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ExxonMobil ProprietaryWATER POLLUTION CONTROL Section Page

ACTIVATED CARBON TREATMENT XIX-A8 1 of 29

DESIGN PRACTICES December, 2001

ExxonMobil Research and Engineering Company – Fairfax, VA

CONTENTSSection Page

1.0 SCOPE .....................................................................................................................................................3

2.0 REFERENCES..........................................................................................................................................3

3.0 DEFINITIONS ...........................................................................................................................................3

4.0 PROCESS DESCRIPTION .......................................................................................................................54.1 ACTIVATED CARBON ADSORPTION THEORY.............................................................................54.2 ACTIVATED CARBON CHARACTERISTICS ..................................................................................54.3 TYPICAL APPLICATIONS OF ACTIVATED CARBON TREATMENT .............................................6

5.0 DESIGN CONSIDERATIONS FOR GAC..................................................................................................75.1 FACTORS WHICH INFLUENCE ADSORPTION .............................................................................7

5.1.1 Nature of the Contaminants........................................................................................................75.1.2 Adsorption Isotherms..................................................................................................................75.1.3 Nature of the Carbon ..................................................................................................................85.1.4 pH...............................................................................................................................................85.1.5 Temperature ...............................................................................................................................8

5.2 WASTEWATER QUALITY AND PRETREATMENT REQUIREMENTS ...........................................95.3 TYPE AND NUMBER OF CARBON ADSORPTION UNITS.............................................................9

5.3.1 Downflow Columns .....................................................................................................................95.3.2 Upflow Columns .......................................................................................................................105.3.3 Packaged Units ........................................................................................................................105.3.4 Number of Units........................................................................................................................10

5.4 CONTACT TIME.............................................................................................................................115.5 SPENT CARBON REGENERATION AND REPLACEMENT .........................................................115.6 BIOLOGICAL GROWTH ................................................................................................................125.7 AIR ENTRAINMENT.......................................................................................................................12

6.0 DESIGN AND FACILITY BASIS SIZING PROCEDURE FOR GAC ......................................................12

7.0 DESIGN EXAMPLE PROBLEM .............................................................................................................15

8.0 POWDERED ACTIVATED CARBON ADDITION TO BIOX ...................................................................188.1 GENERAL DISCUSSION AND BENEFITS ....................................................................................188.2 PATENT ISSUES ...........................................................................................................................188.3 PAC DOSAGE RATE AND MIXED LIQUOR CONCENTRATION..................................................188.4 CONSIDERATIONS .......................................................................................................................19

8.4.1 Clarifier Solids Loading.............................................................................................................198.4.2 Differentiating Between Biomass and PAC...............................................................................198.4.3 Precautions When Adding the PAC to the BIOX.......................................................................198.4.4 Aesthetics of PAC.....................................................................................................................19

9.0 NOMENCLATURE..................................................................................................................................20

Changes shown by ➧

ExxonMobil ProprietarySection Page WATER POLLUTION CONTROL

XIX-A8 2 of 29 ACTIVATED CARBON TREATMENTDecember, 2001 DESIGN PRACTICES


CONTENTS (Cont)Section Page

TABLESTable 1 Properties of Commercially Available Carbons .................................................................21Table 2 Influence of Molecular Structure and Other Factors on Adsorbability ...............................22Table 3 Classes of Materials Readily Adsorbed and Poorly Adsorbed onto GAC..........................23Table 4 Typical Freundlich Isotherm Constants for Petroleum Compounds ..................................23Table 5 Design Parameters for Various ExxonMobil GAC Installations .........................................24Table 6 Process Operating and Design Parameters For Sites Using PAC Additions to the BIOX .25

FIGURESFigure 1 Downflow Fixed-Bed GAC Adsorber .................................................................................26Figure 2 Typical Concentration Profiles and Breakthrough Curves for GAC Columns....................27Figure 3 Two Downflow Carbon Beds in Series ..............................................................................28Figure 4 Straight Line Isotherm Plot (Freundlich) - Sample Problem ..............................................29

Revision Memo

12/01 This update includes new guidance on clean water backwashing and air scourflow rates. Also included is a section that assists in quickly defining facility basesfor Class V Cost Estimation, which is frequently used as a basis for Gate 2 ProjectReviews.





1.0 SCOPEThis practice presents design considerations and recommended process design procedures for facilities to treat water andwastewater using activated carbon. The practice focuses predominantly on the use of granular activated carbon filters whichcan be used to treat wastewater from refineries, chemical plants, and marketing terminals. The principles provided can beapplied to grass roots designs to handle large flows, as well as the selection of vendor-provided packaged units for treatingsmall or remote streams including recovered contaminated groundwater. The practice also includes a discussion on applyingpowdered activated carbon in activated sludge (BIOX) systems. The practice does not address the use of activated carbon fortreating vapors or gases.

2.0 REFERENCES2.1 DESIGN PRACTICESection XIX-A Guidelines for Selecting Wastewater Treatment Systems

2.2 EMRE WATER AND WASTEWATER DESIGN GUIDE (TMEE 080)DG 11-1-2 Activated Carbon Filters

2.3 OTHER REFERENCES1. U.S. Environmental Protection Agency, Process Design Manual for Carbon Adsorption, PB–227 157, (October 1973).2. Water Environment Federation, Design of Municipal Waste Water Treatment Plants, Volume II - Manual of Practice No. 8,

Alexandria, Virginia (1992).3. Eckenfelder Jr., W. W., Industrial Water Pollution Control, 2nd Edition, McGraw-Hill Inc., New York (1989).4. Altemoeller, P. H. and Higinbotham, J. H., Packaged Activated Carbon Adsorption for Toxicity Reduction/Emergency

Applications, ER&E Report No. EE.42E.91 (October 1991).5. Weber, W. J. Jr., Physicochemical Processes for Water Quality Control, John Wiley & Sons, New York (1972).6. Sundstrom, D. W. and Klei, H. E., Wastewater Treatment, Prentice-Hall Inc., New Jersey (1979).7. Metcalf & Eddy Inc., Wastewater Engineering Treatment, Disposal, and Reuse, 3rd Edition, McGraw-Hill Inc., New York

(1991).8. Stenzel, M. H., Remove Organics by Activated Carbon Adsorption, Chemical Engineering Progress, April 1993.9. U.S. Environmental Protection Agency, Carbon Adsorption Isotherms for Toxic Organics, EPA Publication No.

EPA/600/8-80-023 (April 1980).10. Gilbert, S. W., Adsorptive Removal of Contaminants from Terminal Water Discharges, ER&E Report No. EE.2M.92

(January 1992).11. American Society for Testing and Materials, Standard Practice for Determination of Adsorptive Capacity of Activated

Carbon by Aqueous Phase Isotherm Technique, ASTM D 3860 - 89a (Reapproved 1993).12. Eckenfelder, W. W., Argaman, Y., and Miller, E., Process Selection Criteria for the Biological Treatment of Industrial

Wastewaters, Environmental Progress, Volume 8, No. 1, February, 1989.13. Cooney, David O., Adsorption Design For Wastewater Treatment, Lewis Publishers, CRC Press, 1999, ISBN

1-56670-333-6.14. Montgomery, James M., Water Treatment Principles and Design, John Wiley and Sons, Inc., New York (1985).

3.0 DEFINITIONSAbrasion Number - A measure of the ability of a carbon to withstand abrasion, which may be useful in comparing carbons fordurability. This number is related to the crushing strength of the carbon which is important for downflow pressure operation.Active Biomass - The portion of the solids in a biological system composed of microorganisms that are actively degrading thecontaminants.Activated Carbon - A highly porous char made from organic material such as coal, wood, and nutshells which has beenactivated by exposure to an oxidizing gas at a high temperature.Adsorbate - The material or contaminant that is accumulating onto the surface of the carbon and being removed fromwastewater or water.Adsorbent - The solid onto which contaminants accumulate. For the purposes of this design practice, activated carbon is theadsorbent for wastewater contamination.




3.0 DEFINITIONS (Cont)Adsorption - The process of collecting or removing soluble contaminants (adsorbates) from water on the adsorbent, activatedcarbon. The contaminants are collected on the solid/liquid interface.Adsorption Capacity - A measure of the amount of a specific contaminant, or contaminant mixture, that can be removed perunit weight of activated carbon. Ultimate adsorption capacity of an adsorbent is estimated using the adsorption isothermtechnique, and is reported in terms of mass of contaminant removed per mass of activated carbon.Adsorption Isotherm - A measure of the adsorptive capacity of a carbon. The adsorption isotherm is the relationship, at agiven temperature, between the amount of a substance adsorbed per unit of activated carbon and its equilibrium concentrationin the surrounding solution.Apparent Density - A comparison of oven dried spent carbon to oven dried reactivated carbon which gives an estimate ofadsorbate loading on the spent carbon. Also a measure of the degree of reactivation accomplished.BIOX - Abbreviation for BIological OXidation and commonly used to describe an activated sludge system but can be used inreference to other aerobic biological oxidation processes used to treat wastewater.Breakthrough - The point at which the contaminant being removed appears, or breaks through, in the effluent. Determinationof an acceptable level of breakthrough depends on the treatment objective (effluent concentration target or permit limit).Bulk Density - Weight per unit volume of the dry carbon as packed in the bed (supplied by vendor).Bulking - An upset condition in the activated sludge system during which the biomass does not settle effectively, leading tosuspended solids carryover in the effluent. Bulking is usually associated with large quantities of filamentous bacteria in themixed liquor, or oil to the biotreatment system. The result of this bulking is excessive solids remaining in the water todownstream units or effluent, in this case it may overload the activated carbon column.Chemical Oxygen Demand (COD) - A measure of the amount of organic or reduced inorganic compounds in a sample thatcan be oxidized by a strong oxidizer, usually potassium dichromate and sometimes potassium permanganate.Contactor - The carbon column or vessel in which the wastewater comes into “contact" with the activated carbon. Alsoreferred to as a carbon adsorber.Contact Time - The time required for the wastewater to pass through a carbon bed based on the superficial velocity, i.e., thevolume of empty bed divided by the hydraulic flow rate.Conventional Downflow Contactor - A fixed bed contactor which is operated in the downflow mode in service, and isbackwashed in an upflow direction.Desorption - The process where previously adsorbed contaminants are displaced or released back into solution.Downflow Columns - Usually two or more columns in series, which accomplish adsorption of organics and filtration of lowconcentrations of suspended solids in a single step. The influent enters from the top and flows through the bed where it isdischarged. Downflow columns usually require routine backwashing. See conventional downflow contactor definition above.Expanded Bed Column - A column where the influent is introduced at the bottom with sufficient velocity to allow the carbonbed to expand, much as a filter bed expands during backwash.Fixed Bed - Contactors where the carbon bed remains fixed. Usually fixed beds employ downward flow to reduce the chanceof accumulating particulate material in the bottom of the bed that would be difficult to remove by backwashing for solidsremoval.Freundlich Isotherm - An empirically derived relationship commonly used to describe the adsorption characteristics ofactivated carbon for very dilute contaminants.Granular Activated Carbon (GAC) - Generally refers to carbon which has a particle size greater than 100 microns. Twogeneral size ranges of commercial GAC are readily available - 8 x 30 mesh and 12 x 30 mesh - but larger (4 x 6 mesh) orsmaller (20 x 50 mesh) sizes are also available.Hydraulic Retention Time (HRT) - The length of time the influent wastewater is retained in the aeration basin, but notincluding the effect of sludge recycle; i.e., aeration volume divided by influent volumetric flow rate.Iodine Number - This test provides an indication of the adsorptive capacity of a carbon. It is the milligrams of iodine adsorbedfrom a 0.02 N iodine solution at equilibrium under specified conditions.Mesh/Sieve Size - Activated carbon is classified by its ability to be retained by a series of screens or sieves of decreasingmesh size. As the sieve number increases (e.g., 4, 8, 12, 20, 40, 100, 200, 400, etc.), the size of the openings in the meshdecreases. This sieving technique is used to separate and characterize carbon by particle size.Mixed Liquor - The contents of the aeration basin, consisting of the wastewater, cell biomass, inert materials and perhapspowdered activated carbon.Nitrification - A biological process where ammonia is converted to nitrate.Pore Volume - A measure of the total macropore and micropore volume of the carbon which can be of value in screeningactivated carbons for a specific waste constituent relative to molecular weight.





3.0 DEFINITIONS (Cont)Powdered Activated Carbon (PAC) - Generally refers to carbon with a particle size of less than 200 mesh (74 microns). Dueto its small particle size, PAC is not generally used in columns or filters but instead can be added directly to the activatedsludge aeration basin.Powdered Activated Carbon (PAC) Treatment - The PAC Treatment process is a modification of the traditional activatedsludge process which involves the addition of powdered activated carbon to the aeration basin.Pulsed Bed Column - A column where the wastewater flows upwards through a descending fixed bed of carbon. When theadsorptive capacity of the carbon at the bottom of the bed is exhausted, this bottom portion of carbon is removed and anequivalent amount of fresh carbon is added to the top.Solids / Sludge Residence Time (SRT) - The average length of time the solids are held in the biox system expressed in days.SRT is calculated as the total weight of solids in the biox system (aeration basin + clarifier) divided by the total weight of solidswasted each day from the system. Also known as sludge age or MCRT (mean cell residence time).Specific Surface Area - That portion of the total surface area that is available for adsorption. Due to its highly porousmicrostructure, activated carbon has a very high specific surface area, typically in the range of 500-1500 m2/g.

4.0 PROCESS DESCRIPTION4.1 ACTIVATED CARBON ADSORPTION THEORYActivated carbon removes various organic and metal contaminants from water by a process known as adsorption. Adsorptioninvolves the accumulation or concentration of contaminants both at the outer surface of the carbon particle, and within theparticle's pore structure. The contaminants accumulate at the carbon-water interface, but do not actually penetrate the carbon.The material being adsorbed is called the adsorbate, and the activated carbon is termed the adsorbent.Two types of adsorption mechanisms have been demonstrated. The predominant adsorption mechanism for petroleumwastewater contamination is physical adsorption. This occurs when the contaminant molecules are held loosely to the carbonsurface by van der Waals forces. Theory suggests that the molecules are mobile and migrate on the carbon surface. This typeof adsorption occurs largely in the micropores of the carbon particle. The second mechanism is chemical adsorption. Thisoccurs because molecular functional groups of the contaminant and the carbon interact at the particle surface to form a stablecarbon bond. Due to the nature of the carbon surface, physical adsorption rather than chemical adsorption is the predominantforce of importance in wastewater treatment systems.Simply stated, adsorption occurs in three basic steps: outer film diffusion, pore diffusion, and adhesion of the molecules to thecarbon surfaces. Film diffusion is the penetration of the adsorbate through the carbon particle's surface film; pore diffusioninvolves the migration of the molecules through the macropores and micropores to an adsorption site (the rate limiting step);adhesion then occurs when the adsorbate molecules adhere or adsorb onto the carbon particle's surface.Adsorption is a dynamic rather than a static process. This implies that adsorption and desorption occur continuously asdifferent adsorbates approach adsorption sites. A loosely bonded organic structure can be displaced at an adsorption site by amolecule that has a greater affinity for that site. Thus the displaced molecule “desorbs," or is released by the carbon, duringthe adsorption of the preferred species.

4.2 ACTIVATED CARBON CHARACTERISTICSActivated carbons can be made from a variety of carbonaceous materials such as wood, lignin, bituminous coal, lignite, peat,nut shells, and petroleum residues. The material used to manufacture the carbon and the manufacturing process used affectsthe characteristics of the activated carbon. Most activated carbon used in wastewater treating applications comes frombituminous coal and lignite which are relatively inexpensive and readily available.Activated carbon is manufactured by a process consisting of dehydration and carbonization followed by activation. The rawmaterial is dehydrated and carbonized through slow heating in the absence of air. Frequently chemicals are used to create aporous structure of carbon. Final temperatures can be approximately 600°C. The process of carbonization converts theorganic material to primary carbon, which is a mixture of ash, tars, amorphous carbon, and crystalline carbon.The carbonized material is then treated with an activating agent, often steam, at temperatures of 750-950°C. This burns off ofthe tars and other products of decomposition, which opens up, enlarges, and increases the number of pores. In this fashion,the particle becomes highly porous and the active area now available for adsorption is greatly increased. Typical surface areasare 800-1000 m2/g.Activated carbons are broadly classified according to their size, specifically granular and powdered. Granular activatedcarbons (GAC) are those which are larger than approximately U.S. Sieve Series No. 50 (~ 300 microns, or 0.3 mm), whilepowdered activated carbons (PAC) are those with a size lower than No. 100 Sieve, 150 microns. Generally, carbon used incommerical practices for water treatment is about 1.5-1.7 mm, or No. 12 Sieve.




4.0 PROCESS DESCRIPTION (Cont)It is very important to specify the limits on fines (usually 1% maximum) in granular activated carbon purchases. Too many fineswill require an extended period of washing the carbon prior to being put in service, and can cause effluent TSS targetexceedances when put in service.Particle size is generally considered to affect adsorption rate, but not adsorptive capacity. Particle size also affects backwashfluidization and governs the maximum backwash flowrate which can be used in a carbon bed. This is because again the outersurface area of the carbon particle represents only a small percent of the total active adsorptive area. Since adsorptioncapacity is related to total adsorptive area, a given weight of carbon gains little adsorptive capacity when crushed to a smallersize. The rate of adsorption will be increased, however, due to the influence on the pore diffusion process.The properties of several commercially available carbons are given in Table 1. Properties for specific carbons should beobtained from vendors.

4.3 TYPICAL APPLICATIONS OF ACTIVATED CARBON TREATMENTThe use of activated carbon in wastewater treatment has many applications. The most common applications are discussedbriefly below.1. GAC adsorption columns are often used as a tertiary polishing process following conventional biological secondary

treatment. Processes upstream of the GAC are usually designed to remove the free oil, the oily and biological treatmentsolids, and essentially all of the soluble biodegradable organics. This leaves relatively small quantities of refractoryorganics and heavy metals which are the target of the carbon. Polishing with GAC can also be effective in reducingeffluent toxicity. The wastewater treatment facilities at Valero's Benicia Refinery (formerly Exxon) and at EMRE'sCorporate Research facility in Clinton, New Jersey are typical of tertiary polishing following biological treatment.

2. GAC adsorption can be used to remove soluble organics following physical-chemical primary treatment. In this case, thewastewater is first treated by oil/water separation, followed by either chemical coagulation and sedimentation, dissolved airflotation, or dual media filtration. This primary treated wastewater is then passed through GAC columns where the solubleorganic compounds are removed. The physical-chemical system with activated carbon can be designed to produce thesame effluent quality as that achieved with tertiary treatment. For large flows, these systems are rarely cost effective whencompared to biological secondary treatment. The wastewater treatment facilities at the Sakai and Tonen Refineries inJapan are indicative of this application.

3. GAC adsorption is finding increasing application to treat low flow, dilute wastewaters from remote locations such asmarketing terminals and ground water pump-and-treatment situations. Additionally, carbon systems find application inemergency situations, such as a major wastewater treatment plant upset or when temporary additional treatment capacityis required. In these cases, relatively simple packaged units can be used to produce a very high quality effluent.

4. Adding powdered activated carbon (PAC) to a plant's activated sludge or biox system combines activated carbonadsorption with biological secondary treatment. In this process, PAC is added, generally on a continuous basis, to theactivated sludge mixed liquor. This approach is often applied to activated sludge systems which are subjected to shockloadings of toxic or destabilizing organic contaminants. The PAC acts to initially adsorb these surges, then releases thecontaminants over time at a rate at which they can be metabolized by the biomass. PAC Treatment is also used at siteswith stringent effluent toxicity limits. Since PAC increases the specific gravity of the activated sludge floc, PAC canimprove the sludge settling characteristics. This is discussed in more detail in Section 8.0. Valero's Benicia Refinery(formerly Exxon) and EMRE's Corporate Research facility in Clinton use PAC to improve Biox Treatment.

In summary, GAC adsorption should be considered for the following applications:• When the inlet wastewater biochemical oxygen demand (BOD5) is less than 100 mg/L (too low to support suspended

growth activated sludge).• Downstream of biological treatment when necessary to meet an acute or chronic toxicity limit.• Downstream of biological treatment when further polishing is required to remove non-biodegradable organics and selected

heavy metals.• Treatment of small streams for discharge such as terminals, remote tank fields, and ground water cleanup.• Emergency situations when temporary additional treatment capacity is required.• Upstream/in-plant treatment of low contaminant level streams for onsite reuse, or to assist in sewer segregation

modifications.More detailed information on wastewater technology selection can be found in Design Practices Section XIX-A, Guidelines forSelecting Wastewater Treatment Systems.





5.0 DESIGN CONSIDERATIONS FOR GAC5.1 FACTORS WHICH INFLUENCE ADSORPTION

5.1.1 Nature of the Contaminants

The adsorptive capacity of an activated carbon is dependent to a large extent on the properties of the specific contaminant tobe adsorbed. Generally speaking, low molecular weight (< 50), highly water-soluble, highly-polar compounds such as alcohols,aldehydes, glycols, organic acids, and simple amines are poorly adsorbed. Conversely, compounds with lower solubility inwater such as aromatic solvents (e.g., benzene, toluene) and higher molecular weight compounds such as polynucleararomatic, are effectively adsorbed. Generally, carbon adsorbs based on the carbon number of molecules; the higher thecarbon number, the greater the adsorption capacity. The influence of solubility and molecular structure on adsorptioneffectiveness is shown in Table 2. The relative adsorbabilities of various wastewater contaminants are listed in Table 3. Otherreferences are provided for more detailed information on individual compounds.Wastewater contains a mixture of contaminants, mostly soluble but some suspended matter (fine oil droplets or silt / clay), atthe stage where carbon treatment is considered. The various compounds may mutually enhance adsorption, may act relativelyindependently, or may interfere with one another. As there are only a finite number of adsorption sites, the compound with thehighest affinity for the carbon can be expected to be preferentially adsorbed over those with lesser affinities. In general, thisusually results in lower unit loading for any individual compound in a mixture versus as a single contaminant.

5.1.2 Adsorption Isotherms

In addition to knowing which compounds are adsorbable onto activated carbon, it is useful to conduct a series of tests todetermine the applicability of adsorption for the specific wastewater. The prediction of GAC treatment performance bylaboratory testing is at best a rough estimate of what can be expected in the actual application to most wastewaters. Aswastewaters are often complex mixtures which can vary significantly in makeup and concentration over time, long term pilottesting represents the best means of predicting actual carbon performance. That being said, there are relatively quickanalytical techniques which are valuable in determining the effects of contact time and particle size, in defining the ultimateremoval effectiveness, and in obtaining rough estimates of carbon utilization.The most common analytical test for GAC is the adsorption isotherm. This isotherm provides the ultimate capacity of carbon.This test determines the adsorptive capacity of a carbon for a particular contaminant, or contaminant mixture, at equilibriumconditions over a range of carbon loadings. For most petroleum constituents at low concentration in water, this relationship canbe adequately described by the empirical Freundlich equation:

1/n eCkM/X ⋅= = Amount of contaminant adsorbed per unit weight of carbon at equilibrium, mg/g Eq. (5-1)

where: X = Amount of contaminant adsorbed, mgM = Weight of the carbon, gCe = Concentration of the contaminant remaining in solution at equilibrium (after

adsorption), mg/Lk, 1/n = Freundlich constants

The constants in the Freundlich equation can be determined by plotting X / M versus Ce on log-log paper, and making use ofthe logarithmic form of the equation:

log (X / M) = log k + 1/n (log Ce) Eq. (5-2)

When graphed on log-log paper this equation defines a straight line whose slope is 1/n and whose intercept is k at Ce = 1.Freundlich constants for more than 100 single contaminants in water are given in References 4, 9 and 13. Some Freundlichconstants pertinent to petroleum compounds are listed in Table 4. Using these constants and setting Ce equal to the initialstarting concentration of the contaminant can provide an estimate of the carbon maximum usage.As stated above, isotherm analysis should be run on actual wastewater samples. The Freundlich graphical technique for singlecontaminants can also be applied to a complex mixture, typically by using COD or TOC as the measurement parameter.Isotherm analyses are straightforward and can be done in most moderately equipped laboratories in one or two days. Alaboratory test would first combine known quantities of wastewater and carbon, mix them at a constant temperature, and thenmeasure the test parameters's final concentration in the wastewater at equilibrium. This should be repeated several timeswhile varying the carbon loading, then graphing the results as described. A standard methodology for performing the carbonisotherm analysis is contained in ASTM Method D 3860 (Reference 12).




5.0 DESIGN CONSIDERATIONS FOR GAC (Cont)Carbon vendors can also perform Isotherm analyses. Some vendors can also perform short duration dynamic column studiesor laboratory scale pilot tests to better predict carbon usage and performance. Laboratory testing beyond isotherm analysesserves to optimize the facility and investment.

5.1.3 Nature of the Carbon

The physicochemical nature of the activated carbon can have profound effects on both the rate and capacity of adsorption.Manufacturers of activated carbon have become relatively sophisticated in terms of the types and properties of carbons theyproduce, and can offer a variety of carbons depending on the characteristics of the wastewater to be treated.Short descriptions or applications of the important physical properties are provided as follows:1. Surface Area - Adsorption is a surface phenomenon, therefore total surface area is a useful value for screening the

capability of a particular carbon for adsorption.2. Apparent Density - This property is of little value in the initial evaluation and selection of an activated carbon. However, it

is very useful as one of the measures of successful reactivation. Apparent density measurement is a simple test, and thedensity of saturated virgin carbon relative to the density of reactivated carbon indicates the degree of reactivationaccomplished.

3. Bulk Density - Useful in determining the volume requirements of a carbon column after the contact time requirementshave been established.

4. Effective Size, Mean Particle Diameter, and Uniformity Coefficient - Measures of the gradation of carbon particle sizewhich are important in evaluating head loss in flow through the bed.

5. Pore Volume - A measure of the total macropore and micropore volume of the carbon which can be of value in screeningactivated carbons for a specific waste constituent. Pore volume is also related to the strength of the carbon particle.

6. Pore Volume Distribution - The distribution of pore volume among pores of different size or diameter. Of value inscreening activated carbons for removal of a specific waste constituent relative to molecular weight.

7. Sieve Analysis - Useful in checking carbon production, in checking conformance of purchased carbon to specifications,and in evaluating the effects of plant carbon handling procedures on carbon attrition.

8. Abrasion Number - A measure of the ability of a carbon to withstand abrasion, which may be useful in comparing carbonsfor durability.

9. Crushing Strength - Important for downflow operations and is a measure of carbon strength due to weight of carbon andforce exerted by the flowing water.

5.1.4 pH

For a number of reasons, the pH of a solution may influence the extent of adsorption. Because hydrogen and hydroxide ionsare adsorbed quite strongly, the adsorption of other ions is influenced by the pH of the solution. Furthermore, to the extent towhich the ionization of an acidic or basic compound affects its adsorption, pH affects adsorption in that it governs ionization.In general, adsorption is increased with decreasing pH. This may result from neutralization of negative charges at the surfaceof the carbon with increasing hydrogen-ion concentration. This effect can be expected to vary in degree for different carbons,because the charges at the surfaces of the carbon depend on the composition of the raw materials and on the technique ofactivation.Carbon will function satisfactorily over a broad pH range, and a decrease in feed pH during a run is not a problem. However,care must be taken to avoid a substantial increase in feed pH which can result in desorption of contaminants. This is especiallya concern if the bed is nearing exhaustion. In other words, if a GAC column which has been operating at a pH of 7.0 suddenlysees a pH of 8.5 for several hours, desorption of previously adsorbed materials may occur. This pH behavior may change ascarbon becomes exhausted over its run length.

5.1.5 Temperature

Adsorption reactions are normally exothermic; thus the extent of adsorption generally increases with decreasing temperature.Thus adsorption would be expected to be somewhat better in winter with cooler influent water than in summer. Small variationsin water temperature will not have any significant effect.





5.0 DESIGN CONSIDERATIONS FOR GAC (Cont)

5.2 WASTEWATER QUALITY AND PRETREATMENT REQUIREMENTSTo be fully effective, feedwater to the carbon unit should be of relative uniform quality, without large surges in flow, and low infree oil and suspended solids content. This often implies a need for upstream flow equalization, pretreatment for free oilremoval via dissolved air flotation or equivalent, and/or granular media filtration for oil and solids removal.Surges in flow rates can result in entrainment of the carbon in the effluent and premature breakthrough of contaminants.Excessive quantities of free oil in the feed to the carbon bed can quickly reduce the activity/capacity of the carbon.Feed suspended solids levels should be kept below 15 - 30 mg/L. Particle size should be kept below 25 microns (dependingon type of unit), and free oil and grease levels kept below 10 mg/L. These solids and oils can deposit on the carbon granulesand clog the bed, leading to pressure drop, flow channeling and short circuiting, increased carbon losses due to higherlocalized velocities, and an overall decrease in bed life. The presence of colloidal materials can also be a problem. Colloidalmaterial can reduce the adsorptive capacity of the carbon by restricting pore openings which interferes with the diffusionprocess and reduces effective adsorption sites. To avoid or minimize such impairment, pretreated wastewater of high clarity isdesirable. Dual media filtration upstream of the GAC is recommended and is sufficient to insure acceptable feed quality.

5.3 TYPE AND NUMBER OF CARBON ADSORPTION UNITSSeveral types of activated carbon contactor systems are used in plant designs. Generally, adsorption systems are designedaround five basic types of adsorbers: (1) downflow pressure, (2) downflow gravity, (3) upflow fixed bed, (4) upflow expandedbed, and (5) upflow pulsed bed. With the exception of the last type of adsorber, most systems are designed with two or morecolumns in series. The preferred type is conventional downflow pressure contactors, with an upflow backwash and air scour.However, for upflow backwash, velocities should be kept so that carbon does not leave the bed. EMRE's Crude Distillation andSeparation (CDS) Section should be consulted for acceptable velocities through carbon beds to prevent excessive fluidization.Questions on special types of carbon treatment applications should be directed to EMRE's Environmental Technology Sectionor the CDS Section.

