Media Filtration System
- Geoff Davis
- Mukta Sapkota
Media filtration systems are systems that promote the filtration of stormwater through a prescribed filter medium. The type of filter medium determines the effectiveness of the pollutant removal, with material of lower hydraulic conductivity providing the most efficient pollutant removal. Typical examples include sand filters or granular filter media systems. This node is nearly identical to the previous Version 3 bioretention node, and as such, when Version 3 models are imported into Version 5 and the user selects the Version 3 bioretention nodes to be upgraded, the media filtration node is used. This is done to ensure a similar performance between Version 3 and Version 5 models, however this will not account for the current understanding regarding the performance of bioretention systems. When a vegetated filtration system is to be configured in music Version 5, it is strongly recommended that the bioretention node is used. The media filtration node should be used only to represent basic unvegetated filter systems, or proprietary media filtration systems where either the performance is not otherwise known, or if the user has specific algorithms to describe the performance of the system.
Media Filtration System Properties
The media filtration system properties dialogue box displays the parameters that describe the physical characteristics of the media filtration system. It is these characteristics that are used to undertake the hydrologic routing of the stormwater flow as it flows through the media filtration system.
A conceptual diagram of the media filtration system properties in music is presented below:
Conceptual diagram of media filtration system properties.
Inlet Properties
The inlet properties define the flow rate at which the stormwater begins to bypass the media filtration treatment device.
Low Flow Bypass
All of the stormwater that approaches the media filtration system below the user-defined Low Flow Bypass amount will bypass the system. Any flow above the Low Flow Bypass (subject to the presence of a High Flow Bypass) will enter and be treated by the media filtration system.
High Flow Bypass
When the stormwater inflow rate exceeds the user-defined High Flow Bypass amount, only a flow rate equal to the High Flow Bypass (less that specified in any Low Flow Bypass) will enter and be treated by the media filtration system. All of the stormwater flow in excess of the High Flow Bypass amount will bypass the media filtration system and will not be treated.
Tip Box
The Low and High Flow Bypasses are assumed to occur simultaneously. So for a Low Flow Bypass of 2m3/s, a High Flow Bypass of 8m3/s, and inflow of 10m3/s:
Storage Properties
The Storage Properties define the physical characteristics of the surface storage above the infiltration medium of the media filtration system.
Extended Detention Depth
Defines the maximum depth of water ponding above the infiltration medium before flow starts to discharge over the outflow weir.
Surface Area
Defines the surface area of the media filtrationsystem storage (ie. the surface storage above the infiltration medium). For a regular-shaped system such as a rectangle, this will normally be the length multiplied by the width. The hydrologic routing analysis calculates the volume of water in storage during a storm event by multiplying the depth of water in the extended detention storage (ponding) zone by this surface area.
Exfiltration Rate
Exfiltration (also called seepage) from the media filtration system into the underlying soil can be modelled by defining the exfiltration rate. Representative exfiltration rates for different soil types are provided in the table below under Saturated Hydraulic Conductivity. The water that seeps from the media filtration system is lost from the catchment, and cannot re-enter the system downstream. Contaminants in the water that is lost to exfiltration are removed from the media filtration system, along with the exfiltrated water and are also lost from the catchment. Representative exfiltration rates for different soil types are shown in the following table.
| Soil Type | Median particle size (mm) | Saturated Hydraulic Conductivity | |
|---|---|---|---|
| (mm/hr) | (m/s) | ||
| Gravel | 2 | 36000 | 1x10-2 |
| Coarse sand | 1 | 3600 | 1x10-3 |
| Sand | 0.7 | 360 | 1x10-4 |
| Sandy loam | 0.45 | 180 | 5x10-5 |
| Sandy clay | 0.01 | 36 | 1x10-5 |
Filtration Properties
The filtration properties define the physical and hydraulic characteristics of the filtration medium component of the media filtration system.
Filter Area
Defines the plan area of the filtration medium component of the media filtration system. Where the extended detention storage zone does not have vertical sides, the filtration zone will be smaller than the storage zone.
Filter Depth
Defines the depth of the filter medium.
Filter Median Particle Diameter
Defines the median (d50) particle diameter of the filter medium. The median particle size should be available from the system provider or designer.
Saturated Hydraulic Conductivity
Defines the saturated hydraulic conductivity of the filter medium (refer above table).
Outlet Properties
The physical characteristics of the outlet overflow weir are defined in this section of the dialogue box.
Overflow Weir Width
Defines the width of the overflow weir for the surface storage component of the media filtration system. The overflow weir will only start to carry a discharge of water once the depth in the surface storage reaches the Extended Detention Depth defined above.
It is possible to view or save various flows and water quality as below for the media filtration system:
- inflow rate and water quality
- outflow rate and water quality
- low and high flow bypass rate and water quality
- overflow rate and water quality
- total outflow rate (sum of outflow, bypasses and overflow) and water quality
- computed water levels and storage.
You need to select them to record before you run.
Advanced Media Filtration System Properties
The advanced properties section (opened using the ‘More’ button) of the media filtration system properties dialogue box displays the hydraulic characteristics for the overflow weir structure, and the parameters that describe the treatment processes in the media filtration system.
Weir Coefficient
The overflow weir carries a discharge when the water level in the media filtration system extended detention storage zone exceeds the Extended Detention Depth. The overflow weir is modelled as a broad crested weir whose discharge equation is given by:
where
Q Discharge over the weir
Cw Weir Coefficient
L Overflow weir width
H Height of pond above the Extended Detention Depth.
The default Weir Coefficient of 1.7 may be altered if desired.
Voids Ratio
Defines the voids ratio for the filter medium (the proportion of the filter medium which is made up of voids).
For details about CSTR cells, refer to Number of CSTR Cells. For k and C* values, refer to the section below.
k and C* Values
The first order kinetic model adopted in the USTM for the surface storage component of the media filtration system is described by definition of k, the exponential decay rate constant and C*, the background concentration. The rate at which each contaminant is treated, and the background concentration for each contaminant will be different within a media filtration system and different values should be adopted for each contaminant. It is worth noting that in a media filtration system, most of the treatment will occur within the filtration media, rather than in the extended detention zone above it.
NOTE: Advanced Properties are uneditable by default. You can make it editable for each project by clicking on Edit and Project Options. This will present you with a Project Options Screen as displayed below:
Now, click on MUSICX Treatment Node Settings and tick on Editable as shown above.
Pollutant Removal through the Filtration Medium
Pollutant removal through the filtration medium is modelling using empirical equations derived from analysis of data published in the technical literature (see Appendix D - Modelling Filtration Media System Performance). There are two important considerations to note:
- The default empirical equations (presented in Appendix D - Modelling Filtration Media System Performance) are based on a review of filtration system performance. The data come from a range of studies around the world, including systems for gravel, sand and soil, some of which include vegetated systems. The music team believes that these empirical data represent the best estimate of the performance of a typical media filtration system.
- Where possible, data derived from the specific type of system to be used should be used to adjust the calibrate the media filtration node. This can be done by editing the parameters of the pollutant removal equations. To do this, click on
in the Advanced Properties component of the node dialogue box. This will bring up a dialogue box allowing you to enter new pollutant treatment equation parameter values:
