Characterizing Contact Tank Performance
Characterize the performance of a chlorine disinfecting contact tank in an activated sludge process with varying bacteria loads. The model is set up based on a paper by Evans and Kothandramm (2000)1.
FLOW-3D (x) Workflow
An initial simulation computes only the hydrodynamics of the system at a given flow rate. A steady state condition is found at 3000s. Using the flow field from this simulation, a restart simulation is created to introduce the reaction kinetics model. Chlorine and bacteria are introduced via the inlet pipe. At the outlet, a flux surface is placed to measure the concentration of the reactants (chlorine and bacteria).
FLOW-3D (x) uses nodes to construct automated workflows for the optimization. The inflow bacteria concentration variable is given a range from 1.0e9 to 10.0e9 and the chlorine concentration variable is given a range from 1.0e-6 to 9.0e-5. A simulation is executed using a FLOW-3D node. The post-processing node extracts the concentrations of chlorine and bacteria from the results and builds a Pareto front which represents the minimum chlorine required to minimize the bacteria at the outlet.
The budget, or number of simulations allowed, was set to twenty and then fifty to demonstrate the effect of increased budget on the quality of the Pareto front. The runtime for a single simulation is approximately fifteen minutes.
Using FLOW-3D (x)’s data analysis tools, Pareto fronts of the bacteria concentration with respect to chlorine concentration are compiled for both the twenty-budget case and fifty budget case. The larger budget gives the optimization engine more time and data to report for a more accurate Pareto front.
With the Pareto front created by FLOW-3D (x), system designers can quickly assess the amount of chlorine needing to be injected into the contact tank to minimize bacteria at the tank outlet under a given flow rate. For other flow rates, the original simulation defining the tank hydrodynamics would be run at the new flow rate and the same FLOW-3D (x) project would be run, possibly with different ranges on the inlet bacteria and chlorine variables.
1 Evans, R. and Kothandraman, V., “Design and Performance of Chlorine Contact Tanks, “Circular 119 STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION 2000.