The term ‘Waste’ means something that is unwanted and can do without. With industrialization and development in all spheres of life man has created wastes and without realizing the consequences have allowed its accumulation in quantities that have reached a proportion that demands its immediate and systemic disposal for maintaining a healthy and safe environment. Wastes can be categorized into two types: biological and non-biological. Wastes of biological origin due to their high organic contents are easily biodegradable whereas, non-biological ones like plastics and polymers fall under non-biodegradable category.
Waste disposal is one of the major problems being faced by all nations across the globe. The daily per capita solid waste generated in India ranges from about 100 g in small towns to 500 g in large towns. It takes anywhere between three and seven days for the waste to be disposed from the time of its generation. The treatment and disposal of waste by conventional methods is a time consuming as well as energy intensive process and adds to environmental pollution.
In general waste disposal methods include: i) transportation of waste to low lying areas / landfills, ii) burning of waste on site or in the incinerators, iii) composting, iv) briquetting, (v) recycling of waste matter, vi) microbial treatment, aerobic and anaerobic, etc. Each of these methods has its own advantages, and can be employed to certain types of wastes. Hence, for complete disposal of wastes, it is important to take an integrated approach.
Collection of wastes and their transportation to low lying areas on the outskirts of the inhabited areas and localities is a costly affair. Waste production is increasing rapidly and the availability of landfills is decreasing. Since the landfills are being “pushed” away from the inhabited areas, it implies greater distances have to be covered for transportation. It thus leads to increased cost of transportation. Disposal of wastes in unorganized landfills causes environmental pollution due to slow and uncontrolled fermentation, which generates gases like CO2, CH4 and H2S are the major causes of global warming.
Burning of wastes
Burning of wastes on site is a conventional method of disposing wastes. According to various surveys, metropolitan cities generate 4000 to 5000 tonnes of waste per day. Of this 60 to 70% is collected and disposed off at landfills. These wastes have up to 65% of materials, which can be combusted. It results in generation of heat at the rate of around 2700 BTU/ kg waste (1 BTU, British Thermal Unit = 2.9x 10-4 Kilo Watt Hour). Burning of waste in the absence of oxygen at high temperatures results in breakdown of complex organic matter into obnoxious gases, which spread wide apart to distant places. CO evolution reduces the oxygen carrying capacity of our lungs. NH3 generation during burning of dry leaves results in allergic disorders. NO2 affects lungs and nervous system. In addition, SO2 causes asthma, allergy and other respiratory disorders.
Composting is inexpensive, rapidly implemented and a publicly acceptable treatment process. Sewage sludge composting has been fairly successful. However, major problems still occur at some facilities, such as odours, poor product quality, long processing times and excessive moisture and materials handling problems. Composting is a complex biological process. It depends upon a host of physical, biological and chemical factors. Among the physical factors: mixing, heat evolution, temperature, heat flow and control, temperature dynamics, water, available energy density, etc. are some of the important ones. Chemical factors, which affect the processing, are: interstitial oxygen concentration, pH, ammonia, etc.
Briquetting converts highly voluminous and trouble some waste materials in to clean, non-polluting fuel. Being compact, transportation and storage costs are reduced. It is environmentally superior technology for handling different ligno-cellulosic materials. Feedstocks consist of all the forest and agricultural wastes like pine needles, wild bushes, grasses, rice husk, saw dust, tea wastes, etc. The quality and quantity of the fuel produced by this technology depend up on the type of raw materials used and the conditions prevailing during the processing. These briquettes can be used in gasifier production units, industrial boilers and furnaces, ceramic units, brick or lime kilns, bakeries, potteries, etc. It will not only release pressure on non-renewable sources but also control the fire wood demand and consumption considerably and also help protection of the environment.
Microbial treatment of wastes
One more approach to handle this situation may be use of microorganism, via bio-deterioration or bio-disintegration. Recycling and stabilization of waste through anaerobic digestion is a better approach for treatment than aerobic treatment or composting. Unlike uncontrolled and slow fermentation of wastes in a landfill3, biodegradation in a factory system results in energy generation and production of nutrient rich biomanure.
