical junction box couplings but the o-rings are awful); plumbing and boat repair shops tend to have them. You can also try aquarium and aquaponics/hydroponics shops. The best place to look is banjocorp.com and look under Tank Accessories under Polypropylene Bulk Head Fittings

And of course, you will need 3 tanks (one for the digestor, one for the water-displacement gas collector and one for the elevated water pressure vessel)

We get them used, so check Craig's List for your area if you can't find them (restaurants have their food shipped in them so you can also check there). For example, in Los Angeles Craig's List lists them for biodiesel projects under:

"275 Gallon – Biodiesel – Storage Tank Drum Barrel" (this link takes you there)

So you need:

1) 3 IBC Tote HDPE 1000 liter (275 gallon) water tanks, preferably in metal cage frame (as in photo above or you will have to build your own frame)
We use 3 per system - one for the digestor, one for the water-displacement gas collector and one for water pressure.

Sometimes these can be found on Craig's List as "IDB Totes" and range in price from 60 to 150 dollars. the photo shows a system built in Alaska using tanks that have no metal cage, but this meant we had to spend extra time, labor and money building a wooden stand. The metal cage versions simply stack right on top of one another.

2) 3 to 4 meters of 1/2 inch clear plastic tubing (the more the better).

II. Fittings to connect the water displacement gas collector and water pressure tank:

1) 2 2" threaded adapters
2) 2 reduction bushings from 2" to 1 1/4 "
3) 2 reduction bushings from 1 1/4" to 1"
4 2 1" unions
5) 1 1" telescoping union
6) 1 1" valve
7) 1 1" T
8) 1 reduction bushing from 1" to 1/2"
9) 2 1" elbows (90 degrees)
10) 1 1/2 " elbow (90 degrees)
11) 1 1/2" union
12) 1 1/2" valve
13) 1 female threaded 1/2" nipple
14) 1 brass 1/2" thread to 1/2" barbed hose adaptor
15) 1 1/2" hose clamp


15) 3 meters 1" pvc pipe (you won't regret getting a little extra)

Fittings for the digestor feed tube:

(Photo: The cow's mouth and throat)

1) 2 meters 2" pvc pipe
2) 1 2" tank adaptors
3) 2 2" male threaded nipples
4) 2 2" rubber o-ring washers (if they came with the 2" tank adaptor you don't have to worry about this.)

Fittings for the digestor gas output:

1) 1 1/2 " tank adapter (make sure they come with flat washer rubber o-rings) -- you can't go wrong buying extras of these
2) 1 male threaded 1/2" nipple
3) 1 1/2" union
4) 1 1/2 " elbow (90 degrees)
5) 1 1/2 " valve
6) 1 1/2 female threaded nipple
7) 1 1/2" male threaded to 1/2 " barbed brass hose connector.
8) 1 1/2" hose clamp


Fittings for the spent slurry/fertilizer effluent tubes

(Photo: Two feeding tubes upside down (the cow's mouth and top part of throat on bottom, the esophagous that is placed in the stomach on top) next to a digestor tank. To the right of the two feeding tubes you can see the fertlizer effluent tubes -- "the cow's urethra").
1 ) 1 1" tank adapters (with flat washer rubber o-rings preferably)
2) 2 1" o-rings (if they didn't come with the above)
3) 1 1" male threaded nipple
4) 1 1" union
5) 3 1" elbows
6) 1 1" vlave
7) 1 1" female threaded nipple
8) 1 1" male threaded to 1" hose barb (optional; good for connecting a hose in case you want to irrigate your garden with the effluent).
3) 1 meters 1" PVC

A) Flame test valve (between digestor and gas collector) and B) gas collector output valve

A) Flame test valve:
1) 3 1/2" male threaded to 1/2" barbed hose fittings
2) 3 female threaded PVC 1/2 " nipples
3) 1 1/2" T
4) 1 1/2" valve
5) tie wire and screw straps to fix test valve to tank frame
6) 1/2 " plastic tubing
7) 3 1/2 " hose clamps

B) gas collector output valve (ideally you should create two of these, one for each side of the tank so you can bleed out all air and use all gas produced since the tanks have an unfortunate hump on either side. Doing this you don't have to angle the gas collector tank but can keep it level).

