Doctor of Philosophy
Chemical and Biochemical Engineering
George Nakhla, Jesse Zhu
The Circulating Fluidized Bed Biofilm Reactor (CFBBR), a bioparticle technology designed for biological nutrient removal, has been implemented to achieve considerable biodegradation efficiency and low sludge production, compared with activated sludge system and typical biofilm technology. The inherent advantages of bioparticle technology are enhanced substantially by the CFBBR, for example, decoupling of hydraulic retention time (HRT) from solids retention time (SRT), large specific surface area, ideal conditions for biofilm ecosystem.
In this work, bioparticle recirculation, as a novel control method for bioparticle system, was demonstrated in CFBBRs. To verify the impact of bioparticle recirculation on the reactor performance, bio-kinetics and hydrodynamic behavior, three lab-scale CFBBRs were developed and tested for carbon and nitrogen removal from synthetic wastewater as well as municipal wastewater. During all extended experiments, bioparticles are slowly transferred from the Riser (Anoxic column) to the Downer (Aerobic column) for specific bio-reactions, and then recirculating back to the Riser for refreshment. A low shear stress was maintained in order to achieve rich biofilm conditions, where the predation process was encouraged. Furthermore, a novel one-dimensional (1D) bioparticle model successfully combined hydrodynamic parameters with biofilm kinetics to simulate dynamic surface area and dynamic shear stress in bioparticle process.
Two lab-scale CFBBRs fed with synthetic wastewater were applied for extended experimental tests and pseudo-steady-state study of bioparticle recirculation. Over the 285 days of synthetic wastewater experiments in a lab-scale (4 L) CFBBR, over 95% COD removal and 85% TN removal were achieved during slow bioparticle circulation between Riser (Anoxic) and Downer (Aerobic). Furthermore, with sodium acetate as the carbon source, an extremely low net sludge yield of 0.034 mgVSS/mgCOD was observed concomitant with the appearance of macro-consumers and aquatic worms. Another extended (200 days) experiment of a lab-scale (8.5 L) CFBBR demonstrated the feasibility of the pseudo-steady-state for integrated COD, nitrogen removal and worm predation, and the results proved that the worm predation has a significant impact on the pseudo-steady-state performance of the CFBBR, decreasing biomass yield and oxygen concentration while increasing expanded bed height.
Subsequently, Bioparticle Enrichment-Predation circulation (BEP circulation), comprised enrichment (in Riser Bottom section), transportation (in Riser Top section), predator-cultivation (in Downer Top), and deactivation (in Downer Bottom), was proposed as a novel bioparticle recirculation pattern, which effectively improves performance and enhances stability of CFBBR. The bioparticle process involving worm predation proved to be achievable through a self-balancing worm bioparticle process and BEP circulation, and a self-balancing micro-community along with BEP circulation would provide an effective control of the bioparticle system integrated COD and nitrogen removal as well as strong predation.
A CFBBR model was established based on 1D-bioparticle model to investigate hydrodynamic conditions of CFBBR. The model integrated the anoxic riser and aerobic downer, and bioparticle circulation was simulated as a function of expanded bed growth. Experimental data from a 6-L CFBBR fed with municipal wastewater was used to validate the simulation. The CFBBR model can be used to quantify the bed voidage in the riser, and expanded bed height and bed voidage in the downer to achieve good biodegradation performance, optimize the up-flow velocity in both the riser and the downer, then calculate the amount of media for the system. The impact of bioparticle circulation rate (vs) was also studied in the lab, verifyied by three different vs i.e. 50 g bare particle/d, 100 g bare particle/d and 200 g bare particle/d. The range of operational bioparticle circulation rate was calculated by 1D-bioparticle model, which provides crucial control parameter for the CFBBR.
Li, Ming, "Impact of Bioparticle Recirculation in a Circulating Fluidized Bed Biofilm Reactor on Simultaneous Organic and Nitrogen Removal" (2012). Electronic Thesis and Dissertation Repository. 1048.