5.3.1 Downflow Columns

These are fixed beds usually in series. The most commonly applied GAC design for industrial wastewater treatment is thedownflow fixed-bed pressure system. In addition to adsorbing organics, downflow columns will act as filters by removingsuspended solids in the feed water. Unless the water has been pretreated for solids removal to a very high quality (< 2 mg/LTSS), these systems will require backwashing to be included in the design to remove the accumulated particulate material. Anexample of such a unit is shown in Figure 1.The wastewater is introduced above the carbon bed, often without a distributor because the bed will serve to distribute the flow.The water flows down through the bed, and is collected in the bottom of the bed by an underdrain consisting of slotted screencollectors and a header or collection system. Wedge-wire type slotted screens will retain the carbon and allow water to flow outof the adsorber. The underdrain also serves as the inlet for the backwash water. Feed distribution, underdrain, and carbonsupport designs vary from vendor to vendor. Minimum requirements are given in DG 11-1-2.The adsorbers are normally carbon steel pressure vessels that meet the ASME pressure vessel codes. As carbon in thepresence of water will create a corrosion cell due to an oxidation-reduction differential, the interior of the adsorber should havea corrosion resistant lining. This lining needs to be thick enough (nominally 35 mils or greater) to accommodate the abrasivenature of the carbon. The designer is referred to DG 11-1-2 for specifications related to materials and coatings.In a downflow column, the carbon is spent relatively evenly across the vertical cross-section of the bed due to the nature of flowthrough the bed. Completely non-adsorbable compounds immediately pass through a carbon bed, followed by breakthrough ofthe less adsorbable compounds. Eventually even the highly adsorbable compounds break through as the bed becomessaturated, at which time the carbon is removed from the column for regeneration. Breakthrough is not a step change; rather itoccurs slowly at first but depending on the application may quickly advance with the outlet concentration approaching the inletconcentration. A typical breakthrough curve for a single adsorbing component for a downflow GAC column is shown inFigure 2.Downflow systems usually consist of at least two columns in series, typically referred to as a lead bed / polish bedconfiguration. Operating in this design mode becomes particularly important when a wide range of contaminants or widefluctuations in contaminant concentration are present. Highly adsorbable compounds will tend to adsorb on the carbonpreferentially in the top of the bed. This displaces or prevents the adsorption of lower affinity compounds until they reach lower,cleaner portions of the lead bed or break through to the polish bed. Similarly, a significant reduction in inlet concentration(perhaps caused by rainwater temporarily diluting the flow) will alter equilibrium conditions, desorbing some contaminants fromthe top of the bed and carrying them through to less loaded areas lower in the bed or into the polish bed.




5.0 DESIGN CONSIDERATIONS FOR GAC (Cont)In a lead / polish configuration, lead bed breakthrough can be tolerated, allowing the lead bed to stay on-line longer andbecome more heavily and economically loaded with organics prior to regeneration. After the lead bed is spent, it is replacedwith fresh carbon and then becomes the polish bed. This type of operation is shown schematically in Figure 3.

5.3.2 Upflow Columns

Upflow columns are arranged such that the liquid flows vertically upward, with the wastewater inlet at the bottom of the columnand the liquid outlet at the top. These contactors can be of the pressurized fixed-bed type, or designed as fully expanded-beds.Usually fixed-bed systems are designed with downward flow to lessen the chance of accumulating particulate material at thebottom of the bed where it would be difficult to remove by backwashing.In expanded-bed designs, the wastewater flows upward through the carbon bed at a velocity sufficient to suspend the particlesbut not high enough to carry them out of the bed. The upflow velocity is maintained so that the bed is operated at essentially aconstant pressure drop. The advantage of this type of design is the ability to run with higher feed rates and solidsconcentration without excessive pressure loss. This design can also be advantageous if a high level of biological fouling isanticipated in the bed.Upflow columns, especially expanded-bed designs, may have more carbon fines in the effluent than downflow columnsbecause the upflow mode tends to expand, not compress, the carbon. Bed expansion creates fines because carbon particlescollide, causing particle attrition, and allows these fines to escape through passage ways created in the bed. Beds operated inupflow require close attention to flow rate. If the flow rate is too high, excessive carbon particles will be entrained. If the flowrate is too low, then the bed is subject to channel formation.The upflow column has led to the development of the pulsed-bed, in which the wastewater flows upward through a descendingbed of carbon. When the adsorptive capacity of the carbon at the bottom section of the column is exhausted, that portion of thecarbon is removed (usually about 5-20%), and an equivalent quantity of regenerated or virgin carbon is added to the top of thecolumn. This approach seeks to maximize the use of the counter current principle, which contacts the most saturated carbonwith the dirtiest water and therefore the highest capacity for adsorption. This technique results in very efficient utilization of thecarbon.Another advantage of this type of design is that it contains the breakthrough curve within a single adsorber. Consequentlymultiple units operated in-series are not required. The upflow pulsed-bed is often used for wastewaters requiring long contacttimes and having low flow rates, where a very high effluent quality is desired. Since pulsed-bed contactors cannot bebackwashed, residual suspended solids and oil in the influent should be very low to avoid plugging. Note: DG 11-1-2requirements for 2 vessels in series and full filter bed backwash facilities are not applicable for pulse bed units. Onsite freshand spent carbon storage and addition/removal facilities are required.Upflow carbon columns can also have disadvantages such as variable effluent quality and the need for careful backwashing asnot to mix the bed and cause a large breakthrough front for the contamination.

5.3.3 Packaged Units

Packaged carbon adsorption units consisting of one or two pre-engineered downflow adsorber vessels are now available insizes ranging from disposable 55 gal drums to skid-mounted dual 10 ft diameter by 20 ft high steel pressure vessels that readilyfit on normal sized truck trailers. In many instances, these units are ready for immediate delivery and can be either purchasedor leased (with an option to purchase later). Many of these units are completely manually operated but can be automated asrequired. The designer should consider the cost effectiveness of these packaged units versus designing and constructing acustomized unit. More information on the performance and suppliers of these packaged units can be obtained in References 4and 10 and directly from the vendors after consulting with the Environmental Section (ESCMD) of EMRE. Consideration mustbe given to the vendors' vessel design for effective water distribution, via appropriate tower internals, and bed depth vs. emptybed volume, to ensure adequate volume for backwashing and air scour.

5.3.4 Number of Units

For almost all refinery and chemical plant wastewater effluent treatment applications, at least 2 absorbers installed andoperated in series are required to ensure efficient use of the carbon and reliable treatment performance. For these servicesmultiple adsorption trains operated in parallel are also required to ensure continuous operation at the specified design flow witha column out of service for backwashing, carbon replacement, or other repair/maintenance). The actual number of paralleltrains required is a trade off between size of vessels, cost, water flow rate characteristics, and carbon regeneration andtransport requirements. For systems requiring high reliability and effluent quality, DG 11-1-2 should be followed.For some ground water, in-plant or terminal applications with only one or two contaminants at low concentrations (ppb to a fewppm) or where downtime is acceptable, series and/or parallel configurations may not be required. As discussed in Par. 5.3.2above, series operation is not required for the upflow pulse configuration, but parallel trains may be required.






5.4 CONTACT TIMEContact time is calculated by dividing the carbon bed volume by the superficial flow rate of the water through the GAC column.Contact time can be varied by changing bed depth at constant flow or changing flow at constant depth. The effect of contacttime is to alter the time to breakthrough, with shorter relative contact times resulting in shorter breakthrough times or runlengths and ineffective utilization of carbon beds. Longer contact times also result in better carbon utilization. The percentageof exhausted carbon in the bed at breakthrough is higher in a deeper bed. Experience has shown a minimum of 15 minutescontact time per vessel is required to reasonably balance breakthrough time, regeneration frequency and vessel size (cost).For systems with high adsorption rates, light loadings, and high flow rates, contact times of less than 15 minutes per vesselmay be acceptable.

5.5 SPENT CARBON REGENERATION AND REPLACEMENTOnce the activated carbon is spent, it generally can not be regenerated in place but must be removed and replaced with fresh(either virgin or regenerated) carbon. Sometimes light hydrocarbons such as benzene can be stripped from the column via theuse of steam, but eventually the carbon will have to be removed for regeneration.There are two options for handling the spent carbon, but only one is typically practiced within ExxonMobil. This option is toreturn the spent carbon to the carbon manufacturer for regeneration. This offsite regeneration service is usually the mostpractical and economical way of handling the carbon and requires little additional capital investment. The carbon is usuallymixed with spent carbons from other facilities, regenerated, and resold by the manufacturer. The owner is advised to ensurethat the carbon is returned for regeneration, and not just landfilled which could expose the Company to unwanted liabilities.The manufacturer may also be requested to segregate Company regenerated carbon from other companies' carbon, again tominimize potential liability.The second option is onsite regeneration. This is rarely justified for most refinery or chemical plant operations since the volumeof carbon to be regenerated does not support the necessary investment in regeneration facilities. If more information on thedesign of carbon regeneration facilities is needed, the designer should refer to References 1 and 2.Usually the GAC system is designed so that the carbon can be moved into and out of the adsorber as a water slurry underwater or air pressure. This slurry fill-and-discharge design requires a carbon discharge connection (6 in. minimum) at thebottom and a carbon fill connection (4 in. minimum) at the top of the vessel.

➧ Adding dry carbon to an empty vessel is not recommended because it can cause problems with entrapped air and carbon fines.The entrapped air resulting from the addition of dry carbon to an empty vessel will lead to decreased adsorption capacity.Ideally, an empty vessel should be filled with hydrated carbon or a carbon slurry. If this is not possible, the vessel should befirst be filled to ~50% capacity with water. Next, carbon should be added to the vessel and allowed to hydrate. Finally, thevessel should be backwashed with a volume of water equal to or greater than one vessel volume.From an operations perspective, after reloading the filter with fresh GAC, the bed is likely to contain a high concentration ofcarbon fines. The carbon bed should be thoroughly backwashed to remove these fines prior to being brought on line.CAUTION: Activated carbon removes oxygen from the atmosphere and extreme caution should be taken in entering confinedspaces containing activated carbon.In finalizing the system design, the designer should consider the optimum volume of carbon in the adsorber for commerciallyavailable fill-and-exchange services. The standard sized adsorber normally contains about 20,000 lb (9,000 kg) of carbon (on adry basis). This size adsorber has become a widely used design. This is because the spent carbon will contain water andadsorbate within the internal pore structure that cannot be drained, the weight of which is about equal to that of the dry carbon.Consequently, the total weight of the spent carbon in one 20,000 lb adsorber is approximately 40,000 lb, which approaches thelimits for road transport. Smaller volumes may also have logical cutoffs, such as 2,000 or 10,000 lb to accommodate othertransport container limits. The designer needs to take into account the transport of the spent carbon in the sizing of thevessels.In many cases, regenerated carbon is as effective as virgin carbon and can be more cost effective. However, this must first beconfirmed via laboratory isotherm tests. Where water quality requirements are so high that leaching very small quantities ofimpurities from the regenerated carbon is not acceptable, replacement with virgin carbon might be justified.





