Understanding Biosolids Dynamics in a Moving Bed Biofilm Reactor

• Main Author: Goode, Christopher
• Other Authors: Allen, D. Grant
• Language: en_ca
• Published: 2010
• Subjects: Biofilm; Detachment; Moving Bed; Modeling; Multivariate; Calcium; 0542; 0775
• Online Access: http://hdl.handle.net/1807/24758

Biofilm systems such as the moving bed biofilm reactor (MBBR) are finding increased application in wastewater treatment. One important process that governs MBBRs and yet is poorly understood is the rate of biofilm detachment. The detachment of cells from biofilm surfaces controls both the accumulation of biofilm and the quantity of biomass that is suspended in the bulk liquid phase. This changing balance of attached and suspended cells, in this thesis named the biosolids dynamics, can impact the efficacy of MBBRs. The goal of this research was to investigate how the biosolids dynamics are influenced by process changes relevant to applied wastewater treatment systems and suggest new routes to reactor design and optimization. To achieve this goal, the work addresses three separate but interconnected lines of inquiry. First, multivariate analysis (Principal Component Analysis, Partial Least Squares) was used to examine 2 years of historical data from an MBBR operating at a Canadian pulp mill in order to identify key process variables, perform process diagnostics, and act as a predictive tool. Secondly, the effect of calcium concentration on biofilm structure, microbiology and reactor performance was investigated in four laboratory-scale MBBRs operated at a range of calcium concentrations (1 to 300 mg/L Ca2+). It was found that above a threshold calcium concentration between 1-50 mg/L, MBBR biofilms were observed to be thicker with greater density, contain larger anoxic regions adjacent to the carrier substratum, have more proteinaceous EPS, and have altered microbial community structure. The results suggest an important role for calcium that should be considered in the design and operation of MBBRs. In the final line of inquiry, a diffusion-reaction biofilm model was adapted to represent the key processes of the MBBR. The model was found to simulate average trends observed in the lab-scale experiments allowing for quantification of the detachment rate. Transient periods of reactor starvation were also simulated by introducing a novel metabolic state function to account for down-regulation of metabolism as a result of starvation. This approach was found to accurately simulate starvation response when coupled with detachment expressions that were growth-dependant.