With the increasing utilization of nanomaterials, zinc oxide nanoparticles (ZnO NPs) have been reported to induce adverse effects on human health and aquatic organisms. However, the potential impacts of ZnO NPs on wastewater nitrogen and phosphorus removal with an activated sludge process are unknown. In this paper, short-term exposure experiments were conducted to determine whether ZnO NPs caused adverse impacts on biological nitrogen and phosphorus removal in the unacclimated anaerobic-low dissolved oxygen sequencing batch reactor. Compared with the absence of ZnO NPs, the presence of 10 and 50 mg/L of ZnO NPs decreased total nitrogen removal efficiencies from 81.5% to 75.6% and 70.8%, respectively. The corresponding effluent phosphorus concentrations increased from nondetectable to 10.3 and 16.5 mg/L, respectively, which were higher than the influent phosphorus (9.8 mg/L), suggesting that higher concentration of ZnO NPs induced the loss of normal phosphorus removal. It was found that the inhibition of nitrogen and phosphorus removal induced by higher concentrations of ZnO NPs was due to the release of zinc ions from ZnO NPs dissolution and increase of reactive oxygen species (ROS) production, which caused inhibitory effect on polyphosphate-accumulating organisms and decreased nitrate reductase, exopolyphosphatase, and polyphosphate kinase activities.

Volatile fatty acids (VFAs), the carbon source of biological nutrients removal, can be produced by waste activated sludge (WAS) anaerobic fermentation. However, because of high protein content and low carbon to nitrogen mass ratio (C/N) of WAS, the production of VFAs, especially propionic acid, a more preferred VFA than acetic acid for enhanced biological phosphorus removal (EBPR), is limited. After the addition of carbohydrate (rice was used as the model matter) to the WAS anaerobic fermentation system to balance the C/N ratio, the effect of pH on WAS protein conversion and VFAs production was investigated in this paper. Experimental results showed that the addition of carbohydrate matter caused a remarkable enhancement of WAS protein conversion and protease activity, and an apparent synergistic effect between WAS and carbohydrate matter was observed. The study of pH effect revealed that pH influenced not only the total VFAs production but the percentage of individual VFA. The maximal VFAs production (520.1 mg COD per gram of volatile suspended solids (VSS)) occurred at pH 8.0 and a fermentation time of 8 d, which was more than three times that at uncontrolled pH (150.2 mg COD/g VSS). The analysis of the composition of VFAs showed that propionic acid ranked first at pH 6.0-9.0 (around 50%) whereas acetic acid was the dominant product at other pHs investigated. Thus, the suitable conditions for propionic acid-enriched VFAs production were pH 8.0 and a time of 8 d. Further investigation showed that as there was more fermentation substrate consumption with lower biogas generation at pH 8.0, improved VFAs production was observed. Also, the key enzymes relevant to propionic acid formation exhibited the highest activities at pH 8.0, which resulted in the greatest propionic acid content in the fermentative VFAs. The 16S rRNA gene clone library demonstrated that Clostridia, beta-Proteobacteria, and Bacteroidetes were the dominant microbial community when the current anaerobic fermentation system was operated at pH 8.0. With the fermentative VFAs as the additional carbon source of municipal wastewater, the EBPR performance was significantly increased.

The expanding use of titanium dioxide nanoparticles (TiO(2) NPs) in a wide range of fields raises concerns about their potential environmental impacts. However, investigations of the potential effects of TiO(2) NPs on biological nitrogen and phosphorus removal and bacterial community in activated sludge are sparse. This study evaluated the influences of TiO(2) NPs on biological nutrient removal in the anaerobic-low dissolved oxygen (0.15-0.50 mg/L) sequencing batch reactor. It was found that 1 and 50 mg/L TiO(2) NPs had no acute effects on wastewater nitrogen and phosphorus removal after short-term exposure (1 day). However, 50 mg/L TiO(2) NPs (higher than its environmentally relevant concentration) was observed to significantly decrease total nitrogen (TN) removal efficiency from 80.3% to 24.4% after long-term exposure (70 days), whereas biological phosphorus removal was unaffected. Denaturing gradient gel electrophoresis profiles showed that 50 mg/L TiO(2) NPs obviously reduced the diversity of microbial community in activated sludge, and fluorescence in situ hybridization analysis indicated that the abundance of nitrifying bacteria, especially ammonia-oxidizing bacteria, was highly decreased after long-term exposure to 50 mg/L TiO(2) NPs, which was the main reason for the serious deterioration of ammonia oxidation. Further study revealed that 50 mg/L TiO(2) NPs inhibited the activities of ammonia monooxygenase and nitrite oxidoreductase after long-term exposure, but had no significant impacts on the activities of exopolyphosphatase and polyphosphate kinase, and the transformations of intracellular polyhydroxyalkanoates and glycogen, which were consistent with the observed influences of TiO(2) NPs on biological nitrogen and phosphorus removal.