Global context of energy utilization demands nexus of environment and development'. Biogas is one of the most eco-friendly substitute renewable energy sources to meet energy requirement in the development index. In Bangladesh diverse types of bio-degradable waste are producing every day. Utilization of degradable waste produces gas to meet the energy demand of mass people in Bangladesh.

Setting the Scene

Bangladesh is in a favorable condition in respect of the climatic conditions and availability of the raw materials for biogas production. The optimum meso-philic range for biogas generation is 250C -370C. Bangladesh is blessed with this range of temperature almost throughout the year. In Bangladesh, there are about 22 million of cattle which excrete about 220 million kg of dung per day. This can produce an average 2.97x109 M3 of biogas/year which is equivalent to 1.52x106 tons of kerosene. Besides this, a considerable amount of gas and organic fertilizer can be obtained from poultry droppings, human excreta, garbage and water hyacinth.

In Bangladesh, research explains improper use of garbage bins; poor sanitary land filling system creates serious odor, scavenger diseases, and ground water pollution in urban areas. Major portion of solid waste are decomposed through aerobic way and some are processed by scavenger animals and birds. Before and after decomposition, these wastes are scattered through water, air and land. Apart from that-biogas and biochar can be used as an eco-friendly biomass. Fossil fuels (coal, gas and petrol) are the largest emitters of greenhouse gases such as carbon dioxide and methane which are negatively impacting the ozone layer. These greenhouse gases are also responsible for global warming and climate change. With the modernization of our society, our demands for energy are increasing with every passing day.

In today's energy demanding life style, need for exploring and exploiting new sources of energy which are renewable as well as eco-friendly. Bangladesh is one of the most densely populated countries with limited land area and natural resources. Because of increased population pressure, huge amount of natural resources are being used every day and the quality of the environment is deteriorating directly and indirectly through various anthropogenic as well as natural activities.

Air and water pollution have reached such levels that have already resulted serious health hazards and negative impact on the environment as well as inevitably influence the prospects of long-term economic growth. Bangladesh as a country of lower middle income is contemplating more on the renewable energy sources. As an agricultural country Bangladesh has embedded with plenty of biomass which has been used for extracting energy by burning directly or making biogas. Animal manure being available in the rural areas is greatly used in producing biogas to be used for cooking and lighting. Though the elements used in biogas production conventionally are plenty in rural areas, the energy consumption is very high in the urban areas. To meet the growing demand of power in the industries and transportation vehicle in the urban areas some sources are being investigated as a pilot project.

In Bangladesh, high cost of petroleum products, low coverage of the electricity grid, gasification and increasing scarcity of traditional fuel woods due to deforestation created an energy deficit situation in rural Bangladesh. However, long term sustainable development in energy sector requires a gradual shifting towards renewable sources of energy. Contexts demand, Country like Bangladesh need to accept renewable way to meet energy demands, considerably Biogas production.

Biogas Technology and Anaerobic Digestion

The biogas production technology has been available since the early l900's when it was used for the stabilization of organic sludge during the treatment of domestic sewage. It had also been used in India since 1923 and in China for a period of nearly sixty five years. Recently, there has been increasing interest in this technology, especially in the developing world. The governments of Asian countries -- China, India, Nepal, and Thailand had given an attention to biogas technology. More than 90% of presently existing biogas plants are of family size and the rest are at the farm and industrial scale. The potential of biogas technology for the replacement of traditional energy sources is highest in China (80%) and that of India, Nepal and Thailand is about 10%. At present about 120 fixed domes Chinese-type and about 80 floating dome Indian-type biogas plants are operating in Bangladesh. Biogas, which is mainly composed of methane (60-70%) and carbon di-oxide (30-40%) is a combustible gas produced by anaerobic fermentation of organic materials through methanogenic bacteria. Methane is odorless gas and burns with a clear blue flame without smoke. It produces more heat than kerosene, fuel wood, charcoal and dung-cakes. When biogas is used in suitable designed burners; it gives a clean, smokeless, blue flame, which is ideal for cooking. If biogas is used in specially designed lamp it gives a light similar to the kerosene pressure lamps. Biogas can be used for other purposes -- electricity generation, refrigeration, space heating and running engines. Family size biogas plant is appropriate for the domestic use -- cooking and lighting. A total of about 22000 biogas plants constructed throughout the country, out of 30000 family size biogas plant installed in Bangladesh. People are using biogas for cooking, lighting and generation of electricity. Consequently, residues from biogas plants are used as organic fertilizer for agricultural production. Basically, sources of biomass are two; biomass from plant origin is aquatic and terrestrial ones, derived from various sources; biomass from animals are cattle dung, manure from poultry, goats and sheep, and slaughter house and fishery wastes. In most of biogas plants cattle dung is used for gas production.

