The sediment microbial community plays a key role in the cycling of organic matter in marine ecosystems. The characteristics of the sediment microbial community are significantly related to changes in the environment. In this study, we analyzed the diversity and distribution of microbial communities in sediments from different geographical regions (the Liao River estuary, Yellow River estuary, hypoxic zone, and offshore zone) of the Bohai Sea using high-throughput sequencing. The results showed that the community richness and diversity (especially the unique diversity) of the Liao River estuary were larger than those of the Yellow River estuary, hypoxic zone, and offshore zone. The phylum Proteobacteria dominated in the Liao River estuary (46.26%), hypoxic zone (76.19%), and offshore zone (69.79%), while the dominant phylum in the Yellow River estuary was the Bacteroidetes phylum. The genus Gillisia was the dominant genus in both the Liao River estuary and Yellow River estuary. The offshore zone and hypoxic zone shared the same dominant Photobacterium genus. The results of Pearson correlation and redundancy analysis showed that environmental parameters such as nitrite, silicate, nitrate, phosphate, ammonia, dissolved oxygen, pH, and salinity interfered significantly with the structure and diversity of the microbial community. The results of this study will provide support for future research on the interaction mechanism of pollutants and microorganisms in the estuaries and a typical hypoxic zone of the Bohai Sea.

In this study, magnetic CuFe₂O₄, MgFe₂O₄, and ZnFe₂O₄ nanosphere photocatalysts were prepared by the sol–gel method at 300 °C, 400 °C, and 500 °C, respectively (named as CF300, CF400, CF500, MF300, MF400, MF500, ZF300, ZF400, and ZF500). The characterization by X-ray diffraction (XRD) revealed that the optimal calcination temperature was 400 °C. Then, CF400, MF400, and ZF400 were used to treat high-concentration ammonia nitrogen wastewater (HCAW, 1000 mg/L) at different pH levels. The result showed that the optimal pH for CF400, MF400 and ZF400 to degrade HCAW was 9.0, and CF400 required a shorter illumination time (80 min) than MF400 and ZF400 (120 min) to completely remove ammonia nitrogen from HCAW. However, CF400 was unstable and decomposed, and a blue substance was observed during the magnetic recovery experiment. The recovery rate of ZF400 (66.7%) was higher than MF400 (53.2%) with no decomposition phenomenon, and the ammonia nitrogen removal rate of ZF400 remained above 90% after five cycles. Additionally, the ammonia nitrogen removal rate of ZF400 could reach 80.2% when the ammonia nitrogen concentration was as high as 5000 mg/L. Therefore, compared with CF400 and MF400, ZF400 was more suitable for treating HCAW.

As an environment-friendly wastewater treatment process, the anaerobic fermentation process has been widely used for the pretreatment of high-strength wastewater. However, it is rarely applied to treat low-strength wastewater due to low methane recovery. In this study, anaerobic fermentation treating low-strength mariculture wastewater was studied in an anaerobic sequencing batch reactor (ASBR) with a COD removal rate of 75%. Anaerobic fermentation was successfully controlled at the acidification stage by increasing COD loading. As the greenhouse gas emission decreased, the residual organics were enough for biological nutrients’ removal. Fluorescence in situ hybridization results showed that the dominant bacteria in the ASBR were acidogenic bacteria and methanogens, accounting for 39.7% and 46.5% of the total bacteria, respectively. Through the calculation processing of the experimental data, the order of the anaerobic fermentation reaction was a second-order reaction. The kinetic parameters of low-strength organic wastewater treatment were determined by using the Grau second-order substrate removal model, Stover–Kincannon model, Monod model and Haldane model. The maximum rate removal constant U_max, sludge yield coefficient Y and inhibition constant K_i were 1.157 g/(L·d), 0.153 mgVSS/mgCOD and 670 mg/L, respectively. It provided data support for the practical application of the anaerobic fermentation treating low-strength wastewater.

In this study, a sulfur-based autotrophic denitrifying filter (SADF) was developed for the purpose of removing nitrate from simulated recirculating mariculture wastewater. Results showed that over 90% of the nitrate could be effectively eliminated by utilizing thiosulfate as the electron donor, with a molar ratio of thiosulfate-S to nitrate molar ratio of 2:1 or greater. Additional batch tests confirmed that thiosulfate was a suitable sulfur source for nitrate removal even without prior accumulation of the biomass to nitrite. Excess thiosulfate had a minor impact on N-removal efficiency, so an external sulfur source was not required for nitrate removal, however, it could still help to reduce nitrate accumulation and water replacement to some extent. High-throughput sequencing results illustrated that Thiomicrospira and Thioalkalivibrio were the dominant autotrophic denitrifying genera in the SADF, while Thiomicrospira was more significantly affected in the case of insufficient sulfur sources. As the issue of nitrate accumulation in the mariculture recirculating system has been resolved, only a small amount of water needs to be added to the system daily. Therefore, the thiosulfate-based SADF process has the potential to be implemented for nitrate removal in mariculture systems, which could present a promising sustainable solution to the nitrate pollution issue.