A review on the global warming potential of cleaner composting and mitigation strategies

Abstract

With the rapid population growth and industrial development in the fast developing countries, there is a significant increase in the production of waste. For instance, the municipal solid waste in Malaysia is expected to exceed 9 Mt/y by 2020 based on the current production rate of 1 kg−1 person−1. Approximately 90% of the waste is channelled into the landfill that can release a significant amount of greenhouse gases, mainly the methane gas and nitrous oxide gases, due to the anaerobic decomposition of organic matter. Composting is a cleaner alternative to landfilling for managing organic waste. Its main advantage lies in its capability to recycle nutrients through compost utilisation. The life cycle assessment has been widely used as a mean of comparison for impact assessment, such as global warming potential, among different waste management technologies. The common inventories accounted for composting include transportation, operational machineries, fugitive emissions during maturation and curing as well as the end production utilisation. There is inconsistency in the methodology adopted to assess its environmental impact. This study discusses on the variations of life cycle assessment that contributed to the different global warming potential of composting. It also discusses on the mitigation strategies to reduce the global warming potential for composting. The highlight of the study is to examine the variation in the inventory analysis for composting, and its underlying mechanism and the critical inventory for a more representative assessment. The second part of the paper reviews the mitigation strategies for reducing the greenhouse gas emissions, i.e. the use of the bulking agent, aeration system, chemical additives and use of the cover material. This study found that the direct greenhouse gas emissions during composting contribute more significantly to the global warming potential than other direct emissions. The use of bulking agent is desirable to reduce the global warming potential. This study concludes that the assessment of the global warming potential for composting is dependent on the system boundary and the defined functional unit. The environmental impact should be assessed based on the operational mode and the input feedstock to generate a basis with minimised discrepancies among studies. Continuous effort is needed to quantify the long-term benefits of composting on environment, health and soil to further assess its impact as a cleaner process.