The precipitation of struvite (MgNH₄PO₄·6H₂O) is considered to be a promising method for the recovery of phosphate from wastewater. In this review, the kinetic models, which are commonly used to explain the process of struvite crystallization, are described. The mixed-suspension mixed-product removal (MSMPR) model is based on the population balance equation (the size-dependent growth model and the size-independent growth model). Thereafter, the first-order kinetic fitting model that aligned with concentration changes in the substrate is summarized. Finally, the several physical and chemical factors that affected the efficiency of struvite crystallization are determined. The supersaturation ratio, which is seen as the driving force of struvite crystallization, is the main factor that influences crystallization; however, it cannot be used in practical applications of engineering because it is indirectly associated with the following factors: pH, the molar ratio of Mg:N:P, and the interference of foreign impurities. In this study, we present conclusions that should be used to guide further research studies, and encourage the engineering practice of wastewater treatment with struvite precipitation.

The aim of this study is to evaluate the anaerobic digestion and biogas production of plant biomass under high salinity by adopting a theoretical and technical approach for saline plant-biomass treatment. Two completely mixed lab-scale mesophilic reactors were operated for 480 days. In one of them, NaCl was added and the sodium ion concentration was maintained at 35.8 g-Na⁺·L⁻¹, and the organic loading rate was 0.58-COD·L⁻¹·d⁻¹–1.5 g-COD·L⁻¹·d⁻¹; the other added Na₂SO₄–NaHCO₃ and kept the sodium ion concentration at 27.6 g-Na⁺·L⁻¹ and the organic loading rate at 0.2 g-COD·L⁻¹·d⁻¹–0.8 g-COD·L⁻¹·d⁻¹. The conversion efficiencies of the two systems (COD to methane) were 66% and 54%, respectively. Based on the sulfate-reduction reaction and the existing anaerobic digestion model, a kinetic model comprising 12 types of soluble substrates and 16 types of anaerobic microorganisms was developed. The model was used to simulate the process performance of a continuous anaerobic bioreactor with a mixed liquor suspended solids (MLSS) concentration of 10 g·L⁻¹–40 g·L⁻¹. The results showed that the NaCl system could receive the influent up to a loading rate of 0.16 kg-COD/kg-MLSS·d⁻¹ without significant degradation of the methane conversion at 66%, while the Na₂SO₄–NaHCO₃ system could receive more than 2 kg-COD·kg⁻¹-MLSS·d⁻¹, where 54% of the fed chemical oxygen demand (COD) was converted into methane and another 12% was observed to be sulfide.

Sewage sludge is a useful raw material for the production of renewable energy due to its stable annual output. In this study, the enhancement of mesophilic anaerobic digestion of sewage sludge through heat pretreatment at 95 °C for 30 min was tested in an anaerobic moving bed biofilm reactor (hAMBBR). The sludge retention time was set at 20, 15, 10, and 5 days during 300 days of operation and compared to a traditional anaerobic continuous stirred tank reactor (AnCSTR) without pretreatment. Results of this research indicate that the digestion ratio of volatile soluble solids in the hAMBBR process could be improved by 50%, and the average conversion ratio of methane could be increased by 45%. When the sludge retention time (SRT) was shortened to 5 days, the methane production approached twice that of the contrast reactor. The expanded anaerobic digestion model, including activated sludge models, was utilized for operation simulation. The effect of sludge retention time (SRT) shortening on volatile suspended solids (VSS) digestibility and methane production was well reproduced with simulations. The research conclusion reveals the impact of pretreatment and reactor types on anaerobic digestion and provides the scientific basis for improving methane production and process efficiency in anaerobic digestion.

Hydrothermal carbonization technology has attracted increasingly the amount of attention due to its efficient conversion of biomass resources. Such technology benefits both the economy and the environment. In this study, hydrothermal carbonization of reed was conducted in a hydrothermal reactor under laboratory conditions. The factors that control the hydrochar yield and the efficiency of methylene blue adsorption, such as temperature, residence time, and solid-liquid ratio were investigated. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) were used to characterize the structure of the reed hydrochar. The experimental results indicated that the order of the factors affecting hydrochar yield was as follows: residence time > maximum temperature > solid-liquid ratio. The order of factors that determine the extent of methylene blue adsorption was: solid-liquid ratio > maximum temperature > residence time. The optimum condition for hydrochar production was with a reaction temperature at 260 ℃, a residence time of 2 h, and a solid-liquid ratio of 1:10. The optimum condition for the highest amount of methylene blue adsorption was with a reaction temperature at 260 ℃, a residence time of 1 h, and a solid-liquid ratio of 1:10. The SEM imaging revealed a pore structure at the surface of the hydrochar comprising carbon microspheres and multilayer structure. Whereas the FTIR analysis indicated that the hydrochar had a high number of oxygen-containing surface functional groups. The environmental impact of the reed hydrochar preparation process was evaluated using the life cycle assessment approach by the SimaPro 9.2.0.2 software. The results revealed that the hydrothermal carbonization process majorly impacted the toxicity to the environment, the land use and the global warming. Future optimization of the hydrothermal carbonization process should focus on: (1) reducing the energy consumption during this process; (2) evaluating the environmental impact of the hydrochar-producing system, including the resource recycling of the by-products during this process.