1. Waste Treatment, Physical
ENVE 649

2. Physical Separation To remove particles
Particles of contaminant
Particles generated by chemical or biological treatment.
Change phase of contaminant, or
Transfers contaminant from one phase to another

3. Typical Process Flow In Settle Screen Filter Add Chemical Out
Showing three main physical methods: Screening, Settling, and Filtering. Discussed more below. Red indicates where contaminant may be removed.
Screen
Filter
Add Chemical
Out

4. Screening Very coarse material Trash
Screening is used for separation of mixture of particles of different sizes.
Used when more homogeneous material composition is required. Usually two screening materials are used:
perforated sheets (metal, rubber, and plastic)
woven wire mesh
Go to Industrial Screens, then catalog and look at some varieties of screen via the links on left.

5. Sedimentation Particles settle by gravity
Slowed by friction, drag coefficient Cd
Terminal velocity of a particle:
So V depends on square of D
Mu, viscosity, depends on temperature
V AKA Vc = terminal velocity. And g=gravity constant, rho is density of particle and water, D=diameter of particle and mu is viscosity.

6. Sedimentation, zone settling
Supernatant
Hindered
Left is “muddy water” middle after it has been settling for a while, and right is when settling is completed.
Compacted

7. Hindered Settling Interface Height Compaction Transition Time -->
The process might be known by the stages, here the interface (bottom of supernatent layer) height is decreasing with time.
Time -->

8. Remove material at the transition zone to speed settling Clarifiers
Circular
Rectangular
Photos of Photos of some treatment plants and processes
This is municipal sewage, not hazardous waste, but it illustrates the process. The square pools in the background are clarifiers. The tour calls the primary settling and secondary settling.

9. Centrifuges
Increase “g” of the settling velocity equation.

10. Centrifuges See “Sludge Dewatering” for cut-away view of a centrifuge.

11. Flotation Low density particles Hydrocarbon solids
Finely divided air bubbles float material to surface, then skim
Flocculants form flocs with air bubbles
This method is limited to material that is less dense than water (or the matrix medium). It simply relies on the tendency for less dense molecules to float to the top of the solution. This method assumes that when the two substances separate, there is a sharply defined boundary between each substance and there can be no chemical reaction between the two. This is also limited somewhat to the method of skimming the fluid off the top.

12. Filtration Filters plug. Sand filters “Cloth” filters, etc.
How do you backwash?
Filtration: Simple method to remove particles, natural or produced from previous process (ex. Coagulation).
Filters plug: Maintenance is required for all types of filters. They are cleaned, replaced, or back-washed once they are clogged.
Sand Filters: Rapid vs. Slow
Slow sand: Oldest method for treating waste water. You need lots of space and time to treat water. Labor intensive because top layer needs to be cleaned frequently. Not good for “heavy loads”.
Rapid sand: Frequently used to treat water that has previously been treated by sedimentation. Rates are faster and filters are smaller but it’s not as effective for removing bacteria, tastes, and odors. Filters can be cleaned by back-washing.
Cloth Filters: Many other materials used for many different applications. Some boast “100% reclaimed materials” and others “BIG Texas size”

13. Modes of Filtration Mechanical straining adhesion flocculation
sedimentation or inertial impaction

14. Filtration Process Variables
Media grain size, shape, density and composition
Media porosity
Head loss
Bed depth
Filtration rate
Fluid characteristics
Influent characteristics
Solids concentration
particle size distribution
Floc strength
particles charge

15. Removal Mechanisms Mechanical straining Sedimentation
Inertial impaction
Chemical adsorption
bonding
chemical reaction
Physical adsorption
Electrostatic forces
Adhesion and adhesion forces
Coagulation-flocculation
Biological growth

16. Sorption Physical adhesion = adsorption
Penetration of molecules into an absorbent
Adsorption
Carbon (charcoal) filter
organics
Absorption
water absorption of gaseous components
Adsorption is the weak interaction of molecules or particles to a surface. The adsorbent is usually granulated activated carbon produced by roasting wood, coconut shells, or bone. The best material for wastewater treatment is certain grades of coal which have a carbon structure that is hard and dense.

17. Sorption Physical adhesion = adsorption
Penetration of molecules into an absorbent
Adsorption
Carbon (charcoal) filter
organics
Absorption
water absorption of gaseous components

18. Charcoal filtration
Initially, the mass transfer out of the fluid, filtration (absorption), takes place near the entrance of the filter.
As the filtration capacity of a charcoal filter is used up
Mass transfer zone migration away from the entrance
The filter near the entrance is saturated
At “Breakthrough” not all the contaminant will be removed in the filter.

19. Evaporation, Distillation, Condensation
Separate liquids from solids
Separate liquids of different vapor pressure
Recover volatiles
Web sites
Distillation
Evaporation
Evaporation, distillation, and condensation can be used to recover solvent from an aqueous waste stream. The solvent is separated from the waste (e.g., grit, grease, other solvents, paint) and can be reused.
See for a description of a process that leaves the residue in a dry, non-hazardous, form.

20. Stripping
Air stripping (Good overview, you need to click on the cutaway to see the picture better.)
Wastewater trickles down through a packed tower while air bubbles up through it. Volatile contaminants partition out of the water into the air. The contaminant-laden air is captured and treated by another method (e.g., activated carbon adsorption) before release to the atmosphere.

21. Membrane Filtration Systems
Membrane Filtration Systems *PRESSURIZED OR ELECTRICALLY DRIVEN FILTRATION *FOULING A MAJOR PROBLEM
In increasing pore size:
Reverse Osmosis
Electrodialysis (Transport of ions via electric current)
nano-, ultra-, and microfiltration
waste treatment uses
oil-solids-water separation
metals removal/recovery
removal of toxic organics

22. Membrane systems Reverse Osmosis Electrodialysis
Fouling a major problem
Electrodialysis

23. Drying
Freeze drying
desiccants

24. Extraction Transfer between solvents Useful with azeotropes
Step one is contact between the solvent and the waste
Step two is the formation of distinct phases
The hazard containing phase is then removed for further processing
Useful with azeotropes
Supercritical Fluid extraction (CFE)

25. Extraction con’t.
Uses CO2 under high pressure as a solvent
Efficient - improved mass transfer rates
Easy to recover solvent after extraction

26. Stabilization and Solidification
Stabilization makes less toxic or mobile
Solidification changes form
Cement, Lime, and Pozzolans common and cheap
Plastic polymers
Stabilization
A process that renders the substance being treated less toxic, slows the leaching, or the mobility of the toxic substance. It doesn’t change the physical form of the waste, but it does use a chemical reaction.
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Solidification
A process that changes the physical form of a waste not by chemical reaction though. It works in conjunction with stabilization most of the time.