Water Well Journal

July 2016

Water Well Journal

Issue link: http://read.dmtmag.com/i/692787

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Page 41 of 87

D uring the past two months of Engineering Your Business we've introduced you to various technologies associated with removing unwanted solid material from water and wastewater. Although the title has been "Solids Separation," the topic has been expanded to include the removal of undesirable dissolved substances as well as solids from a liquid, usually water. Last month, a more detailed discussion of two of these methods—the straining method using one or more layers of a granular media, and the barrier method using physical impediments such as inline screening devices—to accomplish solids removal were outlined. This month, we'll wrap up this topic with the last two methods of dissolved solids removal: adsorption and chemical alteration. ADSORPTION This certainly won't be the first nor probably the last arti- cle in an EYB column regarding the science of adsorption. As with other water-related technologies, it is an important aspect of water treatment and one that requires detailed discussion and explanation. It is interesting to note both adsorption and chemical alter- ation generally use the additional process of granular media, or straining (described in Part 2), to facilitate their solids re- moval characteristics. Where straining is typically a physical process where the media intercepts and traps particulate mat- ter within the filter media, adsorption and chemical alteration use subtle changes in chemistry to assist removal of fine and dissolved particulates from a fluid. As was outlined in Part 1, adsorption is technically defined as the adhesion of a gas, vapor, or dissolved material onto the surface of a solid. Various dissolved organic compounds in water—herbicides, pesticides, industrial waste products—can pose a significant health threat and affect the taste and odor of drinking water. To remove them, the process of adsorption is often used. Adsorption is a process in which one substance is attached to the surface of another substance. The process of "adsorption" should not be confused with the similar-sounding term of "absorption." Using an analogy of a sponge, a sponge will readily soak up or "absorb" water containing offending substances, such as a taste and odor, and when the water is squeezed from the sponge, the taste and odor will generally remain in the water. This process constitutes "absorption." In an "adsorption" process using an activated carbon media, water with taste and odor substances is brought into contact with the large pore spaces present on and around car- bon particles. The taste and odor constituents "stick to" and remain with the carbon after the water leaves the carbon parti- cles—due to the adhesion between the offending substances and the surface of the carbon media. This results in water free from bad taste and odor. More simply stated, the process of absorption involves the volume of the entire "sponge" to soak up material, while ad- sorption uses the surface or outer area of the "sponge" to soak up material. The ability of a specific material to effect adsorp- tion is known as the affinity of the material. Adsorption is not only used for taste and odor treatment of water, it is also extensively used to remove various other con- taminants, many potentially serious and life threatening, from water as well. This group of contaminants include many of the synthetic and volatile contaminants listed on the "EPA Regu- lated Water Contaminants" list. Technically, adsorption refers to the collecting of mole- cules by the external or internal surface of solids or by the surface of liquids. Occasionally, the word "sorption" is used to indicate the taking up of a gas or liquid by a solid without specifying whether the process is adsorption or absorption. Adsorption can be either physical or chemical in nature. Physical adsorption resembles the physical condensation of gases to liquids and depends on the physical attraction be- tween the solid adsorbent and the adsorbate molecules. There is no chemical specificity in physical adsorption. Any gas will tend to be adsorbed on any solid if the temperature is suffi- ciently low enough or the pressure of the gas sufficiently high enough. In chemical adsorption, gases are held onto a solid surface by the chemical forces that are specific for each surface and each gas. Chemical adsorption occurs usually at higher tem- peratures than those at which physical adsorption occurs. Fur- thermore, chemical adsorption is ordinarily a slower process than physical adsorption and, like most chemical reactions, frequently involves an energy of activation. Adsorption is present in many natural, physical, biological, and chemical systems. It is widely used in industrial applica- tions such as activated charcoal, capturing and using waste heat to provide cold water for air conditioning and other process requirements. Adsorption, ion exchange, and chro- matography are sorption processes in which certain adsorbate- ED BUTTS, PE, CPI ENGINEERING YOUR BUSINESS SOLIDS SEPARATION Part 3: Adsorption and chemical alteration techniques ENGINEERING continues on page 42 waterwelljournal.com 40 July 2016 WWJ

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