Catalytic Wet Air Oxidation of Sewage Sludge: A Review

The self-designed reactor to achieve efficient degradation of polyvinyl alcohol under high-pressure and high-temperature conditions

A huge amount of polyvinyl alcohol (PVA) fabric is abandoned from nuclear power plants every year, the traditional treatment process will occupy land resources and pollute the environment; therefore, a lot of research has been carried out on the chemical treatment of PVA fabric. Herein, the performance of degradation of polyvinyl alcohol under high-pressure and high-temperature conditions is investigated. The effects of the initial pH value, reaction temperature, molar ratio of H2O2/Fe2+, and H2O2 dosage on PVA degradation were evaluated. In the tested ranges in this work, the degradation of PVA fabric via high-pressure and high-temperature method was optimum at the initial pH value of 4, reaction temperature of 300℃, molar ratio of H2O2/Fe2+ as 10, and H2O2 dosage of 13 g/L. The PVA removal rate and TOC removal rate were 99.99% and 97.36%, respectively. Meanwhile, the high-pressure and high-temperature methods also had a great effect on the removal of Rhodamine-B and Reactive Red X-3B, the removal rates of Rhodamine-B and Reactive Red X-3B were 99.83% and 99.76%, respectively. The reaction mechanism of high-pressure and high-temperature methods was also discussed in this study.

Effects of reaction conditions on the wet oxidation of excess sludge from the caprolactam wastewater treatment process

In this study, wet oxidation of excess sludge from the caprolactam wastewater treatment process was investigated. The effects of reaction conditions, including sludge concentration, the amount of sludge, reaction temperature and time and the oxygen supply amount, were discussed. The highest removal rates of chemical oxygen demand (COD) and volatile suspended solid (VSS) that can be attained at 78.6 and 89.3% were acquired separately under 260 °C for 60 min with an initial oxygen gas pressure of 1.3 MPa. The sludge was hydrolyzed and oxidized under hydrothermal conditions, producing small molecule organic acids, such as acetic, formic and oxalic acids, as the main products, which could be used as carbon sources for wastewater treatment. These results indicated that wet oxidation provides a favourable and feasible alternative method for the treatment of excess sludge from the coal chemical industry.

Revisiting China's domestic greenhouse gas emission from wastewater treatment: A quantitative process life-cycle assessment

The wastewater treatment industry could alleviate water pollution but consume a large amount of energy and resources. China has over 5000 centralized domestic wastewater treatment plants and produces an unignorable amount of greenhouse gases (GHG). By considering the wastewater treatment, wastewater discharge, and sludge disposal processes, and employing the modified process-based quantification method, this study quantifies wastewater treatment's on-site and off-site GHG emissions across China. Results showed that the total GHG emission was 67.07 Mt CO₂-eq in 2017, with approximately 57% of on-site emissions. The top seven cosmopolis and metropolis (top 1%) emitted nearly 20% of the total GHG emission, while their emission intensity was relatively low due to the huge population. This means that a high urbanization rate may be a feasible way to mitigate GHG emissions in the wastewater treatment industry in the future. Furthermore, GHG reduction strategies can also focus on process optimization and improvement at WWTPs as well as the nationwide promotion of onsite thermal conversion technologies for sludge management.

Feasibility of using different hydrothermal processes for sewage sludge management in China

Due to its tremendous volume and severe environmental concern, sewage sludge (SS) management and treatment are significant in China. The recent prohibition (June 2021) of reusing SS as organic fertilizers makes it urgent to develop alternative processes. However, there is currently little research analyzing the applicability of using HP for sewage SS treatment in China. The significant difference in SS composition and the much less land supply in urban areas might invalidate most previous localized suggestions. In this paper, the development of emerging hydrothermal processes (HPs) for SS treatment will be reviewed, focusing on their decomposition mechanisms and the benefits of HPs compared with current SS treatment technologies. The SS volume, composition, and regulatory regime in China will also be evaluated. Those efforts could address the potential SS treatment capacity shortage and provide an opportunity to recover nutrients, organics and energy embedded in SS. The results show that HPs' high investment cost is mainly limited by the process scale, while their operating costs are comparable to incineration. Minimizing equipment erosion, ensuring process safety, and designing a more efficient heat recovery system are recommended for the future commercialization of HPs in China.

Performance of high temperature phase-stable high entropy oxide (MgCuMnCoFe)O x in catalytic wet air oxidation of chloroquine phosphate

With the continuous spread of COVID-19, the water pollution problems caused by the abuse of chloroquine phosphate (CQP) as an antiviral drug have attracted wide attention. The cubic Fm-3m spinel high entropy oxide (HEO)-(MgCuMnCoFe)O _x was prepared by coprecipitation method as the catalytic wet air oxidation (CWAO) catalyst to treat CQP simulated wastewater. Through electron spin resonance (ESR) analysis, HEO will stimulate the production of superoxide radical (·O₂ ^-) and hydroxyl radical (·OH) in the wet air oxidation (WAO) process, which accelerates the degradation and mineralization of CQP. Through response surface method (RSM) optimization, the optimal degradation conditions of CQP in CWAO were proposed: initial oxygen pressure of 15 bar, catalyst dosage of 1.4 g/L and temperature of 230 °C. The advantages of HEO in CWAO were analyzed by principal component analysis (PCA). The degradation mechanism of CQP in CWAO by (MgCuMnCoFe)O _x were explored. This work provides a new idea for the rapid development of HEO in the field of environmental catalysis.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10853-022-07271-z.