Cooking Gas Made From Animal & Kitchen Waste

Who can use
1 if there is 1 kg of kitchen waste everyday like family of 4 or more, school or monastery.
2 if you own more than 1or 2 cow or 5 pigs or chickens.

Bio Digester: How it works

When dung from cow, pig and kitchen waste are mixed in ratio of 1 to 5(dung 1:water 5) and exposed to heat without oxygen(air tight) in a container( Bio-Digester), Methane (cooking) gas is produced along with other useless gases. All harmful bacteria such as e-colie are killed during the process and slurry(discharged out, effluent) that comes out of the Bio Digester is organic fertilizer for crops or kitchen garden.

Ratio: Bio waste to Water
Bio waste(dung of cow, pig, chicken, waste food) ratio to Water varies from location to location due to climate dryness, temperature, evaporation rate etc. Most agreed upon ratio of Dung to Water is between 1:2 to 1:5 ie For ONE part of Dung Add TWO part Water; OR for ONE part of Dung ADD 5 part of Water. However, there should not be any solid material floating on the top of the slurry due to less water content.

Watch out for
* Temperature: For optimum result temperature is maintained between 18-27C.
* Starting dung, inoculators, must be fresh. Dung should not be 6 days or older.
* pH level: Methane gas is produce when pH level is from 6 to 8.5; that is slurry is alkaline. If there is gas and no flame, then test the outlet slurry with litmus paper which is sold at any pharmacy shop.
* Do not use Soapy water, Rain water, pesticide and antibiotics in the mix. These will kill bacteria that helps generate Methane gas.
* Avoid direct sunlight or ultraviolet lights, that can kill bacteria.

Tubular Plastic Bio Digester(TPBD) which is made from Plastic in the shape of Tube, not rigid pipe, is the cheapest and easy to construct. Cost of 10meter Tubular Plastic or polythene is less than Rs. 400 Indian. TPBD can last upto 4 years if handled carefully.

Digester Type
There are 3 main types of Bio Digesters - Fixed Dome, Floating Dome and TPBD. TPBD is the cheapest and Do-it-Yourself easy.

1 Balloon type
TPBD was developed by CONDRIT in 80’s.

Note: Tubular Plastic or Tubular Polythene is not same as plastic pipe!

What you need
* Polythene Tube of radius of 80-100cm diameter, 10 meters long and thickness as thick as you can get in the market; thicker the better, any color is good.
* 2 Ceramic or 2 PVC pipes of 1meter length and 4 inch Diameter.
* A PVC pipe, rigid or flexible hose, 1cm or 1.2 cm diameter for carrying gas to kitchen.
* Rubber bands cutout from old Tyre tubes to tie pipes to polythene tube; for air tight assembly.

Mark the trench site with pegs and rope for clear dimensions. Pit must be level. Site must have easy access to water and easy flow for slurry(effluent).

Prepare a Pit 65cm wide, 65cm deep and bottom width is 50cm. Length of the trench depends on number of cows. Length of 8m to 10m long is good for 2 cows or 8 pigs. More animals may need longer trench. Trench walls(side and bottom) must not have roots or stone sticking out. No plants will grow from side of the walls and puncture the Tubular Polythene. Trench must be level. This is technical importance.

Pit is wider at top then bottom to allow for expansion of Polythene Tube as gas builds up.

In some countries due to shortage of land, TPBD can be put out on ponds as it can float too.

From market get hold of Tubular Polythene, any color, 20 meter long, 80 -100cm Diameter and thickness is as much thick as available(thicker the better). Note, if it is really thick, then 10meter long is good else buy 20 meter long and cut it into half; insert 10meter half into the other half; you end up with 10meter long double thick Tubular Polythene.

Both ends of TPBD are fitted with two Ceramic or PVC (4 inch Diameter and 1m long) pipes, pvc-A and pvc-B. PVC-A, Inlet, is from where manure and water mix in a bucket is fed. The other PVC-B, Outlet, is for slurry out. Two ceramic pipes are tied at 45 angle with rubber bands cutout from an old tyre inner tube. No air should escape from TPBD.

PVC-A marked 2 is at higher level than PVC-B marked 4 inside the slurry. This offset is kept between 5-10 cm is an essential design part to push Slurry out by accumulated Gas pressure.

Gas pipe
A small hole(1 cm or 1.2cm diameter, whatever gas pipe size) is punctured at about 1meter from the PVC-A. It is fitted with water pipe now used as gas pipe for gas-line to kitchen. It is made air tight at the joints with pvc assembly as shown in Diagram or some suitable method.

Gas pipe is fitted to kitchen Stove for cooking. To avoid children and animal stepping on gas pipe, gas pipe is hanged overhead!

