PHYSICAL WASTEWATER TREATMENT METHODS

Adsorption and Reverse Osmosis and Other Physical Treatment Methods

Dexby de Guzman (2010)

ADSORPTION WITH ACTIVATED CARBON

Adsorption with Activated Carbon

Adsorption is the process of collecting solublesubstances within a solution on a suitable interface.

In wastewater treatment, adsorption with activatedcarbon—a solid interface—usually follows normalbiological treatment, and is aimed at removing aportion of the remaining dissolved organic matter.

Particulate matter present in the water may also beremoved.

Activated Carbon

Activated carbon is produced by heatingchar to a high temperature and thenactivating it by exposure to an oxidizinggas at high temperature.

The gas develops a porous structure inthe char and thus creates a large internalsurface area.

The activated char can then be separatedinto various sizes with differentadsorption capacities.

Types of AC

Granular Activated Carbon (GAC), which has a diameter greater than 0.1 mm, and

Powdered Activated Carbon (PAC), which has a diameter of less than 200 mesh.

Surface Area of AC

One gram of AC has asurface area =1200 square metersas large as a footballfield.

Adsorption process

It is the vast internal surface that gives activated carbonits unique ability to adsorb a wide range of compoundsfrom both the gas and liquid phases.

Once the targeted compound is in contact with theactivated carbon, it diffuses into its pores and is locked inplace by weak Van der Waals forces.

Regeneration Process

Carbon is regenerated by removing the adsorbed organiccompounds through steam, thermal, or physical/chemicalmethods.

Thermal and steam regeneration are the most commonmethods to regenerate carbon used for wastewatertreatment. These methods volatilize the organiccompounds that have adsorbed onto the carbon.

Physical/chemical regeneration uses a solvent, which canbe a water solution, to remove the organic compounds.

Adsorber Design

A fixed-bed column is often used to bring the waste-water into contact with GAC. The water is applied to thetop of the column and withdrawn from the bottom,while the carbon is held in place.

Backwashing and surface washing are applied to limithead loss build-up.

Adsorber Design

Carbon AdsorberSystems by Siemens Water Technologies

Other Configurations

Expanded-bed and moving-bed carbon contactors havebeen developed to overcome the problem of head lossbuild-up.

In the expanded-bed system, the influent is introducedat the bottom of the column and is allowed to expand.

In the moving-bed system, spent carbon is continuouslyreplaced with fresh carbon.

DESIGN PARAMETERS

1. Properties of Granular Activated Carbon

Particle Size Distribution

Surface Area - In general, the larger the surface area is,the greater is the adsorption capacity; however, thissurface area needs to be effective.

Properties of Granular Activated Carbon

Pore Volume - a measure of the total pore volume within the

carbon particles in cubic centimeters per gram (cm3/g).

Iodine Number - measure of the volume present in poresfrom 10 to 28 Å (10–10 m) in diameter.

Molasses Number - measure of the volume in pores greaterthan 28 Å in diameter.

Properties of Granular Activated Carbon

• Abrasion Number - measures the ability of carbonto withstand handling and slurry transfer.

• Apparent Density - the mass (weight) of aquantity of carbon divided by the volume itoccupies (including pore volume and interparticlevoids, adjusted for the moisture content).

• Bulk Density - the unit weight of the carbonwithin the adsorber.

2. Isotherm

An isotherm is the relationship that shows thedistribution of adsorbate (material adsorbed)between the adsorbed phase (that adsorbed on thesurface of the adsorbent) and the solution phase atequilibrium.

Monolayer or multilayer adsorption

Isotherm Models

Langmuir: qe = Q0KC/(KC +1)

--- monolayer adsoprtion

Freundlich: qe = KFC1/n

--- often used to be the best relation

Brauner, Emmet and Teller:

qe = BCQ0/[(Cs-C)(1+(B-1)(C)/Cs)]

--- multilayer adsorption

3. Breakthrough Curves

The breakthrough curve can be defined as the “S”shaped curve that typically results when the effluentadsorbate concentration is plotted against time orvolume.

The break-through point is the point on thebreakthrough curve where the effluent adsorbateconcentration reaches its maximum allowableconcentration, which often corresponds to thetreatment goal.

4. Mass Transfer Zone

The mass transfer zone (MTZ) is the area within theadsorbate bed where adsorbate is actually beingadsorbed on the adsorbent.

The MTZ typically moves from the influent end towardthe effluent end of the adsorbent bed during operation.

