Constructing a Medium Sized Biogas Plant Using Kitchen Waste

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Long back I have posted an instructable on how to construct prototype of a Biogas plant, using 50 liter capacity tank as digester, which you can see here : http://www.instructables.com/id/Bio-gas-plant-usin…

That was my first instructable and people are still commenting and asking me for guidance on Biogas plants for home use and for demonstration at schools & colleges. I have replied and mailed guidelines to almost all of the queries and I hope that helped them in their quest for building their Biogas plant.

As you can see in the last step of this instructable, I was invited by Hajee Karutha Rowther Howdia College, Uthamapalayam, Theni District in Tamil Nadu, India, to present a keynote address on Biogas and Panchagavya (an organic product made from produce from cow). This event was supported by the Tamil Nadu State Council for Science and Technology, Chennai, attended by local farmers, self-help group members and students. Many of the attendees expressed their interest in Biogas plant using kitchen waste and wanted to see a working plant.

This is my attempt at constructing a medium sized Biogas plant for home use as well for demonstration to students and others using a 750 liter capacity tank as digester and a 500 liter capacity tank as a gas holder, a floating type gas holder method. I have provided easy to understand step-by-step instructions on how to build the plant. Please go through this instructable and feel free to post your comments and queries with respect to biogas.

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Step 1: What is Biogas, What is Biogas Plant and How it Functions…?

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What is Biogas…?

For those who are not familiar with the term Biogas :

Microorganisms who thrive in the absence of air digests the organic material and releases a mixture of gases. The gases thus produced contains mostly methane along with other gases like Carbon dioxide, Nitrogen and Hydrogen Sulphide in small quantities. This process is known as anaerobic digestion.

Methane is a colorless and odorless gas and is highly flammable. (It is Hydrogen Sulphide that smells bad) Methane is not poisonous.

Methane along with other gases occurs naturally in swamps, waste dumps and even in home toilets in the septic tank. Due to its highly flammable quality, it can be used as fuel. But capturing the methane from the atmosphere is very difficult as it is lighter than air. The Biogas Digester or Biogas Plant we see here is a device which helps us in collecting this gas and use it as fuel.

Biogas Plant

You can see the opened-up prototype of a Biogas plant in above photograph.

The Biogas plant consists of a digester tank, where the organic material is stored and the microorganisms work on them and release gas.

The gas thus produced is collected in a tank known as gas collector. In a floating type model, this tank is floating in the slurry and moves up-and-down based on the amount of gas stored in it

A guide pipe helps the gas collector tank to move up-and-down inside the digester tank.

Waste is fed through feed pipe inside the digester tank.

The fully digested slurry drains out through the outlet pipe. This can be collected, diluted and used as fertilizer for plants.

A gas pipe line from the Gas collector tank helps in utilizing the gas for cooking and lighting

Now let’s get down to business and construct a medium sized Biogas plant for home use

Step 2: Selection of Tanks

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Before selection of tanks, I need to consider how much of digestible kitchen and garden waste I could collect everyday for feeding the tank. In my case I can easily collect between 3.5 to 4 kilogram of waste from kitchen and home garden. This quantity will be sufficient for a biogas plant with 700 to 800 liter capacity digester tank. Since only 750 liter capacity tanks are available in the market, I have selected a 750 liter tank to be used as the digester. A simple thump rule for biogas plant for home use is 5 kilograms of waste needs a 1000 liter capacity digester.

Now for the selection of gas holder tank, I need to consider the following before buying the tank:

The model I am building is with a floating type gas holder tank. That means the gas holder will move up and down based on the amount of gas inside. So, the gas holder tank should fit inside the digester and also should have minimum difference between their width as this will reduce in loss of gas through the sides.

During market search, I found that the 500 liters capacity tank will meet the requirement, having a width difference of about 100 mm, that means 50 mm on each side. So, I have decided to use the 500 liter tank as gas holder, which will have an up-and- down movement inside the digester using guides.

