Flow pattern and CFD simulation in wastewater land treatment system with various kinds of net-pipe

The hydrodynamics of wastewater land treatments system has a direct influence on the removal efficiency of pollutants. By studying the effect of net-pipes, a solid basis will be provided for design and optimization purposes. On the basis of residence time distribution (RTD) theory, the flow pattern in a land treatment reactor was analyzed using tracer-response technology. Computational fluid dynamics (CFD) software Fluent was utilized to simulate the flow pattern of water. By combining the results of tracer experiment with CFD simulation, the bear-layer was clearly shown to have the function of distributing water and the water-layer was clearly capable of collecting water from the whole water-carrying section. After flowing horizontally through the bear-layer, the water passed through the packing vertically before finally reaching the collection net-pipe in the water-layer. The net-pipe distribution was shown to have a negligible influence on the flow pattern in the absence of bear-layer and water-layer. However the water distribution of net-pipe holes was uneven. Even though the volume efficiency was all above 90%, there were parts of useless or stagnant zone. The highest volume efficiency was obtained in the reactor with half distribution and collection net-pipe, where the flow pattern was different from the plug flow. The dimensionless mean residence time was 1.09, and the degree of dispersion was only 0.037.