Dropzone Newsletter

By: Jordan Kahlenberg – Sr. Stormwater Design Engineer

As a Sr. Design Engineer with Imbrium Systems, I receive questions every day regarding projects which include specifications for Oil Grit Separator (OGS) devices. Often these questions stem from having to meet a project’s specifications and requirements for pollutant removal performance. In order to provide the best solution, I require great detail about how the device is required to operate and perform, and too often the specifications provided fall short of properly defining the details I desire. Below I touch on one of those desired details: Scaling.

When comparing and contrasting all of the various different OGS devices on the market today, there are several key factors to consider: Testing procedure used to gather performance data, Particle Size Distribution (PSD) used when testing performance, Event Mean Concentration (EMC) of Total
Suspended Solids (TSS) entering and exiting the unit, as well as others. One often overlooked piece is the scaling method used to determine the performance of a full product line.

When testing an OGS device in order to evaluate its performance, manufacturers typically test just one size or model. Testing results typically evaluate various flow-rates (Manufacturer’s Treatment Flow Rates, or MTFRs) and a TSS removal Percentage to the tested unit. In order to assign performance values or ratings to the full range of sizes, the performance must be scaled across the range. The two most common ways to scale performance results for an OGS are by surface area (Surface Area Scaling) or volume (Volumetric Scaling).

Surface Area Scaling looks at the settling area of a given OGS device. By taking MTFRs assigned to the tested unit, and dividing that by the unit’s settling area, the result is a Surface Area Loading Rate. For example, if a unit’s MTFR is 30 l/s, and the settling area is 2.6m2, the Surface Area Loading Rate for that unit is 11.5 l/s/m2. The Surface Area Loading Rate can then be applied to other unit sizes to determine their respective Treatment Flow Rate for which similar performance can be expected (as compared to the tested size).

Volumetric scaling looks at the settling volume of a given OGS device. By taking a MTFR assigned to the tested unit, and dividing by that unit’s settling volume, the result is a Volumetric Loading Rate. For example, if a unit’s MTFR is 30 l/s, and the settling volume is 4.7 m3, the Volumetric Loading Rate for that unit is 6.4 m3. The Volumetric Loading Rate can then be applied to other unit sizes to determine their respective Treatment Flow Rate for which similar performance can be expected (as compared to the tested size). It should be noted that Volumetric Scaling of OGS devices should require test data validation by comparing results from differing size units of the same product line from the manufacture. Volumetric scaling is felt too be a more aggressive method, and likely non defensible without having tested multiple sized units (“Evaluation of Hydrodynamic Separators”, published by Stormwater Equipment Manufacturers Association, September 2010).

Example:

The table below demonstrates the difference between Surface Area and Volumetric Scaling. Notice that the volumetric flow or MTFR and Surface Loading Rate each are double when compared to the MTFR determined based on surface area scaling. Is this exponential increase in a MTFR or Surface loading
Rate justifiable with the treatment unit process of gravity settling, without additional unit size testing for
validation?

Tested
Model
Surface Area
Scaled Model
Volumetric
Scaled Model
Diameter (mm)
1200
2400
2400
Depth (mm)
1200
2400
2400
Surface Area (m2)
1.17
4.66
4.66
Volume (m3)
1.42
11.38
11.38
Manufacturer’s Treatment Flow Rate (L/s)
12.62
50.47
100.94
Surface Loading Rate (L/s/m2)
10.8
10.8
21.7

Table 1: Comparison between Surface Area Scaling and Volumetric Scaling, demonstrating the aggressive nature of Volumetric Scaling

It should be noted that the units are all geometrically proportional to each-other, another important factor to consider when scaling test performance. In order for a scaling method to be valid, the different sizes of a tested OGS device should be geometrically similar, with each successive size proportional to the next. Typically some variance to this is acceptable, usually around 15%.

Next time your project requires an OGS device to meet water quality goals, be sure to consider scaling methods when comparing your options. By defining details such as Scaling (Surface Area or Volumetric), along with PSD, and TSS removal performance, you can make it easier down the road to choose the best option and for engineers like myself to provide the best solution to meet your project’s needs and requirements.



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Posted in: Feburary 2015, 2015

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