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 ) of 0.29 mg/L compared to that of AgNPs (K 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.

No abstract

Nitritation, the oxidation of ammonia to nitrite without subsequent oxidation to nitrate, is a starting step for nitrite-based nitrogen removal approaches. This process can be induced by maintaining specific operating conditions that facilitate ammonia oxidation but deteriorate nitrite oxidation. In recent years, a number of publications have demonstrated the ability of cell immobilization to maintain nitritation and an oxygen-limiting strategy was suggested to be a key to the success of the approach. However, several aspects related to the success and failure of such systems remains unclear and requires further in-depth clarification. This review provides current information on the utilization of cell immobilization in nitritation reactors. Common operating strategies that promote nitritation by controlling environmental conditions are summarized in the first part of the review. The application of cell immobilization, including cell attachment, cell granulation, and cell entrapment systems, as well as microenvironments, and microbial distributions within cell immobilization matrices, are elaborated in the middle part of the review. Problems encountered in the operation of nitritation reactors using cell immobilization are discussed as opportunities for further research at the end of the review.

No abstract

A phosphorylated-polyvinyl alcohol (PPVA) entrapped cell-based reactor was employed to promote partial nitrification for ammonia-rich wastewater treatment. High partial nitrification (66% of nitrite accumulation in average) was achieved along the 165 days of operation indicating that the majority of nitrite-oxidizing bacteria (NOB) activity was suppressed probably as a result of low oxygen environment created within the PPVA gel matrix. However, some portion of nitrate (5-17.6% of the influent ammonia) always appeared in the reactor throughout the operation period. Next-generation sequencing and clone library techniques revealed that NOB with different substrate affinities including Nitrobacter, Nitrospira lineage I and II existed within the gel matrix. The finding speculates that substrate gradient-like microenvironment within the gel matrix probably serves the different physiological groups of NOB to maintain their cells and activities in the reactor. Therefore, instead of using low oxygen environment in gel matrix as a sole control strategy, an additional strategy like promoting free ammonia inhibition in reactor is also needed to affirm the stability of long-term partial nitrification.