Real-life WW Applications

• Nurture Shampoo Manufacturing Co., Ltd. WWTPin India

• Tertiary Treatment

• The main purpose of using the activated carbonadsorption is to remove the color of the effluenttreated water for reusing to the cooling tower,fire fighting, floor washing, gardening, roadcleaning, and GAC back washing.

REVERSE OSMOSIS

Reverse Osmosis (RO)

RO is a membrane-based demineralizationtechnique used to separate dissolved solids,such as ions, from solution (most applicationsinvolve water-based solutions, which is the focusof this work).

Membranes in general act as perm-selectivebarriers, barriers that allow some species (suchas water) to selectively permeate through themwhile selectively retaining other dissolved species(such as ions).

Osmosis

Osmosis is a natural process wherewater flows through a semipermeablemembrane from a solution with a lowconcentration of dissolved solids to asolution with a high concentration ofdissolved solids.

Design Parameters

1. Flow Rating (in gpm)

2. Recovery = permeate/influent

3. Rejection = retained/influent

4. Flux = volumetric flow rate of a fluid through a given area.

5. Concentration polarization - has a negative effect on theperformance of an RO membrane.

6. Fouling is a result of deposition of suspended solids,organics, or microbes on the surface of the membrane,typically on the feed /concentrate side.

Designs: Membrane Modules

Reverse osmosis membranes for industrialapplications are typically modularized usingconfigurations that pack a large amount of membranearea into a relatively small volume.

Economical

Smaller area

Easy installation and replacement

1. Plate and Frame

Plate and frame RO modules are typically usedfor specialty, high suspended solids applicationsand are not generally found in water- purificationfacilities.

These modules consist of flat sheets ofmembrane that are modularized into plates,typically two membranes placed back to backper plate. The plates are then stacked within aframework for support.

Plate and Frame

2. Tubular

Tubular modules are also used for specialty,high-solids applications typically found in thefood and biological processing industries.

These modules essentially resemble a shell-and-tube heat exchanger, with the RO feedwater on the tube side and RO permeate on theshell side.

Tubular

3. Spiral Wound

Spiral wound membrane modules are the mostcommon type of module used for RO today Themajor advantage of a spiral wound module isthat the packing density is fairly high.

Influent that enters the spiral wound moduledoes so tangentially to the membrane surfaceand the reject exits the module at the endopposite of the influent. Water that permeatesthe membrane does so perpendicularly to thesurface of the membrane and is collected .

Spiral Wound

Closer View of Spiral Wound

Why Spiral Wound?

4. Hollow Fine Fiber

• Hollow fine fiber RO modules aremembranes formed into very small-diametertubes, with an outside diameter of about 85microns and an inside diameter of about 42microns.

• The fibers resemble human hair and can be asflexible. The membrane “skin” or thin film ison the outside of the fiber.

Ho

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Fin

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Comparisons

Real-life WW Applications

• In a sewage treatment facility in

15 RO units

1 RO unit = 5 million gallons per day

= 150 pressure vessels

1 pressure vessel = 7 spiral wound elements

P = 150 – 220 psi

Product is said to be distilled quality.

OTHER PHYSICAL TREATMENT METHODS

UV Disinfection

UV spectrum lies between X rays and visible light.

Most bacteria do not have enzymes to repair UV damage.

UV lamps are specially manufactured mercury lamp.

Ion Exchange

• Ion exchange is a unit process in which ions of a givenspecies are displaced from an insoluble exchangematerial by ions of a different species in a solution..

• The most widespread use of this process is in domesticwater softening, where Na ions from a cationic-exchange resin replace the Ca and Mg ions in thetreated water, thus reducing hardness.

• It has been use in wastewater applications for theremoval of nitrogen, heavy metals, and TDS.

Ion Exchange Equipment

Water Softening Cycle

• Service

• Regeneration

a. Backwash

b. Brine

c. Rinse

d. Flush

RegenerationService

Miscellaneous Physical Treatment Methods

Evaporation/Distillation

Dewatering/Drying

Electrolytic recovery

Solvent extraction

References

Kucera, J. Reverse Osmosis: Industrial Applications and Processes. Massachusets, U.S.A.: Scrivener Publishing LLC, 2010.

U.S. Army Corps of Engineers. Adsorption Design Guide. 2001.

Tchobanoglous, G. Wastewater Engineering: Treatment and Reuse. McGraw-Hill Companies, 2003.

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