Enhancing anaerobic degradation of oily sludge using subcritical hydrothermal pretreatment

The methods for improving the biodegradability of oily sludge: a critical review

Biological degradation method, as an environmentally friendly, low-carbon, and clean pollution treatment technology, is widely used for the harmless disposal of oily sludge. The biodegradability of oily sludge with stable emulsification system, high oil, and water content is poor. Therefore, it is necessary to pre-treat the oily sludge to improve its biodegradability, including recover the petroleum resources and remove heavy metals and bio-toxic organic matters. This review systematically summarizes five oily sludge treatment methods and their influences on sludge biodegradability, including pyrolysis, chemical hot washing, solvent extraction, chemical oxidation, and hydrothermal. Pyrolysis at temperatures above 750 °C produces high molecular weight polycyclic aromatic hydrocarbons, chemical hot washing and chemical oxidation would cause secondary pollution, solvent extraction method could not be applied due to the high cost and high toxicity of the extractant, and the oil removal of hydrothermal method is inefficient. Additionally, the principles, advantages, and disadvantages of those treatments and the factors affecting microbial degradation were analyzed, which provide the development direction of pretreatment technology to improve the biodegradability of oily sludge.

Subcritical hydrothermal oxidation of semi-dry ash from iron ore sintering flue gas desulfurization: Experimental and kinetic studies

Realization of low temperature and high efficiency oxidation of CaSO₃ is the key to solve the issue of ecological hazards caused by semi-dry sintering flue gas desulfurization ash. The subcritical hydrothermal technology was employed for the oxidation of CaSO₃, achieving 89.83% of CaSO₃ at 180 °C, 2 MPa for 120 min with a solid-to-liquid ratio of 1:20. The macroscopic oxidation kinetics of CaSO₃ in the subcritical hydrothermal reaction system was investigated. A mathematical model was established, incorporating the intrinsic reaction, CaSO₃ dissolution, oxygen diffusion and CaSO₄ precipitation. It was concluded that the macroscopic oxidation of CaSO₃ was co-controlled by the oxygen diffusion and CaSO₄ precipitation. Subcritical hydrothermal technology promises not only higher efficiency, but more importantly, potentially "one-step" preparation of CaSO₄ whiskers, enabling cost-effective and high value-added resource utilization of the semi-dry FGD ash.

Impact of solid content on hydrothermal pretreatment of municipal sludge prior to fermentation and anaerobic digestion

This study investigated the impact of the solid sludge content concentrations (SC) on hydrothermal pretreatment (HTP) before fermentation and anaerobic digestion. Five different SC of 3.5%, 7%, 10%, 12%, and 16% were investigated in two different scenarios. The first scenario entailed using only the pretreated samples as substrates, whereas in scenario two, the substrates included pretreated samples combined with the supernatant. Results revealed that the highest overall pCOD solubilization (considering HTP and fermentation) of 64% was achieved for the sample with 12% SC combined with supernatant. The maximum volatile fatty acids production of 2.8 g COD/L occurred with 10% SC without supernatant. The maximum methane yield of 291 mL CH₄/g VSS added was attained at 7% SC without supernatant. Furthermore, the results indicated that increasing the SC beyond 7% in scenario 1 and 10% in scenario two led to a decrease in methane yield. Additionally, optimizing for all desired endpoints may be difficult, and there are limits on the increase in SC concerning methane production.

Toxic effect of fracturing flow-back fluid on Vibrio fischeri, Daphnia, and specific industry microorganisms Aspergillus niger and S. cerevisiae

Previous studies have shown that the soil microbial population and soil enzyme activity are seriously affected by fracturing flow-back fluid (FFBF) from the shale gas mining process. However, the toxic effect of FFBF on specific bacteria, fungi, and plankton has not been systematically confirmed in detail. In this paper, a toxic effect evaluation of FFBF was conducted using the representative toxicity test organisms Vibrio fischeri, Daphnia, Aspergillus niger, and S. cerevisiae, indicating that FFBF can significantly decrease the survival rate of these species. The results also showed that there was a significant negative correlation between the concentration of some inorganic toxicity factors and the survival rate when Daphnia was used as the test organism, indicating that the toxicity degree order for these inorganic toxicity factors is Ba²⁺ > Li⁺ > As³⁺ > Cl^- > Cu²⁺ > Rb²⁺ > Ga²⁺ > V²⁺ > Na⁺. In addition, other toxic factors, including polycyclic aromatic hydrocarbons (PAHs), were also determined, and the order of toxic effects with a negative correlation to the Daphnia survival rate was confirmed. These results showed that the biological toxicity of FFBF was caused not only by inorganic toxicity factors such as heavy metals but also by organic compounds such as PAHs. The results not only provide a significant reference value for the systematic assessment of biological toxicity by FFBF, but they also have great significance for developing approaches to appropriate FFBF treatment.

Toxic effects of shale gas fracturing flowback fluid on microbial communities in polluted soil

A large amount of shale gas fracturing flowback fluid (FFBF) from the process of shale gas exploitation causes obvious ecological harm to health of soil and water. However, biological hazard of soil microbial populations by fracturing flowback fluid remains rarely reported. In this study, the microbiological compositions were assessed via analyzing diversity of microbial populations. The results showed significant differences between polluted soil by fracturing flowback fluid and unpolluted soil in different pH and temperature conditions. And then, the microbe-index of biological integrity (M-IBI) was used to evaluate the toxicity of the fracturing flowback fluid based on analysis of microbial integrity. The results showed that polluted soil lacks key microbial species known to be beneficial to soil health, including denitrifying bacteria and cellulose-decomposing bacteria, and 35 °C is a critical value for estimating poor and sub-healthy level of damage to microbial integrity by fracturing flowback fluid. Our results provide a valuable reference for the evaluation of soil damage by fracturing flowback fluid.