To illustrate the use of precipitate removal flux links, consider a simple case involving precipitation of a species from one Cell (a tank) to another Cell (representing a removal pond for sludge). This particular example file, Cell5_PrecipitateRemoval.gsm, can be found in the Contaminant Transport Examples folder in your GoldSim directory (accessed by selecting File | Open Example... from the main menu).
Suppose that the treatment tank contains 10 m3 of water. Water cycles through the system at a rate of 1 m3/day (flowing to an equal sized tank). The incoming water contains Fe at a concentration of 1 mg/l (equivalent to a mass rate of addition of 1 g/day). Chemicals are added to the tank such that the Fe has a low solubility in the tank water (0.01 mg/l), causing it to precipitate out.
The precipitated solid is immediately collected (via a filter) and removed to a sludge tank. The treated water is then discharged to another tank, which subsequently discharges to a sink.
To simulate this system, you would do the following:
1. Define a single Species called Fe.
2. Specify a solubility of Fe in Water of 0.01 mg/l.
3. Create a Cell called Treatment_Tank containing 10 m3 of Water;
4. Define a Cell called Sludge containing 1 m3 of water (the amount of Water is not important, as the Sludge Cell is simply acting as a sink for the precipitate);
5. Create a Cell called Next_Tank containing 10 m3 of Water;
6. Create a Cell called Sink (the amount of Water is not important, as it is simply acting as a sink);
7. Create an Input Rate for Treatment_Tank equal to 1 m3/day * 1 mg/l (equivalent to mass entering at a rate of 1 g/day);
8. Create an advective flux link between the Treatment_Tank and the Next_Tank with a flow rate of 1 m3/day.
9. Create an advective flux link between the Next_Tank and the Sink with a flow rate of 1 m3/day.
10. Create a precipitate removal flux link between the Treatment_Tank and the Sludge Cell with a high (100 day-1) transfer rate (so that the precipitate is removed immediately).
11. Specify the simulation settings (i.e., duration and timesteps), and run the model.
The output of this simulation, in the form of time histories of the concentration in the Next_Tank and the amount of mass in the Sludge Cell is shown below:
Note that the concentration of Fe increases in the Next_Tank (since it starts out empty), until it eventually approaches the concentration in the Treatment_Tank (which never exceeds the solubility limit). The amount of precipitated mass that has been removed to the Sludge tank increases linearly at a rate of just under 1 g/day (nearly all of the Fe that enters the Treatment_Tank precipitates, since it enters at a concentration of 1 mg/l, and the solubility limit is 0.01 mg/l).