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226                            Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological


                 3. Second mixing tank. For the second mixing tank,  9.7  Conversion—moles=Ltomg=L
                   the detention time recommended is 10–30 min.     Given=required
                   Let t (second tank) ¼ 30 min. Then,
                                                                    The problems are stated as (a) and (b). The solutions
                                                                    are straightforward and are freshman chemistry type con-
                 V(second tank) ¼ Q(first dilution)   t(second tank)
                                                                    versions (mentioned so that the problems are seen as not
                             ¼ 0:080 L=s   1800 s
                                                                    too involved). Example 9.3 illustrates the conversion
                             ¼ 144 L                                procedure.
                                                               9.8  Metering Liquid Alum
              From the Clarifloc A-210P data sheet, ‘‘polymer solution
                                                                    Given=required
              should be aged for 15–60 min for best results. Solution shelf
                                                                    In treating the low turbidity in the water of a mountain
              life is 8–16 h.’’ The 30 min detention time in the second
              tank takes care of the aging requirement and consumes the  stream (e.g., the Cache La Poudre River in Colorado)
              polymer before the 8–16 h shelf life has occurred.    the lowest effective alum concentration was determined
                                                                    to be 26 mg Al 2 (SO 4 ) 3   14H 2 O=L (from a pilot plant
              Discussion                                            study). Assume the flow of raw water to the plant is
              Since the tote tanks provide for continuous mixing of  Q(raw water) ¼ 0.76 m =min (200 gpm), which is a
                                                                                       3
              the emulsion and would involve less handling, tote tank
              delivery would be preferred. To comply with the 6-month  small plant.
              shelf life, the delivery schedule should be about three tanks  9.9  Alum Manufacture
              every 6 months. As a second point, the flow of diluted  Given=required
              polymer, Q(first dilution) ¼ 0.080 L=s, compares with  Describe the manufacture of alum. Describe sources,
                                        4
              Q ¼ 1000 L=s, a factor of about 10 , compared with about  manufacturing steps, annual production in the United
                6
              10 for the neat solution. Thus, the mixing requirement  States, how it is packaged and delivered, the distribution
              remains a major issue, but is less severe than if the neat  infrastructure, and costs.
              emulsion was used.
                                                               9.10 Historical Persons
                                                                    Given=required
            PROBLEMS                                                Outline the contributions of A. P. Black to knowledge
                                                                    of coagulation chemistry. W. F. Langlier. W Stumm,
            9.1 Shulze-Hardy Rule
                                                                    and former students.
                Given=required                                 9.11 Alum in History
                Describe the logic of the Shulze-Hardy rule.
                                                                    Given=required
            9.2 Theories of Coagulation
                                                                    Outline coagulation practice in water treatment from
                Given=required                                      antiquity.
                Coagulation theory has evolved to include such ideas as  9.12 Wastewater Coagulation
                ion exchange, double layer, and sweep-floc. Describe
                                                                    Given=required
                briefly some of the key aspects of the evolution of the
                                                                    What could be expected if coagulation is used as a unit
                current theory.
                                                                    process in treatment of raw wastewater in reduction of
            9.3 Potential Barrier
                                                                    BOD (or COD or TOC) and suspended solids? Verify
                Given=required                                      with laboratory jar tests.
                Describe methods to reduce the magnitude of the poten-  9.13 Tertiary Wastewater Coagulation
                tial barrier in the double layer theory.
                                                                    Given=required
            9.4 Bonding Energy between Particles—from DVLO
                                                                    What could be expected if lime coagulation is used as a
               Theory
                                                                    unit process in treatment of secondary treated waste-
                Given=required
                                                                    water in reduction of BOD (or COD or TOC) and
                Determine the order of magnitude of the bonding energy
                                                                    suspended solids? Verify with laboratory jar tests. You
                between two particles, expressed in kJ=mol. (1) Charge-
                                                                    may wish to consider other coagulants, for example,
                neutralization occurs and (2) DVLO theory applies.
                                                                    ferric iron or alum.
                Hint: Section 9.4.1.7 may serve as a starting point.  9.14 Selection of Coagulants
            9.5 Equilibrium Constants—Aluminum
                                                                    Given=required
                Given=required                                      How would you go about selection of coagulation
                Generate the lines of the alum concentration versus pH  chemicals for a new water treatment plant installation?
                diagram by means of a spreadsheet.                  Outline important considerations.
            9.6 Equilibrium Constants—Ferric Iron              9.15 Issues in Wastewater Coagulation
                Given=required                                      Given=required
                Generate the lines of the ferric iron concentration versus  Outline the issues for a program to determine if coagu-
                pH diagram by means of a spreadsheet.               lation should be used in treating a raw wastewater.
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