Chemical Engineering Department Government Engineering College Bhuj Prepared by: Kumar Satyam Guided By: Prof. Sejal J. Solanki Chemical Engineering Department Filtration And Sedimentation B.E. (Chemical)- Sem-V Subject : Mechanical Operations Subject Code:

Unit Processes Designed to Remove Particulate Matter  Screening  Sedimentation  Coagulation/flocculation  Filtration  slow sand filters  rapid sand filters  diatomaceous earth filters  membrane filters

Conventional Surface Water Treatment Screening Coagulation Flocculation Sedimentation Filtration Disinfection Storage Distribution Raw water Alum Polymers Cl 2 sludge

Sedimentation  the oldest form of water treatment  uses gravity to separate particles from water  often follows coagulation and flocculation reservoirs

Sedimentation the vital one is the sedimentation process, the process conventionally used is the tank sedimentation process as described below Sludge Zone Inlet Zone Outlet Zone V o = Q/A s The overflow rate H

Sedimentation: Effect of the particle concentration  Dilute suspensions  Particles act independently  Concentrated suspensions  Particle-particle interactions are significant  Particles may collide and stick together (form flocs)  Particle flocs may settle more quickly  Particle-particle forces may prevent further consolidation

projected Sedimentation: Particle Terminal Fall Velocity Identify forces

Sedimentation Basin: Critical Path Horizontal velocity Vertical velocity L H Sludge zone Inlet zone Outlet zone Sludge out A = WH Q = flow rate (property of the particle) (property of the tank)

Sedimentation Basin: Importance of Tank Surface Area L H W Want a _____ V c, ______ A s, _______ H, _______ . small large Time in tank smalllarge

Conventional Sedimentation Basin  long rectangular basins  4-6 hour retention time  3-4 m deep  max of 12 m wide  max of 48 m long Settling zone Sludge zone Inlet zone Outlet zone Sludge out What is V c for this sedimentation tank?

Settling zone Sludge zone Inlet zone Outlet zone Design Criteria for Sedimentation Tanks Minimal turbulence (inlet baffles) Uniform velocity (small dimensions normal to velocity) No scour of settled particles Slow moving particle collection system Q/A s must be small (to capture small particles) This will be one of the ways you can improve the performance of your water treatment plant.

Typical design overflow rates All particles with a settling velocity greater than or equal to the overflow rate will be 100% removed. Particles with a lesser settling velocity will be removed to a fractional extent. Those that happen to enter the settling zone near the bottom will be completely removed and those that enter near the top will not. for settling tanks are: gpd/ft 2 for settling tanks are: gpd/ft 2

Sedimentation Tank Configurations  Rectangular Clarifiers  most common  Circular clarifiers  Center feed  Peripheral feed  Flocculator-Clarifiers

Sedimentation Tank Design  Side water depth: about 12 ft.  settling velocities: 2-6 ft/hr  retention time: 4-8 hr  overflow rates: gpd/ft 2 or gpm/ft 2  linear velocities: less than 0.5 ft/min

Typical rectangular clarifier

Sedimentation Tank Configurations  Rectangular Clarifiers  most common  Circular clarifiers  Center feed  Peripheral feed  Flocculator-Clarifiers

Filtration  A “polishing” solid/liquid separation step  Intended to remove particles  Other impacts  biodegradation  organics adsorption (especially to GAC)  Mn and Fe adsorption

Types of Filtration  Granular media filters  slow sand filters  rapid sand filters  high-rate granular media filters  Membrane filters  microfiltration, ultrafiltration, nanofiltration  Cake filtration  diatomaceous earth

Slow Sand Filtration  First filters to be used on a widespread basis  Fine sand with an effective size of 0.2 mm  Low flow rates ( cm/hr)  Schmutzdecke (_____ ____ ) forms on top of the filter  causes high head loss  must be removed periodically  Used without coagulation/flocculation! filter cake

Diatomaceous Earth Filters  Diatomaceous earth (DE) is made of the silica skeletons of diatoms  DE is added to water and then fed to a special microscreen  The DE already on the microscreen strains particles and DE from the water  The continuous DE feed prevents the gradually thickening DE cake from developing excessive head loss  Was seriously considered for Croton Filtration Plant

Membrane Filters  Much like the membrane filters used to enumerate coliforms  much greater surface area  Produce very high quality water (excellent particle removal)  Clog rapidly if the influent water is not of sufficiently high quality  More expensive than sand and DE filters

Rapid Sand Filter Sand Gravel Influent Drain Effluent Wash water Anthracite Size (mm) Specific Gravity Depth (cm) 30 45

Particle Removal Mechanisms in Filters Transport Attachment Molecular diffusion Inertia Gravity Interception Straining Surface forces

Sand Gravel Influent Drain Effluent Wash water Anthracite Backwash Only clean water should ever be on bottom of filter

Ways to Improve Filtration  Filter to waste  Extended Terminal Sub-fluidization Wash  Alum feed directly to filter  Potato starch

Deposition in a Filter

Filter Design 4.9 m/hr = 2 gpm/ft 2 From Tobiason, 1995

References-  Tobiason, edition 1995, sterling publications  Monroe L. Weber Shrink, Cornell University  Korich et al., 1990  Venczel et al., 1997  Finch et al., 1994  Owens et al., 1994  waterwastemanagement waterwastemanagement 