A gross pollutant trap is a treatment device designed to capture coarse sediment, trash and vegetation matter carried in the stormwater. music assumes that gross pollutant traps have very small pool volumes, and therefore, no hydrological routing is simulated within the GPT node.
In MUSIC-XMUSICX, Generic Treatment DevicesNode is used for GPT. music MUSIC requires you to describe the performance of the GPT (entered in a table) for each pollutant type, and does not provide default performance figures. The reason for this is that there are many GPTs available, including several proprietary products, which may perform very differently. Appendix C provides a summary of one study of GPT performance, undertaken by the CRC for Catchment Hydrology (Allison et al., 1996; Walker et al., 1999).
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Gross Pollutant Trap Properties
Please refer Generic Treatment Devices.
Transfer Functions
For each pollutant (TSS, TN, TP and gross pollutants), performance of the GPT can be described using one of three options:
- A concentration-based capture efficiency (defined by a table) - define the outlet concentration as a function of the inlet concentration;
- A flow-based capture efficiency (defined by a table) - Entered in a table, this allows you to define a percentage of capture of each pollutant as a function of the flow. A 100% capture represents a total reduction of the pollutants, whereas 0% capture means that the node has no effect on the pollutant. Note that if your table does not cover the entire hydrograph then music extrapolates linearly using the last two values of the table; or
- A combination of the above options - When the option Both is selected, the concentration-based transfer function is applied on the captured percentage of the flow. This provides greater flexibility and more options in your model. For example, you can model reduced efficiency with higher inflows into the GPT. The concentration-based reduction can now be altered by the flow-based efficiency, thus providing higher accuracy.
The Properties dialog shown above is used in the example described next.
This table shown below has been extracted from the flux file. Notice the following:
- At 19:54, the inflow is 0.237 m3/s, so the flow based capture efficiency will be 100%. Then, the concentration based transfer function will apply to 100% of the incoming flow, i.e. 234 mg/L • 0.4 = 93.5 mg/L.
- At 20:00, the inflow is 1.63 m3/s, so the capture percentage is 50% (as defined in the table of the previous figure). Then, the concentration based transfer function will apply to the captured flow (i.e. 50% here).
- 50% of the flow with 50.2 mg/L reduced to 50.2•0.4 = 20 mg/L and 50% of the flow not captured, at 50.2 mg/L gives the final value of 0.5•20+0.5•50.2 = 35.1mg/L.
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Time
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Inflow (m3/s)
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Totaloutflow (m3/s)
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TSSinflow(mg/l)
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TSSoutflow(mg/l)
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TSStotaloutflow(mg/l)
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25/04/1970 19:54
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0.237
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0.237
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234
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93.5
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93.5
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25/04/1970 20:00
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1.63
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1.63
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50.2
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35.1
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35.1
To use the function editor:
- Select the required pollutant using the radio-buttons adjacent to each pollutant. For each pollutant, you can choose how to display the transfer functions: as a concentration based transfer function only, as a flow-based capture efficiency only, or both. This is done using the appropriate radio buttons above the Transfer Function Tables and Graphs.
- A concentration-based capture efficiency (defined by table) – you can define output concentration for each input concentration.
- A flow-based capture efficiency (defined by table) - this allows you to define a percentage of capture of each pollutant as a function of the flow. A 100% capture represents a total reduction of the pollutants, whereas 0% capture means that the node has no effect on the pollutant.
- For both concentration and flow based capture efficiency table, you can add new points in the table manually. You can also import the file.
If you wanted to simulate a GPT which removed 90% of all gross pollutants, you would create a straight line with a slope (ie. output/input) of 0.1, as expressed by the form:
- Gross Pollutant Output = 0.1 • Gross Pollutant Input.
Using the above example, this would be created by entering an output value of 100 for an input value of 1000.Node.