Some designs cater for a water seal between the digester and gas holder, but in my case that will considerably reduce the capacity of the digester. However, the gas loss through the sides will be very marginal with respect to providing a water seal and reducing the capacity of the digester.

In the above photographs, you can see the selection of my tanks. They are of very good quality three layered tanks that can withstand exposure to sunlight and acidic condition of the slurry inside.

Step 3: Other Materials Required

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In addition to the tanks, I have utilized the following PVC parts

  • PVC Door elbow 120 mm dia one number to be used for feeding waste
  • PVC pipe 50 mm dia 300 mm long to be fitted with digester for slurry outlet
  • PVC pipes 32 mm dia 250 mm long 4 pieces to be fitted with digester for guide system
  • PVC pipes 32 mm dia 1000 mm long 4 pieces for guide system
  • PVC pipes 12 mm dia 1000 mm long 4 pieces for guide system and stabilizing gas tank
  • PVC pipe 120 mm dia one piece to be used for waste feeding
  • PVC cap 120 mm dia for the waste feed pipe
  • PVC pipe 50 mm dia about 5 meters for the slurry outlet system
  • PVC bend 50 mm dia one piece for the slurry outlet system
  • PVC 32 mm dia threaded couplers 4 pieces to be fitted with gas tank for guide system
  • PVC 32 mm dia plain couplers 4 pieces to be fitted with digester for guide system
  • PVC Elbow reducer 32 mm to 12 mm 4 pieces for the guide system

Items required for the Gas pipe line is given separately in Step 10 below

Step 4: Adhesives Used

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For joining the parts of the gas plant, I have used the following adhesives

  • Araldite Epoxy Adhesive
  • M-Seal Epoxy Compound
  • PvC Solvent Cement

Step 5: Tools Required

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You can see here that construction of this plant does not require many tools. These are list of tools I have used:

  • A hacksaw with frame
  • A single sided hacksaw blade
  • A sharp knife
  • A medium sized hammer
  • Set of spanners to tighten the gas pipe connectors

For crimping the connectors with the ends of gas pipes, I got assistance from the shop from where I bought the gas pipes. They helped me with their Hand crimping equipment as per my requirement. (please see Step 10)

Step 6: Preparation of Gas Holder Tank

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The 500 liter capacity tank is required to be cut at the top. The visible top ridge will be used as guide line to cut the tank

  • Using a sharp knife make a slot along the line
  • Now you can insert a hacksaw blade in the slot and cut along the ridge
  • The hacksaw blade gets very hot. Wrap the end with a piece of cloth
  • Cut through the ridge and remove the top cut portion from the tank

Step 7: Preparation of Digester Tank

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The top portion of the digester tank also needs to be cut and removed. However, the width of cutting should be just enough for the free movement of gas tank

  • Place the removed top portion of gas holder on top of digester tank
  • Leave about 20 mm on all sides and mark the guide line for cutting
  • Using a hacksaw, cut slots on top of projected portion of digester tank
  • Now use a hacksaw blade to cut along the guide line and remove the top
  • Finish the cut edges with sand paper

Step 8: Fixing the Feed Pipe to the Digester Tank

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The 120 mm dia door elbow need to be fixed at the bottom of the digester tank

  • Place the elbow and mark the cutting line
  • make a slot along the line with a sharp knife
  • Insert the hacksaw blade in the slot and cut along the guide line
  • Insert the Elbow in place
  • Seal with M-seal epoxy compound on both outer and inner sides of the tank

Step 9: Fixing Slurry Outlet Pipe With The Digester Tank

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The 50 mm dia 300 mm long pipe is to be fitted at the top of digester for the slurry outlet

  • Place the pipe on top portion of digester tank opposite to the feed pipe
  • Mark the cutting line
  • Using a sharp knife cut along the guide line and remove the cut piece
  • Insert the slurry outlet pipe and seal with M-seal epoxy compound from both sides of the tank
  • You can cut and remove the extra projection inside tank if required. Otherwise leave as it is

Step 10: Fixing The Guides Over Digester for the Movement of Gas Holder Tank

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Some sort of guides are to be provided for the easy up-and-down movement of gas holder tank. We will use the 250 mm long 32 mm dia pipes for this.There are six projected portions on top of the digester. The slurry outlet pipe and waste input pipe are in alignment with two them opposite to each other. Leaving these two we will fix 32 mm pipes on other four projections. These pipes will be extended after placing the gas holder tank.

