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Effective Headworks for Grit Removal

Tuesday, August 11th, 2015

Hazen and Sawyer

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In the past, grit removal of raw sewage influent wasn’t given much thought

■ Design guidelines and performance an afterthought● Wasn’t strictly regulated● Designers weren’t experiencing end results● Clients weren’t expressing disappointment

■ Manual removal once considered part of the job

■ Demand for higher removal rates of “sugar sand” prompted new technologies

■ Demand to prove performance a more recent development

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Why care? Downstream processes strip the organics so inorganics settle somewhere!

Loss of treatment volume could be a regulatory concern

Process becomes less efficient if covered in grit

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And eventually that grit has to be cleaned out. Many times that’s a huge task!

With Primary Settling –You have a digester cleaning problem!

Without Primary Settling –You have an aeration tank cleaning problem!

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Basics of grit removal have two steps:

GRITREMOVAL

GRITSEPARATION

Collected GritVolatile Content = < 20%Solids Content = > 60%

Raw Sewage Influent

DegrittedEffluent to Main

Process

Degritted Drain Return

High Capture,Clean & Dry

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Common Types of Grit Removal Equipment

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Manual removal in process tanks

■ Simple■ Labor intensive■ Concentrated cost■ Best in “wide spots”

in the process

Removal and separation are done at the same time

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With automated systems both removal and separation have to be efficient

RemovalTransfer

Separate

Dispose

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Detritor Grit Removal

■ Similar to a primary clarifier

■ Simple design

■ Removal/Separation in one

■ Good removal at ADF

■ Large area

■ Difficult to elevate

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Air assist grit removal

■ Good removal

■ Large, long footprint needed to settle particles targeted

■ Difficult to elevate

Air In

Grit Out

Improvement to:

■ Rectangular Flow

■ Horizontal Flow

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Vortex Type

■ Very common for years

■ Smaller foot print

■ Easy to elevate

■ Effective in smaller sizes but reduced performance for larger flow and smaller particles

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Smith & Loveless has modified the design of their vortex unit to get higher grit removal

Flow enters the unit by a downward sloping ramp that increases the downward velocity above simple gravity settling

A patented baffle (referred to as the “V-Force Baffle” separates the grit from the wastewater flow.

Another patented feature is the flat bottom plate separating the upper chamber from the lower grit collection chamberBased on these modification the manufacturer claims higher grit

removals than conventional mechanically induced vortex units

Forced Vortex Type

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Hydraulically Induced Vortex

■ Similar footprint to other vortex types

■ Better efficiency for smaller particles

■ Sensitive to minor adjustments

■ Sole Source Costs

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Multi-Tray Gravity Type

■ Smallest footprint

■ Easy to elevate

■ Good removal efficiency

■ No moving parts

■ Higher head loss

■ Continuous pumping

■ Sole source costs

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Grit Removal Equipment Comparison

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Grit Removal Equipment Comparison (cont.)

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Turndown considerations important because of typical collection system tendencies

Effective grit removal systems must function at peak flow and peak grit load

■ Grit remains in the collection system until transported to the plant

■ Flow/velocity suspends grit

■ Low flow moves only small and light grit

■ First flush significantly increases grit load

SuspendedLoad

Bed Load

Deposited Load

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Grit Separation Devices

Cyclones and Classifiers

Grit Washers

TeaCup

SlurryCup

Grit Snail

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Grit Separation by Cyclone and Classification

■ Very Common

■ Cyclone to remove organics

■ Classifier to settle grit and remove by screw conveyor

■ Dual cyclones for backup

■ Oversize classifier for improved performance

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Grit Separation by Grit Washer

■ Consists of an oversized circular classifier

■ PW or NPW is used to create a fluidized bed of settled grit

■ Fluidized bed creates contact between grit particles breaking away organic material

■ Clean grit is removed through the screw conveyor

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Grit Separation by TeaCup

■ Uses a free vortex concept to capture, classify and remove grit

■ Naturally removes organics reducing odors

■ Proprietary technology

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Grit Separation by SlurryCup

■ Uses a open free vortex to capture, classify and remove grit

■ Grit underflow passes through a hydraulic valve providing secondary grit washing

■ Proprietary Technology

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Grit Separation by Grit Snail

■ Uses step cleats in lieu of a screw conveyor (problematic)

■ Slow moving screw conveyor now available (SpiraSnail)

■ Oversized and slow moving to reduce overflow of grit back to the system

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Grit conveyance by pumping(from removal unit to separation unit)

Self-Priming (Installed

above)