Anaerobic digestion is a series of process in which microorganisms break down biodegradable material in the absence of oxygen, used for industrial or domestic purposes to manage waste to release energy. There are four key biological and chemical steps of anaerobic digestion process --hydrolysis, acidogenesis, acetogenesis and methanogenesis as shown in Figure 2.

A. Hydrolysis

In most cases sludge contains lots of large organic polymers. In order to access the energy potential of the materials, the long chains must firstly be broken down to their smaller components such as sugar being readily available for other bacteria. This process is so called hydrolysis. The hydrolysis step degrades both insoluble organic material and high molecular weight compounds such as lipids, polysaccharides, proteins and nucleic acids, into soluble organic substances.

B. Acidogenesis

The components formed during hydrolysis are further split during acidogenesis. Volatile fatty acids are produced by acidogenic bacteria along with ammonia, carbon dioxide, hydrogen sulphide and other by-products.

C. Acetogenesis

The acetogenesis, where the carbonic acids and alcohols are further digested by acetogens to produce mainly acetic acids as well as hydrogen and carbon dioxide.

D. Methanogenesis

Methanogenesis produces methane by two groups of methanogenic bacteria: the first group splits acetate into methane and carbon dioxide and the second group uses hydrogen as electron donor and carbon dioxide as acceptor to produce methane.

Bacteria Isolation from Cow Dung: For the screening of methanogenic bacteria, about 1g of cow dung sample was weighed and poured into a test tube containing 9ml of autoclaved nutrient media and incubated under anaerobic condition for 72h at 35ºC to enhance the growth of the anaerobic methanogenic bacteria. Following incubation, the test tube was then manually agitated to form a uniform solution and allowed to stand for some seconds for the larger particles to settle down. The obtained dilution of 10-1 (w/v) was further serially diluted to 10-5 (w/v) and 100μl of the 10-5 dilution was then spread plated on the nutrient agar plates and further incubated at 35°C up to 24h. Following incubation, distinct pure colonies obtained on the plates which were isolated and purified using streak plate method and transferred to nutrient agar slants as stock. The bacterial isolates were then used as inoculums in the anaerobic digester containing cow dung as the substrate to check their individual potential for the biogas fabrication (Pratiksha Pradhan, 2012).

Municipal Waste Management in Bangladesh

Municipal waste generally compose of food scrap, packaging materials, used plastic materials, tire etc. Due to the increasing growth of urban population in Bangladesh this municipal waste is getting high concerns from the treatment perspective. The amount of municipal waste produced in different cities in Bangladesh is shown in the table 1:

(Source: Waste Concern, 2008)

Table 1: Urban Population Growth and Waste Generation in Bangladesh.

The amount of produced municipal waste is about 7900 tons daily in 8 major cities in Bangladesh. The composition of the entire waste stream is about ˗ 74.4% organic matter, 9.1% paper, 3.5% plastic, 1.9% textile and wood, 0.8% leather and rubber, 1.5% metal, 0.8% glass and 8% other waste. This large portion of municipal waste being organic can contribute to the production of biogas. Besides it is a well management system of organic waste. The organic matters are mainly food waste and agricultural waste (Waste concern, 2005). Figure 3, analyses the composition of typical municipal waste in Bangladesh:

Concluding Remarks

If facultative anaerobes and other bacteria ˗ responsible for anaerobic digestion and biogas formation, are separately cultured and these garbage's are separately decomposed, then this experiment will be resulted in more accurate in investigation and increase the production of biogas and decrease time consuming in digestion process. However, there are some socio-economic aspects. We can mitigate the energy crises by using low cost technology and produce biogas for our daily using purpose then it will be very much acceptable which will give us a way of sustainable resource management in Bangladesh.

Shishir Reza is an Environmental Analyst and Associate Member, Bangladesh Economic Association.