An Important Note

Note that the use of metal salts for coagulation may increase the quantity of dissolved solids. One must consider the downstream impact of these dissolved solids. In addition, the impact of carryover of suspended Al ... and Fe... compounds and their related effect on downstream processes must be considered.

Aluminum salts are most effective as coagulants when the pH range is between 5.5 and 8.0 pH. Because they react with the alkalinity in the water, it may be necessary to add additional alkalinity (called buffering) in the form of lime or soda ash. Use the values in Table 3 to guide you. Iron salts, on the other hand, are most effective as coagulants at higher pH ranges (between 8 and 10 pH). Iron salts also depress alkalinity and pH levels; therefore, additional alkalinity must be added. Sodium aluminate increases the alkalinity of water, so care must be taken not to exceed pH and alkalinity guidelines. As is evident from the reactions discussed above, a working knowledge of the alkalinity relationships of water is mandatory. By using inorganic coagulants we can wind up producing a voluminous, low-solids content sludge the is difficult to dewater and dries very slowly. The properties of the sludge to be generated and estimated quantities needs to be carefully determined, in part from pilot-scale and bench testing prior to the design and construction of a plant.

Polymers are often described as long chains with molecular weights of 1,000 or less to 5,000,000 or more. Along the chain or backbone of the molecule are numerous charged sites. In primary coagulants, these sites are positively charged. The sites are available for adsorption onto the negatively charged particles in the water. To accomplish optimum polymer dispersion and polymer/particle contact, initial mixing intensity is critical. The mixing must be rapid and thorough, Polymers used for charge neutralization cannot be over-diluted or over-mixed. The farther upstream in the system these polymers can be added, the better their performance. Because most polymers are viscous, they must be properly diluted before they are added to the influent water. Special mixers such as static mixers, mixing tees and specially designed chemical dilution and feed systems are all aids in polymer dilution. Static or motionless mixers in particular are popular for this application. Refer to Figure 20 for an example of an in-line static mixer.

Table 3. Recommended Alkali and Lime 1 ppm Equivalents.

Chemical -lppm

Formula (1 ppm)

Alkalinity Increase

(1 ppm)

Hardness as CaCOj Increases

Sodium bicarbonate


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