Drinking Water Filtration Using Granular Activated Carbon

Major Contributors
Ed Ford, Robinson Township Municipal Authority
Kevin Walsh, The Connecticut Water Company
Teresa Trott, Augusta Water District
Bill McCarthy, City of Lawrence, Massachusetts

Introduction:

Granular Activated Carbon (GAC) is commonly employed as an adsorption media in many surface water treatment plants. Most plants, however, also rely on GAC to provide effective filtration, as turbidity reduction is an essential element in maintaining desired water quality. Often used in conjunction with sand and gravel, GAC provides many additional benefits when utilized as the filtration media. These benefits derive from the adsorption properties of GAC as opposed to other filtration media.

Proper application of the GAC is critical to obtain the desired performance. For filtration performance, mesh size, manufacturing techniques and starting materials play the largest roles. Reagglomerated bituminous coal-based GAC provides the common selection for its superior strength, higher density and resistance to abrasion. The different particle sizes each have their specific niche dependent upon the application.

Use of GAC

Filtration of surface water for use as drinking water has been a common practice since the inception of the water treatment plant. The earliest designs utilized combinations of sand and gravel which serve to greatly reduce the turbidity in the water. Traditional treatment practices employed chemical coagulation of influent raw water, settling the bulk of the sediment in clarifiers, filtration through sand/gravel media, treatment with chlorine and distribution to the customers.

In the 1960's, the increased emphasis on water quality promoted the use of granular activated carbon (GAC) in conjunction with the other media to facilitate the adsorption of taste and odor-causing compounds. In addition to removal of taste and odor compounds, GAC also provides a means for removal of other organic compounds. In groundwater treatment, for example, man-made contaminants can often be effectively removed utilizing GAC. For both surface water and groundwater, this is of great significance when considering the health effects of certain compounds such as pesticides.

In addition to the importance of the adsorption benefits of GAC, the filtration benefits are also significant, as GAC serves a dual role of being a highly effective filtration media. As GAC particles are typically larger than the traditional sand media, design and performance considerations are necessary to properly employ the benefits of GAC in a surface water plant.

The particle size of GAC is typically described by its mesh size. Industry standards use the U.S. Sieve designation to denote particle size. This designation denotes two sieve sizes which describe the maximum and minimum size for the bulk of the material. For example, a 12x40 mesh GAC indicates that the bulk of the material would fall through a 12 mesh screen but be retained on a 40 mesh screen.

Initially, a 12x40 mesh GAC was the standard, as this size most closely approximated the effective size of sand. In an effort to retain the filtration properties but reduce the head loss, the 8x30 product was introduced. This allowed for higher filtration rates and remains the largest type of GAC supplied to the municipal drinking water market. This move to the larger mesh size prompted the development of even larger GAC products such as the 8x16 and 8x20 products. These became highly effective solutions in direct filtration plants, as they allowed deeper beds and higher filtration rates with acceptable head loss.

The Manufacturing Process - Reagglomeration vs. Direct Activated

Coal is the most common starting material for GAC, and the three primary are bituminous, sub-bituminous or lignite. The selection of the starting material greatly impacts the quality of the finished product as it relates to adsorption and filtration parameters. Equally important is the manufacturing process itself which is either a reagglomeration process or a direct activation process.

The reagglomeration process for the manufacture of activated carbons consists of the following steps:

1) A high grade metallurgical coal is pulverized to a powder approximately 50 micron in size.
2) A coal tar or petroleum base binder is added.
3) The product is reagglomerated into briquettes under several tons of pressure.
4) The briquettes are crushed to the desired size.
5) The carbon is baked to remove VOC's at temperatures up to 800 oF in a controlled atmosphere.
6) Finally, the carbon is thermally activated by exposing it to temperatures approaching 1900 oF in a controlled atmosphere.

The alternative to reagglomeration is direct activation, which skips the initial steps and proceeds directly to crushing, sizing, baking and activation. Due to the fact that you must activate a relatively large granule completely from the outside, direct activation produces a granule with high surface activity on the outer shell of the granule and little surface area in the center of the granule. This is contrasted by the reagglomeration process, which produces a large number of man-made pores evenly distributed throughout the granule. The individual powder granules can then be accessed by the high temperature activation and thus a more uniform activation can occur.

These two different activation processes produce GAC with different performance characteristics. The reagglomerated process produces an activated carbon that is much denser and more resistant to abrasion. Also, the addition of the man-made pores and the well distributed activity of the reagglomerated granule allow increased adsorption performance in many applications.

GAC Description and Specifications:

The GAC in a water filter is initially backwashed to segregate the granules. Following this backwash, the smallest granules will be at the top of the filter and the largest at the bottom. The particle size distribution is further described by two parameters: effective size and the uniformity coefficient (table 2). The effective size is typically measured in (mm) and equals the diameter of the smallest 10 percent of the GAC granules. From a mechanical standpoint, the size of the GAC granules in the top part of the filter relates to the pressure drop and the filtration efficiency. The smaller the particle, the higher the pressure drop and the higher the filtration efficiency.

