16593 06 tủ tài liệu bách khoa
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16593 06 tủ tài liệu bách khoa
CHAPTER 6COAGULATION AND FLOCCULATIONRaymond D. Letterman, Ph.D., P.E.Professor, Department of Civil and Environmental Engineering, Syracuse Universit 16593 06 tủ tài liệu bách khoaty, Syracuse, New YorkAppiah Amirtharajah, Ph.D., P.E.Professor, School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GeorgiaCharles R. O'Melia, Ph.D., P.E.Abel Wolman Professor. Department of Geography and Environmental Engineering, The Johns Hopkins University, 16593 06 tủ tài liệu bách khoaBaltimore, MarylandCoagulation is a process tor increasing the tendency of small particles in an aqueous suspension to attach to one another and to at16593 06 tủ tài liệu bách khoa
tach to surfaces such as the grains in a filter bed. ĨI is also used to effect the removal of certain soluble materials by adsorption or precipitationCHAPTER 6COAGULATION AND FLOCCULATIONRaymond D. Letterman, Ph.D., P.E.Professor, Department of Civil and Environmental Engineering, Syracuse Universit 16593 06 tủ tài liệu bách khoaional water treatment systems in which the processes of coagulation, sedimentation, filtration, and disinfection are combined to clarify the water and remove and inactivate microbiological contaminants such as viruses, bacteria, and the cysts and oocysts of pathogenic protozoa. Although the removal 16593 06 tủ tài liệu bách khoaof microbiological contaminants continues to be an important reason for using coagulation, a newer objective. the removal of natural organic material16593 06 tủ tài liệu bách khoa
(NOM) to reduce the formation of disinfection by-products, is growing in importance.Aluminum and ferric iron salts have long been used to remove colorCHAPTER 6COAGULATION AND FLOCCULATIONRaymond D. Letterman, Ph.D., P.E.Professor, Department of Civil and Environmental Engineering, Syracuse Universit 16593 06 tủ tài liệu bách khoaand sediments, and released by plankton and bacteria. Natural organic material adsorbs on natural particles and acts as a particle-stabilizing agent in surface water. It may be associated with toxic metals and synthetic organic chemicals (SOCs). Natural organic material includes precursor compounds 16593 06 tủ tài liệu bách khoathat form health-related by-products when chlorine6.2CHAPTER SIXlation in water treatment, even when color removal is not the principle objective. A t16593 06 tủ tài liệu bách khoa
reatment technique requirement in the US. Environmental Protection Agency's (USEPA's) Stage 1 Disinfection By-Products Rule requires NOM removal in coCHAPTER 6COAGULATION AND FLOCCULATIONRaymond D. Letterman, Ph.D., P.E.Professor, Department of Civil and Environmental Engineering, Syracuse Universit 16593 06 tủ tài liệu bách khoa United States since the 1880s. Alum and iron (III) salts have been employed as coagulant chemicals since the beginning, with alum having the most widespread use. In the 1930s. Baylis perfected activated silica as a “coagulant aid." This material, formed on site, is an anionic polymer or a small, ne 16593 06 tủ tài liệu bách khoagatively charged colloid. Synthetic organic polymers were introduced in the 1960s. with cationic polymers having the greatest use. Natural starches we16593 06 tủ tài liệu bách khoa
re employed before the synthetic compounds. Polymers have helped change pretreatment and filtration practice, including the use of multimedia filters CHAPTER 6COAGULATION AND FLOCCULATIONRaymond D. Letterman, Ph.D., P.E.Professor, Department of Civil and Environmental Engineering, Syracuse Universit 16593 06 tủ tài liệu bách khoation). and in-line filtration (rapid mixing and filtration only).Coagulants are also being used to enhance the performance of membrane microfiltration systems (Wiesner Ct al.. 1989) and in pretreatment that prolongs the bed life of granular activated carbon (GAC) contactors (Nowack and Cannon. 1996) 16593 06 tủ tài liệu bách khoa. The development of new chemicals, advances in floc removal process and filter design, and particle removal performance standards and goals have stim16593 06 tủ tài liệu bách khoa
ulated substantial diversity in the design and operation of the coagulation process, and change can be expected to continue into the future.In evaluatCHAPTER 6COAGULATION AND FLOCCULATIONRaymond D. Letterman, Ph.D., P.E.Professor, Department of Civil and Environmental Engineering, Syracuse Universit 16593 06 tủ tài liệu bách khoaration is more critical to success than the physical facilities al the plant."Their report recommends that plant staff use a well-defined coagulant chemical control strategy that considers variable raw-water quality, lhere is no question that high-rate (rapid sand) filtration plants are coagulant-ba 16593 06 tủ tài liệu bách khoased systems that work only as well as the coagulants that are used.DEFINITIONSCoagulation is a complex process, involving many reactions and mass tran16593 06 tủ tài liệu bách khoa
