Improving Wastewater Systems by Coupling Biokinetic Models and Hydrodynamics using Computational Fluid Dynamics

Fine bubble diffusers are often used for aeration systems in large wastewater treatment works. The bubbles created are a source of momentum for mixing and provide oxygen required for the micro-organisms to feed on the organic matter and nitrogen in the wastewater. The energy needed to aerate the system can account for 25-60% of the total energy consumption of a wastewater treatment plant and therefore, large savings can be gained with an energy efficient aeration system. Biokinetic models have been developed (ASMs, ADM1, etc.) that describe the growth and decay of micro-organisms and their dependence on oxygen in wastewater treatment systems. These models consist of a system of rate equations and are a useful tool for understanding the biological processes occurring in wastewater systems which can be used to improve design and operating conditions. However, a critical assumption made by these models is that the reactors are well-mixed such that the parameter concentrations are uniform throughout the reactor. If we reduced the aeration, this would reduce the amount of available oxygen for the reactions and also affect the mixing flow regime. By coupling the biokinetic growth models with the hydrodynamics using computational fluid dynamics, we can analyse the impact of the hydrodynamics on the biokinetic growth models in the case of reduced aeration. These enhanced predictions can aid in improved operating conditions of the aeration systems, leading to more efficient processes which reduce the overall cost of the systems.

To achieve this, a multiphase Eulerian-Eulerian approach has been adopted to simulate the hydrodynamics under a range of initial conditions. An experimental large scale transparent aeration reactor has been used that is a 2/3 replica of a working pilot-plant to validate the model. The model has been validated using ultrasonic techniques, dissolved oxygen and other experimental measurements which has shown that the model can reliably reproduce the hydrodynamics and oxygen distribution over time. The biokinetic models have then been coupled to the model to investigate the affect of the initial aeration conditions on the biological performance of the pilot-plant reactor. Validating the coupled model using biological measurements from the pilot-plant reactor will provide a means to better understand the affect of changing operating conditions on the overall biological performance of the reactor.