The Occoquan Reservoir is a public water supply in northern Virginia. The Occoquan Watershed has developed over the years from rural land uses to metropolitan suburbs within easy commuting distance from Washington, DC. Due to this urbanization, the Occoquan Reservoir is especially vulnerable to hypereutrophication, which results in problems such as algal blooms (including cyanobacteria), periodic fish kills, and taste and odor problems. In the 1970's, a new management plan for the Occoquan Reservoir called for the construction of the Upper Occoquan Sewage Authority (UOSA), an advanced wastewater treatment plant that would take extraordinary measures for highly reliable and highly efficient removal of particulates, organics, nutrients, and pathogens. Eliminating most of the water quality problems associated with point source discharges in the watershed, this state-of-the-art treatment is the foundation for the successful indirect surface water reuse system in the Occoquan Reservoir today. A limnological analysis of thirty years of water quality monitoring data from the reservoir and its two primary tributaries shows that the majority of the nutrient and sediment load to the reservoir comes from nonpoint sources, which are closely tied to hydrometeorologic conditions. Reservoir water quality trends are very similar to trends in stream water quality, and the tributary in the most urbanized part of the watershed, Bull Run, has been identified as the main contributor of sediment and nutrients to the reservoir. Despite significant achievements in maintaining the reservoir as a source of high quality drinking water, the reservoir remains a phosphorus-limited eutrophic waterbody. / Master of Science

Reverse osmosis (RO) is widely used for water reclamation and is one of the most feasible technologies for addressing water scarcity around the world. RO membrane fabrication procedures are continually being optimized and modified to enhance the treatment performance and efficacy of the RO process. A review of the existing literature published on membrane fabrication revealed that a detailed and reproducible methodology consistent among prior studies was not available. Therefore, the primary objective of this study was to utilize techniques from prior research to develop a reliable lab-scale membrane fabrication process for studying the potable reuse applications of TFC RO membranes. Phase inversion was used to create a polysulfone (PSF) support layer on a non-woven fabric sheet. Then, the process of interfacial polymerization (IP) between amine and acyl chloride monomers was utilized to form a highly selective and ultrathin polyamide (PA) layer on the PSF support surface. The resulting membrane composition and performance was dependent on a wide range of parameters during the fabrication process. The optimal support materials, reactant types and concentration, and reaction conditions were determined through trial and error. The best performing membranes utilized N-methyl-2-pyrrolidone (NMP) as the solvent, Novatexx-2471 non-woven fabric for mechanical support, and 15 wt% PSF concentration for phase inversion. The optimal immersion duration was five minutes for the aqueous amine monomer solution during the IP process. The flux for membrane triplicates was 20.2  3.6 liters per square meter per hour (LMH) while the salt rejection was 96.8  2.0%. The relatively low standard deviation for flux and salt rejection indicates that the fabrication method developed herein is consistent. A commercial Dow Filmtec BW30 flat sheet PA-TFC RO membrane was tested for comparison and exhibited a flux of 44.9 LMH and a salt rejection of 98.5%. Thus, the membranes developed in this study achieved salt rejection on par with commercial membranes but exhibited a flux that was significantly lower. Furthermore, this study investigated modifications to the traditional TFC membrane using engineered silica nanomaterials with the goal of enhancing the membrane flux while maintaining high salt rejection. Two types of nonporous silicon dioxide nanoparticles (SDNPs), non-functionalized and amine functionalized, were dispersed in the aqueous and organic IP solutions. Ultrasonication of the non-functionalized SDNPs in the aqueous solution was observed to produce the most stable dispersion. Compared to the unmodified TFC membranes, the average flux of the SDNP-modified (TFC-NP) RO membrane triplicates was higher at 25.4  2.0 LMH with 0.1% (w/v) SDNPs incorporated in the PA layer. The salt rejection was lowered to 92.3  0.1% for the TFC-NP membranes. In addition, the membranes fabricated in this study were characterized using scanning electron microscopy (SEM), Fourier Transport Infrared Spectroscopy (FTIR), atomic force microscopy (AFM), and goniometry measurements. SEM images showed that the TFC-NP membranes contained larger spaces between ridges and valleys of the PA pore structure. FTIR confirmed the PA layer formation on the membranes fabricated herein but a spectral peak from the SDNPs was not observed for the TFC-NP membranes. AFM measurements indicated an increase in surface roughness of the modified membranes, likely because of the incorporation of SDNPs. The surface of TFC-NP membranes was found to be more hydrophilic than the unmodified TFC membranes based on contact angle measurements. Further optimization of the fabrication method developed herein is warranted before pursuing additional RO research topics, such as the disinfection byproduct precursor removal of TFC membranes.

