ENG421 (6abc) – Screens and Grit Removal Screens and Bar Racks Screening : design considerations Screening : design example Grit removal : design considerations Grit removal : design example

General Water Treatment Technologies (Week 4) Treatment technologies (unit operations and processes) used determined by what needs to be removed, inactivated or modified

Screens and Bar Racks (1 of 7) Screening is first step in water treatment Usually a simple device at surface water intake Grades or separates particulate matter by size Particulate matter may : - interfere with proper working of pumps, pipes, or mechanical equipment - damage pumps, pipes, or mechanical equipment Water intake points often located below surface of lake/river to minimise : - intake of floating objects - damage from ice - shore vegetation

Screens and Bar Racks (2 of 7) Suspended matter : - sticks - branches - leaves - plastics - cloth - paper - dirt, soil, mud, fine particles causing turbidity Removal via two steps : 1. large solids removed by screens (opening 0.8 – 100 mm) coarse screens medium screens fine screens 2. separated suspended matter (called screenings) disposed via : comminution (grinding/blending) incineration then landfill composting for agricultural use

Screens and Bar Racks (3 of 7) Types of screens - Bar or rack screens (most common) sets of parallel bars predetermined spacing between bars stationary or vibrating may be self-cleaning

Screens and Bar Racks (4 of 7) Types of screens (cont) - Revolving screens (trommel screen) revolving cylindrical frame surrounded by wire cloth open at both ends

Screens and Bar Racks (5 of 7) Types of screens (cont) - Shaking screens rectangular frame lined with wire cloth may be used in conjunction with conveying system

Screens and Bar Racks (6 of 7) Types of screens (cont) - Vibrating screens high capacity and high efficiency mechanically or electrically powered

Screens and Bar Racks (7 of 7) Types of screens (cont) - Oscillating screens low speed oscillation often used with silk cloth

Screening Screens and racks strain out floating and suspended matter material removed is larger in size than opening Strainer Rack : parallel bars or rods Screen : wire mesh and perforated plates coarse screens remove bulky material fine screens remove smaller matter leaves, twigs, fish microscreens remove algae and plankton

Screening – design considerations (1 of 5) factors to consider : type of raw water size of suspended and floating objects location of intake entrance location of screen type of screen area of screen based on flow velocity and headloss during screening cleaning arrangements screenings (i.e. collected material) disposal

Screening – design considerations (2 of 5) area of screen flow velocity across screen must avoid carry-through of retained material (aka wash off) manually cleaned screens : max velocity 0.61 m/s mechanically cleaned screens : max velocity 1.0 m/s more reliable, more frequent, less build up of material may be kept below 0.1 m/s to protect small fish design flow rate : maximum hourly rate x 1.1 (1.1 factor allows for fluctuation and waste) flow velocity in rack chamber must avoid grit settling 0.4 – 0.75 m/s

Screening – design considerations (3 of 5) area of screen headloss across screen function of open, blocked and submerged areas of screen usually not significant compared with headloss across entire plant

Screening – design considerations (4 of 5)

Screening – design considerations (5 of 5)

Screening – design example (1 of 7)

Screening – design example (2 of 7)

Screening – design example (3 of 7)

Screening – design example (4 of 7)

Screening – design example (5 of 7)

Screening – design example (6 of 7)

Screening – design example (7 of 7) Design Summary : Typical values Design Value Flow velocity in chamber (m/s) 0.4 – 0.75 0.59 Flow velocity through the screen (m/s) 0.6 – 1.0 0.83 Headloss (mm head) 150 max. 25

Grit Removal Grit includes : sand heavy particulate objects, e.g. silt, gravel. Shells Grit chambers shallow elongated basins (up to 20 m) collects particles > 0.2 mm diameter specific gravity > 2.65 very low flow rate enables high density particles to settle at bottom also used in wastewater treatment

Grit Removal – design considerations (1 of 8) factors to consider in grit chamber design : size of grit to be collected size of basin shape of basin number of basins removal and disposal of grit Located as close as possible to raw water intake usually immediately after the screens

Grit Removal – design considerations (2 of 8) Types of grit chambers : - velocity controlled uses horizontal flow pattern low horizontal velocity gives sufficient time for particles to settle increase theoretical design length by 50% allows for inlet and outlet turbulences, and shock loads

Grit Removal – design considerations (3 of 8) Types of grit chambers : - velocity controlled (cont)

Grit Removal – design considerations (4 of 8) Velocity Controlled Grit Removal – design data

Grit Removal – design considerations (5 of 8) Velocity Controlled Grit Removal – ideal design

Grit Removal – design considerations (6 of 8) Types of grit chambers (cont) : - aerated uses injection of compressed air to create turbulence separates organic and inorganic solids lighter organic material kept in suspension heavier inorganic grit falls to bottom due to rolling motion of water turbulence

Grit Removal – design considerations (7 of 8) Aerated Grit Removal – design data

Grit Removal – design considerations (8 of 8) Types of grit chambers (cont) : - constant level short term sedimentation circular and shallow rapidly rotating scraping devices grit and organics not separated not common due to poor efficiency

Grit Removal – design example (1 of 4)

Grit Removal – design example (2 of 4)

Grit Removal – design example (3 of 4)

Grit Removal – design example (4 of 4) Design Summary : Typical values Design Value cross flow velocity (m/s) 0.04 – 0.1 0.05 surface loading rate (m/h) 10 – 25 25 hydraulic retention time (min) 5 – 15 8 total length to width ratio 3:1 – 8:1 3.7:1 total length to depth ratio 6:1 – 10:1 10:1

References Droste, R.L., 1997, Theory and Practice of Water and Wastewater Treatment, John Wiley and Sons, New York (TD430D76 1997), pages 181 - 207, 219 – 227 Hendricks, D., 2006, Water Treatment Unit Processes, CRC, New York (TD430H46 2006) , pages 121 - 138, 201 – 231 Nemerow, N.L. et al, 2009, Environ Eng : Water, Wastewater, Soil and Ground, 6th ed., John Wiley and Sons, New York (TD430 .E58 2009), pages 208 - 209 Qasim, S.R., 1999, Wastewater treatment plants : planning, design, and operation, Technomic, Lancaster, USA (TD746 .Q37 1999), pages pages 185 - 214, 291 - 322 Viessman, W. et al, 2009, Water Supply and Pollution Control, 8th ed., Pearson, Upper Saddle River, pages 322 - 324