sfer steps. As practiced in water treatment the process is essentially three separate and sequential steps: coagulant formation, particle destabilizatCHAPTER 6COAGULATION AND FLOCCULATIONRaymond D. Letterman, Ph.D., P.E.Professor, Department of Civil and Environmental Engineering, Syracuse Universit 16593 06 tủ tài liệu bách khoa after chemical dispersal in rapid mixing: interparticle collisions that cause aggregate (floc) formation begin during rapid mixing but usually occur predominantly in the flocculation process. For example, using the aluminum sulfate salt known as alum [AỰSOẠy 14HX")] in coagulation involves formatio 16593 06 tủ tài liệu bách khoan of an assortment of chemical species, called aluminum hydrolysis products. that cause coagulation. These species are formed during and after the tim16593 06 tủ tài liệu bách khoa
e the alum is mixed with the water to be treated. Coagulants are sometimes formed (or partially formed) prior to their addition to the rapid-mixing unCHAPTER 6COAGULATION AND FLOCCULATIONRaymond D. Letterman, Ph.D., P.E.Professor, Department of Civil and Environmental Engineering, Syracuse Universit 16593 06 tủ tài liệu bách khoa chloride (PAC1) and polyiron chloride (PIC1).The terminology of coagulation has not been standardized. However, in most of thứ water trùOtmùrít litoroturo i'AOftiilatinn rùfút-c oil tliii rùo/'tOtirl tntii'hoCOAGULATION AND FLOCCULATION6.3coagulant formation (where applicable), particle destabiliza 16593 06 tủ tài liệu bách khoation, and physical interparticle contacts. The physical process of producing interparticle contacts is termed flocculation.These definitions of coagul16593 06 tủ tài liệu bách khoa
ation and flocculation are based on the terminology used by early practitioners, such as Camp (1955). However, in the colloid science literature. LaMeCHAPTER 6COAGULATION AND FLOCCULATIONRaymond D. Letterman, Ph.D., P.E.Professor, Department of Civil and Environmental Engineering, Syracuse Universit 16593 06 tủ tài liệu bách khoa them. LaMer defined destabilization by simple salts such as NaCI (a so-called indifferent electrolyte) as “coagulation." Destabilization of particles by adsorption of large organic polymers and the subsequent formation of particlc-polymer-particlc bridges was termed “flocculation."Ihc water treatme 16593 06 tủ tài liệu bách khoant literature sometimes makes a distinction between the terms “coagulant" and “flocculant." When this distinction is made, a coagulant is a chemical u16593 06 tủ tài liệu bách khoa
sed to initially destabilize the suspension and is typically added in the rapid-mix process, in most cases, a flocculant is used after the addition ofCHAPTER 6COAGULATION AND FLOCCULATIONRaymond D. Letterman, Ph.D., P.E.Professor, Department of Civil and Environmental Engineering, Syracuse Universit 16593 06 tủ tài liệu bách khoalocculants arc often used to increase filter performance (they may be called “filler aids" in this context) and to increase the efficiency of a sludge dewatering process. In any case, depending on how and where it is used and at what dosage, a coagulant is sometimes a flocculant and vice versa. In t 16593 06 tủ tài liệu bách khoahis chapter, no distinction is made between coagulants and flocculants. The term “coagulant” is used exclusively.Coagulants and Treatment WasteThe typ16593 06 tủ tài liệu bách khoa
e and amount of coagulant or coagulants used in a water treatment facility can have a significant effect on the type and amount of residue produced byCHAPTER 6COAGULATION AND FLOCCULATIONRaymond D. Letterman, Ph.D., P.E.Professor, Department of Civil and Environmental Engineering, Syracuse Universit 16593 06 tủ tài liệu bách khoan most water treatment facilities, coagulation is the process that generates the bulk of the residual materials, their handling and disposal processes and costs must be considered in the selection and use of coagulants. The use of enhanced coagulation is an important example of this, because the hig 16593 06 tủ tài liệu bách khoaher coagulant dosages may produce residuals that are much more difficult to dewater. Water treatment plant waste handling, treatment, and disposal are16593 06 tủ tài liệu bách khoa
covered in Chapter 16.CONTAMINANTSNatural Organic MaterialHumic substances are typically the major component of NOM in water supplies. They are derivCHAPTER 6COAGULATION AND FLOCCULATIONRaymond D. Letterman, Ph.D., P.E.Professor, Department of Civil and Environmental Engineering, Syracuse Universit 16593 06 tủ tài liệu bách khoamic substances are anionic polyelectrolytes of low to moderate molecular weight: their charge is primarily caused by carboxyl and phenolic groups: they have both aromatic and aliphatic components and can be surface active: they are refractive and /-•Oti Aùrcict tVir nũntnriac Innoar LI 11 tilir- ('i 16593 06 tủ tài liệu bách khoaiki tùriPùC ủrù z-lofirizizl MtiLirititirioIIw Kt'6.4CHAPTER SIXthe more soluble fulvic acids (FAs) and the less soluble humic acids (H As), with FAs16593 06 tủ tài liệu bách khoa
predominating in most waters (Christman. 1983).The concentration of NOM in water is typically expressed using the amount of organic carbon. Organic caGọi ngay
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