Impact of Metal Oxide Nanoparticles on Wastewater Nitrification

The use of metal oxide nanoparticles in electronics, textiles, cosmetics and food packaging industry has grown exponentially in recent years, which will inevitably result in their release into wastewater streams in turn affecting the important biological processes in wastewater treatment plants. Among these processes, nitrification plays a critical role in nitrogen removal during wastewater treatment, however, it is sensitive to a wide range of inhibitory substances including metal oxide nanoparticles. Therefore, it is indispensable to systematically asses the effects of metal oxide nanoparticles on nitrification in biological wastewater treatment systems.

In this dissertation, we have discussed the present scenario of metal oxide nanoparticles and their impact on biological wastewater treatment processes, specifically nitrogen removal through nitrification. We also summarized the various methods used to measure nitrification inhibition by metal oxide nanoparticles and highlight corresponding results obtained using those methods. For specific studies, we investigated the impact of zinc oxide nanoparticles (ZnO NPs) and manganese (III) oxide nanoparticles (Mn2O3 NPs) on the physiological and genetic activities of nitrifying bacteria under different conditions.

In the first study, the effects of ZnO NPs were examined for nitrifying bacterial enrichments by measuring substrate (ammonia) specific oxygen uptake rates (sOUR) in conjunction with the transcript level of functional genes involved in nitrification quantified by reverse transcriptase – quantitative polymerase chain reaction (RT-qPCR). Samples from nitrifying bioreactor were exposed in batch vessels to ZnO NPs (1, 5 and 10 mg/L) for either 3 or 6 h. There was considerable increase in sOUR-based nitrification inhibition with increasing dosages of ZnO NPs. At 10 mg/L ZnO NPs, the inhibition was about 35% and 50% for 3 and 6 h exposure, respectively. As the ZnO NPs dosage increased, the transcript levels of amoA, Hao and nirK for 6 h exposure samples were decreased which corresponded well with sOUR data.

In the second study, the nitrification inhibition of nitrifying bacteria upon exposure to Mn2O3 NPs were also investigated. Samples from the nitrifying bioreactor were exposed in batch vessels to Mn2O3 NPs (1, 5 and 10 mg/L) for 1 and 3 h under two cases (no additional aeration-low DO and 0.25 L/minute aeration- high DO). The results showed that there was an increase in nitrification inhibition as determined by sOUR with increasing dosages of Mn2O3 NPs for both cases- low DO and high DO. Based on the RT‐qPCR data, there was notable reduction in the transcript levels of amoA, Hao and nirK for increasing Mn2O3 NPs dosage for the case of high DO and 3 h exposure, which corresponded well with sOUR.

The correspondence between the relative expression of functional genes and sOUR shown in these studies demonstrates the effectiveness of using transcriptional responses in conjunction with physiological activity for better understanding the effect of inhibitory compounds on nitrification activity for different environmental circumstances.