Air Out
Two pipes and Gas pipe elbow are fitted before TPBD is placed in the trench. Also push out all the air from outlet pipe by closing the inlet and gas pipe and using gentle squeeze. Best way to push out air is to lay down the TPBD on a flat surface free from nails or sharp edges. Run a smooth wooden plank across the TPBD. When all the air is out, close all the opening temporarily and transfer TPBD to its resting place-trench.

10meter TPBD is laid down with inlet(closed), outlet(closed) and Gas pipe(closed) facing up.

Feed the TPBD
Initially, fresh dung not older than 6 days and water mix is fed through PVC-A, which is open now, until TPBD is filled 2/3 or some slurry content leaks out the other end- outlet is also open. However, gas pipe is closed at this stage.

Both ends of PVC-A (2)and PVC-B(4) must be submerged inside the slurry or else gas will escape through them. Dung to water ratio can go from 1:2 to 1:5 depends on how much water content is left inside the TPBD chamber. In hot and dry climate, water evaporates at faster rate.

Gas will fill up the remaining TPBD in a day or two. However, everyday, 1 to 2 buckets (20 liter) of dung and water mix poured into the TPBD through inlet.

Other issue
Since it is Polythene Sheet, it must be protected from Sun (ultraviolet will kill bacteria), animals, sharp objects, kids and rats.

To increase pressure, if it is low due to cooking, put a small weight(2-4 kg) on TPBD.

5 cubic meter of TPBD requires 19 kg of Cow dung(1 or 2 cows) and 47 liters of water every day.

Ideal working temperature is between 35-40C but must be above 10C.

Retention Time(RT)
Time elapsed since manure is fed into Digester until slurry comes out at the other end; 40 days is ideal time for fermentation, Do not confuse RT with Methane Gas production. Methane Gas is ready to use in 2 days time since initial “startup manure feed”.

High altitude and cold climate like Tibet and Bolivia, above TPBD is housed within a Green house. Put up a frame(F1) higher than TPBD on one side along the length of the TPBD and another frame(F2). Throw a transparent plastic sheet over frames F1 and F2. That will keep TPBD within 25-35C. Put stone on side to make Green house air tight. Alternatively, use one side of the house wall as frame F1.

Such TPBD should last for 3-4 years if maintained well.

Soapy water, Rain water, pesticide and antibiotics.

Beside cooking Gas, slurry is good organic fertilizer for kitchen garden or crops.

Data provided here are collected from other field research. There is no precise data that can be used optimally. However, most data points to a general working data. For instance, 20kg of dung from Thailand may have different outcome than Bangladesh as cows are fed differently.

Besides providing cooking gas, it eliminates indoor smoke and related lung disease. Fire wood are gathered by children, who skip class, and women. They are exposed to danger of rain, snake and cold.

ARTI – Appropriate Rural Technology Institute, India:
This device is modified to feed kitchen waste only and left over food. Concept is the same. Inlet to feed kitchen waste; Outlet for discharge; Digester unit and a Gas Chamber.

In India and Nepal there are big PVC Drums and Bitumen Containers (Drums) that are fabricated as Digester and Gas Chamber.

ARTI digester use Two Drums; Drum A is Digester which is in upright position; Drum B, bit smaller, is inserted into Drum A upside down to collect gas. Drum B is effectively Gas Chamber. Gas chamber, Drum B, must move up and down freely as gas pressure built. Whole structure depends on how these two drums are made AIR TIGHT at a) Inlet b) Outlet c) Gas pipe connection d) Drum A and Drum B fits snugly.

Note: In ARTI design outlet is at higher position than the inlet. Inlet is at the very bottom of the Drum A.

From market get one 1000 Liter capacity Plastic Tank and one 750 Liter plastic Tank. PVC pipes for connections, inlet and outlet.

1-1.5 kg of kitchen waste and 15 Liter of water everyday. Mix with hand thoroughly before feeding the digester.

Very Watery effluent fit for crops, flower and kitchen Garden.

Retention time
48 – 72 hours

ARTI Drum design Bio Digester are fed only Kitchen waste-bread and cake can also be used. This is a major difference and critical in working of the digester. NO animal waste or dung are used. ARTI Bio digesters has less foot print and therefore can be put out on Roof top or backyard.

Arti tecnichian charge Rs. 2500 to 7000 and depends on material cost.

Another configuration of above design is to seal air tight 3 Drums.

Above two BIO Digesters are now more common due less skill requirement, less cost and fast-whole project can be done in a day.

2 Fixed Dome Digester:

It is considered as an investment. It can last upto 20 years.
Investment and skill required is significant. Cheap and unskilled labour can fail this project and waste of investment.

