Biodegradable material, primarily composed of food waste, accounts for 40-70 wt% of municipal solid waste (MSW) in developing countries. Therefore, to establish a sustainable waste management system, it is essential to separate and recycle biodegradable organic material from the municipal waste stream. Of all the recycling methods, composting is recommended due to its environmental and economic benefits. However, compared with readily recyclable materials (e.g., paper, metals, etc.), recycling/composting biodegradable MSW presents a great challenge to furthering the promotion of waste recycling. This review provides a systematic analysis of organic waste sorting and recycling/composting practices in several countries: the UK, US, Japan, and China. Compared to the great efforts that have been made in developed countries for the promotion of waste composting, much less has been achieved in developing countries. For example, the MSW composting rate in China decreased from 10% to less than 2% in the past 15 years, and similar trends may exist in other developing countries. Therefore, it is essential to identify the barriers that impede waste composting and predict developing trends. This article emphasizes environmental challenges (i.e., odor, bioaerosols, and heavy metals), focusing on their generation and control strategies, in an effort to identify barriers hindering MSW composting. Successful practices in several European countries suggest that source-separated composting presents many advantages over mechanical-separated composting. This may partially be ascribed to the fact that source separation of organic waste can prevent contact with heavy metal-bearing items, resulting in the production of highquality compost. Mixed collection MSW normally contains significantly higher concentrations of heavy metals, which could affect the marketing of composting products. Moreover, source separation of organic waste can minimize waste pretreatment operations, leading to lower bioaerosol and malodor generation during composting. Implementing source separation of MSW in more countries would increase the amount of organic waste available for composting. In addition, attention should focus on secondary pollutant production for proper composting management. Finally, setting standards for end product quality control is highly recommended for controlling both marketing and environmental risks.

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Owing to the high Ca content but low concentration of Si and Al in fly ash (FA) generated from municipal solid waste incineration (MSWI), hydrothermal treatment (HT) of MSWI FA directly could not result in the synthesis of calcium-containing aluminosilicate minerals zeolites such as tobermorite, an important material for the hydrothermal solidification of heavy metals. According to the research results from the effect of the extra silicon–aluminum source added, with the higher amount (30%) of the composite additive in which the ratio of quality for coal fly ash (CFA) and diatomite is 1:1, tobermorite could be successfully synthesized, which is mainly attributed to the proper element proportion of Ca to Si and Al and Al to Si and Al, adjusted by additives. Furthermore, the tobermorite seed added also could not help the formation of tobermorite, even at a higher reaction temperature of 200 °C, due to the improper element proportion with a 10% mass of the composite additive. With a 30% mass of the composite additive and seed at a higher reaction temperature of 200 °C, the tobermorite seed added could not only promote the formation of tobermorite as early as the first 3 h but also inhibit the hibschite generated from tobermorite. Due to the leaching toxicity of Pb decreasing to the lowest level (0.25 mg/L) being related to the tobermorite largely formed, it can be concluded that the addition of a 30% mass of composite additive (including CFA and diatomite) and a 3% mass of tobermorite seed at a higher reaction temperature of 200 °C would be the recommended technological parameter for seed-assisted hydrothermal solidification of MSWI fly ash.