As the population in water stressed areas increases, it is critical that wastewater treatment plants (WWTPs) continue to replenish depleted water supplies, and serve as an alternative water source. WWTPs depend on microorganisms in activated sludge to remove pollutants from wastewater and therefore an understanding of how these microorganisms are affected by various conditions and pollutants is needed. Also, as consumer products and industrial processes evolve, so do the pollutants they discharge to wastewater. In order to keep pace with these changes, understanding the effects of emerging contaminants to WWTP processes is essential. The research herein assesses microbial community dynamics of the response of nitrifying microorganisms in activated sludge to variation in ammonia concentration and evaluates the impact of engineered nanoparticles on activated sludge microbial communities and other emerging pollutants, such as antibiotic resistance genes and disinfection by-products.
In order to assess microbial community dynamics of the response of nitrifying microorganisms to removal of ammonia in the feed, nitrifying activated sludge reactors were operated at various relevant temperatures and the nitrifying microbial community was characterized using activity assays and bio-molecular techniques. We found that Nitrospira spp. were the dominant nitrifying microorganisms, exhibiting stable relative abundance across multiple trials and over a range of temperatures. These results indicate the possibility of comammox bacteria in the system and highlight the complexity of nitrifying microbial communities in activated sludge relative to past understanding.
Both microbial and chemical impacts of engineered nanoparticles on WWTP processes were also investigated. Metagenomic analysis of DNA extracted from activated sludge sequencing batch reactors dosed with gold nanoparticles with varied surface coating and morphology indicated that nanoparticle morphology impacted the microbial community and antibiotic resistance gene content more than surface coating. However, nanoparticle fate was controlled by surface coating more than morphology. Disinfection by-product formation in the presence of nanoparticles during WWTP disinfection was assessed using silver, titanium dioxide, ceria, and zero valent iron nanoparticles. Silver nanoparticles were found to enhance trihalomethane formation, which was attributed to the citrate coating of the nanoparticles. These studies both raise concern over the relationship between engineered nanoparticles and other emerging concerns in WWTPs, and take a step towards informing nanoparticle design in a manner that limits their associated environmental impact. / Ph. D. / Wastewater treatment plants (WWTPs) are crucial to protect human and environmental health by removing pathogens and pollutants in sewage before they are released into aquatic environments used for recreation and drinking water. As populations living in water stressed areas continues to rise, the continued recovery of clean water from WWTPs is essential to both replenish water supplies and serve as an alternative water source. WWTPs depend on a complex mixture of microorganisms called activated sludge to remove pollutants from water. In order for WWTPs to continue discharging acceptable water in the future, a greater understanding of how these important microorganisms respond to environmental changes such as temperature and sewage content is needed. Sewage flowing into WWTPs is also evolving as advances in technology and chemicals used in consumer products and industrial settings discharge new pollutants into waste streams. Therefore, an understanding for how these new pollutants affect WWTP processes is also needed. In this dissertation, two challenges facing WWTPs were evaluated: 1) how bacteria responsible for nitrogen removal in WWTPs respond to the stress of starvation, and 2) how engineered nanoparticles in sewage impact the microorganisms in activated sludge and disinfection in WWTPs.
Nitrogen removal is important because it can cause algal blooms when treated wastewater is discharged and because some forms, like ammonia, are toxic. The first step of nitrogen removal in WWTPs involves forming nitrate from ammonia, performed by nitrifying bacteria and archaea. This nitrate is then transformed into nitrogen gas by other microorganisms and therefore removed from the wastewater. How nitrifying microorganisms responded to decreased ammonia concentrations in the feed was determined using nucleic acid based techniques. Traditionally it is thought that in wastewater treatment, ammonia is oxidized to nitrite by one group of microorganisms, and nitrite is then oxidized to nitrate by separate microorganisms. However, in this study only microorganisms from the latter group were detected, which demonstrates the possibility of microorganisms capable of both ammonia and nitrite oxidation present in our system (as has been found in other environments).
Also, the increased use of engineered nanoparticles in consumer products and industrial processes has led to their increased presence in wastewater. Nanoparticle are particles that are 1-100 nm in one dimension and have unique properties compared to larger forms of the material they are made of. These particles are sometimes utilized for their antimicrobial activity and therefore may impact the microorganisms used in WWTPs. Using activated sludge bioreactors dosed with gold nanoparticles with various morphologies and surface coatings, implications of these nanoparticle properties on activated sludge microorganisms was assessed. We found that nanoparticle morphology was more important than surface coating in affecting the activated sludge microbial communities. However, gold nanoparticle fate in the bioreactors was determined more by surface coating than morphology. These results and further research on how nanoparticle properties affect WWTPs and the environment may inform nanoparticle design that can be tailored to decrease environmental impact.
The impact of nanoparticles on WWTP disinfection processes was also evaluated. WWTPs often use chlorine and/or ultraviolet (UV) disinfection in order to inactivate pathogens in wastewater. Chemical reactions between organics in the wastewater and chlorine produce disinfection by-products which can be toxic. Nanoparticles are used to enhance desired chemical reactions in industry, and therefore may enhance the undesired reactions of disinfection by-product formation in WWTPs. Here several types of nanoparticle (silver, titanium dioxide, ceria, and zero valent iron) were dosed to WWTP effluents and then subjected to chlorine and/or UV disinfection, then this was analyzed for trihalomethanes (a common type of disinfection by-product). It was found that the citrate coating on silver nanoparticles led to increased trihalomethane formation. More research is needed to determine the mechanisms involved with this phenomenon, and to determine other nanoparticle-coating combinations that may have similar effect.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/85257 |
| Date | 13 April 2017 |
| Creators | Metch, Jacob W. |
| Contributors | Civil and Environmental Engineering, Pruden, Amy, Vikesland, Peter J., Badgley, Brian D., Edwards, Marc A., Novak, John T. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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