Relative Performance of Grit Removal Devices Matthew Bodwell Hydro 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/ /18/ /19/ *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/ /28/ /29/ *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/ /18/

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/ /21/ /22/

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/ /19/

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/ /19/

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 µm and larger, 2.65 SG 66 to 7157 to 68 AGB66 to 100Unknown35 to 7032 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 5243 to 50 Stacked Tray100 95% removal of 75 µm, 2.65 SG 91 to to 90 Structured Flow Vortex 66 to % removal of 106 µm, 2.65 SG 90 to 9587 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 Bodwell Hydro International Wastewater Division (207)