Solid waste refers to any refuse resulting from human or animal activities that has been discarded on purpose or because it is no longer useful. In layman’s terms, nearly anything we leave behind is some kind of waste. Solid waste can be broadly categorised into residential, industrial and commercial waste. It can also be categorised on the basis of its contents such as plastic, paper, organic and metal waste, or its hazard potential, such as flammable, radioactive, infectious, toxic, or non-toxic material.
Too much waste is being generated
Solid waste management has become a major environmental concern for countries around the world. With the rapid increase in population, industrialisation and urbanisation, the generation rate of municipal solid waste (MSW) in cities and towns has skyrocketed.
The past century was marked with growing urbanisation and affluence, leading to a tenfold increase in waste generation. In the 1900s, the world had only 220 million urban residents, who accounted for 13 per cent of the total population. These residents produced less than 300,000 tonnes of solid waste per day. But by 2000, urban residents grew at an accelerating pace and reached 2.9 billion (approximately 49 per cent of the world’s population), producing nearly 3 million tonnes (mt) of solid waste per day. By 2025, urban residents are expected to reach twice the current figure, which would approximately increase MSW generation levels to 2.3 billion tonnes per year. According to a World Bank 2012 report, it is estimated that global annual solid waste management costs will increase from today’s $205.4 billion to about $375.5 billion in 2025.
If we talk about India alone, the Central Pollution Control Board of India estimates that nearly 0.14 mt of waste is being generated per day (approximately 50 mt per year). Of the total waste generated, only 83 per cent is collected every day and only 29 per cent of the collected waste is being treated. As a result of the country’s rapid economic growth, there has been a substantial increase in solid waste generation in major metropolitan cities such as Delhi, Chennai, Mumbai and Kolkata. Urban areas in India alone generate more than 100,000 metric tonnes of solid waste per day, which is higher than many countries’ total daily waste generation. Without an effective and efficient solid waste management programme, the waste generated from various human activities, both industrial and domestic, can result in health hazards and have a negative impact on the environment.
How can the situation be improved
Various studies reveal that about 90 per cent of MSW is disposed of unscientifically, in landfills and open dumps. This contaminates waterbodies, causes land pollution and generates hazardous gases resulting in adverse environmental impact, public health problems and other socio-economic issues. This problem can be mitigated by adopting suitable waste-to-energy (W2E) technologies. There are many technologies available that can be utilised for extracting energy from waste. These include incineration, gasification, pyrolysis, refuse-derived fuel (RDF) and biomethanation. Some of these are discussed below.
The heart and soul of any W2E facility is fuel, where the higher quality of fuel is directly proportionate to higher output. Talking about India in particular, most of its cities have insufficient infrastructure for handling waste, and with poor waste management systems as well as lack of awareness, it is difficult to mitigate solid waste issues. In the following paragraphs, various technologies for handling waste have been discussed in detail.
- Thermal treatment/Incineration: Thermal treatment is a traditional way of producing heat or power by burning waste. It may also be referred to as mass burning, where all the wastes combust, leaving behind non-combustible material. Hence, it is unsuitable to directly burn the waste without removing the non-combustible material from it. To overcome this issue, various technologies such as gravitational, magnetic and mechanical separation are available, which can remove the non-combustible material from the waste.
- RDF: This is generated from combustible ingredients of MSW. Raw MSW has low heating value, and high ash and moisture content. The MSW collected from households comprises solid, semi-solid, inert or non-combustible components. RDF is obtained when the combustibles are segregated from non-combustible waste and dried. Treatment of raw waste such as segregation is done using various mechanical, gravity-based and magnetic-based methods. The RDF, which has a smaller volume and higher calorific value than raw waste, is then sent to specially designed boilers for generating electricity. These boilers are able to incinerate MSW at a higher temperature to avoid harmful emissions. This method, being a costlier affair, has been adopted only at the MW scale.
- Gasification: This is another form of thermal treatment but it is carried out in a closed environment and in limited supply of oxygen, thus resulting in the generation of producer gas in temperature ranges of 480-1,650 °C. The producer gas thus generated is further utilised for heating applications and to generate electricity. One of the most mature technologies among W2E, it requires a complex system for cleaning producer gas. It also needs special gas cleaning equipment to deal with tar and other harmful emissions.
- Pyrolysis: This is another thermochemical conversion process for converting organic materials into synthetic gas (syngas), but in the absence of oxygen. Pyrolysis occurs at elevated temperatures, greater than 400 °C. The syngas thus obtained from the above-mentioned reactions is converted into liquid hydrocarbon or bio-oil/biodiesel as a final product. Other by-products such as charcoal and ash are also obtained. Although a few models have been developed on a small scale, the aim is to speed up technology development for large commercial use.
- Biomethanation: This involves the biological treatment of organic wastes under anaerobic conditions. Organic wastes microbiologically disintegrate into biogas under controlled anaerobic conditions, leading to its main constituent, methane, which finally gets converted into heat or electricity with the help of gas engines. This technology is regarded as one of the most efficient W2E conversion technologies as it minimises the waste that gets generated after the process.
Is change possible?
The waste generation situation is alarming, especially in countries like India and China, where waste generation is expected to increase approximately 3.5 and 2.7 times respectively by 2025 as compared to current generation levels (see Graph).
Although India has taken a major initiative to sanitise the country under the Swachh Bharat Abhiyan, waste generation is still a matter of major concern and needs greater attention to fight the problems associated with it. Moreover, by 2021 an average of 32,000 people will be added to urban India every day. This number is alarming and with increasing waste generation and improper waste management, Indian cities will hit rock bottom. With the problems related to closure of existing landfills, finding new waste dumping sites in and around cities has become nearly impossible. Before the situation gets out of hand, we must focus on developing more efficient technologies such as RDF to handle huge quantities of wastes. Policy intervention and appropriate planning for promoting such technologies would be the most efficient way to manage this crisis.