The objective of the study was to characterize the effect of shortening hydraulic retention time in a bench-scale activated sludge wastewater treatment system on 1) ammonia oxidation, 2) viral-particle attachment, and 3) the bacterial, viral, and fungal communities. The goal of wastewater treatment has evolved over time and now includes the desire to create a circular economy. By harvesting nutrients like ammonia, local governments and utilities can sell recovered products as raw materials to manufacturers and recuperate operating costs, but this requires changes to the activated sludge process, specifically to the hydraulic retention time (HRT). It is important to consider how these process changes could affect the quality of treated effluent. Pathogens like viruses are of concern in wastewater. The association of bacteriophages as a virus indicator, specifically phage that infects E. coli, with particles in these systems at different HRTs is largely unknown. Particle association affects virus removal by treatment. In this study, a bench-scale activated sludge reactor was set up in triplicate and seeded with sludge from the East Lansing wastewater treatment plant (ELWWTP). The experimental data show that recoverable ammonia in reactor effluent rose from 0.05 to 7 to 8.9 mg/L in the 24-, 16-, and 8-hour HRTs, respectively. In the influent sewage, 62.1% of somatic coliphage and 83.3% of f-specific coliphage were associated to particles of 0.45 μm or larger. Association to particles in reactor effluent decreased with HRT. The percent of somatic coliphage that was associated was 66.6, 51.4, and 47.2% of with particles of 0.45 μm or larger at the 24-, 16-, and 8-hour HRTs, respectively. The percent of f-specific coliphage was associated with particles of 0.45 μm or larger was 88.2, 66.2, and 74.2% of at the 24-, 16-, and 8-hour HRTs, respectively. For f-specific coliphage, the difference in association profiles were most evident between the 100 and 3 μm filter pore sizes. The main finding of this work was that as HRT decreased, particle attachment decreased for both somatic and f-specific coliphage. There were differences in particle attachment profiles between the two coliphage. The functional gene count of ammonia oxidation related genes fell as HRT decreased. Nitrosomonas was the dominant ammonia oxidizing bacteria at all HRTs in this system, while Nitrospira was the main nitrate oxidizing bacteria. The dominant genera of fungi did not change across HRT, and only one known fungal nitrifier was found in sludge samples at all HRTs, Aspergillus flavus. Its percentage relative abundance did not change with HRT. Similarly, the dominant viral families remained stable across HRTs. However, the percent relative abundance of Siphoviridae increased from 30 to 32 to 38% at the 24-, 16-, and 8-hour HRTs, respectively. Gokushovirinae emerged as a dominant family in the 8-hour HRT samples and was not seen in the 24- or 16-hour HRT. Future work will use R to find correlations between the decrease in nitrifying bacterial populations and fluctuations in the smaller fungal populations in the hopes of identifying possible symbiotic relationships.
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