5.6 BIOLOGICAL GROWTHBiological growth in a carbon bed is normal and partially desirable if it is aerobic. The biomass can adsorb and biologicallyremove certain dissolved substances from the wastewater resulting in higher actual organic removal rates across the GACcolumn than predicted in laboratory tests. However, excessive biological growth in the carbon column can cause high-pressuredrop across the bed, decreasing flow capacity and increasing backwash requirements. In most cases where the oxygendemand/organic loading is relatively low (< 100 mg/L BOD5) and contact time is reasonably short (< 30 minutes), biologicalfouling is not a significant problem. An anaerobic growth that can generate sulfides is undesirable and requires careful analysiswhen selecting corrective measures.Care should be taken to prevent “seeding" carbon systems with fecal coliform (sewage) bacteria if this parameter is importantto the effluent criteria. These bacteria can thrive in carbon beds and result in significantly elevated effluent bacteria levels. Ifthis is a concern, consideration should be given to providing facilities to disinfect either the carbon bed influent or effluent.

5.7 AIR ENTRAINMENTEntrapped air or other gases in the influent water can build up as trapped bubbles in the carbon bed. These trapped bubblescan cause excessive pressure drop and/or channeling of flow through the bed, as well as excessive biological growth.Excessive pressure drop will reduce flow capacity, and channeling can cause poor contacting and loss of removal efficiency.Provisions should be made to disengage air from the feed upstream of the carbon bed or to ensure that suction is from aquiescent feed source. Care also needs to be taken not to introduce air during feed pumping.If air bubbles form in the bed, backwashing can be used to dislodge and carry the bubbles out of the unit. If entrained air isanticipated to be a concern, sufficient vessel freeboard above the carbon bed should be provided to allow the higher backwashrates sometimes necessary to dislodge the trapped air and solids from the bed.

6.0 DESIGN AND FACILITY BASIS SIZING PROCEDURE FOR GACStep 1. Determine the Design Flow Rate.The design flow rate chosen for equipment sizing should take into account equalization tankage, process unit operation andvariability, stormwater management, load growth for expansion, pre-investment philosophy for infrastructure for future facilities,contingency, as well as overall operating philosophy and regulatory compliance requirements and penalties.The design flow rate is usually equal to the design flow rate of upstream treatment units unless a new equalization step isprovided upstream of the GAC. In principal, the feed system should be designed to minimize surges in flow and load.

Step 2. Characterize the Wastewater, Determine the Need for Pretreatment, and Determine Breakthrough Concentration (Cb).Prepare a comprehensive analysis of the wastewater to be treated. Parameters that will influence the performance of the GACtreatment include BOD/COD/TOC, oil and grease, total suspended solids, pH, and any specific constituents which must beremoved to meet effluent discharge requirements such as BTEX, MTBE, heavy metals, etc.Although GAC filters can be designed to handle suspended materials in the influent, in most wastewater applications primarytreatment followed by filtration is advised as the minimum treatment prior to carbon adsorption. Free oil and grease (O&G),suspended solids (TSS), and other solid materials can cause plugging, bridging and expansion of the carbon beds whichdecreases the removal performance. The feed water to fixed bed downflow or upflow, and pulsed-bed GAC adsorbers shouldbe pretreated to ensure that the following are not exceeded:• Free Oil and Grease/Oil in Water - 10 mg/L• Total Suspended Solids - 20 mg/LExpanded bed upflow units are more tolerant of solids and can accept feeds with solids up to 50 mg/L. Most of these solids, aswell as some carbon fines will end up in the GAC effluent.The breakthrough concentration (Cb) is determined based on the required GAC effluent quality. A safety factor of ~ 20% isused to allow sufficient time to change out the carbon prior to exceeding the required effluent quality. Larger or smaller safetyfactors might be warranted depending on the critical nature of the effluent requirements or other treatment plant characteristicssuch as blending or storage capabilities. For units operating in series, monitoring for breakthrough should be conductedbetween the units.





6.0 DESIGN AND FACILITY BASIS SIZING PROCEDURE FOR GAC (Cont)Step 3. Determine the Activated Carbon Adsorption Capacity.An adsorption isotherm analysis as discussed in Par. 5.1.2 should be conducted to determine the activated carbon adsorptionefficiency for the wastewater and whether the treatment requirements can be met. In the field, the breakthrough adsorptioncapacity, (X/M)b, of the GAC in a full-scale column will be some percentage of the adsorption capacity found from the isotherm.The difference in capacity is attributable to fouling by oil, solids, and biological growth, trapped air bubbles, and the fact that thecarbon will probably be removed from the vessel prior to being completely saturated/spent.If only a single carbon adsorber is used, then (X/M)b can be assumed to be approximately 50 percent of the isotherm capacity.However, multiple columns operated in a rotating lead/polish configuration can improve the effectiveness of this process and for2 or more units in series 75% the isotherm capacity can be used. If laboratory column testing is performed down rating isgenerally not necessary.

Step 4. Select the Type and Configuration of GAC Columns.The type (e.g., downflow, upflow fixed / expanded / pulsed-bed) of adsorber should be selected based on the informationpresented in the Design Considerations Section. Conventional downflow contactors is the preferred type. In general, mostrefinery and chemical plant applications can be expected to use multiple (at least two) fixed-bed adsorbers operated in series ina rotating lead/polish configuration. Multiple adsorption trains operated in parallel are usually required, again to ensurecontinuous operation at the specified design flow with units out of service for backwashing, carbon replacement, or othermaintenance.

Step 5. Determine the Hydraulic Loading Rate, Area, and Diameter for each Filter.The hydraulic loading rate is simply the flow rate divided by the filter area. The units are gpm/ft2 or m/hr. The unit m/hr isderived by dividing the flow rate (m3/hr) by the filter area (m2).For downflow filters typical design values are 3 - 5 gpm/ft2 (7.9 - 13.2 m/hr) with a maximum rate of 8 - 10 gpm/ft2 (21.1 - 26.3m/hr) with one filter out of service for backwashing, etc. At hydraulic loadings less than 3 gpm/ft2, the carbon bed is subject tochanneling. For upflow and expanded bed filters allowable design rates are higher in the 6 - 10 gpm/ft2 (15.8 - 26.3 m/hr) witha maximum rate 12 - 15 gpm/ft2 (31.6 - 39.5 m/hr) with a filter out of service. Exceeding the 12 gpm/ft2 (31.6 m/hr) ratemaximum hydraulic rate in DG 11-1-2 should only be done for upflow filters where specific carbon characteristics are knownand in concert with vendor design information.Once the hydraulic loading rate has been selected, the area of each filter can be determined. This is done by dividing the flowrate by the selected hydraulic loading rate:

A = Q / HLR Eq. (6-1)

where: A = Filter area, ft2 (m2)Q = Flow rate to each filter, gpm (m3/hr)HLR = Hydraulic loading rate, gpm/ft2 (m/hr)

The diameter of the filter can then be determined:D = [A (4) / 3.14]1/2 Eq. (6-2)

where: D = Filter diameter, ft (m)A = Filter area, ft2 (m2)

Step 6. Determine the Quantity of Activated Carbon Required.The quantity of activated carbon required per day can be calculated as follows:

ACR = [(Co – Cb) / (X/M)b] x Q x 8.34 lb/gal x 1L/106 mg x 1440 min/d (Customary) Eq. (6-3)

or

ACR = (Co – Cb) / (X/M)b] x Q x 24 hr/d x 103 L/m3 x 10–6 kg/mg (Metric) Eq. (6-3)M

where: ACR = Carbon required, lb/d (kg/d)Co = Initial COD concentration, mg/LCb = Design breakthrough COD concentration, mg/L(X/M)b = Breakthrough adsorption capacity, lb COD/lb carbon (kg/kg)Q = Flow rate to each filter, gpm (m3/hr)




6.0 DESIGN AND FACILITY BASIS SIZING PROCEDURE FOR GAC (Cont)Step 7. Determine the Filter Run Length and the Volume of the Carbon Bed.The run length of the filter (time to breakthrough), and hence the necessary volume of the carbon bed, should be chosen tostrike a balance between required contact time, operating convenience, size of the filter, and transport and regenerationrequirements. Once a filter run length has been established, the depth and volume of the carbon bed can be determined asfollows:

d = (Tb x ACR) / (A x ACρ) Eq. (6-4)

where: d = Carbon bed depth, ft (m)Tb = Time to breakthrough, daysACR = Carbon required, lb/d (kg/d)A = Filter area, ft2 (m2)ACρ = Activated carbon density, lb/ft3 (kg/m3)

VC = A x d Eq. (6-5)

where: VC = Carbon bed volume, ft3 (m3)A = Filter area, ft2 (m2)d = Carbon bed depth, ft (m)

A minimum carbon bed depth of about 10 ft (3 m) in each vessel is desirable.

Step 8. Check the Empty Bed Contact Time.The empty bed contact time is simply the volume of the carbon bed divided by the flow rate to each filter:

TC = (VC / Q) x 7.48 gal/ft3 or TC = (VC / Q) x 60 min/hr Eq. (6-6)

where: TC = Contact time, minutesVC = Packed carbon bed volume, ft3 (m3)Q = Flow rate, gpm (m3/hr)

As discussed in the DESIGN CONSIDERATIONS section, typical design empty bed contact times are 15 to 30 minutes. DG11-1-2 calls for a minimum empty bed contact time of 30 minutes, 15 minutes per filter.

Step 9. Check that the Wet Weight of the Carbon is Compatible for Transportation and Regeneration Purposes.The wet weight of the carbon that is dumped from the filter for regeneration is about 1.5 to 2 times the dry carbon weightdepending on the carbon. Use 2 for design to account for incomplete drainage and excess water. If critical, contact vendor forspecifics based on carbon type, vessel design, and unloading procedures.

