Study of Particle Size Distribution in Activated Sludge Processes: Impacts of Solids Retention Time and Process Configurations

Particle size distribution of the particulates is an essential characteristic of the wastewater quality. Particle size distribution has been used to predict COD, suspended solids, color, and turbidity. The understanding of particle size distribution contributed to the better understanding of soluble and particulate COD fractions and benefited the modeling of activated sludge process. Particle size distribution of wastewater particles was used to improve the understanding of both primary treatment and secondary treatment. Particle size of activated sludge flocs may affect key sludge handling processes including sedimentation, thickening, digestion, and dewatering. Particle size distribution of secondary effluent is also an important consideration for the design of tertiary treatment such as filtration and disinfection.Several design and operational parameters, e.g. mixing, aeration, flocculation, and SRT, may affect particle size distribution of activated sludge. Previous results strongly suggest that SRT is an important parameter affecting particle size distribution in activated sludge process. However, direct comparison of different wastewater treatment plants could not rule out possible confounders such as sheer force in aeration basin, doses of coagulants, and variation of organic loadings. The objective of this study is to investigate particle size distribution of activated sludge flocs under different SRTs and treatment processes. Particle size distribution of lab-scale MLE reactor and IFAS reactor were studied under various SRTs and carbon sources. Five full-scale wastewater treatment plants were surveyed for detailed understanding of the change of particle size distribution from raw wastewater to secondary effluent.Chapter 2 investigates the impact of SRT on particle size distribution, sludge settleability, effluent turbidity, and removals of COD and NH4+-N. A MLE reactor is established with 16L of operational volume. Settling test, water quality analyses, and microscopic examination are applied to evaluate the impact of different SRTs. Particle size of activated sludge flocs are analyzed at different controlled SRTs. Particles with different size ranges were evaluated at various SRTs.Chapter 3 focuses on the impact of SRT on particle size distribution, sludge settleability, effluent turbidity, and removals of COD, NH4+-N in a lab-scale Integrated Fixed Film Activated Sludge (IFAS) reactor. Chapter 3 further investigates the impact of difference carbon sources (Glucose vs. Sodium Acetate) on particle size distribution and reactor performance in the IFAS reactor. Settling test, water quality analyses, and microscopic examination are applied to evaluate the impact of different SRTs. Particle size distribution of the mixed liquor in the IFAS reactor is compared with that in MLE reactor operated at similar SRTs for suspended solids.Chapter 4 surveys particle size distribution in 5 full-scale WWTPs with different SRTs and treatment processes in the Los Angeles County. Particles size distribution profiles from primary influent to secondary effluent are fully evaluated. The relationship between SRT and particle size of activated sludge in biological process and sedimentation process are studied in detail.