Relative Performance of Grit Removal Devices

Matthew BodwellHydro International

November 16, 2015

Objective

• Summarize published industry data on grit removal• Grit Management• Impact of Poor Grit Management• Grit Removal Objectives• Grit Basics

• A unique study comparing performance of most common technologies• Detritus Tank• Aerated Grit Basins (AGB)• Mechanically Induced Vortex (MIV)• Stacked Tray Separator (STS)• Structured Flow System (SFS)

Grit Management

• Grit removal has historically been one of the most unvalued processes at a treatment plant

• Current trend is to place higher value on grit removal• Following same trend as fine screening• Smaller plants – less capacity to inventory grit• Reduced staff – less manpower to remove grit• Reduced budget for repair/maintenance of grit related wear

• Still many challenges:• No industry standard for sampling method• No industry standard for required performance• Conflicting info regarding performance by Manufacturers

• A light at the end of the tunnel:• WEF has shown a renewed interest in Grit and implemented a task force.

Impact of Poor Grit Management

• Takes up volume in process tanks, reduces treatment efficiency• Primaries, anoxic, aeration, digesters, incinerators• Plugs piping

• Accelerates wear on equipment, reduces performance• Collectors and screws• Sludge transfer pumps• Sludge dewatering feed pumps• Digester mixing components• Centrifuges

• Maintenance: manual labor, parts and disposal costs (time & cost)

Grit Removal Objectives

• Prevent unnecessary abrasion and wear• Prevent deposits and accumulations

• Produce a clean/dry product for landfill

Grit Basics - Understanding Grit

• Grit is often assumed to behave (settle) like clean sand in clean water

• Reality – municipal grit DOES NOT behave (settle) like clean sand

• Understanding grit behavior is key to a successful grit removal system design.

• Grit is a common and serious problem for many Wastewater Treatment Plants

Settling velocity affected by:• Size/Shape• Specific Gravity• Fats, Oils & Grease

Grit Basics - Settling Velocity Assumed vs. Measured

Clean Sand

Technology Overview

• Most common technologies• Detritus Tank• Aerated Grit basin (AGB)• Mechanically Induced Vortex (MIV)• Stacked Tray Separator (STS)• Structured Flow System (SFS)

Detritus Tank

• Typical Performance• Removal of 150 micron

• Surface overflow rate based • Square tank with circular scraper

James River TP – Detritus Tank

 % Removal Efficiency

Particle Size

50 Mesh(297 µm)

70 Mesh(211 µm )

100 Mesh(150 µm)

Total % Removal 150 µm and up

Total % Removal 106 µm and up

6/17/07 81.8 72.6 41.7 66.2 57.3

6/18/07 76.9 77.2 66.6 73.2 67.7

6/19/07 82.6 74.7 55.3 71.2 64.2

*Source: McNamara, 2009 WEF

Aerated Grit Basin (AGB)

• Typical Performance• Removal of 212 micron

• Retention time based • Rectangular tank with diffused air to

create rolling motion

Columbus, GA South WRF - AGB

 % Removal Efficiency

Particle Size

50 Mesh(297 µm)

70 Mesh(211 µm )

100 Mesh(150 µm)

Total % Removal 150 µm and up

Total % Removal 106 µm and up

1/27/08 81.8 49.8 42.2 70.5 67.2

1/28/08 53.0 13.5 21.7 35.6 32.5

1/29/08 66.3 60.0 44.4 58.7 53.1

*Low flows seen during HRSD testing, insufficient grit quantities to accurately test.

