Investigation of hydraulic selection in activated sludge and its effects on floc characteristic, settling velocity, and microbial community, An
AuthorMaltos, Rudy Alexander
AdvisorCath, Tzahi Y.
activated sludge settling velocity
aerobic granular sludge
sludge volume index
solid liquid separation
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AbstractWastewater facilities in urban communities are quickly approaching their maximum treatment capacity due to increased influent wastewater flows. Thus, expansion is needed, but facilities located in urban settings may not have access to additional land for development. The major bottleneck of these traditional facilities tends to be the secondary clarifiers, which often span 100 ft in diameter each, and are responsible for the solid-liquid separation process, allowing the dispersed microbial community known as activated sludge (AS) to separate from the effluent via gravity. The AS that settles to the bottom of the clarifier is then collected and partially returned to the start of the wastewater treatment process, and the effluent is disinfected and released to the environment. If the overflow rate, the vertical flow velocity of water in the clarifier, is higher than the settling velocity of the AS, the AS will not completely separate from the effluent and a fraction escapes to the disinfection basin, leading to higher consumption of chlorine, formation of carcinogenic disinfection byproducts, and a potential discharge permit violation. Lower overflow rate in the secondary clarifier reduces the risk of AS washout, but it also increases the hydraulic retention time and reduces the treatment capacity of the facility. The settling velocity of the AS (the rate at which the AS separates itself from the effluent) determines the maximum overflow rate of the clarifier; fast settling sludge allows the overflow rate and treatment capacity to increase. However, the majority of wastewater facilities experience slow settling velocity of less than 10 m/h due to the current operation practices of AS treatment trains and the wasting methods used to control AS concentration. Technology such as membrane bioreactors promise to deliver fast and effective physical separation, but would require major capital investments and alterations to the treatment train. In addition, membranes do not resolve the core problem of the AS process, the accumulation of poor settling floc. Aerobic granular sludge (AGS) is a dense community of microorganisms with an anaerobic core and aerobic outer layer. This unique structure allows denitrifying and phosphorus accumulating organisms to grow in the core while nitrifying and heterotrophic microorganisms thrive on the outer layers, resulting in improved nutrient removal abilities compared to conventional AS. AGS also has enhanced settling characteristics due to its circular shape and high density, settling at a rate between 15 and 60 m/h. However, AGS has not been adopted by wastewater facilities due to an 18 ft reactor height requirement, this height allows for the stratification and separation of slow settling floc. Thus, the objective of the research described in this dissertation was to develop a new AS wasting system through rapid prototyping and bench scale testing. The developed technology, aka the hydraulic selector, was long-term tested at the pilot scale in sequencing batch reactors, which were able to increase AS settling velocity above 10 m/h and maintain constant nitrification and carbon removal throughout the experiments. Subsequently, the feasibility of developing AGS when combining hydraulic selection technology with traditional wasting practices was tested for 238-days. The effects of the hydraulic selector on the AS microbiome and the communities present in various floc diameter ranges were evaluated over time using 16S-rRNA gene and 18S-rRNA gene. Abundance of filamentous bacteria, phosphorus accumulating bacteria, ammonia-oxidizing bacteria, and nitrite-oxidizing bacteria were also quantified in both hydraulic selector discharge and AGS. Lastly, computational fluid dynamics were used to quantify and visually express the velocity flow field and separation of poor settling solids under turbulent conditions. This model may influence future selector geometry and selector operation to maximize the removal of poorly settling floc.
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Mechanisms of diversity maintenance, resilience, and niche differentiation in activated sludge microbial communitiesDrewes, Jörg E.; Munakata Marr, Junko; Vuono, David Charles; Navidi, William Cyrus; Spear, John R.; Landkamer, Lee L.; Saikaly, Pascal (Colorado School of Mines. Arthur Lakes Library, 2014)Scarcity of potable water is a major threat to human civilization, particularly in semi-arid and arid regions of the world. As a consequence, water reclamation and reuse of treated wastewater has received more attention. Local reuse of reclaimed water that is tailored to local needs will be essential in cities because of greater population densities. The activated sludge process was chosen to investigate if new operational strategies could facilitate tailored reuse of reclaimed water because it has been applied under many operational configurations. However, alterations of operational conditions within the same biological nutrient removal treatment plant have not been tested for the intentional retention of nutrients in treated wastewater. Thus, the consequences of changing treatment conditions on biological and engineered operational parameters in a treatment plant are unknown, specifically the resilience of functionally important microorganisms involved in the removal of nutrients or trace organic chemicals. In this dissertation, a variety of genomic molecular techniques such as massively-parallel DNA/RNA sequencing are used to phylogenetically reconstruct activated sludge microbial community assemblages in a full-scale active sludge wastewater treatment bioreactor that is configured as a sequencing batch membrane bioreactor (SBMBR). The SBMBR system, located at the Colorado School of Mines' Water Reclamation Test Site Mines Park, was operated under different bioreactor set-points through time to test fundamental and applied questions in the fields of microbial ecology and environmental engineering. The key findings of each research chapter are as follows: Chapter 2) Adjustments in aeration strategy can be used to elevate nutrient concentrations in the effluent of an SBMBR without adverse effects on bioreactor operation. Chapter 3) Bioreactor performance in an SBMBR is resilient to long-term disturbances. Abundant bacterial populations recover from the disturbance more than rare populations. Chapter 4) A long-term disturbance allows for immigrating bacteria to colonize and establish into an activated sludge community. In some cases, newly established colonizers become abundant community members post disturbance. Chapter 5) Bacterial populations within a stable treatment process exhibit distinct activity signatures that correspond to fluctuations in redox conditions.
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Hydraulic selection to transform and improve activated-sludge based wastewater treatmentBlair, Emily; Maltos, Rudy; Holloway, Ryan; Vuono, David Charles; Cath, Tzahi Y.A majority of wastewater plants in the United States use conventional activated sludge (CAS) in their treatment process. While CAS is a common practice, it is not without faults. One of its drawbacks is the difficulty in maintaining the process, as it is often prone to bulking, which is caused by an undesired build up of filamentous bacteria and results in sludge that does not settle. Additional drawbacks include limited space for WWTPs to expand and the energy costs required for aeration during CAS treatment. Because CAS infrastructure is so common, optimizing this technology is crucial for the prevention of water pollution. Some solutions for these problems have been developed including aerobic granulation, which allows the plant to have a smaller footprint; however, these advanced wastewater technologies are not easily implemented in existing WWTPs.