Notes on Activated Carbon Treatment - Wastewater Treatment Systems | PAPR 3531, Study notes of Chemistry

Download Notes on Activated Carbon Treatment - Wastewater Treatment Systems | PAPR 3531 and more Study notes Chemistry in PDF only on Docsity! ACTIVATED CARBON TREATMENT PURPOSE Usually activated carbon (AC) is used for the removal of dissolved organic compounds. AC can remove organic compounds that are poorly biodegraded. FUNCTION AC works by adsorption. Adsorption is the removal of a substance from one phase, e.g., a liquid, by concentrating the substance on the surface of another phase, e.g., solid (accumulation at an interface). Adsorption can occur by physical (van der Waals) forces or by chemical reactions. Physical adsorption is reversible. TERMINOLOGY Adsorbent – The solid causing the adsorption (the AC). Adsorbate or sorbate – The substance (solute or pollutant) being removed. AC MANUFACTURE Raw Material: Coal (most common) Wood Nut shells Rice hulls Bone Resins Preparation: 1. Dehydration @ 170 °C (3 steps) 2. Carbonization (pyrolysis) @ 400-700 °C (absence of O2) 3. Activation @ 400-1000 °C (steam and O2) Carbonization drives off volatiles, and a char is formed. Activation burns out additional material, and a very porous structure is formed. 2 AC CHARACTERITICS High surface area because of porous structure. 1 gram has ~1000 m2 of surface area ~4.5 grams has the surface area of a football field ~99% of surface area is internal (within the pores) Surface chemistry and pore size distribution are functions of the raw material and conditions of manufacture. Types based on particle size: Powdered (PAC) <200 mesh (<0.074 mm) Granular (GAC) >200 mesh GAC 5 6 TECHNICAL CONSIDERATIONS 1. Kinetics Four steps – bulk transport → film transport → pore transport → adsorption (attachment) M&E Fig. 11-52 (pg. 1140) 2. Capacity Adsorption capacity AC has a finite capacity to adsorb organic compounds. Capacity is a function of: The AC The organic compound(s) (adsorbates) Adsorbate concentration Temperature Possibly other factors (e.g., pH) 7 Capacity can be expressed in terms of an “isotherm,” which is an expression relating capacity to adsorbate concentration in solution. Data collection: Set up a series of bottles to which the same volume of water containing a known concentration of adsorbate AND varying amounts of AC are added. Agitate the bottles until the adsorbate concentration in solution no longer changes. Calculate capacity: plot capacity versus solution concentration: x = mass of adsorbate removed (adsorbed) = (Co – Ce)V where Co = initial absorbate concentration in solution Ce = equilibrium concentration of adsorbate in solution (concentration in water after adsorption is complete) V = volume of water or wastewater x/m = AC capacity where m = mass of activated carbon Plot capacity versus equilibrium concentration (isotherm plot), not in M&E: 10 GAC VS. PAC PAC has lower capital cost (possibly use existing tank/clarifier) but higher operating cost (AC is thrown away) GAC in a bed or column more efficiently uses its “capacity” than PAC dosed into the wastewater. Why? A higher x/m. That is, the GAC equilibrates with the influent or starting concentration Co while the PAC equilibrates with the effluent or ending concentration Ce. See isotherm below. PAC use, though a continuous feeding into the wastewater flow, is analogous to batch treatment. 11 Typical GAC-bed breakthrough curve (M&E Fig. 11-53, pg. 1146): In the ideal, the bed effluent concentration would go from “0” (zero) to Co instantaneously when the entire amount of GAC reached capacity (equilibrated with Co). This instantaneous breakthrough would occur at about where C/Co = 0.5 on the above figure. Because of kinetics and dispersion, in reality the breakthrough curve is “S” shaped with bleeding occurring before the idealized instantaneous breakthrough would occur. (Also see Fig. 11-54 on pg. 1147 in M&E for breakthrough curves related to various types of organic constituents.) 12 To make full use of the GAC capacity, the design can include two or more beds in series. See figure below. Three carbon beds in series operation: If properly designed, as the first column becomes saturated (its effluent = Co), the effluent from the third column is still quite low. The first bed can be removed from service and the carbon replaced or regenerated. With flexibility in piping design, the first column with “clean” carbon can then be put back in service as the third column (staggered regeneration). EXAMPLE PROBLEM How much AC is required to treat a 5000 L batch of wastewater containing 1000 mg/L of the chemical know as “CARP” if the treated effluent must contain no more than 10 mg/L of CARP? Assume that the treatment duration is sufficient for equilibrium to be reached and that adsorption data conform to the Langmuir isotherm. The Langmuir constants (for concentration expressed as mg/L) are: a = 0.3 and b = 0.1