carbon adsorption techniques for tertiary treatment of...

Activated Carbon for Future

Carbon Adsorption

Techniques for

Tertiary Treatment of

Industrial & Municipal Effluents By T M Swamy GCARBON


Activated carbon is a product which is highly porous and has special properties like adsorbing odouring, coloring and removing certain carcinogens. Primarily there are three different types of activated carbon. •Powdered activated carbon •Granular activated carbon and •Palletized activated carbon.

Basics


The main factor that influences the grade or activity of any activated carbon are the presence of pore structures. The coloring & odouring matter and other organic matters are adsorbed or held temporarily in these pores thereby the liquid gets purer.

What is Activated Carbon?


These pores can be classified as micro pores, meso pores and macropores. They are in order of their increasing sizes. While meso pores and macro pores do not directly contribute to the adsorption of contaminants, they are necessary since they create passage to the micro pores.


Micropores generally contribute to the major part of the internal surface area. Macro- and mesopores can generally be regarded as the highways into the carbon particle, and are crucial for kinetics.


1. Highly effective at removing non-polar organic chemicals from water (removal of COD and BOD).

2. Applicable to a wide variety of organic compounds 3. Very effective at removing colours from waste streams. 4. Effective at removing low levels (ppb range) of inorganic

pollutants. 5. Thermal regeneration of the carbon destroys the adsorbed waste

solute. 6. Very flexible system allows rapid start-up and shut down as

needed. 7. System can be designed so that it is portable, to be taken to waste

sites. 8. Can handle surges in contaminant levels due to unexpected leaks.

Benefits of Activated Carbon Filtration


1. Limited to wastes with low organic concentrations (< 5%). 2. Limited to wastes with very low inorganic concentrations

(< 1%). 3. Unable to remove highly soluble organics, or those with

low molecular weights. 4. Systems cannot tolerate suspended solids in the influent

stream (due to clogging). 5. High operating costs due to carbon costs system

requirements. 6. Disposal of contaminated carbon can be problematic if it is

not regenerated.

Limitations of Activated Carbon Treatment


In all these processes the wastewater is contacted with granular activated carbon (GAC)typically in a semi-batch or continuous operation. Processes that utilize this type of carbon include: 1. Fixed-bed or expanded-bed adsorption 2. Moving-bed adsorption 3. Fluidized-bed adsorption

Adsorption Processes Utilizing Granular Activated Carbon for Waste Water Treatment


Fixed-Bed Expanded-Bed

Waste Water In

Waste Water In

Waste Water Out

Waste Water Out

Fixed Bed and Expanded Bed Adsorption Systems

GAC Bed

GAC Bed


1. This is the most common type of adsorption column for wastewater treatment.

2. These columns must be provided with a system for the removal of spent carbon and the addition of fresh or regenerated carbon.

3. Because or their construction and operation down flow fixed-bed adsorbers also acts as depth filters for particles that can be contained in the wastewater.

4. Therefore this adsorption column must also be provided with facilities for backwashing (including air scouring, if necessary.

Down-flow Fixed-Bed Adsorbers


Height of packing 3 - 9 m (10 - 30 ft) Particle size 8 - 40 mesh Hydraulic loading 1.4 - 6.8 L/m2 s (2 -10 gpm/ft2) Residence time 10 - 60 min (typically 20 -30 min) Typical carbon requirements – pre-treatment – tertiary treatment (in g carbon/m3 wastewater) 60 - 20025 – 50 Operating pressure < 20 KPa/m of bed

Characteristics of Commercial Adsorbers


1. In general, most up flow adsorbing columns operate in the expanded-bed mode

2. Expanded-bed adsorbers are used when the wastewater fed to the column contains a significant fraction of suspended particles

3. Since the bed is always expanded the column does not act as a filter for the suspended particles

Up-flow Expanded-Bed Adsorbers


1. GAC beds are sometimes used as deep-bed filters (as well as adsorbers)

2. the capital cost associated with dual purpose adsorber-filter beds is typically lower than separate beds

