Thermal treatment of plastic wastes for sustainable environment

Already, the rapid growth in population and economic activities have caused depletion in natural resources. In order to meet the enormous demand for cheap products, plastic has become an alternative source. Since the beginning of the 20th century, plastic has been considered the most common material in modern civilisation.
Big countries to produce and export plastics are China, Germany, United States, Japan and France. China manufactures 23.9 per cent of the world's plastic products. The per capita consumption of plastic products corresponds to 32 kilograms per year for 109,706,545 people in the United States, Japan, EU, China and Brazil. Between 1950 and 2012, the world's plastic production has increased from 1.5 Mt to 288 Mt. In FY-2011, total consumption of polymers including imported polymers and recycled plastic wastes (PW) was 0.75 Mt. At present, there are 3,000 plastic manufacturing units in Bangladesh while per capita consumption is 5 to 8 kilograms per year against the international average of 30. Plastics are used for different purposes - household products, toys, electric products, packaging materials, construction, transport, and manufacturing of plastic bags.
However, the advantage of using plastics - durability, light weight and low cost - also makes it problematic since expired plastics turn into wastes. Roughly, 33 per cent of the plastic products are manufactured for temporary use. Eventually, plastic wastes are mixed with municipal solid wastes (MSW) - at a variable rate between 5.10 and 12.3 per cent. Unlike the MSWs, plastic wastes are not biodegradable.
Scientific researches have suggested that the life span of plastic products is roughly 1 to 30 years while plastic packaging materials and plastic bags are often thrown away even after using them once. In addition, plastic is a long and artificial chain of polymeric molecules with basic elements including carbon, silicon, hydrogen, nitrogen, oxygen and chloride whereas conventional raw materials are petrol, diesel, coal and natural gas. Some additives (heavy metals) like Pb, Cd, Cr, Hg, Br, Sn and Sb are often used in plastic materials to make sure the required appearance, strength, pigments, fillers, ultraviolet (UV) stabilisers, and flame retardants. These additives are inserted into plastics, although encapsulated in polymer matrix even with no chemical effect to polymer molecules. Gradually, it is released into the environment due to plastic expiration. These heavy metals affect the environment and human body in various ways. Chemically, plastic products comprise certain types of plastic contents - PE, HDPE, LDPE, LLDPE, PVC, PET, PP, PS, ABS, PUR, and Nylon (PA) - staying in the conventional dumping sites and sanitary landfills for thousands of years. Besides, PVC contains Cl which is responsible of producing toxic chemicals during incineration - dioxin and furan. So, the developed countries are searching for alternate technology in PW treatment.
Bangladesh generates 336,000 tons of PWs per year and about 17,000 tons per year end up in the landfill. Among all types of PWs, the primary recycling of plastic bottles for plastic resin or plate recovery has become quite popular in Bangladesh. Currently, about 80-90 per cent of plastic bottles are recycled in Dhaka, Chittagong, Sylhet, Rajshahi and Khulna. In Bangladesh, the recycled plastic wastes are used either for producing low-quality plastic products or these are exported to industrialised countries. By recycling both PWs and MSWs, Dhaka saves more than Tk.10 billion per year. But, the city's PWs are recycled in unhygienic conditions leading to future hazards. All the plastic wastes have a high-heating value of 18 MJ to 46 MJ per kilogram - almost equal to conventional fuel. Only 8.0 per cent of PWs are recycled in the United States followed by 15 per cent in EU. The United States, EU and Japan are generating electricity through thermal treatment or incineration by utilising such high-heating value of plastic materials.
This facilitates the reduction of pollutants as well as ensures a sustainable source of energy. Currently, Bangladesh has a huge demand of electricity and fuel, although the government is failing to meet such need of large population. By using both the daily-produced and the landfill-going PWs, between 4,603 to 11,763 MWh per day and 232.9 to 595.1 MWh per day of electricity can be generated through incineration. Roughly between 920,548 litres per day and 46,575 litres per day, fuels can be collected in Bangladesh through pyrolysis.
Developed countries like the United States, Japan and EU use the technique of incineration for the thermal treatment of MSWs to recover energy resources. Their scientists have confirmed that PWs are a major source of releasing pollutants into environment during incineration. Therefore, pyrolysis should be utilised in energy recovery and fuel collection from PWs. Besides, pyrolysis is  a closed process with less possibility of toxic release. In Bangladesh, the thermal treatment of PWs should be considered a sustainable solution for the environmental protection as well as an alternative source of renewable energy.
The writer teaches at the Institute of Waste Treatment and Reclamation (IWTR) at Tongji University in Shanghai, China.
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