Filtration in water treatment is the last process that removes suspended solid and other substances after sedimentation process. The most critical process in filtration is backwash which removes the deposit in the filtration media. The basic factors of backwash efficiency are size of the media, shape of the media, density of the media, quality of the influent water, temperature of the water and use of coagulant. Many researchers have studied the backwash mechanisms to develop theories of media expansion according to the backwash rate, optimum backwash velocity to produce optimum shear force, etc.
The mudballs formed during the filtration process are not easily removed even by hard backwash. To cope with this problem, a new method of air-scoured backwash was introduced. In spite of its disadvantages of media loss and abrasion, its implementation is growing because of its advantages such as reduced wastewater and no need for re-stratification.
Recently, as the numerical calculation techniques improve, CFD(Computational Fluid Dynamics) methods are introduced into environmental engineering. The analysis of filtration process must be based on 2 or 3 phase formulation, so CFD is useful in analyzing the filtration and backwashing process.
In this thesis, multi-phase model and buoyant model in CFX version 4.2 were used to simulate the expansion model which takes the particle size distribution into account. And from the result of this simulation, predictions were made for other various conditions. To interpret the experimental result, the behavior of the bubbles in the bed was speculated.
The modeling and experimental results conclude as follows:
① Through modifications and evaluations, CFX codes were used to predict the expansion of the media in the bed during the backwash process. The model which incorporates the particle size distribution produced results closer to the experimental data.
② As the result of this CFD analysis, it was found out that current backwashing process is operated in excessive condition.
③ Sensitivity analysis on the model reveals that viscosity plays a critical role in the media expansion.
④ In the case of air-scoured backwash, smaller bubbles produced larger effect, and a model was developed to explain this.