Watch out for
Beside Dung, pH, temperature and general warnings, Dome Construction suffers from:
* Cement in the market may be adulterated.
* Skilled and technical labour is essential.
* Bricks are not standard and porous.
* initial investment is substantial.
* Good foundation is necessary. May have to cast in Iron.
* Dome is made air and water tight.
* Whole project depends on; Air and Water tight structure; any leakage will fail the project.
* Weak and Strong Ring concept is understood well and taken care of. If fixed Dome is caste in reinforced concrete(iron rods and cement mix) with substantial increase in investment may eliminate Weak link.

First lay and compact 10cm thick of 4X6 cm size broken bricks.
Next lay 3 cm thick layer of 1X2cm size stone.
Add concrete mix of 5cm thick; concrete mix ratio is 1 cement:2 sand:3 small stones(1X2cm diameter).

If foundation earth is soft and wet(bad), Above concrete mix must have Iron bars(8mm diameter) cast in checked pattern with gap of 20cm.

If mountain water or ground water is likely then plastic sheet or proper drainage is dug out to drain water out from foundation.

Brick technical
Dome Bio Digester walls are 10cm thick, same thickness as the bricks. High quality Bricks are used with ratio of 1 cement to 4 sand mix. It is to ensure Dome walls can withstand high pressure Gas and Slurry material. Dome is made water and air tight.

Inner wall coating: Technical
Cement and sand ratio, mortar, is kept strict; 1 cement to 3 sand ratio; Sand must be washed first;
After first coat is done; wait for 1-2 hours and do a second coat. Do not ignore waiting time. It is part of the procedure. In the mean time do some other work . This will ensure water tight and air tight. Any leakage will render Dome(investment) useless.

In fixed Dome Digester construction, slurry(manure, water mix) and Gas Chamber are not separate but housed in one.

Main points for FDD constructions are:
Skilled construction is very important. There is a chance for crack at the line where Manure Slurry and Gas chamber meet due to Gas pressure which is upward and slurry gravity acting downward.

Weak-Ring and Strong-ring Concept
Weak Ring and Strong Ring constructions technique are applied at Slurry and Gas junction. Intake(manure feed) tube must sit within the weak ring. Dome wall submerged inside slurry will not let air escape because Slurry itself coat the dome wall. However since Gas Chamber is pushed upward by accumulation of Gas and slurry push Dome foundation down due to gravity, junction where Slurry and Gas Chamber meet is subject to opposite force and therefore it cracks. Cracks compromise air tight and water leak. FDD failure is eminent and failure rate is also high.

Inlet tube and outlet tube must be fitted during Dome construction. Once Dome is constructed making holes for inlet and outlet later will introduce cracks in the Dome.

Inner wall submerged under the slurry may be plastered with water-proof paint with help of wooden trowel instead of steel. Wooden Trowel leaves imperfection which is good for bacteria growth.

Vacvina is fixed Reinforced Concrete construction and inner corners are made angular with 5cm thick mortar and cement to make Vacvina air and water proof. Investment is high.

3 Floating Dome Digester (FLD)

In Floating Dome Digester, Gas chamber floats up and down as gas accumulates. Gas chamber is made of metal or reinforced concrete. In FLD Weak-Strong Ring concept is eliminated and therefore possibility of crack is eliminated. However, air and water tight must be ensured for success of the project. Any leakage or cracks will render project failure.

How to make Gas Chamber float yet air tight
Gas chamber should float either in a separate water tank surrounding the Digester or in the slurry itself. Such technique requires skill and extra investment.

Quite a few Tibetan settlements in South India have installed Floating and Fixed Dome Bio Digester and almost all failed due to poor workmanship, ill design. Gas leaked or cracks appeared.

Note: failure rate for fixed and floating Dome Digester is high due to
* substandard cement,
* wrong concrete mix to save money or lack of knowledge,
* Startup feed did not work and therefore abandoned,
* Weak-ring and Strong Ring concept did not follow during construction that result cracks at later stages. Weak link is where gas chamber and slurry meets.
* inlet and outlet pipes are installed during the Dome Construction and not after.
*Floating Gas Chamber is too heavy and Gas leaked.

Improved Chulla(Stove for wood burning)

For those who are not interested in Bio Gas digester, yet want to save fire-wood and tree can use improved Chulla or Stove.

These improved Chulla are efficient way of cooking meals as they do not require wood as its main source of fire-wood. Any small twigs, dung, rice hulls, Chili twigs, corn cob etc. can work as fire-wood.

It is easy to construct. Follow the diagram.

Ref: Search for
Bio Digester
Improved Chulla
Smokeless chulla

Ganeshnagar, Phaltan-Baramati Road,
Phaltan 415 523, Maharashtra, INDIA .
Phone: 091-02166-249874