Applications of Aerobic Granular Sludge and Source-Separated Urine for Enhanced Nutrient Removal and Recovery

Thesis (Ph.D.)--University of Washington, 2022Nutrient removal is one of the main goals of wastewater treatment plants (WWTP) to prevent eutrophication in the environment. The growing population and increasingly stringent discharge limits impose a need for intensification of existing wastewater infrastructure to maximize treatment capacity in the available space. The secondary goals of wastewater treatment are related to sustainability, such as reduced chemical usage and resource recovery. This thesis aims to find actionable maps for compact wastewater treatment solutions and new concepts for nutrient recovery with four specific objectives. (1) Aerobic granular sludge (AGS) is an emerging technology that can enhance biological nutrient removal with reduced footprint. The current AGS technology is implemented in sequencing batch reactors (SBR) and its integration into existing full-scale continuous flow activate sludge (CFAS) systems is the next challenge to make this technology widely accessible. This thesis aims to address this research gap by understanding the selection factors contributing to granule growth in CFAS systems, investigating the partitioning of microbial community between granules versus flocs, and exploring strategies for cultivating granules in a WWTP. The major conclusions from this work are the following: (i) Aerobic granules were discovered to naturally occur at 13 surveyed CFAS plants and their abundance were related to key design features such as high anaerobic food to mass ratios and influent soluble carbon fraction. Molecular analyses indicated that process configurations that select for slow-growing heterotrophs may also play an important role. (ii) In a hybrid granular activated sludge reactor, flocs offered better growth conductions for polyphosphate accumulating organisms while ammonia-oxidizing bacteria benefitted from the longer SRT in the granules. A complex relationship of microbial competition/cooperation between granules and flocs was demonstrated, highlighting the important role of small particles in maintaining the nutrient removal capacity of a hybrid granule/floc system. (iii) Full-scale application of a novel biocarrier technology for intensifying a CFAS plant was demonstrated with an all-organic media made of kenafs to facilitate biofilm attachment. The biofilm aggregates developed were comparable to SBR-type aerobic granules in terms of physical characteristics and activities of key microbial functional groups. (2) Phosphorus recovery is gaining importance as the global phosphorus reserves is a finite resource. Struvite (MgNH4PO4· 6H2O) is a proven slow-releasing fertilizer that can be obtained via precipitation. Struvite precipitation at WWTPs uses reject water from anaerobic digesters, but sludge thickening prior to digestion is needed to obtain high enough P concentrations in the struvite reactor as necessary to drive precipitation kinetics. This research aims to utilize the high thickening characteristics of AGS to demonstrate the production of a P-rich stream by simple anaerobic holding of the waste granular sludge. This concept was tested with aerobic granules cultivated on aquaculture waste, achieving a P-rich stream without the need for sludge thickening equipment or anaerobic digester. (3) Resource recovery at a WWTP can be limited by existing infrastructure and diluted concentrations. On the other hand, urine collected at the source (source-separated urine) offers an alternative stream for struvite recovery and is more favorable to centralized WWTP in terms of sustainable nutrient management. Struvite recovery from urine is well-demonstrated but due to the disproportionally high N/P ratio, significant amount of ammonia is remained after the struvite recovery step and requires additional treatment. The industrial production of nitrogen fertilizer via the Haber Bosch process consumes 1 – 2% of the global energy usage, and thus recycling of nitrogen from wastewater is desired. This thesis tested a chemical/physical process that combines air stripping and acid scrubbing at pilot-scale and demonstrated 93% N removal, from which 85% was recovered in the form of ammonium sulfate fertilizer. (4) While nitrogen recovery via chemical/physical processes is a viable option (as identified in objective 3), the high energy demand is a major drawback. Biological nitrification offers a promising alternative as its energy input is lower than chemical/physical processes and the product (nitrified urine) is a good fertilizer. Urine can consist of varying compositions of ammonia and urea and in some cases (e.g., fresh urine), microbial hydrolysis of urea (ureolysis) is the first step towards obtaining a nitrified urine product. However, little is known about the ureolytic metabolism of nitrifying organisms and how their cellular regulations control the selective use of different nitrogen substrates. This thesis characterized the ureolytic physiology of five ammonia oxidizer isolates and showcased their varying regulatory responses to alternative nitrogen substrates