Biogas composition I Plant of biogas composition

Biogas typically refers to a gas produced by the biological breakdown of organic matter by bacteria in the absence of oxygen [].Although the composition of generated biogas varies a lot for different feed materials. The typical composition can be summarized as in Table 2.1.

Table 2.1 Typical composition of biogas [9]

Compound	Concentration (by volume)
Methane (CH4)	50-70 %
Carbon dioxide(CO2)	25-50 %
Nitrogen(N2)	0-10 %
Hydrogen(H2)	0-1 %
Hydrogen sulfide(H2S)	0-3 %

Principle of Anaerobic Digestion

Biogas production is a microbial process.Microbes involved in biogas productiongrow in the absence of oxygen. The most important organisms are tiny bacteria.Different groups of bacteria act upon complex organic materials in the absence of oxygen toproduce biogas. The process involved combined action of threegroups of bacteria, in three stages in the biogas plant. The first stage is thefermentation of high molecular weight substances like cellulose, starch, protein, fats, etc.present in organic materials into small molecular weight compounds like fatty acidsamino acid, carbon dioxide and hydrogen, this is brought about by a hydrolytic group ofbacteria, in the second stage the end products of the first stage are converted into acetic acid,hydrogen& carbon dioxide by hydrogen producing acetogenic bacteria. In the third stage, the methane producing bacteria convert the substances produced in the first and second stage into CH₄&CO₂.[ ]

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Design & Fabrication of Bio gas Introduction

In today’s energy demanding life style, need for exploring and exploiting new sources of energy is becoming essential. Untilnow, the growing energy demand is met mainly by fossil fuel based sources. There are two immediate concerns raised for these sources: rapid depletion of their reserves and the associated environmental pollutions. Therefore, the searching of new and alternative energy sources is directed towards renewable sources which are in general environmentally friendly. Among the different types of renewable sources, biogas is distinct one because of its characteristics of using, controlling and using of organic wastes and at the same time producing fertilizer and water for use in agricultural irrigation. Biogas does not have any geographical limitations and it does not require advanced technology for producing energy and also it is very simple to use and apply.

Deforestation is a very big problem in developing countries like Bangladesh as fuel wood is a major source of cooking energy. Also, due to deforestation, it leads to decrease the fertility of land by soil erosion. Direct combustion of dung, firewood in the kitchen is also harmful for the health due to the pollutants generated there. Therefore,it is very important to have an eco-friendly substitute for cooking energy.

Kitchen waste from households and restaurants can be estimated to be 23% of municipal solid waste in Bangladesh. The heating value of kitchen waste is very low and not adequate for incineration. Also in most of cities and places, kitchen waste is disposed in landfill or discarded which causes the public health hazards and disease like malaria, cholera, typhoid. Inadequate management of wastes like uncontrolled dumping bears several adverse consequences. It does not only lead to polluting surface and groundwater through leaching and further promotes the breeding of flies, mosquitoes, rats and other disease bearing vectors. Also, it emits bad odor& methane which is a major greenhouse gas contributing to global warming. Therefore, a preferred strategy is to recover the kitchen waste and convert it to useful material[1]. According to the estimation by IftekharEnayetullah Director of Waste Concern, 13,332.89 ton of total waste is generatedper day in Bangladesh [2], most of which was kitchen waste. Thus there is no question of the availability of raw materials, if kitchen waste is considered as raw material for biogas generation.

Among the various types of kitchen wastes, potato waste and onion waste are the two major kinds which are available in every house. The potato waste is content some water which is not constant and it is ranged from 72 to 83% [3] and the percentage of water content of onion waste is about 89.9%.That means, less water is added to the onion waste than potato waste.

Kitchen waste is an organic material having the high calorific value and nutritive value to microbes, that’s why efficiency of methane production can be increased by several orders of magnitude. It means higher efficiency and thus size of digester and cost of biogas production is reduced.

In most parts of the developing world where biogas digesters are used, digesters are constructed out of cement, stone, and sand. Compared to portable biogas digester, cement biogas digesters are resource and labor intensive and can be more expensive than portable digester.Moreover, the portable biogas digester is less resource intensive, lighter (enabling convenient transportation), and requires less construction time. This digester is fabricated with mild steel sheet .The fabrication time of portable biogas digester is very less; whereas the construction time for a cement biogas digester is 7-10 days. Additionally, portable biogas digester is well sealed and airproof, while cement biogas digester is prone to methane leakage due to the molecular spacing of cement and the molecular diameter of methane. That’s why to reduce the environmental pollution and to overcome the crisis of energy in the world,this kitchen waste based portable bio-gas digester project was selected as a thesis.

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NOMENCLATURE

TS	Total solid: It is a solid part after removing water content from a feed material.
TD	Total discharge: It is the sum of total solid and water content inside the materials.
AD	Anaerobic digestion
HRT	Hydraulic retention time: It is a measure of the average length of time that a soluble compound remains in a constructed bioreactor.
Vgs	Volume of gas storage: It is the top area of the digester where gas is stored.
Vs	Volume of sludge layer: It is the bottom area of the digester where sludge is accumulated.
Vf	Volume of fermentation: It is the middle area of the digester where feed materials are fermented.
Q	Total influent: It is the mixture of water and total discharge.
COD	Chemical oxygen demand: It is the mass of oxygen consumed per liter of solution.
CO2	Carbon dioxide
CH4	Methane
H2S	Hydrogen sulfide
m3	Cubic meter

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