1) 1/2" tank fitting
2) 1/2" male threaded nipple
3) 1/2 union (not shown in photo above)
4) 1/2" elbow
5) 1/2" valve
6) 1/2" male threaded to 1/2" barbed hose fitting
7) 2 1/2" hose clamps

III) Gas input to Gas collector (on side of gas collector)

This fitting is nearly identical to the gas output of the digestor but we put the 1/2" union before the valve leading to the tank (correct), whereas in the digestor we had the union before the valve (incorrect, though we did it for stability and it works). It actually makes better sense to always have the union AFTER the valve relative to the tank, in other words you should be able to valve seal the tank before opening the union.

1) 1 1/2" brass hose adapter
2) 1 1/2" female threaded PVC nipple
3) 1 1/2" union
4 1 1/2" valve
5) 1 1/2" elbow
6) 1 1/2" male threaded nipple
7) 1 1/2" tank fitting
8) 2 1/2" hose clamps

IV) Connecting to water pressure tank:

1 1 80 liter (20 gallon) plastic bucket (trash can will do fine) with lid for sump

2) 1 garden sump pump 1/12 HP

3) 1 float switch with daisy chain plug

4) 1 rechargeable power station to run the pump if you are far from a standard electrical socket.

5) 1 extension cord with three outlets

To get food into the digestor:

1) Insinkerator garbage disposal (in sink food grinder/blender) (The 1/3 HP Badger for about 90 dollars works fine)

2) 1 large funnel

Summary of what you need to buy:

3 IBC Tote HDPE 1000 liter water tanks in metal cage frame (3 per system).
4 meters of 1/2 inch clear plastic tubing
2 2" threaded adapters with reduction bushings to 1 1/4", to 1 " and 1/2"
2 meters 2" pvc pipe
2 meters 1" pvc pipe

2 meters 1/2" pvc pipe
PVC primer
PVC glue
1 2" tank adaptor
1 1" tank adaptor
5 1/2 " tank adaptors
2 2" rubber o-ring washers (if tank fittings don't have them)
6 1/2" male threaded nipples
6 1/2 " female threaded nipples
8 1/2 " male threaded to 1/2" hose barb adaptors
8 1/2 " rubber o-ring washers (if tank fittings don't have them)
6 1" tank adapters with 1 " hose barb
6 1" o-rings (if tank fittings don't have them)
3 meters 1" plastic tube
5 1/2 " ball valves
2 1 " ball valves
1 80 liter plastic bucket with lids
1 garden sump pumps 1/12 HP
1 float switch
1 rechargeable power station (optional)
1 extension cord with three outlets
1 funnel 1 Insinkerator garbage disposal (in sink food grinder/blender)




Area required:
Each system requires an area of 1.5 x 3 meters. Three systems in a row would require 1.5 x 9 meters or, placed side by side, 4.5 x 3 meters.

Bacteria culture starters for the system
(first day only, thereafter feed ground up kitchen waste and water slurry, about 1 to 2 kg per day -- but don't start feeding until first flammable methane)

You can use regular old cow manure or horse manure or pig manure for your starter culture of methanogens. Add about 40 kg (20 lbs, or 2 or 3 5 gallon buckets) of manure to each digestor the first day. These digestors then need to be kept above 15 C (55 F) to produce gas; ideal is 37 C (98 F) but they give good output at 25 C (80 F).
For colder temperatures use Alaskan psychrophilic bacteria from lake mud.
Psychrophilic bacteria belong to four phylogenetic groups, the alpha and gamma subdivisions of the Proteobacteria, the Flexibacter-Bacteroides-Cytophaga phylum and the gram-positive branch. Isolates from cold environments include Arthrobacter sp., Psychrobacter sp., and members of the genera Halomonas, Pseudomonas, Hyphomonas, and Sphingomonas, these are some of the extremophil bacteria that scientists believe are like those that might exist on Mars on Jupiter's moon Europa. They are anaerobic Archaea and pose no threat to humans.

What to feed:
Any kitchen waste (ground up in a blender with water) except for bones, corn cobs, tea bags or coffee filters that could clog up the blender. (Wait until first flammable methane production before feeding! This can take between 43 hours and 3 three weeks!).