3. the regeneration of the adsorptive capacity of the bed should be followed by the removal of solids via bed fluidization

4. most heavy metals are also adsorbed on activated carbon beds

Adsorption Beds as Filter Beds and Heavy Metal Adsorbers


1. The presence of organic material in a typical activated carbon bed coupled with the presence of micro organisms in the wastewater makes the bed an ideal breeding ground. This can have both negative and positive effects:

2. The microorganisms may contribute to the breakdown of pollutants adsorbed on the bed, thus improving its performance

3. The presence of excessive organic material and the typical lack of oxygen may result in anaerobic growth (associate with the potential for odor generation) and the plugging of the bed due to excessive growth

Biological Reactions in Adsorption Beds


Adsorber Kinetics


GAC Bed

Down Flow Fixed Bed Adsorbers in Series

Waste Water Inlet

Waste Water Outlet

GAC Bed

GAC Bed


GAC Bed GAC Bed GAC Bed

Waste Water Inlet

Waste Water Outlet

Down Flow Fixed Bed Adsorber in Parallel


Waste Water Inlet

Waste Water Outlet

Up-Flow Fixed Bed Adsorbers in Series

GAC Bed GAC Bed GAC Bed


Moving Bed Adsorber

Waste Water Out Carbon In

Waste Water In Carbon Out

GAC Bed


Fluidized Bed Adsorber

Waste Water Out

Waste Water In


1. Particle size 2. Diameter of column 3. Flow rate of incoming wastewater (or

residence time) 4. Height of adsorption bed 5. Pressure drop 6. Time required to achieve breakthrough (Time of exhaustion)

Important Consideration for Designing Carbon Bed Adsorber


As the wastewater moves through a fixed bed of carbon the pollutant to be adsorbed will move from the wastewater to the carbon bed. Several steps are involved in the overall adsorption process of a single molecule of pollutant:

Mass transfer step. Mass transfer from the bulk of t wastewater to the surface of the carbon particle through the boundary layer around the particle

Diffusion step. Internal diffusion through a pore

Adsorption step. Adsorption on to the surface of the particle

Adsorption Process


In most wastewater treatment applications the overall adsorption process is dominated by mass transfer, especially intra-particle mass transfer. A qualitative ranking of the magnitude of the resistances is:

External interparticle mass transfer step slow to not-so-slow, depending on the operation

Intraparticle diffusion step typically slow

Adsorption step typically fast

Relative Magnitude of Rates involved in Adsoprtion


Break Point & Break thorough Curve

Breakpoint

Breakthrough Curve

Exhaus tion Point

Ads oprtion Zone

Co Co Co Co

C C

Co

Cb Ce

Cumulative Wastewater Flow


Many different approaches are available. In general, partial differential equations can be written, incorporating the different mass transfer and adsorption mechanisms. Typically these models are complex and require numerical solutions.

Other models rely on experimental data and the use of simpler models to interpret them so as to produce satisfactory designs.

These models can be used to size the column by determining the depth of the adsorption zone, the shape of the breakthrough curve, the time at breakpoint and the pollutant removed at breakpoint.

Approach to Design Considerations


A common way to collect information on the performance of an adsorption column and to scale up the results consists in setting up a column as high as that eventually to be used in the full scale application, but of much smaller diameter. Data are then collected by determining the time at breakpoint, tB, at different points (ports) along the column

Interpretation of Experimental Data


Case Study


Gowrishankar Chemicals (P) Ltd in their effort to implement continual improvement programs (CIP), an special effort was made in house to explore new grades of activated carbon suitability to one of their select client M/s Reliance Industries Ltd at Jamnagar (RILJ). Presently RILJ are using GS850-6x12 grade of activated carbon for their ETP primarily to bring down COD level in their final polishing stage. The purpose of implementing CIP was to chose a better quality of carbon with different size to see any improvement over the existing grade of carbon in terms of higher adsorption thereby reducing replacement frequency. For this GSPLused poilet plant test to evaluate and compare adsorption capacity/characteristics of two grades of granular activated carbon. The two grades of activated carbon selected were GS850-6x12 and GS1000-8x30.

