Introduction

The problem of waste management runs across geographies and its gravest causal agent, i.e. urbanism, is a global phenomenon. However, its ramifications are relatively more pronounced in developing nations on account of improved standards of living and changing consumption patterns. The growing population and increasing consumer demand is leading to excessive consumption of available resources and generation of tremendous amount of different kind of wastes, which is emerging as a chronic problem in urban societies. Their efficient management is needed at the earliest to avoid numerous problems related to public and environmental health.
 

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 significantly contributed to the growth and economic development of the country, apart from 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 something to be considered with utmost diligence, since it is the urban areas, which mostly contribute to the waste generation. The situation grows even starker from the observation that 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 at the same time, reiterates the huge business potential that ensues the proper utilization of such a widely available potential energy/power resource.
 

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.
 

 

Figure 1: Benefits of Anaerobic Digestion

 

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, wherein it is mixed with the recirculated digestate to bring it in contact with some of the wore out/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.

 

Figure 2: Layout of a typical food waste-to-biogas power plant

 

The feed is then directed into the anaerobic digester. The most commonly used biogas plants for power generation using biogas are the Continiuous Stirred Tank Reactors (CSTR). These reactors involve anaerobic digestion at mesophillic temperaturres and generally have a retention time of about 20-25 days. For smaller scales and other domestic and thermal applications of biogas, other reactors are also commercially available like the floating drum KVIC model, fixed dome type model by TERI, the Janata model or the TERI Enhanced Acidification and Methanation (TEAM) setup which is essentially Upflow Anaerobic Sludge Blanket Reactors (UASB).
 

The quality and quantum of biogass depends on a variety of factors like the technology used, type of waste, quantity and quality of waste, waste parameters like moisture content, volatile matter, ash content, the C/N ratio, heating and calorific value and other factors. An important consideration while generating power using biogas is the desulphurization of the gas. Anaerobic process results in the formation of H₂S which on combustion generates SO₂. It is not only corrosive to the gas engine but also harmful to the environment. To tackle this situation, chemical or biological desulphurization is carried out. The chemical desulphurization involves the use of FeCl₂ in which chloride is replaced by sulphur owing to the higher affinity of the latter with Iron. Biological desulphurization on the other hand, utilizes the sulphur oxidizing bacteria and converts hydrogen sulphide into elemental sulphur, in the presence of air. However, utmost care needs to be taken while introducing air to biogas, since introduction of air, more than 2-5% of total biogas volume, might result in its implosion.
 

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.
 

Although the Municipal Solid Waste Management Directive (2000) mandates source segregation of waste which is easily biodegradable in nature, in reality it has not been able to find widespread implementation till now. The major reasons include lack of proper implementation and reporting mechanism, and lower degree of awareness among the people in general. In addition, urban waste in India is also mixed with a huge amount of rubble, construction and demolition waste and other such wastes, which render the food-waste unsuitable for subsequent conversion to energy.
 

Most of the organic waste generated in the country is either being dumped into the landfills or composted or sent to piggeries. It is a sheer waste of such biodegradable waste capable of generating energy to be sent into the landfills. There it is not only responsible for large scale green house gas emissions, but also becomes a health hazard and creates terrestrial pollution.
 

There are numerous places which are the sources of large amounts of food waste and hence a proper food-waste management strategy needs to be devised for them to make sure that either they are disposed off in a safe manner or utilized efficiently. These places include hotels, restaurants, malls, residential societies, college/school/office canteens, religious mass cooking places, airline caterers, food and meat processing industries and vegetable markets which generate organic waste of considerable quantum on a daily basis.
 

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.
 

About the Author

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 setu.goyal@gmail.com