Air Lift Pump (Installed Above)

Recessed Impeller (Installed Below)

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Effective Grit Removal Design

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Steps necessary for successful and effective grit removal design

■ Speak with client’s staff

■ Conduct grit characterization

■ Select appropriate process

■ Follow guidelines for the selected process

■ Incorporate performance criteria specs

■ Include grit removal tasks in contractor’s scope

■ Review plans for tests during submittal review

■ Witness testing and assist in interpreting results

■ Follow-up after system in operation for some time

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Client’s staff sets the agenda for the design

■ Practices already in place for controlling grit

■ Impacts of unremoved grit on down stream equipment

■ Range of flow and when is grit load the highest

■ Budget limits

■ Space constraints

■ Level of knowledge on available options

■ Preferences for processes and equipment

■ Relationships with equipment suppliers

LISTEN THEN DELIVER = SUCCESS!

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Not all grit is the same …

“…grit, consisting of sand, gravel, cinders, or other heavy solid materials that have subsiding velocities or specific gravities substantially greater than those of the organic putrescible solids,… Grit also includes eggshells, bone chips, seeds, coffee grounds, and large organic particles, such as food waste.” (Metcalf & Eddy 2nd Edition 1979)

“ … grit is traditionally defined as particles larger than 0.21 mm (0.008 in) (65 mesh) and with a specific gravity of greater than 2.65” (U.S. EPA, 1987).

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Wastewater grit is not sand – poor design could lead to as little as 6% removal!

Clean Sand Real Grit

SES

Settling Rate

Same Size –Slower Settling Rate

Settles Like a smaller particle

Grit Characterization is essential for good design

Particle Size (microns)

Aggregate Class Time  Required to Settle 1’SG = 2.65

Time Required to Settle 1’SG = 1.35

100 Very Fine Sand 38 Seconds 2 min. 48 sec.

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Two methods of grit characterization

■ Black Dog Analytical, Illinois, (815)715-3123

■ Grit Tech, Kansas, (913)894-6144

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Recent grit characterization study

■ Not expensive■ Does not take much time

For 95% removal –

must design for >105

micron particles

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Grit Classification

Raw sewage only

Raw with RAS

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Target Grit Removal

95% removal at 105 microns

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Select a process that meets removal goals based on actual grit type and load

■ Larger grit – more options available

■ Smaller grit – options limited to meet goals

■ Consider automated versus manual removal

■ Select equipment that meets goals at each step● Removal from process flow● Separation and cleaning equipment● Recycle of drains from separation step

■ Identify potential suppliers and begin communications

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Follow applicable guidelines for selected process – get buy-in from suppliers

■ Channel minimum and maximum velocities

■ Hydraulic head calculations up and down stream

■ Materials of construction

■ Chamber and channel geometry

■ Minimum pressure and range for NPW lines

■ Control of process equipment and conditions

■ Coordination of equipment sizing

■ Access ports for covered areas

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Incorporate good grit pumping and piping design practices

■ Keep suction piping as short (10’ to 15’)

■ Pipe Velocity: 6 to 8 fps, min. 4”

■ Provide cleanouts and flushing connections

■ Avoid check valves

■ Pinch valves for isolation (full port plugs ok)

■ Glass lined piping

■ Long radius bends

■ Victaulic type couplings

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Utilization of 3D CADD

■ Grit piping■ Screenings discharge■ Drains■ NPW■ Odor Duct■ Aeration

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Develop performance specifications

■ Specify removal rate: 95% of xxx micron particles

■ Require for both removal and separation units

■ Specify minimal criteria for organic separation

■ Specify means of testing expected● Where to take samples● How to take samples● How to test for compliance

■ Get manufacturers to review and agree with criteria and testing requirements

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Other grit system design considerations

■ Odor control ventilation● Even stainless will corrode with H2S

■ Electrical classification● Location of electrical gear● LEL gas detection in grit pump rooms

■ Corrosion resistant coatings for concrete

■ Grease removal

■ Specs for removal of grit in existing basins?

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More grit design considerations

■ Equipment redundancy

■ Flow sequencing for peak flows

■ Flow equalization

■ Flow splitting

■ Channel aeration to keep grit suspended

■ Separation of recycle flows after raw sampling

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Your work doesn’t stop at design …

■ Measurement and payment for grit removal

■ Thorough review of submittal – last chance to get it right

■ Contractor’s proposed plans for tests in the submittal

■ Witness testing and assist in interpreting results

■ Follow-up after system in operation for some time

■ Learn and do the next presentation!

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Questions