Waste Treatment, Physical ENVE 649

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

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

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.

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.

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

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 -->

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.

Centrifuges Increase “g” of the settling velocity equation.

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

Flotation Low density particles Hydrocarbon solids Finely divided air bubbles float material to surface, then skim Flocculants form flocs with air bubbles http://www.muddyriv.com/icsep.html http://www.abanaki.com/ 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.

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”

Modes of Filtration Mechanical straining adhesion flocculation sedimentation or inertial impaction

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

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

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.

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

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.

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 http://www.sawyerandsmith.com/faq21.htm for a description of a process that leaves the residue in a dry, non-hazardous, form.

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.

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

Membrane systems Reverse Osmosis Electrodialysis Fouling a major problem Electrodialysis

Drying Freeze drying desiccants

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)

Extraction con’t. Uses CO2 under high pressure as a solvent Efficient - improved mass transfer rates Easy to recover solvent after extraction http://www.appliedseparations.com/Supercritical/default.asp

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. http://www2.wateronline.com/content/productshowcase/product.asp?DocID={1A88F54C-2E3F-11D3-9A4B00A0C9C83AFB} &Page =1&Search =stabilization http://www2.wateronline.com/content/productshowcase/product.asp?DocID={83BCB02B-6853-11D3-9A64-00A0C9C83AFB}&Page=1&Search=stabilization 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. http://www2.wateronline.com/content/productshowcase/product.asp?DocID={611C990E-1068-11D3-9A43-00A0C9C83AFB}&Page=1&Search=solidify