Mechanically Induced Vortex (MIV)

• Mechanical vortex• Paddle maintains vortex

• Power required • Low headloss• Typical Performance:

• 95% removal of 300 micron particle• 85% removal of 212 micron particle• 65% removal of 150 micron particle

Chesapeake–Elizabeth TP – MIV*Source: McNamara, 2009 WEF

 % Removal Efficiency

Particle Size

50 Mesh(297 µm)

70 Mesh(211 µm )

100 Mesh(150 µm)

Total % Removal 150 µm and up

Total % Removal 106 µm and up

5/17/07 72.6 19.1 7.0 48.1 45.8

5/18/07 77.8 28.9 14.7 52.1 50.9

Virginia Initiative Plant – MIV *Source: McNamara, 2009 WEF

 % Removal Efficiency

Particle Size

50 Mesh(297 µm)

70 Mesh(211 µm )

100 Mesh(150 µm)

Total % Removal 150 µm and up

Total % Removal 106 µm and up

5/20/07 57.7 29.8 22.7 45.3 44.3

5/21/07 60.5 26.8 23.2 45.1 43.7

5/22/07 59.3 33.2 27.9 43.3 43.3

Stacked Tray Separator

• All hydraulic induced vortex system• No power requirements• No moving parts• Surface overflow rate based sizing • Performance

• As low as 95% removal of 75 micron

Army Base TP – Stacked Tray *Source: McNamara, 2009 WEF

 % Removal Efficiency

Particle Size

50 Mesh(297 µm)

70 Mesh(211 µm )

100 Mesh(150 µm)

Total % Removal 150 µm and up

Total % Removal 106 µm and up

12/17/07 95.8 90.4 81.5 91.9 88.8

12/19/07 95.7 93.0 85.6 92.5 89.3

Structured Flow System

• All hydraulic induced vortex system• No power requirements• No moving parts• Flow stabilizing internal components• Performance

• As low as 95% removal of 75 micron

Army Base TP – Structured Flow *Source: McNamara, 2009 WEF

 % Removal Efficiency

Particle Size

50 Mesh(297 µm)

70 Mesh(211 µm )

100 Mesh(150 µm)

Total % Removal 150 µm and up

Total % Removal 106 µm and up

12/17/07 93.6 89.4 78.7 90.3 87.5

12/19/07 97.4 94.3 89.0 95.0 92.7

Relative Performance of Grit Removal Devices

Technology% of Design Flow When

Tested

Design Removal Efficiency at

100% of Flow

Observed Total % Removal

150 µm and up

Observed Total % Removal

106 µm and up

Detritus Tank 66 150 µm and larger, 2.65 SG 66 to 71 57 to 68

AGB 66 to 100 Unknown 35 to 70 32 to 67

Mechanically Induced Vortex

27 to 90

95% removal of 270 µm, 2.65 SG

65% removal of 150 μm, 2.65 SG

43 to 52 43 to 50

Stacked Tray 100 95% removal of 75 µm, 2.65 SG 91 to 92.5 89 to 90

Structured Flow Vortex 66 to 100 95% removal of

106 µm, 2.65 SG 90 to 95 87 to 93

General Testing Observations

• Testing method consistent for devices which produced repeatable & accurate results

• Performance:• All systems saw reduced efficiency as flow rate increased

- Indicates gravity is the prevailing force in all devices

• AGB & MIV units had lowest removal efficiencies despite operating well below rated flows

• Stacked Tray & Structured Vortex units had highest capture efficiencies; 20 to 55% higher than any other technology

• Grit settling velocity should be considered when designing grit removal systems

• Observed grit quantities increased significantly due to wet weather conditions

30 – 50% Removal is not Sustainable

Owners/Operators expect better performance

Summary

• Grit Removal System Design Recommendations • Know your grit!

- Consider performing grit characterization prior to selecting a grit removal technology or utilizing regional grit gradation data

• Design grit removal systems for peak flow and peak grit load

• Select grit removal technology which yields the highest value - What are the plant drivers?

- Footprint?- Grit Quality?- O&M Cost?

Where can you find more information??

• Paper

• WEFTEC 2014, Session 407 – Attendee Access Available

• NC AWWA-WEA 2015 Annual Conference Proceedings

Matt BodwellHydro International

Wastewater Divisionmbodwell@hydro-int.com

(207) 776-1949