Certification Action Line – Questions – December 2013

Dec. 1, 2013

Questions about scale formation

1.    In evaluating potential for scale formation, the inorganic salts of _________________ will exhibit the least solubility at pH of seven to eight.

    a.    calcium carbonate (CaCO3)
    b.    ferrous silicate (FeSiO3)
    c.    ferric hydroxide (Fe(OH)3)
    d.    silica (Si(OH)4)

2.    What concentration of salts might one expect to find at an RO membrane surface, when 1,000 milligrams per liter (mg/l) are in the feedwater, the RO system operates with a 50 gallons per day (gpd) feedwater flow rate and a 40 gpd concentrate or reject flow rate and the concentration of salts at the membrane surface (concentration polarization) can be assumed to be 20 percent higher than that in the bulk concentrate stream because of the less than immediate diffusion back to the bulk stream of the salts left behind as water permeates through the membrane?

    a.    2,000 mg/l
    b.    1,500 mg/l
    c.    1,250 mg/l
    d.    1,200 mg/l

3.    The type of scale that precipitates and forms most quickly once its solubility limit has been reached is:

    a.    calcium carbonate, CaCO3
    b.    calcium sulfate, CaSO4
    c.    silica, Si(OH)4
    d.    barium sulfate, BaSO4

4.    A means to prevent calcium carbonate scale formation in a RO system is to:

    a.    operate at a low permeate flow recovery
    b.    remove water hardness in RO pretreatment
    c.    inject acid in the RO feed stream
    d.    inject a scale inhibitor in the RO feed stream
    e.    all of the above

5.    The cycle in a water softener operation that is used to remove suspended solids and resin fines that have been accumulated in the resin bed is called:

    a.    brining
    b.    slow displacement rinse
    c.    fast rinse
    d.    backwash

6.    The desired expansion for a softener resin during backwash is _________ percent of the bed depth.

    a.    10
    b.    20
    c.    50
    d.    200

7.    The time required for filter and water softener backwashing is usually around _____________.

    a.    10 minutes
    b.    30 minutes
    c.    60 minutes
    d.    90 minutes

8.    If a set backflow rate (e.g., 5 gpm) causes a bed expansion of 50 percent of bed depth when the water temperature is 40º F, what will the bed expansion likely be at the same flow rate setting if the water temperature warms to 60º F?

    a.    50 percent of bed depth
    b.    100 percent of bed depth
    c.    30 percent of bed depth
    d.    10 percent of bed depth

9.    The two water softener operational cycles that utilize the same flow rate of about one gpm or less per cubic foot of resin, are:

    a.    brining and slow rinse
    b.    backwash and fast rinse
    c.    fast rinse and service
    d.    service and backwash

10.    Water softening may offer advantages over acid injection to prevent scale formation in RO membrane elements, including the:

    a.    elimination of the possibility of precipitating other insoluble salts with polyvalent cations, such as sulfates and phosphates
    b.    removal of iron
    c.    removal of manganese
    d.    all of the above

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