Understanding the Interplay Between Fluid Mechanics and Particle Size Distributions for Filtration of Flocculated Mixtures
Master of Engineering Science
Mechanical and Materials Engineering
Christopher T. DeGroot
The breakdown of nutrients in biological processes in wastewater treatment require a sub- stantial amount of energy, contributing to greater greenhouse gas (GHG) emissions. Floc- culation can be used to reduce the loading on a biological process by increasing removals during primary treatment in devices such as a rotating belt filter (RBF). The relationship between shear conditions, defined by velocity gradients (G-values) and floc size is crucial. Understanding G-value magnitudes and distributions within an RBF, corresponding trans- port network, and the effects on floc size degradation are needed to maximize removals. G-values were calculated using a newly developed approach, improving accuracy com- pared to traditional expressions. Floc strength was quantified at calculated G-value ranges, where flocs were found to resist breakage as G-values increased. The effects of varying PSD on RBF modelling constants were explored, where a strong response to PSD size fractions at or above the nominal pore size of the filter was found.
Summary for Lay Audience
Wastewater treatment is an essential service needed to remove pollutants from our water system. Wastewater treatment plants (WWTP) remove pollution in stages. The first stage removes large items such as, sticks, leaves, cooking grease, etc. Next, biological treatment is done to remove nutrients such as, phosphorous and nitrogen. These biological process require a lot of energy to be completed. In order to meet these increased energy demands more energy needs to be generated resulting in more greenhouse gases being produced.
Reducing the load placed on these biological treatment operations can reduce the overall energy consumption of a WWTP. This can be accomplished by increasing removals during primary treatment. One method to increase removals in primary treatment is to use a process known as flocculation. Flocculation involves the addition of coagulants and flocculants (chemical additives) to bring smaller particles together forming large clusters referred to as flocs. This grouping of particles is referred to as a shift in the particle size distribution (PSD).
As flocs are transported from the mixing vessel where they are grown to their respective removal device the shear conditions within the fluid can cause the flocs to break. To estimate the amount of breakage we can calculate the velocity gradients, referred to as G-values, within a geometry of interest. Current methods for calculating G-values have been found to have limited applicability to the small volume open system nature of pipe network geometries used with an RBF. Additionally, there is limited literature regarding the strength of flocs formed from primary influent.
The research presented herein will evaluate the G-value distributions within pipe network geometries where G-values are predicted to be large. The strength of flocs formed from primary influent will presented with respect to G-values found in pipe network components. Finally, the effects that varying PSD has on numerical modelling constants used to simulate flow through an RBF will be explored with the goal to be able to model the effects of flocculation inside an RBF and the impact it has on performance.
Remler, Brooke, "Understanding the Interplay Between Fluid Mechanics and Particle Size Distributions for Filtration of Flocculated Mixtures" (2019). Electronic Thesis and Dissertation Repository. 6655.