Fresh water scarcity is an alarming issue for communities across the globe. The development of water recycling and reuse technologies has become crucial in expanding the limited water resources. Reverse osmosis (RO) is among the key processes that can treat wastewater to meet potable water reuse standards. Despite the advancements in RO membrane technologies, many challenges persist regarding the operation and maintenance of RO membranes, such as membrane fouling. Extensive research investigations have focused on developing RO membrane modifications to combat the decreased performance due to fouling. Polyethylenimine (PEI) is a promising polymer used for enhancing the anti-fouling properties of thin film composite (TFC) RO membranes. PEI, a positively charged polymer with high charge density, is commonly grafted on TFC RO membrane surfaces to produce smoother, more hydrophilic membranes to minimize fouling. However, little research is available on the optimal PEI placement within the composite RO membrane layers for enhancing antifouling properties. The current study aimed to investigate whether alternative positions within the membrane layers could yield better anti-fouling performance compared to incorporation PEI on the membrane surface. Unmodified (i.e., control) and PEI-modified TFC RO membranes were fabricated in the laboratory. The PEI-modified membranes were produced in two variations with regards to the position of PEI in the composite membrane layer. The first variation, named PEI-1, involved immersing the polysulfone (Psf) support layer of the membrane in an aqueous PEI solution, before the active polyamide (PA) layer was formed. The second variation, named PEI-2, consisted of immersing the fully formed TFC RO membrane in an aqueous PEI solution to incorporate PEI on the surface of the active PA layer. The PEI used in the study for membrane modification had branched configuration with molecular weight of 1200 g/mole. The laboratory-scale TFC RO membranes produced herein were characterized and tested for water flux, salt rejection, and fouling behavior. The water flux and salt rejection, commonly referred to as permselectivity, of all the membranes produced were evaluated in a crossflow filtration unit. On the other hand, the fouling tests were conducted in a dead-end membrane filtration unit because of operational limitations of the crossflow unit. The PEI-1 membrane produced a water flux of 18.7 LMH (L/m2hr) and a stable salt rejection of 82.1%. The PEI-2 membrane resulted in a water flux of 22.4 LMH and a salt rejection of 85.2%. These results indicate that incorporating PEI on the membrane PA active surface layer achieved better permselectivity compared to PEI-1, which is the membrane with PEI incorporated inside the structure (i.e., incorporated on the Psf support layer). However, both PEI-modified membranes exhibited lower permselectivity performance compared to the unmodified control membrane, which produced a water flux of 23.9 LMH and salt rejection of 88.2%. To test fouling of the unmodified and PEI modified RO membranes, bovine serum albumin (BSA) was chosen as a model foulant based on preliminary investigations conducted herein to compare BSA to sodium alginate. After the foulant was introduced in the feed, the unmodified membrane exhibited a 31.8% total fouling ratio, the decrease in flux from the foulant solution compared to running clean DI water. However, a 90.7% flux recovery ratio was achieved when a final DI water rinse was performed. The PEI-1 membrane had a 39.7% total fouling ratio and a 81.6% flux recovery ratio after rinsing with DI water. The PEI-2 membrane showed a 43.1% total fouling ratio as a result of BSA fouling and a 94% flux recovery ratio when rinsed with DI water at the end of the fouling test. Water contact angle (WCA) analysis confirmed that the PEI-2 membrane had the most hydrophilic surface (WCA 25.1°) compared to the control membrane (WCA 52.9°). The higher hydrophilicity of PEI-2 aligns with its higher flux recovery results, which indicated reduced membrane fouling. Furthermore, the PEI-2 membrane had a drastically lower WCA than those reported in the literature for PEI-modified membranes, which ranged from (63° – 80°). In conclusion, the increased flux recovery and surface hydrophilicity of the PEI-2 membrane indicated that the best anti-fouling performance would likely be obtained when PEI is grafted onto the surface of the active PA membrane surface. Future research is warranted to optimize the PEI-2 membrane by exploring the effect of PEI concentration, molecular weight, and structural configuration (i.e., branched versus linear), on anti-fouling performance of the membranes.