The evaluation of greenhouse gas emissions from sewage treatment with urbanization: Understanding the opportunities and challenges for climate change mitigation in China's low-carbon pilot city, Shenzhen

Sewage treatment provides a pathway for anthropogenic water purification that can address the growth in domestic sewage volumes due to urbanization and protect the aquatic environment. However, the process can also generate greenhouse gases (GHGs), which are sometimes termed "unrestricted" GHG emissions and are neglected by low carbon policies. A combination of a life cycle analysis (LCA), data envelopment analysis (DEA), and questionnaire survey was used to evaluate sewage treatment related GHG emissions and assess the GHG emission reduction efficiencies during 2005-2020, as well as determine the opinions of environmental managers regarding the threats to climate change mitigation posed by sewage treatment in the low carbon pilot city of Shenzhen, China. There were four main results. (1) GHG emissions from sewage treatment plants (STPs) in Shenzhen increased gradually from 0.22 Mt. CO₂-eq in 2005 to 1.16 Mt. CO₂-eq in 2020 with an emission intensity ranging from 0.41 to 0.58 kg CO₂-eq/m³, mainly due to the indirect emissions from sludge disposal (35-57 %). Longgang administrative district was the hotspot of these GHG emissions during the study period. (2) Reductions in GHG emissions were achieved in most years since 2012 with the greatest efficiency observed in 2020. (3) Beyond the environmental managers' perceptions of the challenges in GHG mitigation, future sewage treatment may create the potential for more substantial GHG emission growth compared to the emissions from energy combustion, due to policy deficiencies, growth in sewage volumes, and the enforcement of stricter effluent quality control. (4) Several opportunities to overcome these barriers were considered including innovational environmental management, planting of constructed wetlands, and the promotion of water-saving behavior. This case study of Shenzhen has valuable implications for the synergistic governance of water pollution and climate change mitigation in megacities in China and elsewhere, enabling a move towards a future carbon-neutral society.

Magnetic nanocomposite microbial extracellular polymeric substances@Fe3O4 supported nZVI for Sb(V) reduction and adsorption under aerobic and anaerobic conditions

The extracellular polymeric substances coating magnetic powders-supported nano zero-valent iron (nZVI@EPS@Fe₃O₄) was synthesized, using reduction and adsorption to treat Sb(V) wastewater. The adsorption performance and mechanism were investigated under aerobic and anaerobic conditions. The adsorption capacity of nZVI@EPS@Fe₃O₄ (79.56 mg/g at pH = 5) was improved compared to that of the original materials (60.74 mg/g). The spectral analysis shows that both nZVI and EPS@Fe₃O₄ in nZVI@EPS@Fe₃O₄ played an important role in reducing Sb(V) to Sb(III) and adsorbing Sb. The reducibility and adsorption capacity of nZVI@EPS@Fe₃O₄ towards Sb(V) remained strong under aerobic condition (62% Sb(III), 79.56 mg/g), although they were slightly weaker than those under anaerobic condition (74% Sb(III), 91.78 mg/g). nZVI@EPS@Fe₃O₄ showed good performance in regeneration experiments. nZVI@EPS@Fe₃O₄ is promising as a cost-effective and highly efficient material for Sb(V)-contaminated water. This study is meaningful in understanding the redox behaviour of nZVI composites in aerobic and anaerobic conditions.

Nitrogen oxide reduction through absorbent solutions containing nitric acid and hydrogen peroxide in hollow fiber membrane modules

Emissions of nitrogen oxides such as NO and NO₂, which are commonly known as NOx, are threats to human existence and cause environmental problems. Mainly, two techniques have been developed to drastically reduce these emissions, which are dry and wet processes. The wet process has several advantages, major identifiable advantages are the adaptability to the flue gas, low operating temperatures and no poisoning and inactivation catalyst. Also, a mixture of hydrogen peroxide and nitric acid are used as absorbents solution for NOx reduction in the wet process. The advantages of using this mixture include the ability to reduce the negative effect of NOx and does not contaminate the scrubbing solution. In addition, nitric acid has an economical advantage in the process considering the fact that it is produced in the process. Finally, it can be conducted at ambient temperature. This study furthermore used a mixture of hydrogen peroxide and nitric acid solutions as an absorbent to reduce NOx in hollow fiber membrane modules. The hydrogen peroxide oxidized HNO₂ to nitric acid, while enhances the oxidation through an autocatalytic reaction. The effects of the feed gas flow rate, hydrogen peroxide concentrations and number of fibers on the NOx reduction, absorbed NOx and flux were varied to study. The experimental results showed that the increase in the feed gas flow rate from 100 to 200 mL/min decreased NOx reduction from about 98 to 94% but increased the absorbed NOx and flux from about 0.13 to 0.255 mmol/h and 0.85–1.63 mmol/m².h, respectively The increase in proportion of NO_x in the feed gas effect was dominant than the increase in absorbed NOx. An increase in hydrogen peroxide concentration from 0.5 to 10 wt.% in the absorbent solutions increased NOx reduction, absorbed NOx and flux from about 94 to 98%, 0.257–0.267 mmol/h and 1.09–1.13 mmol/m².h, respectively. Additionally, the H₂O₂ plays an important role in enhancing HNO₂ oxidation to HNO₃. Furthermore, an increase in the number of fibers from 50 to 150 in the membrane module increased NOx reduction and absorbed NOx from 86 to 97% and 0.23–0.27 mmol/h. Flux decreased from 2.98 to 1.13 mmol/m².h due to increment in the gas-liquid contact surface area.

Two selenium tolerant Lysinibacillus sp. strains are capable of reducing selenite to elemental Se efficiently under aerobic conditions

Microbes play important roles in the transport and transformation of selenium (Se) in the environment, thereby influencing plant resistance to Se and Se accumulation in plant. The objectives are to characterize the bacteria with high Se tolerance and reduction capacity and explore the significance of microbial origins on their Se tolerance, reduction rate and efficiency. Two bacterial strains were isolated from a naturally occurred Se-rich soil at tea orchard in southern Anhui Province, China. The reduction kinetics of selenite was investigated and the reducing product was characterized using scanning electron microscopy and transmission electron microscopy-energy dispersive spectroscopy. The bacteria were identified as Lysinibacillus xylanilyticus and Lysinibacillus macrolides, respectively, using morphological, physiological and molecular methods. The results showed that the minimal inhibitory concentrations (MICs) of selenite for L. xylanilyticus and L. macrolides were 120 and 220 mmol/L, respectively, while MICs of selenate for L. xylanilyticus and L. macrolides were 800 and 700 mmol/L, respectively. Both strains aerobically reduced selenite with an initial concentration of 1.0 mmol/L to elemental Se nanoparticles (SeNPs) completely within 36 hr. Biogenic SeNPs were observed both inside and outside the cells suggesting either an intra- or extracellular reduction process. Our study implied that the microbes from Se-rich environments were more tolerant to Se and generally quicker and more efficient than those from Se-free habitats in the reduction of Se oxyanions. The bacterial strains with high Se reduction capacity and the biological synthesized SeNPs would have potential applications in agriculture, food, environment and medicine.