The first use of activated carbon for water treatment dates back to more than 2000 years ago when the product was used in char form. Powdered Activated Carbon (PAC) became commercialized during the 20th century for water treatment to remove taste and odor contaminants. Granular Activated Carbon (GAC) came into use after World War II.
Major grades supplied by Carbochem for municipal water treatment:
• Carbochem® LQ-325: PAC
• Carbochem® LQ-900S: GAC for regular filter operation
• Carbochem® BAC-820: Biologically activated GAC for deep bed filters
Primary Treatment Objectives
• TOC reduction
• Taste and odor removal, such as Geosmin and MIB
• Pesticide removal, such as Atrazine
• Removal of disinfection byproduct precursors
• Reduction of turbidity (filtration mechanism)
Operating conditions that make GAC cost-effective:
• Year round treatment to achieve high quality water
• Dual filter media
Operating conditions that make PAC cost-effective:
• Seasonal demand for specific treatment (such as removing MIB, Geosmin)
Activated Carbon Evaluation and Cost Optimization
• Requires performance testing which is specific for the contaminant involved (such as MIB, Geosmin) and reliable analytical techniques
• Pitfalls producing unreliable results:
1) Threshold Odor Test has limitations in terms of reproducibility due to the “fatigue factor” and the subjective nature of the test. In addition, it does not provide an absolute result for a specific compound and therefore has a limited degree of accuracy.
2) The use of TOC as a surrogate for MIB or Geosmin is not meaningful or reliable since the PAC needs to be filtered before the measurement can be taken, which removes the undissolved organic carbon component and thus effectively only the DOC is measured. DOC consists of a wide range of molecular weight compounds with different functionalities to MIB and Geosmin, so the resulting adsorption result will not be representative of MIB and Geosmin.
3) The same applies to Tannin Value, as the Tannin Value is only an indicator of tannin removal and has no meaningful correlation to any other performance criterion. Tannin Value is highly dependent on the source of tannic acid used and the correct correlation of the isotherm data to the definition of tannin value given in the Test Protocol. As a result of these limitations, the tannin value is unreliable and inconsistent which does not make it a valid standard.
Design Parameters for GAC Use
1) Type of filter: Gravity – surface water, high volume/flow rate
2) Pressure – ground water, low volume/flow rate
3) Empty Bed Contact Time (EBCT = GAC volume/flow rate): 10 minutes minimum (producing a shorter mass transfer zone [MTZ] and thus more efficient use of the GAC)
4) Particle Size:
a) Effective Size: 0.8 -1.0 mm Uniformity Coefficient: 2.1 maximum
This combination maximizes the adsorption characteristics of the GAC but promotes a larger pressure drop due to stratification. This size is larger than conventional filter sand but comparable to average size anthracite. Recommended to improve performance of filters that have a limited EBCT due to greater rate of adsorption. Typical bed depth: 1 meter
b) Effective Size: 1.0 – 1.2 mm Uniformity Coefficient: 1.5 maximum
This combination optimizes filtration performance and produces a lower pressure drop with a longer cycle time. Recommended for filters with a high treatment capacity/flow rate that require deep bed filters (2 meters deep).
5) Backwash flow rate: achieve a bed expansion of 30% minimum
6) Service life: typically 3 years depending on the quality of the water
7) Ash consideration: Historically, Total Ash was considered to be a key quality parameter but Water Soluble Ash has replaced this for the following reasons (supported by the fact that AWWA no longer lists Total Ash as a specification requirement):
a) For water treatment applications, it is the water extractable component of the carbon which could have the greatest impact on the quality of the water and therefore has the greatest significance;
b) The non-extractable component of the ash is basically inert and thus the difference between 8% total ash vs. 10% total ash (for example) is insignificant and in most cases the difference in performance is undetectable.
Note: High ash carbon (> 15%) is not recommended as it is indicative of a lower quality carbon and will cause problems if the GAC needs to be reactivated.
8) Bituminous coal GAC vs. Coconut shell GAC
Key Quality Properties of GAC Related to Performance (refer to chart):
• Iodine Number: indication of micropore content and surface area
• Abrasion Number: Hardness and degree of attrition including reactivation yield
• Apparent Density: Volume activity and design parameters
• Water Soluble Ash: Level of extractables which can impact water quality
• Pore Size: Needs to be compatible with the dimensions of the contaminant
Conditions that justify replacement of traditional filtration media, such as Anthracite, with GAC:
1. Taste and Odor (T&O) control can be more than a seasonal problem
2. Elevated TOC levels
3. Treatment with powdered activated carbon (PAC) required for more than 60 days per year to control T&O
4. Effectiveness of PAC treatment is limited due to inadequate contact time
5. Poor/inconsistent quality feed water
6. Presence of organics such as herbicides, pesticides and THM
7. Planning to change out current filter media within 2 years
GAC can provide the dual function of a filter media and a cost effective means of removing organic contaminants for T&O control, etc. GAC also provides the benefit of more effective turbidity reduction than anthracite.