Anaerobic digestion is one of the most sustainable and promising technologies for the management of organic residues. China plays an important role in the world's biogas industry and has accumulated rich and valuable experience, both positive and negative. The country has established relatively complete laws, policies and a subsidy system; its world-renowned standard system guarantees the implementation of biogas projects. Its prefabricated biogas industry has been developed, and several biogas-linked agricultural models have been disseminated. Nonetheless, the subsidy system in China's biogas industry is inflexible and cannot lead to marketization, unlike that of its European counterpart. Moreover, the equipment and technology levels of China's biogas industry are still lagging and underdeveloped. Mono-digestion, rather than co-digestion, dominates the biogas industry. In addition, biogas upgrading technology is immature, and digestate lacks planning and management. China's government subsidy is reconsidered in this work, resulting in the recommendation that subsidy should be based on products (i.e., output-oriented) instead of only input subsidy for construction. The policy could focus on the revival of abandoned biogas plants as well.Sustainability 2020, 12, 1490 2 of 21 effectively and widely implemented in countries where governments and institutions are involved in the subsidy, planning, design, construction, operation, and maintenance of biogas plants [7,8]. A number of countries in Asia and Africa have launched massive campaigns to popularize biogas technology via government support and international aid (Table 1) [9,10]. Globally, it is estimated that 50 million micro-digesters (family size), 132,000 biogas engineering projects (about 15,000 in Europe [11]) and 700 biogas upgrading plants are operating [12]. Promotion of biogas technology has several opportunities and obstacles. Numerous studies have focused on these aspects to discuss the country scenario. Many case and field studies have aimed to assess biogas technology to verify its multiple benefits [13,14] and identify potential barriers [8,[15][16][17][18][19][20][21]. However, these studies are limited to individual country scenarios. The progress and prospect of the biogas industry in different nations vary widely. Nearly no crosswise comparisons have been made for different countries. The current study intends to fill this gap.China leads the world in domestic biogas technology. With the expansion of the biogas industry, lessons learned, whether positive or negative, would be valuable and abundant for countries whose biogas industry is still at the initial stage, including nations mainly in South and Southeast Asia, Africa, and Latin America. Meanwhile, along with the development of medium-and large-scale biogas plants (MLBPs), progress is not trouble-free [22]. (According to the Chinese biogas standard NY/T 667-2011 Classification of Scale for Biogas Engineering, the thresholds of medium-scale and large-scale biogas plant are 300 m 3 and 500 m...

Cement is the most widely used construction material in the world. However, its manufacture is high energy consumption and high carbon emission owing to the high temperature calcination process. Geopolymer is an ideal alternative material for cement because it has a similar structure and performance to cement. In addition, it can be synthesized at room temperature and thus has the advantages of energy saving and carbon emission reduction. Sewage sludge (SS) can be used as raw material for geopolymer synthesis. However, the high organic matter content in SS lowers the mechanical strength of geopolymer. Although the organic matter in SS can be removed by incineration at high temperature, this consumes energy and emits carbon dioxide, which diminishes the advantages of geopolymer. In this study, the Fenton method was applied for the removal of organic matter in SS at room temperature. The parameters of the Fenton method, including the dosages of hydrogen peroxide (H 2 O 2 ) and Fe 2+ reagent (FeSO 4 ·7H 2 O), reaction time, and initial pH value, were investigated. The results indicated that 83.7% of the organic matter in SS could be removed at room temperature by using 5.15 M H 2 O 2 and 5.15 mM FeSO 4 ·7H 2 O at pH 7, which suggested the possibility of sewage sludge reclamation through geopolymer synthesis as an alternative material for cement toward sustainability.

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Abrasive–glass polishing waste is generated from the polishing process of glass components by using cerium oxide abrasive, which contains the cerium oxide abrasive and the polished glass. This study attempted to recover the cerium oxide abrasive from the abrasive–glass polishing waste through removing the polished glass by alkaline roasting using sodium hydroxide (NaOH) followed by water leaching. The experimental results indicated that the polished glass in the abrasive–glass polishing waste could be fully removed under the optimal alkaline roasting and water leaching conditions of roasting temperature of 450 °C, mass ratio of polishing waste to NaOH of 1:1, roasting time of 30 min, leaching pH of 3, leaching temperature of 25 °C, and liquid–solid ratio of 25 mL/g. The characteristics including elemental composition, particle size distribution, mineralogical phases, and morphology of the recovered cerium oxide abrasive obtained under the optimal conditions were similar to those of the original unused one, which was suitable to be reused for polishing again.

The novelty of this study is to rapidly reduce hazardous lead leachates from solid waste using microwave digestion treatment, which is an energy-saving and low greenhouse gas emission technology. The article presents the reduction of toxic characteristic leaching procedure-extractable lead concentration in the municipal solid waste incineration fly ash by the microwave digestion treatment in HNO3/H2SO4 combination, and focuses on the effects of treatment time and temperature. The results obtained from this study indicated a significant reduction efficiency of toxic characteristic leaching procedure-extractable lead concentration and showed sufficient reduction in leaching levels to render the treated fly ash safe in lead compound leaching characteristics. The reduction efficiency of toxic characteristic leaching procedure-extractable lead concentration can reach 98% in 15 minutes of treatment time. This is equivalent to the original toxic characteristic leaching procedure-extractable lead concentration of 46.2 mg L(-1) in raw fly ash being reduced down to less than 1.0 mg L(-1). Based on the experimental data obtained in this study, a useful correlation between reduction efficiency and treatment conditions is proposed. For engineering applications, the necessary minimum treatment time is solved using a graphic illustration method, by which the minimum treatment time (t(min)) is obtained if the desired reduction efficiency (η) and treatment temperature (T) are known. The effects of treatment time and temperature are discussed. Some problems caused by the microwave digestion treatment method are also delineated in this article.