Dropzone Newsletter

By: Ben Farrell - Imbrium Stormwater Design Engineer

Sediment (TSS) in urban stormwater runoff causes many water quality, habitat and aesthetic problems in our waterways. Various pollutants of concern, such as metals, nutrients, hydrocarbons, and bacteria, are typically adsorbed onto the sediment particles. Filtration devices are often used to greatly reduce the pollutant loads in runoff. When sizing a filter it is important to consider both the water quality flow rate and the annual sediment mass load, which can vary widely depending on land use.

For the purposes of demonstrating the effect of annual sediment mass load on filter sizing, four Jellyfish® Filter sizing simulations were analyzed. All sizing parameters, except for the annual sediment mass loading rate (kilograms per hectare per year), have been held constant for each of the four sizing simulations. The sediment loading rates were determined by using data from land use research published by US EPA. Four land use types (Commercial, Highway, High Density Residential (HDR), and Low Density Residential (LDR)) and their corresponding annual sediment mass loads were selected. These simulations did not account for a treatment train design (for example, the use of an oil-grit separator upstream of the filter system), but rather sized the filter as a stand-alone treatment system.

The results of the simulations highlight the importance of selecting the correct land use parameter for the project, assuming the associated annual sediment mass load is reasonably representative of actual site conditions. Three of the four simulations resulted in the final filter design being dominated by the annual sediment mass load, rather than the water quality flow rate. We call these three systems “mass limited”. In a “mass limited” system, the size of the system and number of cartridges required are selected to ensure at least a twelve month maintenance interval based on annual sediment load; the system size and number of cartridges are larger than that required to simply treat the water quality flow rate. Conversely, only the Low Density Residential land use type resulted in a “water quality flow limited” system due to the low sediment loading rate. The “WQ flow limited” system results when the filtration device is sized based on the required treatment flow rate, or the flow rate associated with 90% average annual runoff volume in this case. The results of the four sizing simulations can be found in Table 1.

As stormwater knowledge continues to advance, so must the implementation and enforcement of sizing and design methodologies of proprietary and non-proprietary stormwater treatment systems. Sizing filtration devices based on water quality flow rate alone has generally been the industry standard, but in many cases, sizing solely for the water quality flow rate only provides half the picture, and can lead to an increased maintenance frequency. In addition, various types of filtration systems function differently, so they are largely not interchangeable once specified. The sediment mass loading rate is the critical parameter that completes this picture. Both treatment flow rate and sediment mass loading rate parameters must be considered when designing filtration systems.

Table 1:

Constant Parameters
Rainfall Station
Toronto Central ON 100
Site Area
1 hectare
Imperviousness
90%
Annual Runoff Volume Capture
90%
Treatment Flow Rate
25.7 L/s

Experimental Parameter
Land Use

Commercial

Highway

High Density Residential
Low Density Residential
Sediment Loading Rate (kg/ha/yr) *
1120
986
480
11

Jellyfish® Filter Sizing Results
Land Use Type

Commercial

Highway

High Density Residential
Low Density Residential
Annual Sediment Capture Required
1120 kg
986 kg
480 kg
11 kg
Annual Sediment Capture Provided **
1138 kg
1024 kg
512 kg
313 kg
Water Quality Flow Rate Capacity Provided ***
101 L/s
90.9 L/s
45.4 L/s
27.8 L/s
Mass Sediment Limited or WQ Flow Limited
Sediment Limited
Sediment Limited
Sediment Limited
Flow Rate Limited

*Source: Nationwide Urban Runoff Program (US EPA 1983)
**Annual Sediment Capture based on Sediment Design Capacity of 54" Cartridges
***Water Quality Flow Rate Design Capacity based on Treatment Rate of 54" Cartridges


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Posted in: 2015, May 2015

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