Ww = VC x ACρ x 2 Eq. (6-7)

where: Ww = Wet Weight of the carbon, lb (kg)VC = Carbon bed volume, ft3 (m3)ACρ = Activated carbon density, lb/ft3 (kg/m3)

Step 10. Determine the Total Filter Volume.The filter vessels need to be large enough to accommodate the expansion of the bed during the backwash cycle, or an upflowexpanded bed operation. Bed expansion is a function of carbon density, backwash rate and temperature. A bed expansion ofabout 30 to 40% is typical. Additional volume may be needed to accommodate piping, etc., and this is usually determined bythe manufacturer of the vessel. Exact bed expansion requirements may be outside the typical 30% - 40% requirementdepending on the application.

➧ Step 11. Determine the Backwash Rate.The design backwash rate depends on the application of the activated carbon. For treating relatively clean streams (such asgroundwater w/ only trace organics present), a backwash rate of 4 gpm/ft2 (10.5 m/hr) is typical. Higher upflow backwashflowrates in excess of 4 gpm/ft2 can lead to excessive bed expansion followed by carbon entrainment which should be avoided.For wastewaters higher in solids, backwash rates can be higher (8 - 10 gpm/ft2 (21.1 - 26.3 m/hr)). In such cases, thefreeboard present must be large enough to accomodate the higher backwash rate. The frequency of the backwash dependslargely on the nature and concentration of suspended solids in the wastewater, carbon particle size, and adsorber type (that is,fixed or expanded bed). Backwash frequency may be specified arbitrarily (each day at a specified time), or determined basedon operating criteria such as head loss or effluent turbidity. Backwash duration is typically 10 to 15 minutes.





6.0 DESIGN AND FACILITY BASIS SIZING PROCEDURE FOR GAC (Cont)In addition to providing facilities for full filter bed backwash, facilities to permit air or gas scour are also recommended. Theminimum recommended design air/gas scour rate is 3 scfm/ft2 (59 m/hr) at standard reference conditions of 60°F (15°C) and 1atm (101 kPa). This scour rate is sufficient to dislodge suspended matter and reduce the potential for sulfide generation in thecarbon bed. Consult EMRE's Crude Distillation and Separation Section for issues related to the fluidization and expansion ofcarbon beds.The process design parameters for several ExxonMobil GAC wastewater installations are listed in Table 5.

Step 12. Determine the Valve and Piping Requirements.Valve and piping requirements for upflow and downflow contactors are similar. Upflow units are piped to operate either asupflow or downflow units and to allow backwashing. Downflow units are piped to operate downflow and in series. Each columnis valved to be backwashed individually. Furthermore, as discussed in Par. 5.3.1, downflow series contactors are valved andpiped so that the respective position(s) of the individual contactors may be interchanged in a lead / polish configuration (seeFigure 5).The designer is referred to DG 11-1-2 for additional valve and piping specifications.

Step 13. Determine the Instrumentation, Analyzer, and Controller Requirements.Each filter should be equipped with a differential pressure gauge to monitor the head loss across the combined inlet and outletheaders. Adjustable flow controls and flow indicators are necessary to control and indicate service, air/gas scour, backwashand flow through each filter. Flow measurements are used to equalize flows through the individual carbon filters and todetermine the actual carbon contact time. The designer is referred to DG 11-1-2 for additional specifications oninstrumentation, analyzers, and controllers.

Step 14. Determine Carbon Transport and Regeneration Requirements.An air or water pressure slurry system shall be provided to facilitate the transfer of carbon into and out of the filters. Thesefacilities and the air and water requirements are typically specified by the vendor. Generally the most economical means ofhandling the spent carbon is to return it to the manufacturer for regeneration.

7.0 DESIGN EXAMPLE PROBLEMThe water to be treated is a mixture of oily process water, desalter water, ballast water, crude tank water bottoms, and strippedsour water bottoms coming from a 100,000 B/D refinery. The COD concentration of this water must be reduced from 90 mg/Lto 55 mg/L (30 day average). You have been asked to design a GAC adsorption system capable of meeting this newspecification of 55 mg/L COD.

Step 1. The design flow rate is determined to be 400 gpm (90.8 m3/hr), which is the maximum peak hourly flow expected.

Step 2. The wastewater is currently treated by oil/water separation, dissolved air flotation, and dual media filtration. Based onthe analysis below, no additional pretreatment is necessary.

PARAMETERMAXIMUM

CONCENTRATION, mg/LCOD 100O&G 10TSS 15pH 6.5 - 8.5

The effluent specification is 55 mg/L. To ensure a margin of safety, we will design for a breakthrough concentration(Cb) of 45 mg/L COD in effluent.

Step 3. To determine the adsorptive efficiency of the carbon, samples of the wastewater were taken into the lab andadsorption isotherms were performed using three different carbons. The raw data from the best performing carbon ispresented below. When running isotherm tests, the laboratory wastewater COD concentration should be similar to theinlet COD of the full-scale wastewater process. The laboratory wastewater COD concentration should also be highenough relative to activated carbon mass such that a measurable COD concentration exists at the conclusion of thetest.




7.0 DESIGN EXAMPLE PROBLEM (Cont)

Co, (mg/L) INITIAL COD Ce, (mg/L) FINAL COD Co - Ce, (mg/L), X

M, (g), WEIGHT OFCARBON IN 1 L

VOLUME X/M, (mg/g)100 100 0 0.00 —100 83 17 0.05 340100 71 29 0.10 290100 52 48 0.20 240100 26 73 0.40 183100 14 86 0.80 108100 7 93 1.20 78

The isotherm data have been plotted in Figure 4. The fact that the data fit a straight line suggests that the Freundlichmodel provides a good representation of these data.Determine the Freundlich constants, using Figure 4 and Eq. (5-2):

When Ce = 1.0, (X / M) = 26, therefore k = 26 (the y-axis intercept).

Using data from the graph at Ce = 90:log(350) = log(26) + (1/n)·log(90)

1/n = 0.58

Therefore, the theoretical equilibrium capacity at an influent COD of 90 mg/L can be found using Eq. (5-1):X / M = k · Ce1/n

X / M = (26) (90) 0.58

= 354 mg/g = 0.35 g COD removed / g carbon, or 0.35 lb COD/lb carbon

Step 4. The laboratory isotherm data indicated that the contaminants in the water were readily adsorbable. Therefore thedesign can accommodate typical loading rates, and excessive contact times, transfer zones, or difficulties with carbonutilization are not expected. Furthermore, as the water is well pretreated, excessive fouling is not anticipated.Consequently, we will select a design consisting of two parallel trains of two downflow pressure contactors operated inseries (four units total).

Step 5. As stated in Step 1, the design flow rate is 400 gpm (90.8 m3/hr). The design concept is that each train will normallyhandle 50% of the flow or 200 gpm (45.4 m3/hr). However, the design will provide that all the flow can be handled inone train during periods when the other train is out for backwashing, carbon change out, etc.We will specify the design hydraulic loading rate with 2 trains in service as 4 gpm/ft2 ((10.5 m/hr). This is a typicalrecommended design value for hydraulic loading of downflow units in end of pipe effluent compliance service.From Eq. (6-1):

Adsorber area A = 200 gpm / 4 gpm/ft2 = 50 ft2 (5.4 m2)

From Eq. (6-2):Adsorber diameter D = [50 (4) / 3.14]1/2 = 8.0 ft (2.6 m)

With one train out of service, the maximum hydraulic loading rate on each filter is:400 gpm/50 ft2 = 8 gpm/ft2 (21.1 m/hr) which is well within the recommended design values for maximumhydraulic loading of downflow units.

Step 6. In Step 3, the adsorption capacity of the carbon was determined to be 0.35 lb COD/lb carbon. This is the theoreticalmaximum and probably will not be achieved in practice. With two adsorbers in series, it is estimated that the removalefficiency will be about 75% of the theoretical isotherm measurements.

0.35 lb COD/lb carbon x 0.75 = 0.26 lb COD/lb carbon (0.26 kg/kg)





7.0 DESIGN EXAMPLE PROBLEM (Cont)We can now calculate the carbon required to remove 45 mg COD/L (Co – Cb) (90 - 45 mg/L)from the wastewater usingEq. (6-3):

45 mg COD/L x 400 gpm x 8.34 lb/gal x 1L / 106 mg x 1440 min./d

= 216 lb COD/d, or

216 lb COD/d x kg COD / 2.2 lb COD = 98 kg COD/d

So, 216 lb COD/d x 1 lb carbon/0.26 lb COD = 830 lb (377 kg) carbon/d, or approximately 210 lb (95 kg) carbon/dayfor each of the four filters.

Step 7. We initially will select a desired run length per filter of 90 days. We therefore want to calculate the volume of thecarbon bed in each reactor. The bulk density of the dry carbon to be used is 28 lb/ft3 (450 kg/m3) (this value will beprovided by the carbon manufacturer).Using Eq. (6-4):

Tb = (50 ft2 x depth ft x 28 lb/ft3) / 210 lb/d = 90 d

Solving for the depth of the carbon bed = 13.5 ft (4.0 m)Using Eq. (6-5), the volume of the carbon bed in each filter is:

50 ft2 x 13.5 ft = about 675 ft3 (18.75 m3)

Step 8. The contact time per filter can now be calculated using Eq. (6-6):675 ft3 x 7.48 gal/ft3/ 200 gpm = 25 minutes

This is an acceptable contact time, but is on the high side compared to a typical range of 15 to 20 minutes. Thisindicates a relatively low carbon capacity for the organics present in the wastewater. Conducting column testing tosupplement the isotherm data is warranted in this case to better define carbon capacity and breakthrough. This willpotentially reduce the size of the carbon bed and the vessels.

Step 9. The dry weight of the carbon bed in each filter is:675 ft3 x 28 lb/ft3 = 19,000 lb (8,600 kg)

Now determine the weight of the wet carbon in each filter using Eq. (6-7):675 ft3 x 28 lb/ft3 x 2 (to account for the weight of the water) = 38,000 lb (17,200 kg)

These values are anticipated to be near the maximum volume of spent carbon that can be transported via truck foroffsite regeneration.

Step 10. We can now determine the total height of the filters themselves. The vessels need to be large enough toaccommodate the expansion of the bed during the backwash cycle. A bed expansion of about 30 to 40% is typical.Additional volume may be needed to accommodate piping, etc., and this is usually determined by the manufacturer ofthe vessel.

