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Investigation of hydraulic selection in activated sludge and its effects on floc characteristic, settling velocity, and microbial community, An
Maltos, Rudy Alexander
Maltos, Rudy Alexander
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2021
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2022-10-14
Abstract
Wastewater 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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