Substrate loading rates conducive to nitritation in entrapped cell reactors: performance and microbial community structure

This study aimed to elucidate the boundaries of ammonia and organic loading rates that allow for nitritation in continuous flow phosphorylated-polyvinyl alcohol entrapped cell reactors and to clarify the community structure of microorganisms involving nitrogen transformation in the gel bead matrices. At operating bulk dissolved oxygen concentration of 2 mg/L, nitritation was accomplished when the total ammonia nitrogen (TAN) loading rate was ≥ 0.3 kgN/m³/d. At TAN loading rates of ≤ 0.2 kgN/m³ /d, complete oxidation of ammonia to nitrate took place. Nitritation performance dropped with increased chemical oxygen demand (COD) loading rates indicating limitation of nitritation reactor operation at some COD loading conditions. 16S rRNA gene amplicon sequencing revealed that the uncultured Cytophagaceae bacterium, Arenimonas, Truepera, Nitrosomonas, Comamonas, unclassified Soil Crenarchaeotic Group, and uncultured Chitinophagaceae bacterium were highly abundant taxa in the reactors' gel bead matrices. qPCR with specific primers targeting amoA genes demonstrated the coexistence of ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea, and Comammox in the gel bead matrices. AOB was likely the main functioning ammonia-oxidizing microorganisms due to the amoA gene being of highest abundance in most of the studied conditions. Nitrite-oxidizing microorganisms presented in less relative abundance than ammonia-oxidizing microorganisms, with Nitrobacter rather than Nitrospira dominating in the group. Results obtained from this study are expected to further the application of nitritation entrapped cell reactors to real wastewater treatment processes.

Partial nitrification in entrapped-cell-based reactors with two different cell-to-matrix ratios: performance, microenvironment, and microbial community

In this study, we investigated the effect of different cell-to-matrix ratios (1% and 4%) on the partial nitrification of phosphorylated polyvinyl alcohol-entrapped-cell-based reactors and evaluated the microenvironment, microbial community, and microbial localization within the gel matrices. The results indicated that the reactor with a 1% cell-to-matrix ratio required 184 days of operation to reach partial nitrification that produced anaerobic ammonium oxidation-suitable effluent. In contrast, partial nitrification was achieved from the beginning of the operation of the reactor with the 4% cell-to-matrix ratio. The oxygen-limiting zone (dissolved oxygen = 0.5-1.5 mg L^-1), where nitrite-oxidizing activity has been suggested to be suppressed and ammonia-oxidizing activity was reported to be maintained, occurred at 10-230 µm from the gel matrices surface. In addition, the layer of ammonia-oxidizing bacteria observed in this zone is likely to have played a role in obstructing oxygen penetration into the inner region of the gel matrices. The next-generation sequencing results indicated that members of the family Nitrosomonadaceae accounted for 16.4-20.7% of the relative abundance of bacteria at the family level, while members of the family Bradyrhizobiaceae, to which the genus Nitrobacter belongs, accounted for approximately 10% of the relative abundance of bacteria at the genus level in the gel matrices.

Influence of silver nanoparticles and liberated silver ions on nitrifying sludge: ammonia oxidation inhibitory kinetics and mechanism

Silver nanoparticles (AgNPs) are widely used in commercial products because of their excellent antimicrobial activity. Entrance of AgNPs and its released Ag ions (Ag⁺) into wastewater treatment plants could harm ammonia oxidation (AO) process resulting in environmental problems. This study investigated inhibitory kinetics and mechanism of AO from nitrifying sludge influenced by AgNPs and Ag⁺. The findings demonstrated that AgNPs and Ag⁺ adversely influenced on AO. Silver ions were more toxic to AO than AgNPs, which was indicated by the lower inhibitory constant (K _i ) of 0.29 mg/L compared to that of AgNPs (K _i of 73.5 mg/L). Over the experimental period of 60 h, AgNPs at 1, 10, and 100 mg/L released Ag⁺ in the average concentrations of 0.059, 0.171, and 0.503 mg/L, respectively. Silver nanoparticles of 1-100 mg/L inhibited AO by 45-74%, whereas Ag⁺ of 0.05-0.50 mg/L inhibited AO by 53-94%. This suggested that the AgNP toxicity mainly derived from the liberated Ag⁺. Scanning electron microscopy results revealed that AgNPs attached on microbial cell surfaces, and both AgNPs and Ag⁺ induced cell morphological change from rod shape to shorter rod shape. Transmission electron microscopy showed that AgNPs and Ag⁺ diminished the thickness of the outer layer and reduced the density of internal parts of the exposed microbial cells, which could be the reasons for the morphology change. Live/dead results also confirmed that AgNPs and Ag⁺ damaged membrane integrity of cells in the nitrifying sludge. This study suggested that the primary mechanism for toxicity of AgNPs was the liberation of Ag⁺ and then both of silver species caused cell death.