Height: 13.5 ft x 1.40 = 19.0 ft (6.2 m)

Step 11. The backwash rate will be set at 4 gpm-ft2 (10.5 m/hr) using treated effluent water. Therefore the total volumetric flowto each filter is:

4 gpm/ft2 x 50 ft2 = 200 gpm (45.4 m3/hr)

➧ The minimum recommended design air scour rate of 3 scfm/ft2 (59 m/hr) shall also be specified.The valving, piping, instrumentation, analyzer, and carbon transport requirements shall be are consistent with thosedescribed in Par. 6.0, Steps 12-14.




7.0 DESIGN EXAMPLE PROBLEM (Cont)

PROCESS DESIGN/FACILITY SIZING BASIS SUMMARY TABLE

DESIGN PARAMETER VALUE

Flow 400 gpm (90.8 m3/hr)Number of Filters 4Bed Type Fixed, downflowOperating Mode Two parallel trains with two filters in series.Treatment Rate per Filter 200 gpm (45.4 m3/hr)COD Inlet 90 mg/LCOD Outlet 45 mg/LHydraulic Loading Rate/Filter 4 gpm/ft2 (10.5 m/hr)Contact Time 25 min per filter, 50 min per trainAdsorption Capacity 0.26 lb COD/lb carbonCarbon Bed Volume 675 ft3 (18.75 m3) per filterCarbon Bed Dry Weight 19,000 lb (8,600 kg)Carbon Bed Wet Weight 38,000 lb (17,200 kg)Filter Dimensions (Dia. x Height) 8.0 ft x 19 ft (2.6 m x 6.2 m)Backwash Rate 4 gpm/ft2 (10.5 m/hr) 200 gpmAir Scour Rate 3 scfm/ft2 (59 m/hr)Regeneration Frequency 90 daysRegeneration Type Offsite

8.0 POWDERED ACTIVATED CARBON ADDITION TO BIOX8.1 GENERAL DISCUSSION AND BENEFITSAdding PAC to a plant's activated sludge/biox unit combines activated carbon adsorption with biological secondary treatment.In this process, PAC is added, generally on a continuous or daily batch basis, to the activated sludge mixed liquor.This approach can be beneficially applied to activated sludge systems which are subjected to shock loadings of toxic ordestabilizing organic contaminants. The PAC acts to initially adsorb these surges; then it is thought to release thecontaminants over time at a rate at which they can be metabolized by the biomass. This can be particularly useful for bioxsystems which have trouble maintaining nitrification due to toxicity to the nitrifier microorganisms.Along these lines, PAC can be useful when priority or hazardous pollutant removal is required. Similarly, PAC can help if acuteor chronic bioassay monitoring indicates a problem with effluent toxicity. (Reference 19)In addition, because PAC increases the specific gravity of the activated sludge floc, PAC can improve the settlingcharacteristics of the sludge. For example, the settling characteristics of sludges that are susceptible to filamentous bulking orsubject to dispersed flocs due to rapid microbial growth rates can potentially be improved via PAC additions. PAC can also beused to adsorb surfactants, hence reducing problems associated with aeration basin foaming.PAC Treatment can be applied as either an upgrade to an existing Biox system or as a grassroots facility. The improvement inperformance achieved with PAC Treatment over a traditional Biox unit is not predictable and pilot testing or a full scale trial isrequired.

8.2 PATENT ISSUESIt is important to note that the process of adding PAC to activated sludge systems has been the subject of legal discussionsrelating to patents associated with this technology. While the original “PACT patents" have expired, one can not be sure thatany use of PAC in wastewater treatment can be done without impunity. Any proposed application of PAC into activated sludgesystems should be reviewed beforehand by legal counsel for potential patent issues.

8.3 PAC DOSAGE RATE AND MIXED LIQUOR CONCENTRATIONThe concentration of PAC to be added to the biox system, and hence the desired concentration of PAC in the mixed liquorinvolves a fair bit of guess work. Typically, PAC dosage rates in the biox feed are in the range of 20 to 200 mg/L.





8.0 POWDERED ACTIVATED CARBON ADDITION TO BIOX (Cont)Depending on the sludge age, the percentage of PAC in the mixed liquor can reach 50% or higher. The sludge age affects thePAC efficiency with higher sludge ages enhancing the organic removal per unit of carbon, and establishes the equilibriumbiological solids level in the aeration basin.The PAC dosage and the PAC mixed liquor solids concentrations (MLSS) are related to the sludge age as follows:

Xe = [Xi · SRT] / HRT Eq. (8-1)

where: Xe = Equilibrium PAC MLSS content, mg/LXi = PAC dosage concentration, mg/LSRT = Sludge retention time, daysHRT = Hydraulic retention time, days

8.4 CONSIDERATIONS

8.4.1 Clarifier Solids Loading

The addition of PAC to the biox system will naturally increase the solids loading to the clarifier. While hydraulic loading is a keyclarifier design and operating parameter, the solids loading to the clarifier with PAC should be calculated and checked againstthe clarifier design solids loading to ensure that the clarifier will not be overloaded. The fact that PAC often improves the solidssettleability helps to offset the impact of the additional solids loading.

8.4.2 Differentiating Between Biomass and PAC

With PAC additions, the mixed liquor is now comprised of active biomass, PAC, and residual inert solids such as dirt, sand, andthe remains of dead microbes. PAC additions make it difficult to determine what percentage of the mixed liquor is activebiomass. While Eq. (8-1) gives an estimate for the concentration of PAC in the system, it is not as reliable as taking actualsamples. The difficulty arises because the PAC combusts at 550°C, and therefore is measured as volatile suspended solidswhen using the standard drying and combustion method. In order to differentiate between PAC and active biomass, an aciddigestion procedure is commonly used. This method removes the biomass by a digestion process which does not destroy thePAC. The difference between the weights of the digested and undigested samples gives the percentage of biomass in themixed liquor. More information on this test can be provided by contacting EMRE's Environmental Technology Section.

8.4.3 Precautions When Adding the PAC to the BIOX

The PAC should be added in a fashion which minimizes the potential for the PAC to become airborne. PAC is highly respirableand a severe lung, eye, and skin irritant. PAC should not be handled dry without proper personal protective equipment such aseye goggles, respirator, and gloves. This means that the PAC should not be dumped on the surface of the biox unless it hasfirst been well wetted. Also, if the carbon is not well wetted before entering the biox, it will tend to float on the surface which isnot desirable.

8.4.4 Aesthetics of PAC

Aesthetically, PAC has the undesirable effect of turning the biox facilities, and the mixed liquor, black. The system tends toremain black for quite a long period of time even after PAC additions are stopped. The effluent color is not affected negatively.




9.0 NOMENCLATUREA = Filter area, ft2 (m2)ACρ = Activated carbon density, lb/ft3 (kg/m3)ACR = Carbon required, lb/d (kg/d)BOD5 = Five day biochemical oxygen demand, mg/LCb = Breakthrough chemical oxygen demand concentration, mg/LCo = Initial chemical oxygen demand concentration, mg/LCe = Concentration of the contaminant remaining in solution at equilibrium, mg/LD = Filter diameter, ft (m)d = Carbon bed depth, ft (m)GAC = Granular activated carbonHLR = Hydraulic loading rate, gpm/ft2 (m/hr)HRT = Hydraulic residence time, daysk = Freundlich constantM = Weight of the carbon in the one liter of solution, gMLSS = Mixed liquor suspended solids, mg/L1/n = Freundlich constantPAC = Powdered activated carbonQ = Flow rate to each filter, gpm (m3/hr)SRT = Solids residence time, daysTb = Time to breakthrough, daysTC = Contact time, minutesTSS = Total suspended solids, mg/LVC = Carbon bed volume, ft3 (m3)VSS = Volatile suspended solids, mg/LWw = Wet weight of the carbon, lb (kg)X = Amount of contaminant adsorbed, mgXe = Equilibrium PAC MLSS concentration, mg/LXi = PAC dosage concentration, mg/L(X/M)b = Breakthrough adsorption capacity, lb COD/lb carbon (kg/kg)

Note:gpm = Gallons per minuteKBD = Thousands of barrels per dayscfm = Standard cubic ft per minute





TABLE 1PROPERTIES OF COMMERCIALLY AVAILABLE CARBONS, (Reference 2)

PROPERTIES AND SPECIFICATIONSICI AMERICA

HYDRODARCO 3000

CALGONFILTRASORB 300

(8 x 30)

WESTVACONUCHAR WV - L

(8 x 30)WITCO 517

(12 x 30)

Surface area, m2/g 600 - 650 950 - 1050 1000 1050

Bulk density, kg/m3 430 480 480 480Density, backwashed and drained, lb/ft3 22 26 26 30Real density, g/cm3 2.0 2.1 2.1 2.1Particle density, g/cm3 1.4 - 1.5 1.3 - 1.4 1.4 0.92Effective size, mm 0.8 - 0.9 0.8 - 0.9 0.85 - 1.05 0.89Uniformity coefficient 1.7 1.9 1.8 1.44Pore volume, cm3/g 0.95 0.85 0.85 0.60Mean particle diameter, mm 1.6 1.5 - 1.7 1.5 - 1.7 1.2

Sieve size (U.S. standard series)Larger than No. 8, maximum percent 8 8 8 —Larger than No. 12, maximum percent — — — 5Smaller than No. 30, maximum percent 5 5 5 5Smaller than No. 40, maximum percent — — — —Iodine No. 650 900 950 1000Abrasion No. minimum Not available 70 70 85Ash, % Not available 8 7.5 0.5Moisture as packed, maximum percent Not available 2 2 1

lb/cu ft x 16.02 = kg/m3

1000 g/cm3 = kg/m3




TABLE 2INFLUENCE OF MOLECULAR STRUCTURE AND OTHER FACTORS ON ADSORBABILITY, (Reference 3)

1. An increasing solubility of the solute in the liquid carrier decreases its adsorbability.2. Branched chains are usually more adsorbable than straight chains. An increasing length of the chain decreases solubility,

increasing the adsorbability.3. Substitute groups affect adsorbability:

SUBSTITUENT GROUP NATURE OF INFLUENCE

Hydroxyl Generally reduces adsorbability. Extent of decrease depends on structure of host molecule.