Anaerobic digestion process is a multiple stage process. It involves the conversion of complex organic matter into soluble compounds. This activity is carried out by the consorted action of a wide range of microorganisms in the absence of oxygen or other strong oxidizing chemicals. Methane and CO2 are the principal products and minor quantity of N2, H2, NH2 and H2S also generated. Thus this process consists of complex sequences of biological reactions, during which the product by one group of organisms serves as the substrate for the next and the methanogens are the terminal organisms in the microbial food chain. It is generally accepted to be effective and economical method of stabilization and waste volume reduction of biological wastes. At present anaerobic treatment is successfully implemented for various types of industrial as well as domestic wastes.
At present 90% world energy needs are fulfilled by fossil fuels, which is regarded as an endless source of cheap energy. But it is now realized that earth possesses a finite amount of fossil fuels The indiscriminate use of fossil fuels has caused extensive damage to human health and this planet. World efforts are focused on the development of non-polluting and sustainable energy sources, which can replace the fossil fuel in post fossil fuel era. Among the various primary energy sources like solar, wind, hydropower, geothermal, ocean current, tide and wave, thermonuclear, nuclear, water temperature energy (solar ponds) and fossil fuels (coal, petroleum and natural gas) except the latter, others cannot be directly used as a fuel.
The various competitors for the future fuel are gasoline, ethanol, methanol, methane and hydrogen. On comparing all the candidates, H2 stands out as best possible fuel with unmatched unique. Hydrogen combustion results largely in the production of water (with very small amount of nitrous oxide), which can be re-utilized so as to realize a benign cycle of nature.
Biological Hydrogen production
Hydrogen is also an important source of energy, which represents a highly efficient energy carrier. It is an efficient fuel (122KJ/g) and can be converted to steam, heat, electricity, etc. and therefore it is the most versatile fuel. It has the potential to be used as a fuel for automobiles, trucks, buses, aircraft, etc. Many chemotrophs produce hydrogen by the use of different carbohydrate. Among the various wastes employed for hydrogen generation, bio-waste could produce 40L to 100L of H2/ kg dry matter.
The production of hydrogen by different microorganisms is intimately linked with their energy metabolisms. In aerobic microorganisms, the released electrons from substrate oxidation are transferred to oxygen as ultimate oxidant while in anaerobic organisms, where the supply of energy is limited; electrons released from the anaerobic catabolism use many terminal oxidants such as nitrate, sulphate. However, obligate and facultative anaerobes use as the terminal electron acceptor. Thus here hydrogen production is one of the specific mechanisms to dispose of excess electrons through the activity of enzyme hydrogenase present in H2 producing microorganisms.
Biological hydrogen production is limited by feedback inhibition, partial pressure of hydrogen and the presence of hydrogen consumers in a wide range of habitat. The theoretical maximum yield through fermentation is reported to be 4.0 moles H2 per mole of glucose. In practice, hydrogen yield by pure or mixed cultures has been reported to range from 0.37 to 3.3 moles H2 per mole of glucose. Considering these high theoretical values, it is worth exploring approaches towards increasing the yields. A challenging problem in establishing hydrogen as a source of energy is the renewable and environmentally friendly generation of large quantities of hydrogen gas.
We would like to thank Alan Bartulovic for being part of our research and development team. He has implemented many of his great ideas and shared alot of knowledge among us. His expertise in programming and software development made it possible for this project to go through.
In last 25 years hydrogen energy has moved in all fronts making roads in all areas of energy. And in next 20 years the progress will be many fold greater and hydrogen energy system will provide the planet earth with the energy system, she deserves, which is hospitable to life, clean and efficient. The future of biological hydrogen production depends not only on research advances (i.e. improvement in efficiency through computational genomics, genetically engineering microorganisms and/or the development of bioreactors) but also on economic considerations (the cost of fossil fuels), social acceptance and the development of hydrogen energy systems. Biomass-derived hydrogen is likely to become a competitive fuel tomorrow. Thus, processes that are presently conceptual in nature, or at a developmental stage in the laboratory need to be encouraged, tested for feasibility and otherwise applied toward commercialization.
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