Kitchen Waste Based Biogas Plant

S. P. Kale and S. T. Mehetre
Nuclear Agriculture and Biotechnology Division
Bhabha Atomic Research Centre

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An organism that can happily grow in an extreme environment is an extremophile. The extreme envir-onments include physical extremes like pressure, temperature and radiation and geochemical extremes like desiccation, salinity, pH and low redox potentials.

Biogas Plant at Trombay. The plant produces biogas from kitchen waste by using thermophilic microorganisms that flourish in extreme environment. The biogas plant has following components: A mixer/pulper (5 HP motor) for crushing the solid waste, Premix tanks, Predigester tank, Solar heater for water heating, Main digestion tank (35 m3), Manure pits, Gas lamps for utilisation of the biogas generated in the plant.

The thermophiles are the extre-mophiles that can thrive superbly at high temperatures. They have developed such enzyme systems that can help the organisms not only to survive at higher temperatures but also grow and reproduce. They have ability to use sulphurous waste and convert it into non-toxic products. Since the environment for such microorganisms sustains higher temperatures, many spoilage and pathogenic organisms cannot survive in such extreme conditions. Therefore it would be ideal if we can make use of these organisms to degrade the kitchen waste to remove more toxic elements and then subject it to the traditional biogas plant for methane generation. What we need to do is to maintain the high temperature in the predigestor tank.

 

Precautions may be taken while collecting the kitchen waste :

l A separate container for coconut shells, coir, egg shells, onion peels and bones. These will not be processed in the biogas plant.

l Separate containers of small volumes (5litre capacity) to collect the wet waste (spoilt or stale cooked food, waste milk products etc.). The vegetables refuse like peels of various vegetables, rotten potatoes, and tomatoes, coriander leaves etc. may be collected in garbage bags of 5-kilo capacity. It must be noted that such segregation is of utmost importance for smooth running of the biogas plant.

 

Sunlight is available almost throughout the year except for some days in the months of July-August. This natural source of energy can be effectively used for providing the thermophilic microorganisms their natural environment. This energy is used to heat water and by controlling the proportion of hot water in the predigestor tank, one can achieve the desired temperature that can be easily sustained for about a day. This would provide favourable surroundings for the potential use of thermophiles to degrade the waste and sustain the culture. Thus the system is self-sustainable and effective.

Another important aspect in smoother running of a biogas plant based on solid waste is how effectively one can avoid the choking of the plant. This choking may occur due to thick biomass that may be inaccessible to the microorganisms to digest it. The logical solution to such a problem is to convert the solid waste into slurry that would be far more accessible for the microbial action. A high power mixer to convert the solid waste into slurry can achieve this purpose. These two modifications certainly improve design of the traditional biogas plant.

A kitchen waste based biogas plant has been installed at Nursery site for environmental friendly disposal of the waste generated in kitchens of various canteens in BARC premises. It is expected that the plant can process all the waste generated in these canteens. This plant works on similar principles of traditional gobar gas plants with the exception of type of feed with the above modifications.

The waste generated in kitchen in the form of vegetable refuge, stale cooked and uncooked food, extracted tea powder, waste milk and milk products can all be processed in this plant. Based on the understanding of thermophilic microorganisms in particular and microbial processes in general, there are two important modifications made in the conventional design of the biogas plant in BARC :

l Introduction of a 5 HP mixer to process the waste before putting it into predigestor tank. The waste is converted in slurry by mixing with water (1:1) in this mixture.

lUse of thermophilic microbes for faster degradation of the waste. The growth of thermophiles in the predigestor tank is assured by mixing the waste with hot water and maintaining the temperature in the range of 55-60oC. The hot water supply is from a solar heater. Even one-hour sunlight is sufficient per day to meet the needs of hot water.

After the predigestor tank the slurry enters the main tank where it undergoes mainly anaerobic degra-dation by a consortium of archae-bacteria belonging to Methanococcus group. These bacteria are naturally present in the alimentary canal of ruminant animals (cattle). They produce mainly methane from the cellulosic materials in the slurry.

The undigested lignocellulosic and hemicellulosic materials then are passed on in the settling tank. After about a month high quality manure can be dug out from the settling tanks. There is no odour to the manure at all. The organic contents are high and this can improve the quality of humus in soil, which in turn is responsible for the fertility.

As the gas is generated in the main tank, the dome is slowly lifted up. It reaches a maximum height of 8 feet holding 35 m3 of gas. This gas is a mixture of methane (70-75%), carbondioxide (10-15%) and water vapours (5-10%). It is taken through GI pipeline to the lamp posts. Drains for condensed water vapour are provided on line. This gas burns with a blue flame and can be used for cooking as well.

The gas generated in this plant is used for gas lights fitted around the plant. The potential use of this gas would be for a canteen. The manure generated is high quality and can be used in fields.

Success of this biogas plant depends a great deal on proper segregation of the kitchen waste. The materials that can pose problems to the efficient running of plant are coconut shells and coir, egg shells, onion peels, bones and plastic pieces. Steel utensils like dishes, spoons etc. are likely to appear in the waste bags from canteens. While bones, shells and utensils can spoil the mixer physically, onion peels, coir and plastic can have detrimental effects on microbial consortium in the predigester and main digestion tanks which could be disastrous for the plant.

Thus the efficient disposal of kitchen waste can be ecofriendly as well as cost effective. While calculating the cost effectiveness of such waste disposal one has to consider more than monetory aspects. The dumping of uncooked food in unmanned area may not be very civilized. It can also lead to population growth of nuisance animals. It is undoubtedly unhygienic and can pose threat to the habitat. These factors will add to the value of such plants. Using the natural friends in the form of thermophiles, methanogenic micro-organisms and their consortiums we can certainly handle the kitchen waste and may be other biodegradable waste like paper.