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Downsize Your Design:Filtering vs. Settling for TSS Removal
Ian Jewell
NCAPWA Stormwater Management Division Conference
October 23, 2018
CONTENTS
• The Challenge of TSS Removal
• Filtering vs. Settling Overview
• Case study: Fine Sediment Removal at Industrial Facility in Coastal Plain, NC
THE CHALLENGE OF TSS REMOVAL
• INTERNATIONAL STORMWATER BEST MANAGEMENT PRACTICES (BMP)
DATABASE (2011) :
Whereas most particles with diameters greater than 75µm and densities similar to sand
are easily removed through sedimentation and filtration in stormwater BMPs, fine
particles and dissolved solids are more challenging to remove.
Most SCMs do not remove 85% of TSS, especially at
lower concentrations of TSS in the influent.
• FROM NCDEQ STORMWATER DESIGN
MANUAL (2017) :
TSS REMOVAL APPROACHES
Settling
• Construction/Post-Construction for removal of
most solids
• Combine with flocculant can be very
effective
• Sediment basin or skimmer basin -construction
Detention pond- post-construction
Pros:
• Relatively inexpensive
• Ease of maintenance and access
Cons:
• Large footprint needed for maximum
efficiency
• Finest particles not sufficiently removed
Source: NC DEQ Erosion and Sediment Control Planning and Design Manual
TSS REMOVAL APPROACHES
Filtering
• Particles physically filtered
• Higher sediment removal efficiency
• In “treatment train”
• Space limitations
• Sand Filters, Proprietary Devices
Pros:
• High TSS Removal rate
• Smaller Footprint
Cons:
• More expensive
• Construction
• Access and maintenance
Source: NC DEQ Stormwater Design Manual (2017)
SETTLING VS. FILTERING CASE STUDY
INDUSTRIAL FACILITY IN COASTAL PLAIN NC
DESIGN CONSIDEATIONS/CONSTRAINTS
• Shipping container
lot
• Removal target:
Very fine gravel
sediments
• No new BUA or
disturbed area
CONSTRAINTS
• Minimize footprint
needed for SCM
• No new “waste
stream”
• Surrounded by
wetlands
• Ease of
maintenance
SETTLING APPROACH• Relatively large basin
needed for removing
fine sediments
• Removal efficiency
still low
FILTERING APPROACH• “Treatment train”
filtering concept
• Series of
progressively smaller
filters
• Open basin concept
for maintenance
and cost
DESIGN OVERVIEW
SEDIMENT
BAG
OUTLET
FOREBAY
SKIMMER
• “Hybrid solution”
• Elements of construction and post-
construction SCMs
• Basin downsized to capture “first
flush” volume for filtration, not
settling
HORIZONTAL
MANIFOLD
FILTER 1: SLOTTED INTAKE MANIFOLD
https://www.rstinstruments.com/PVC-
Wellscreen-Pipe.html
FILTER 2: SKIMMER
FILTER 3: SEDIMENT BAG
Source: http://www.erosionsupply.com/images/site_photos/sed1.jpg
COMPONENTS: FOREBAY
MAINTENANCE CONSIDERATIONS
• Manifold and skimmer wrapped in filter
fabric
• Replace fabric
• Concrete maintenance shelf for
sediment cleanout
• Dual sediment bag approach
• Bags reusable or disposable
TAKEAWAYS
• Filtering good approach to downsize SCM while still maintaining
removal efficiency
• “Treatment Train approach” of filtration required ¼ size of settling
basin
• Elements of construction and post-construction SCMs can be
hybridized to target specific pollutants
• Think “outside the manual” in specialized applications