  • Place the 32 mm dia pipe on top of projections and make a mark
  • Using a hacksaw cut and removea square portion of the marked area.
  • Insert 32 mm dia pipes on all these cut portions and fix with M-seal epoxy compound
  • After the compound is cured cut and remove excess leaving about 25 mm projection

Step 11: Providing Guide Support on the Gas Holder Tank

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As we have seen in the previous step, 4 numbers of 32 mm dia guide pipes have been provided on top of the digester. In order to keep the gas holder tank in alignment, we will provide a sort of guide system on the sides of the gas tanks. Here we can use the 40 mm dia threaded couplers for this. Fix only two numbers opposite to each other. We can fix the other two once we place gas tank inside the digester and observing the movement of gas holder.

Step 12: Preparation of Gas Pipes

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I spent one full afternoon in shopping and preparing the Gas pipes. There will be three sets of gas pipes as below

  • From Gas Holder tank to the Link Pipe mounted on wall
  • Link pipe between gas holder tank pipe and kitchen
  • From the link pipe to Biogas stove

Parts required

  • Three pieces of gas pipes, all about 2.5 meters long
  • Ball valve 2 numbers one with gas outlet on gas tank and another with the end of link pipe
  • Bend 1 number over the gas tank
  • Nipple 1 number over gas tank
  • Adapter 1 number inside gas tank connecting nipple with the bend
  • Three numbers of barbs with male threaded ends crimped with clips at one end of pipes
  • Two numbers of barbs with female ends crimped with clips at other end of pipes (One end of the pipe is left free as this will connect with the stove)
  • Inner and outer washers
  • One roll of teflon tape
  • Few 12 mm size U-clamps for fixing the link pipe on the wall

At the shop itself I have measured all the pipes and used their hand-crimping tool to crimp all the ends of pipes. The pipes are tested for leakage by fixing the closed ball valve at each end separately, immersed in water and blowing air from the other end.

Step 13: Fixing Gas Outlet from the Gas Holder Tank

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  • Assemble all parts required for the gas outlet.
  • Using a sharp knife, make a small hole at the center of the Gas holder tank. You can make a neat round hole by just twisting the knife all around. This hole should not be larger than the threaded portion of the accessories
  • Use suitable inner and outer washers at all joints. All threads must be covered with teflon tape.
  • Connect the gas pipe with the bend
  • Connect the bend with the nipple
  • Connect the nipple with the adapter inside the tank. Make sure all joints are tightened properly
  • Provide a leak proof joint by applying epoxy resin over the joint from outside and inside the tank

Step 14: Placing Tank and Fixing Slurry Outlet Pipe

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Now we can move our digester tank to the desired location. Select a site where,

  • The unit gets more sunlight
  • Easier access for feeding waste
  • Easy to remove the slurry and use it properly
  • Minimum distance from the Biogas unit to the place of utilization

Here, I have place the unit on the terrace which gets maximum sunlight throughout the day. I have also provided a pipeline from the biogas plant for easy collection of slurry down. The kitchen is very adjacent to the plant and the gas line can be taken through the ventilator opening on the wall.