Objective


GS850-6x12 is the present grade of activated carbon now being supplied. The properties of the same are; Macro Properties Iodine Adsorption Value 840 mg/g ( on dry basis) Moisture 5% max Size 2 mm to 4mm Ash Content 5% max Base material Coconut Shell Charcoal Micro Properties Total Pore Volume mL/g 0.73 Average Pore Size (oA) 12-14


The properties of new grade selected (GS1000) for the test are; Macro Properties Iodine Adsorption Value 1000 mg/g ( on dry basis) Moisture 5% max Size 8x30 ( 3.44 mm to 0.7 mm ) Ash Content 2% max Base material Coconut Shell Charcoal Micro Properties Total Pore Volume mL/g 0.82 Average Pore Size (oA) 14-18


Firstly adsorption isotherm tests were conducted using both the grades of activated carbon as described below; Both grades activated carbon was ground so that at least 60% passes through 325 mesh and 100% passes through 100 mesh. Various weight sample of carbon was treated with 50mL of effluent waste water and allowed to stand for 60s. The slurry was then filtered through and clear filtrate was collected and its COD analyzed.


Test No 1 2 3 4 5 6 7 8 Grade of Activated Carbon GS850

Vol of Effluent used 50mL ( sol pH=7.83 ) 30 deg C Weight of carbon, g 0.10 0.11 0.12 0.14 0.16 0.18 0.20 0.22 % reduction in COD 33 37 43 45 49 51 52 52

Table 1 Effluent Stream 761

Test No 1 2 3 4 5 6 7 8 Grade of Activated Carbon GS1000

Vol of Effluent used 50mL ( sol pH=7.83 ) 30 deg C Weight of carbon, g 0.10 0.11 0.12 0.14 0.16 0.18 0.20 0.22 % reduction in COD 62 64 69 72 76 81 84 84

Table 2 Effluent Stream 761


Testing Apparatus Details; Secondly a test set up consisting of an 100mm ID column and a bed height of 3000mm with pumping arrangement along with flow controls was set up. The following characteristics were maintained at inlet conditions; Flow 12 cu.m/hr/sq.m Temperature 28 to 31 oC pH 7 to 7.5 Effluent form RILJ site Stream 761 was used.


Time INLET COD, ppm OUTLET COD, ppm

0 min 162 0

+120 min 162 45

+ 360 min 162 48

TRIAL A Carbon Details used: Grade GS850 Size2-4mm (Effluent Sample S1)

Time INLET COD, ppm OUTLET COD, ppm

0 min 162 0

+120 min 162 18

+ 360 min 162 18

TRIAL B Carbon Details used: Grade GS1000 Size8x30 (Effluent Sample S1)


Conclusion; GS1000-8x30 grade performed better than the GS850 2-4mm. The reason for better adsorption are;

•GS1000 has larger average pore size (by 29%) •GS1000 has higher pore volume (by 12%) •GS1000 was tested in smaller particle sizes i.e., 8x30 (adsorption is inversely proportional to particle size).


Thermal regeneration consists of processing the spent carbon at high temperature to volatilize and/or burn the adsorbed materials. Thermal regeneration typically involves some loss of carbon (typically between 5 and 10%) and some changes in the adsorption characteristics of the carbon itself (mainly caused by the reactivation process producing larger pores than those present in the original material). Thermal regeneration can be conducted on site or, more often, off-site.

Thermal Regeneration of Spent carbon


Thermal Regeneration Rotary Kiln


Thank You

carbon adsorption techniques for tertiary treatment of...

Documents