The uniformity coefficient is a dimensionless value that indicates the degree of uniformity of the GAC. A value of (1) would indicate all particles are identical in size with greater values relating to a higher degree of variation. AWWA Standard B604-96 specifically defines uniformity coefficient as follows: "A ratio of the size opening that will just pass 60 percent of a representative sample of the filter material divided by that opening that will just pass 10 percent of the same sample."

Density, measured in g/cc, determines the amount of carbon that is needed to occupy a specific filter volume. Higher density GAC is preferred for several reasons. First, high density GAC products have more carbon structure. In addition, a denser product indicates that for each cubic foot of volume, more GAC can be installed. The denser material also provides a stronger material which is better able to withstand frequent backwashing. With the limited space available in filters originally designed for sand filtration only, this becomes critical. Bituminous coal based GAC provides a much denser material as compared to lignite and sub-bituminous alternatives.

The abrasion number defines the GAC's resistance to abrasion with a higher number indicating greater resistance. This parameter is important in municipal drinking water applications due to the rigors of routine backwashing that may rapidly degrade a softer GAC. When GAC is used for filtration, adsorption, and as a biological support media, this benefit is even more pronounced, as backwashing is required on a more frequent basis. Bituminous coal-based GAC offers the greatest resistance against abrasion as indicated by this measurement.

Design Considerations for GAC in Public Water Treatment

The amount of GAC to be installed for surface water treatment must provide sufficient contact time with the water to facilitate adsorption and provide enough bed depth to allow for proper filtration. Both adsorption and filtration performance of the GAC are closely linked to the particle size.

As previously stated, a smaller particle size provides a higher degree of filtration. In addition, the deeper the GAC bed, the greater the filtration ability of the filter. One would think that the optimum solution for filtration would be a very deep bed of very fine particles. This does not, however, take into account the increased pressure loss associated with this approach. Certainly, the preferred solution would be the selection of bed depth and media size that provides sufficient filtration at a minimal head loss across the bed. This relationship between bed depth, particle size and filtration efficiency can be best expressed by the ratio: (depth of media)/(effective size of filter media), or L/de. Based upon the specific requirements of the application, we can look to maintain empirically derived ratio guideline (Kawamura, 1991):

Minimum for dual media beds with polymer as filter aid
L/de > 1,000

Recommended for dual media beds with polymer as filter aid
1,200 > L/de > 1,500

Minimum for dual media beds without polymer as filter aid
L/de > 1,500

Challenging Waters
L/de > 1,800

L = depth of media
de = effective size of filter media

Typical surface water treatment systems require a gravity flow rate through the GAC bed. Maximum flow rates in shallow bed filters are typically in the 2-3 gpm/ft2 range but can exceed 9 gpm/ft2, especially with deep bed monomedia filters. The operational flow rate needs to provide the predetermined contact time for both adsorption and filtration and at the same time be rapid enough to maintain the economics of the plant.

Due to the build-up of filtered materials on the filtration media, periodic backwashes need to be performed. The common triggers for a backwash are: a) effluent water quality as measured by turbidity, particle count or both, b) timed basis, or c) head loss basis. Operator experience also contributes heavily to the start of the backwash cycle.

The backwash consists of the filter taken off-line and an aggressive countercurrent wash of the filter media bed. Although the rate and duration of the backwash vary depending upon the specifics of the installation, there are common practices. In plants which utilize a surface wash, it is common to backwash at a low rate to allow the surface wash to scrub the top of the filter. The backwash rate is then increased to 15-17 gpm/ft2 for 10-15 minutes to wash the filtered particles from the filter.

In deep bed filters and those in difficult filtration situations, an air scour is used. Typically, the filter is drained to the surface of the filter media. Air is then injected at 4-8 scfm/ft2 at pressures of 4-10 psi. The air is allowed to scour the entire filter from the bottom to the top. When this is complete, the air is turned off and the filter is backwashed at 15-17 gpm/ft2 for 10-15 minutes to wash all removed particles from the filter.

It should be pointed out that as the temperature affects the viscosity of the water, the backwash rate must be adjusted to compensate for this temperature difference. For example, if the water temperature is 35 oF, a backwash rate of 10 gpm/ft2would expand an 8x30 bed 22%. If the water temperature increased to 75 oF, this backwash rate would expand the bed to only 10%. In the summer months with the increased temperature of the water, the backwash rate typically needs to be increased to compensate for this viscosity effect.

