Hydroxide precipitation is the most widely used heavy metal wastewater treatment method. However, the metal hydroxide sludge produced must be treated concurrently. In this paper, an integrated treatment procedure, which modifies hydroxide precipitation with oxidation treatment and sulfide precipitation, is proposed to sequentially precipitate Fe, Cu, and Zn from wastewater for metal recovery. An industrial wastewater specimen containing Cu ($290\pm 6.25\mathrm{mg}/\mathrm{L}$ ), Fe ($\mathrm{2,092}\pm 12.3\mathrm{mg}/\mathrm{L}$ ), and Zn ($\mathrm{4,648}\pm 15.4\mathrm{mg}/\mathrm{L}$ ) was used. First, hydroxide precipitation using lime was applied to study the precipitation characteristics of Cu, Fe, and Zn, following which ${\mathrm{H}}_2{\mathrm{O}}_2$ oxidation treatment and NaHS sulfide precipitation were conducted and integrated with hydroxide precipitation to selectively precipitate Cu, Fe, and Zn from the wastewater specimen. The Fe, Cu, and Zn were sequentially removed from the wastewater by introducing 0.1% (v/v) ${\mathrm{H}}_2{\mathrm{O}}_2$ , altering the pH of the wastewater to pH 3.5–4.0 using lime, adding $1\mathrm{g}/\mathrm{L}$ NaHS, and subsequently altering the pH to 9.0. The Fe, Cu, and Zn were subsequently separated as individual precipitates with recoveries of $99.8\pm 0.1$ , $94.0\pm 0.3$ , and $96.1\pm 0.7\%$ , respectively, which can facilitate metal recovery and render sludge disposal unnecessary.

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.

In recent years, with the increasing research and development of the light-emitting diode (LED) industry, which contains gallium nitride (GaN), it is expected that there will be a large amount of related wastes in the future. Gallium has an extremely high economic value, therefore, it is necessary to establish a recycling system for the GaN waste. However, GaN is a direct-gap semiconductor and with its high energy gap, high hardness, and high melting point, these make it difficult to recycle. Therefore, this study will analyze the physical characteristics of LED wastes containing GaN and carry out various leaching methods to leach the valuable metals from the waste optimally. Different acids are used to find out the best reagent for gallium leaching. Different experimental parameters are discussed, such as the effect of the different acid agents, concentration, pressure, liquid-solid mass ratio, temperature and time, which influence the leaching efficiency of gallium. Finally, acid leaching under high pressure is preferred to leach the GaN waste, and hydrochloric acid is used as the leaching solution because of its better leaching efficiency of gallium. Optimally, the leaching efficiency of gallium can reach 98%.

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 (H₂O₂) and Fe²⁺ reagent (FeSO₄·7H₂O), 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 H₂O₂ and 5.15 mM FeSO₄·7H₂O at pH 7, which suggested the possibility of sewage sludge reclamation through geopolymer synthesis as an alternative material for cement toward sustainability.