Water Well Journal
Issue link: https://read.dmtmag.com/i/692787
organic compounds attach to the surface of the powder gran- ules. The granules of PAC have irregularly-shaped surfaces, which gives PAC a very large surface area to attract organic compounds. It is estimated 1 pound of PAC has a surface area of up to 100 acres. The carbon can then be removed by filtra- tion, taking the unwanted organic compounds with it. CHEMICAL ALTERATION The process involving chemical alteration is the last process of solids separation we will discuss in this series. Although this process is not technically the same solids removal process as the barrier or strainer methods, it is often used to alter the characteristics of water to make the removal of solids or substances in the liquid much easier to remove— thus the connection to our theme. Chemical alteration typically involves one or more of the following processes: (1) precipitation, (2) oxidation, (3) ion exchange. Precipitation Chemical precipitation in water and wastewater treatment is the change in form of materials dissolved in water into solid particles. Chemical precipitation is used to remove ionic con- stituents from water by the addition of counter-ions to reduce the solubility. It is used primarily for the removal of metallic cations, but is also used for the removal of anions such as fluoride, arsenic, cyanide, and phosphate as well as organic molecules such as the precipitation of phenols. The precipitation process starts with the addition of chemi- cal precipitants, including coagulants and polymers, followed by a second process, flocculation, that is used to increase the particle size through a third process called aggregation. The specific precipitation process can change very fine particles that are normally held in suspension by electrostatic surface charges into larger and heavier particles that readily settle or are removed by filtration due to their increased mass. These initial electrostatic forces cause clouds of counter-ions to form around the particles, giving rise to repulsive forces that pre- vent aggregation and reduce the effectiveness of subsequent solid-liquid separation processes (filtration). Therefore, chem- ical coagulants are often added to overcome the inherent re- pulsive forces of the particles. The three main types of coagulants are inorganic elec- trolytes (common water treatment agents such as alum, lime, ferric chloride, and ferrous sulfate); organic polymers; and synthetic polyelectrolytes with anionic or cationic functional groups. Typically, the addition of coagulants is followed by a low-shear and gentle mixing device called a flocculator, used to promote and enhance contact between the particles, allow- ing larger particle growth through the sedimentation phenom- enon referred to as flocculant settling. Flocculant settling refers to a dilute suspension of particles that coalesce, or flocculate, during this operation. As floccula- tion occurs, the particles continue to combine and increase in mass and settle at an increasing rate. Following flocculation, water is directed to another downstream basin, referred to as a sedimentation basin. Here, the water is allowed additional time needed to complete the process of gravity settling of the larger particles before the water is directed to the final process—filtration, usually a form of granular media or strain- ing filtration where the remaining finer and aggregated parti- cles are removed through this final solids separation process. The entire process is shown in graphical form in Figure 2. The amount of particle removal that occurs depends on the opportunity for coagulant and flocculator contact, sedimenta- tion, and filtration, which varies with factors and specific de- sign criteria—such as the coagulant selection and injection rate, overflow rate, the depth and cross sectional area of the basins, the velocity gradients in the system, the raw and fin- ished water concentrations and range of particle sizes, the time provided for sedimentation, and the filtration rate. The effects of these variables can only be accomplished by theo- retical or practical determinations in jar, sedimentation, and pilot tests. Precipitation or a variable of the process is the most com- mon form of water treatment for surface water supplies and is rapidly becoming one of the most widely selected methods for the removal of heavy metals from groundwater in pump-and- treat operations. It is also used as a pre-treatment process with other chemical alteration technologies (such as chemical oxi- dation), where the presence of metals could possibly interfere with treatment. After water is pumped to the surface, precipi- tation converts the soluble heavy metals to insoluble metals that readily settle or are filtered out of the water. ENGINEERING continues on page 44 Figure 2. Chemical precipitation. WWJ July 2016 43 Twitter @WaterWellJournl