Compatibility with other media must be taken into account during the backwash step. Although some filters are monomedia utilizing GAC only, many filters utilize a combination of sand and GAC for filtration. During the backwash process, the media bed will become fluidized and if proper attention is not paid to the compatibility of the sand with the GAC, intermixing and ineffective backwashing will occur. In order to maintain backwash compatibility, the following relationship should be adhered to (Kawamura, 1991):

ESs X UCs ASGg - pw 2/3
-------------- = -------------
ESg X UCg ASGs - pw

ES = Effective Size
UC = Uniformity Coefficient
ASG = Apparent Specific Gravity
pw = density of water

Experience with GAC

GAC has become an essential part of the drinking water treatment process as both a filtration and adsorption media. Although many different sizes and types of GAC are employed, by far the most common is the bituminous based 8x30 mesh GAC. This has proven to provide effective filtration performance for most surface water applications. Four specific plants utilizing bituminous coal based GAC were investigated to illustrate the difference in operation as it relates to mesh size (Table 1).

Robinson Township, PA, utilizes 5 filters containing a combination of 24" of sand and 24" of 8x30 GAC. This would be considered a shallow bed system as the total GAC depth is less than 36". The average influent turbidity in the early part of February of 1999 was .57 NTU (Figure 1) with filtration providing an effluent water quality of .022 NTU. Flow rates during this period were maintained at 1.5 gpm/ft2 and backwashing typically occured at 100 hour intervals. During this period between backwashes, the head loss across the filter typically rises from 0.5' head loss to 3.0' head loss due to the build-up of filtered particles. The operators are also instructed to wash if the head loss reaches 7.0'. A surface wash is employed during the initial low rate backwash to scrub the top of the filter. As typical with shallow bed systems, an air scour is not needed.

Lawrence, MA, like Robinson Township, also employs shallow bed filtration. Unlike Robinson Township, however, Lawrence utilizes 33" of the smaller 12x40 granule on 15" of gravel in 10 filters. Lawrence, MA has the most challenging waters of the four plants discussed with an average influent turbidity of 1.01 NTU (Figure 2) for period discussed. At a flow rate of 2 gpm/ft2, the turbidity is effectively reduced to an average effluent of .31 NTU. The backwash procedure at this facility does not utilize air scour or surface wash. The backwash itself consists of quickly increasing backwash rate to 15 gpm/ft2 and holding it for 8 minutes.

Augusta, ME and Chester, CT, give examples of deep bed filtration. Both plants use the larger granules of 8x16 and 8x20. This requires the use of deeper beds to maintain effective filtration as illustrated by the bed depths of 48" and 60". Unlike the shallow beds, both installations are monomedia, which is typical of deep bed filters. The deeper beds do require the use of air scour to effectively wash the filter beds.

Augusta, ME consists of 3 filters containing 60" of 8x16 mesh carbon with flow rates typically maintained at 2.5 gpm/ft2. For this process, either turbidity, time or headloss trigger a backwash. Normal operation produces backwash cycles every 72 hours. This backwash process is preceded by a 10 minute air scour followed by 10 minutes of a low wash, 8 minutes of a high wash and a final low wash. The turbidity is effectively controlled (figure 3) as the average influent turbidity is reduced from .29 NTU to .019 NTU during period described.

Chester, CT, achieved effective filtration utilizing three filters with 48" depth of the 8x20 mesh carbon at a flow rate of 1.9 gpm/ft2. Influent turbidities averaged .40 NTU with effluent turbidities averaging .1 NTU (figure 4). Like Augusta, Chester requires an air scour and has a similar backwash rate.

It is also important to note that all plants achieved desired filtration with each different type of GAC. Filtration and adsorption characteristics of each type and size of GAC must be properly evaluated in order to ensure the optimum water quality results. When utilizing smaller mesh size GAC's such as 8x30 and 12x40, a shallow bed needs to be employed to maintain acceptable head loss across the filter. The larger mesh sizes such as 8x16 and 8x20 allow the use of deep beds, but the deep penetration of the filtered particles requires the initial air scour step prior to backwashing.

Summary

The three primary starting materials for GAC are bituminous, sub-bituminous and lignite coals. There are two types of manufacturing processes that further define the GAC, either reagglomeration or direct activation. The reagglomerated bituminous coal-based product produces an activated carbon that is denser, more resistant to abrasion, and which has more uniform activation throughout the granule.

There are four primary mesh size carbons that can be used for this application; 8x30, 12x40, 8x16, and 8x20. There is a trade-off between head loss and filtration efficiency as the smallest particle creates the highest filtration efficiency as well as the highest head loss. For this reason, shallow bed designs utilize either the 8x30 or 12x40 GAC, while high rate deep bed filters utilize either the 8x16 or 8x20 filters. Specific design ratios exist for determining bed depth based upon size of granule and influent water quality as well as ensuring compatibility between media in dual-media filters. The four plants discussed illustrated the effective application of bituminous based GAC as a filtration media for the four different sizes that are available.

In addition to its adsorption properties, GAC provides surface water drinking processes the additional benefit of effective filtration. This dual functionality of GAC as an adsorbent and as a filtration media allows more efficient filter design as both benefits can be employed in the same filter volume. In addition, the filtration abilities of GAC allow for the suitable retrofit of existing plants currently utilizing other filtration media but desiring the use of GAC for its adsorption properties.