Downsize Your Design: Filtering vs. Settling for TSS...

Downsize Your Design:Filtering vs. Settling for TSS Removal

Ian Jewell

Ian.jewell@freese.com

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

Questions?

Ian JewellIan.Jewell@freese.com