The waste management hierarchy suggests that reduce, reuse and recycling should always be given preference in a typical waste management system. However, these options cannot be applied uniformly for all kinds of wastes. For examples, organic waste is quite difficult to deal with using the conventional 3R strategy. Of the different types of organic wastes available, food waste holds the highest potential in terms of economic exploitation as it contains high amount of carbon and can be efficiently converted into biogas and organic fertilizer.
A consistent growth rate of 8 to 10 percent for India is symbolic of its increasing production and consumption trends. The main reasons for such trends have been the increasing disposable incomes and the growing consumerism and urbanism. All this has contributed to the growth and economic development of the country, and also the tremendous increase in waste generation across the country.
The amount of waste generated by any country is directly proportional to its population and the mean living standards of the people. As per the last census of India, the Indian population was 1027 million with about 5161 urban cities and towns contributing up to 28% of the total population. A constant rate of increase of about 30% per decade in the number of town/cities urbanized is important, since it is the urban areas, which mostly contribute to the waste generation. The situation grows even starker because the per capita waste generation in India has been rising by about 1-1.3% annually over the past few decades and the population itself has been rising at an annual rate of 1.2-1.5%.
With organic or food waste being one of the main constituents of the total urban waste generated, it not only makes it essential to seek means for its safe disposal but it also provides a huge business potential.
Anaerobic digestion is a proven and commercially available technology to handle wastes having high carbon content. It is widely acknowledged as the best means to deal with organic waste in rural as well as urban areas. One of the major benefits of anaerobic digestion is its almost negative impact on the environment since it saves on emissions which would have been caused if the organic waste was dumped into landfills or an equivalent amount of power would be generated using conventional fossil fuel based resources. Another important feature is its scalability and ability to accept varied types of biomass. World over, the technology has been reaching newer and higher scales, with plants of capacity 300 tonnes per day and above already in operation in countries like Austria, Germany, Sweden and Italy.
The feedstock to be utlized, e.g. organic waste from various sources, is first collected and then passed through a shredder to reduce the minimum particle size. The homogenated mass is then moved to a mixing tank, where it is mixed with the recirculated digestate to bring it in contact with some of the used microbial biomass to increase the rate of biochemical degradation in the subsequent steps and also to make the input feed more acclimatized to the system or process requirements. This homogenate along with the recirculated digestate from the mixing tank, which is responsible for maintaining the adequate solid content in the feed in terms of volume, is then transferred to a storage tank. The main purpose of placing another tank in between the mixing tank and main bio-digester is to maintain an input reservoir in order to account for a few days of unavailability in feedstock. In certain cases of large-scale power application of this technology, waste heat is utilized from the gas engine exhaust and fed to the storage tank to double it up as a pre-digester by facilitating the growth of thermophilic bacterias and elimination of any pathogens.
The feed is then directed into the anaerobic digester. An important component of a typical biogas facility is the gas holder which is used to maintain a buffer between the production and consumption rates of the biogas. The gas is drawn into the gas engine from the gas holder and the waste heat generated is utilized to improve the overall efficiency of the system by directing it through the pre digester and the main digester.
Since the water effluent from such a process is expected to possess high BOD and COD characters, the need of a dedicated effluent treatment plant is ineluctable. This waste water is mainly obtained after the dewatering of the slurry obtained from the above process. The solid content in the slurry increases after going through the de-watering stage in multiple stage screw-presses and it can be sold as high quality compost in the market.
The anaerobic digestion technology is highly apt in dealing with the chronic problem of organic waste management in urban societies. Although the technology is commercially viable in the longer run, the high initial capital cost is a major hurdle towards its proliferation. The onus is on the governments to create awareness and promote such technologies in a sustainable manner. At the same time, entrepreneurs, non-governmental organizations and environmental agencies should also take inspiration from successful food waste-to-energy projects in other countries and try to set up such facilities in Indian cities and towns.
Setu Goyal is pursuing Masters Program in Renewable Energy Engineering and Management at the TERI University (New Delhi), and has an entrepreneurial zeal to improve waste management and renewable energy scenarios in developing countries. He can be reached at firstname.lastname@example.org