Kitchen Waste Bio-Gas
Bio-gas systems, with the help of bacteria, break down organic material/waste in an air-tight tank into bio-gas. The gas generated during this process is mainly methane with some carbon dioxide, and it can be used as fuel for cooking or other purposes, while the solid residue can be used as organic compost.
Methane burns with a blue flame without producing any smoke or soot. It is, therefore, an environmentally-friendly cooking system. Thus, it not only helps urban households in utilising their domestic wet waste, but also helps in preventing diseases caused by the smoke and soot from burning wood. Further reductions in pollution and energy arise from not having to transport liquefied petroleum gas (LPG) cylinders to be re-filled.
Replacement of fossil fuels reduces the emission of greenhouse gases. As per estimates in developing countries, for a typical urban household, bio-gas saves about 100kg of LPG or 250ltrs of kerosene per year, which is equivalent to 300kg to 600kg of carbon dioxide (CO2) per year. A rural family could save about 3 tonnes of wood per year, which would generate about 5 tonnes of CO2 if burnt.
 How Does it Work?
The compact plant is made from cut-down high-density polythene (HDPE) water tanks. The standard plant uses two tanks, with volumes of typically 0.75m3 and 1m3. The smaller tank is the gas holder and is inverted over the larger one. The larger tank holds the decomposing feedstock mixture and water (slurry). An inlet is provided for adding feedstock, and an overflow for removing the digested residue. The overflow mixture can be recycled into the plant. A pipe takes the bio-gas for consumption.
The gas holder gradually rises as gas is produced, and sinks down again as the gas is used. Weights are placed on the top of the gas holder to increase the pressure of the gas.Comparison with Conventional Bio-Gas Plants
|Conventional Bio-gas Systems||Kitchen Waste Bio-gas System|
|Amount of required feedstock||40kg + 40ltr water||1-1.5kg + 15ltr water|
|Nature of required feedstock||Dung||Any starchy material|
|Amount and nature of slurry to be disposed of||80ltr, sludge||15ltr, watery|
|Reaction time for full utilisation of feedstock||40 days||48 hours|
|Standard size for household||4,000ltr||1,000-1,500ltr|
In a kitchen waste bio-gas system, a 1.5kg feedstock of kitchen waste produces about 500gm of methane, and the reaction is completed in 48 hours. Conventional bio-gas systems use cattle dung or human excrete, and 40kg feedstock is required to produce the same quantity of methane. The reaction time in conventional bio-gas systems is 40 days. Thus, kitchen waste bio-gas systems are 800 times more efficient than conventional bio-gas systems.
The feed can be waste flour, vegetable residues, waste food, fruit peelings and rotten fruit. Feedstock with large lumps (more than 20mm) needs to be broken down for faster reaction. Most of the kitchen waste, including leftover food, can be used for decomposing.
A bio-gas plant can become acidic and fail if it is over-fed, and this is a particular challenge with a plant using highly digestible organic materials. If this happens, reduce the feeding rate.
 References and Useful Websites
- Appropriate Rural Technology Institute
- Advancing knowledge for human security, peace, and development
- Gujarat Energy Development Agency