The couplers required for fixing guide pipes for the gas holders also added at this point. Place the couplers over the already fixed 32 mm dia pipes and lightly tap them down in place using a hammer

Step 15: Fixing the Waste Feed Pipe

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  • Clean the inner surface of door elbow and bottom outer surface 120 mm dia waste feed pipe
  • Apply PVC solvent cement over both the cleaned surfaces
  • Join them together immediately before the solvent cement dries out
  • Place the cap on top of the feed pipe

Step 16: Place the Gas Holder Tank and Finish

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Gently lift the gas holder tank and place it over the digester tank so that the 40 mm dia guide couplers fixed on the sides of the gas tank sits over the 32 mm dia couplers on the digester

Place one set of guide pipes through the 40 mm dia coupler as seen in the photograph

Now our Biogas plant is ready…

Step 17: Feeding the Biogas Plant for the First Time

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Initially you need to feed fresh cow dung to start the process. On the same day this plant was made ready, I collected about 10 kilograms of cow dung from nearby farm and another 20 kilogram the next day.

  • Dilute the cow dung with water. Use Chlorine-free water. If only chlorinated water is available, keep a bucket of water open to atmosphere overnight and use
  • Add about 100 grams of country-made jaggery. This will help the microorganisms to multiply fast
  • Stir and mix well using a ladle
  • Feed the mix into the plant through the feed pipe
  • Wash the pipe and replace cap

Step 18: Gas Formation

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Gas formation will start after about two days of feeding the digester with cow dung slurry. You can see here in these photographs that the gas holder tank started raising along the guide pipes.

But, this gas contains lot of impurities and will not burn. Moreover, there was air present in the gas tank when we placed it over the digester.

Once the gas tank is full, this gas will be released to the atmosphere.

Step 19: Type of Material whicn can be used for Feeding the Biogas Plant

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Once the gas formation starts, you can feed waste material to the digester. The following are some of the wastes commonly available in a household

  • Rotten vegetables
  • Vegetable peels
  • Fruit skins
  • Left-over spoiled food which are unfit for consumption
  • Very sour curd unfit for consumption
  • Over-fermented Dosa batter
  • Left-over vegetable oil used for frying
  • Grass and weed clippings
  • Dried flowers
  • Tender banana stems
  • Cattle waste like cow dung, goat’s dung and poultry waste
  • Leftovers coffee grounds and leftover tea leaves
  • Water used for rinsing rice and pulses. You can use this water for diluting the feed material

This list is endless… there are so many organic waste material you can use in the anaerobic digester. Make sure you chop them into pieces as small as possible. However, avoid certain items given in the next step which are unfit for feeding the biogas plant.

Remember, food is precious. Do not waste food. Use only spoiled food which is unfit for consumption

Step 20: Materials unfit for Biogas Digester

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Avoid following type of materials in a Biogas plant

  • Dry skins of Onion and Garlic
  • Egg shells
  • Fibrous materials like coconut husk
  • Bones, raw or cooked

Step 21: A Word of Caution : Never Try This

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Real Story… One student who got guidance from me over telephone, constructed a mini biogas plant for school exhibition and was overenthusiastic… She opened the gas valve from the gas holder and tested whether the gas burns or not by holding a burning match stick in front of the gas pipe. The entire gas tank exploded. Fortunately, nobody was injured but they had to spent lot of time to clean up the mess.

Never do this… only use a Biogas stove or a Bunsen burner or a suitable nozzle arrangement to test the gas.

Step 22: My Presentation on Biogas and Panchagavya

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A Science awareness Program was organised at Hajee Karutha Rowther Howdia College, Uthamapalayam, Theni District in Tamil Nadu, India, during 15th and 16th July 2014. The event was supported by the Tamil Nadu State Council for Science and Technology, Chennai. I was invited to present a Key Note Address on Biogas and Panchagavya. Many local farmers, ladies from Self-Help groups and Staff & Students of Biology Department attended the presentation.

My presentation covered all aspects of Biogas plant including construction, feeding and functioning. As you can see in the photographs above, the local farmers were very much interested in the simple technology which can convert organic waste from the kitchen in to useful biogas and the composted slurry as fertilizer for the plants. They were also very much eager to visit a working unit of a plant which uses only wastes to make fuel. My attempt in constructing this model is for home use as well for demonstration to students, farmers, Self-help groups and all those who are interested in this technology

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