Hydrodynamic assessment of different UASB reactor’s influent distribution systems to improve granulation

Abstract

Wastewater treatment systems are implemented to remove wastewater pollutants before discharge into receiving water bodies. Wastewater can negatively affect the ecosystem of the receiving water body if proper treatment is not conducted. Despite its importance, small coverage of wastewater treatment systems has been achieved worldwide mainly due to the related high construction and operating costs. According to the Food and Agriculture Organization, about 80% of the world’s wastewater generation is discharged into the environment without any treatment, especially in low-income countries. Anaerobic biological wastewater treatments could be an answer to reduce treatment costs. Anaerobic technologies offer advantages over competing technologies such as reduced land footprint, small reactor volume, reduced excess sludge production, and the ability to recover energy through methane capture. The most widespread anaerobic technology worldwide is the Upflow Anaerobic Sludge Blanket (UASB) reactor. The UASB reactor uses an upward flow to produce granular sludge capable of treating high organic loads. Although there is extensive information on the microbiology of these granules and their efficiency in treating different wastewater qualities, further research is required to better understand the relationship between granule formation and reactor hydrodynamics. Flow hydrodynamics, almost entirely controlled by the reactor's Influent Distribution System (IDS), is key to consider during the UASB reactor’s design since it modules the substrate distribution inside the reactor and the formation of stagnant and short-circuited zones. The IDS role is critical, especially during the reactor's start-up stage when the granular sludge starts to form. This thesis aimed to advance our understanding of the flow hydrodynamics impact on the operation and efficiency of the UASB reactor during its startup stage. The research was divided into two main stages. The first stage was dedicated to physically modeling the reactor using a Froude dynamic similitude scaled reactor and developing an automated tracer testing system. This system allowed us to determine the importance of controlling the test water's conductivity, temperature variation, and surface tension during the tracer tests. The second stage was devoted to numerically modeling the hydrodynamics of the UASB reactor using computational fluid dynamics (CFD) simulations. Initially, the research focused on finding the turbulence closure model that best reproduced the reactor’s hydrodynamics. Thus, CFD simulations were conducted using the realizable k-epsilon model to assess the potential volume of granule generation for IDS configurations commonly used in the literature. The simulations confirmed that the IDS configuration recommended by the design guidelines has a high performance in reducing stagnant and short-circuited zones. This research proposed a novel IDS configuration that generated a granulation volume 22% larger than the recommended IDS configuration, potentially reducing UASB reactor start-up time. The research demonstrates the potential of using physical and numerical techniques as a basis for the model-based design approach to solve problems specific to UASB reactors, an approach that could be extrapolated to other types of reactors.