Amino Effect similar to that of hydroxyl but somewhat greater. Many amino acids are not adsorbed to anyappreciable extent.

Carbonyl Effect varies according to host molecule. Glyoxylic acid more adsorbable than acetic but similarincrease does not occur when introduced into higher fatty acids.

Double bonds Variable effects as with carbonyl.

Halogens Variable effects.

Sulfonic Usually decreases adsorbability.

Nitro Often increases adsorbability.

4. Generally, strongly ionized solutions are not as adsorbable as weakly ionized ones, i.e., undissociated molecules are ingeneral preferentially adsorbed.

5. The amount of hydrolytic adsorption depends on the ability of the hydrolysis to form an adsorbable acid or base.6. Unless the screening action of the carbon pores intervene, large molecules are more sorbable than small molecules of

similar chemical nature. This is attributed to more solute carbon chemical bonds being formed, making desorption moredifficult.

7. Molecules with low polarity are more sorbable than highly polar ones.





TABLE 3CLASSES OF MATERIALS READILY ADSORBED AND POORLY ADSORBED ONTO GAC, (References 4, 10, 14)

CLASS READILY ADSORBED MATERIAL POORLY ADSORBED MATERIAL

ORGANICS: Aromatics:• Benzene, phenol, toulene, nitrobenzenes, etc.Chlorinated aromatics:• PCBs, chlorobenzenes, choloronaphthalene, and

cholorophenolsChlorinated non-aromatics:• Carbon tetrachloride, chloroalkyl ethers,

hexachlorobutadiene, etc.Polynuclear aromatics:• Acenaphthene, benzopyrenes, naphthalenes, etc.Pesticides and herbicides:• DDT, aldrin, chlordane, BHCs, heptachlor, etc.Higher MW hydrocarbons:• Dyes, gasoline, higher weight amines, humics

AlcoholsLow-MW ketones, acids, amines, and aldehydesSugars and starchesVery-high-MW or collodial organicsLow-MW aliphatics, e.g., propaneMTBE (moderately adsorbable - see Reference 10)

METALS: • Antimony• Arsenic• Bismuth• Chromium

• Cobalt• Mercury• Silver• Tin• Zirconium

• Barium• Cadmium• Copper• Iron• Lead• Manganese• Molybdenum

• Nickel• Radium• Selenium• Titanium• Tungsten• Vanadium• Zinc

OTHERINORGANICS:

• Bromine• Chlorine

• Fluoride• Iodine

• Bromide• Chloride

• Iodide• Nitrate• Phosphate

TABLE 4TYPICAL FREUNDLICH ISOTHERM CONSTANTS FOR PETROLEUM COMPOUNDS, (Reference 10)

CONTAMINANT INTERCEPT, k, mg/g SLOPE, 1/nBenzene 14.3 0.51Ethyl Benzene 53.0 0.79Naphthalene 132 0.42Phenol 21.0 0.54Toluene 26.1 0.44




TABLE 5DESIGN PARAMETERS FOR VARIOUS EXXONMOBIL GAC INSTALLATIONS

DESIGN PARAMETER SAKAI REFINERY SAKAI REFINERY BENICIA REFINERY CLINTON RES. LAB TONEN REFINERY TONEN REFINERY

Equipment Number GZ-AC-11 & 12GZ-AC-3901/3902

GZ-AC-A, B & C — — WWWJ-9 WWWJ-10

Date Installed 2/74 (AC-11 & 12)9/84 (3901 & 3902)

11/81 May/June 1996 Early 80's Mid 70's Mid 70's

Wastewater Source Desalter, ballast,crude tanks, sourwater stripper

Process water,cooling tower, rainwater

Refinery processwater

R & D facilityprocess water

Ballast water Refinery processwater

PretreatmentFacilities

Dissolved airflotation, dualmedia filters

Parallel plateseparator, dualmedia filters

PAC Biox, dualmedia filters

PAC Biox, dualmedia filters

Parallel plate sep.,sand filter, polymerabsorber

Parallel plate sep.,sand filter,polymer absorber

No. of GAC Filters 4 3 6 2 1 8

Bed Type Fixed Fixed Fixed Fixed Fixed Fixed

Flow Direction Upflow Upflow Downflow Downflow Downflow Downflow

Operating Mode Two parallel trainswith two filters inseries

Two filters in parallelwith one as spare

Three parallel trainswith two filters inseries

Two filters in series Single filter Four parallel trainswith two filters inseries

Treatment Rate/Filter 60 m3/hr (des) 250 m3/hr (des) 530 gpm (des) 100 gpm (des) 110 m3/hr (des)

40 m3/hr (op)

100 m3/hr (des)

50 m3/hr (op)

Wastewater Influent 83 mg COD/L (des) 18 mg COD/L (des) 27 mg TOC/L (des) 9 mg TOC/L (des) 4 mg COD/L (des)

9 mg COD/L (op)

20 mg COD/L(des)

9 mg COD/L (op)

Waste Effluent 52 mg COD/L (des) 9.5 mg COD/L (des) — 7 mg TOC/L (des) 2 mg COD/L (des)

8 mg COD/L (op)

10 mg COD/L(des)

6 mg COD/L (op)

Hydraulic LoadingRate/Filter

11 m3/m2/hr (des) 16 m3/m2/hr (des) 6.8 gpm/ft2 (des) 1.3 gpm/ft2 (des)

5.0 gpm/ft2 max

18 m3/m2/hr (des)

6.5 m3/m2/hr (op)

16 m3/m2/hr (des)

7.7 m3/m2/hr (op)

Contact Time(Minutes/Filter)

30 (des) 27 (des) 11 (des) 30 (des) 8.5 (des)

23 (op)

11 (des)

22 (op)

GAC BedVolume/Filter

30 m3 111 m3 785 ft3 390 ft3 15 m3 18 m3

Dry GAC wt./Filter 10,000 kg 30,000 kg 20,000 lb 10,000 lb 6,750 kg 8,000 kg

Filter Dimensions(Dia. x Height)

2.6 x 11.6 m 4.5 m x 11.6 m 10 ft x 12 ft 10 ft x 10 ft 2.8 m x 3.0 m 2.8 m x 4.5 m

Filter Area 5.3 m2 15.9 m2 78.5 ft2 78.5 ft2 6.2 m2 6.2 m2

Total Filter Volume 62 m3 185 m3 942 cu ft 785 cu ft 18.5 m3 28 m3

Adsorption Capacity(lb COD/lb AC)

0.3 0.05 — 0.01 — —

AC Regeneration Offsite Offsite Offsite Offsite Offsite Offsite

AC RegenerationFrequency

83 days design; 6 -12 mos. in practice

46 days design; 6 -12 mos. in practice

— — 1 yr design; 6 - 8yrs. in practice

1 yr design; 6 - 8yrs. in practice

Conversion Factors: lb = 0.45 kg gal = 3.79 x 10–3 m3 ft = 0.305 m ft3 = 0.028 m3gpm/ft2 = 2.45 m3/hr/m2 gpm = 0.227 m3/hr ft2 = 0.093 m2





TABLE 6PROCESS OPERATING AND DESIGN PARAMETERS

FOR SITES USING PAC ADDITIONS TO THE BIOX

DESIGN PARAMETER VALERO'S BENICIA REFINERY EMRE'S CLINTON RESEARCH FACILITYCarbon Addition Rate 300 lb/day/cell

900 lb/day total (160 mg/L)Designed for 130 mg/L of carbon. However, as theydo not waste any sludge, they do not add carbon.

HRT 9 hrs Designed for 24 hrs. Flow has been extremely low,so HRT is usually > 100 hrs.

SRT >100 days Designed for 20 days. However, with the lowflow/organic load, they never intentionally waste anysludge. Any TSS in effluent is taken out by DMF &sent back to the head of the plant.

MLSS% Biomass% Carbon% Inerts

7800 mg/L473716

4800 mg/L76132

Influent TOC 170 mg/L Design: 80 mg/L. Recent average is 11 mg/L

Effluent TOC 29 mg/L Recent average is 2.7 mg/L

Basin Size 3 @ 47 ft x 47 ft x 15 ft; coarse bubble aeration 4 @ 18 ft x 18 ft x 15 ft; coarse bubble aeration




FIGURE 1DOWNFLOW FIXED-BED GAC ADSORBER, (Reference 8)

ASME Code SteelPressure Vessel

Fresh CarbonSlurry Fill

MaintenanceAccess Manway

Spent CarbonSlurry Discharge

Stainless SteelWater Collector/Backwash Distributor

Corrosion ResistantLining

Treated water

Backwash

Slotted Nozzleor Johnson Screen

Packed Bed GranularActivated Carbon

Space forBed Expansion

Influent Backwash

DP19A8f01

Stainless SteelInlet Distribution





FIGURE 2TYPICAL CONCENTRATION PROFILES AND BREAKTHROUGH CURVES FOR GAC COLUMNS, (Reference 4)

Operating Time, T

CompleteExhaustion

Breakthrough

Saturated Zone

Absorption Zone

(C,In)

0

C, Out = Effluent Concentration

C, In = Influent Concentration

C, In C, In C, In C, In C, In

C, OutC, OutC, OutC, OutC, Out

DP19A8f02

C, Out

C, In

C, O

ut




FIGURE 3TWO DOWNFLOW CARBON BEDS IN SERIES, (Reference 4)

Col.A

Col.B

Col.A

Col.B

Piping Diagram

Then, Flow A to B, Cycle is Complete

First Flow A to B, Renew Carbon in A

Then, Flow B to A, Renew Carbon in B

DP19A8f03

CarbonColumn

B

CarbonColumn

A

Effluent

Bypass

Influent





FIGURE 4STRAIGHT LINE ISOTHERM PLOT (FREUNDLICH) - SAMPLE PROBLEM

10070 90806050403025201.51098765432.521.5110

15

20

25

30

40

50

60

708090

100

150

200

250

300

400

500

600

700800900

1000

X / M

, mg

CO

D/g

Car

bon

DP19A8f04

(1,26)

Extrapolate

(90,350)

Extend toY-Axis

Equilibrium Concentration, Ce, mg/L

dp19a8

Documents

carbon regeneration

influence adsorption

adsorption isotherms

number of units

packaged units

exxonmobil research

design practices

design considerations