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Pretreatment


Pretreatment Selection

1. Remove suspended solids.

2. Control calcium scaling.

             * Antiscalant Injection

             * Softening

      * pH Adjustment (Acid Injection)

3. Other treatment as dictated by water analysis and desired recovery.

 

Particulate 10 to 25 micron cartridge filter.  Sand or diatomaceous earth filter (Optional)
pH correction Acid addition to lower pH to about 6.  Membrane hydrolysis function of pH
Calcium Carbonate pH adjustment prevents precipitation
Calcium Sulfate Precipitation inhibited with scale inhibitors
Iron & Manganese Soluble state no problem, but dander is oxidation to insoluble form at or near membrane.  Oxidation, reduction, scale inhibitor possible methods
Colloids Coagulation/Flocculation; Ultrafiltration
Organic Deposits Filtration, Activated Carbon, Polyelectrolytes
Microbiological Organisms Chlorination
Silica Dependent on form, soluble, crystalline amorphous
Temperature Product flux temperature; 1 to 1.5% per degree

 

Pretreatment Techniques

Feed Water Characteristics Potential Treatment
Turbidity (Suspended solids) Forms Deposits in lines and equipment Coagulation, settling, filtration
Color Stains and causes foaming Coagulation, filtration absorption
Odor (see dissolved gases)   Chlorination, Absorption
Taste   Chlorination, Absorption
     
Chemical Characteristics Potential Treatment
Hardness (Ca and Mg salts) Forms Scale pH, softening, conversion control
Minerals (Na, K, CI, SO4, HCO3) Contribute to TDS RO
Mn++ Forms Deposits Oxidation
Fe++ Upon Oxidation Filtration
Silica Forms Scale Lime softening process
Gases (O2 , H2 S, CO2 , NH3) Cause Corrosion Aeration Degasification
     
Biological Characteristics Potential Treatment
Bacteria Forms organic Chlorination
Algae Deposits UV Irradiation
Viruses    

 

Summary of Pretreatment Options

This table summarizes the pretreatment options when specific risks for scaling and fouling are present.  It is a quick reference for "possible" and "very effective".


Dechlorination

  • Sodium Bisulfite (Na H SO3)

        -10% Solution

        -3 Lbs. of Na H SO3 per lb. of Cl2

  • Activated Carbon

         -1 million PPM gallons per cubic ft.

  • Solution life of Sodium Bisulfite

 

SODIUM BISULFITE SOLUTION

WEIGHT %

MAXIMUM SOLUTION LIFE

 

 

2

3 Days

 

 

10

1 Week

 

 

20

1 Month

 

 

30

6 Months

 

Activated Carbon

  • 1 cubic foot of Carbon will remove 1 ppm of Chlorine from 1 million gallons of water

  • Design Flow Rate

        -Chlorine                     15 gpm/ft2

        -Organics                    6 gpm/ft2

 

 

Removal of Suspended Solids

Technique Equipment Options
   
Gravity Separation

Primary Sedimentation

Secondary Sedimentation

Chemical Sedimentation

Flotation

Clarification

Settler

Physical Straining Process

Screening

Diatomaceous Earth Filtration

Ultrafiltration

Granular Media Filtration

Upflow Sand Filters

Downflow Sand Filters

Green Sand Filters

Multi Media Filters

Special Design Filters

 


 

Granular-Media Deep-Bed Filtration

Table V. Minimum Fluidization Velocities for Various Uniform-Sized Media

(Observed Empirically)9

Between U.S. Std. Sieves

Passing Mm  Retained Size Mean Flow Rate to Achieve 10%
      (mm) Expansion at 25C, gpm/ft2
        Coal Sand Garnet

7

2.830

8

2.59

37.0

 

 

8

2.380

10

2.18

30.0

 

 

10

2.000

12

1.84

24.0

41.0

 

12

1.680

14

1.54

20.0

33.0

 

14

1.410

16

1.30

15.7

27.0

49.0

16

1.190

18

1.09

12.5

21.0

40.0

18

1.000

20

0.92

9.9

16.4

32.0

20

0.841

25

0.78

8.4

12.6

27.0

25

0.707

30

.65

7.0

9.0

22.0

30

0.595

35

0.55

 

6.3

18.0

35

0.500

40

0.46

 

5.4

13.7

40

0.420

45

0.38

 

4.0

11.3

50

0.297

  60 (.25mm)

0.27

 

 

6.3

Specify Gravity

 

 

 

1.7

2.65

4.1

 

 

 

 

 

 

 

Table VI. Temperature Correction: Approximate Correction Factors to be Applied for Temperatures Other Than 25C

Temperature (C)

Multiply the 25C Value by

 

 

30

1.09

25

1.00

20

0.91

15

0.83

10

0.75

5

0.68

 

 

Typical Pressure Filter Media Beds For RO Pretreatment

A.  SAND FILTERS

Top Layer                -12 to 24 fine sand            -0.45 to 0.6 mm e.s.

Middle Layer            -6 to 12 coarse sand          -0.8 to 1.2 mm e.s.

Bottom Layer            -4 find gravel                     -1/4 to 1/8 diam.

 

B.  DUAL - MEDIA FILTERS

Top Layer                -6 to 24 No. 1 anthrafilt        -0.65 to 0.76 mm e.s.

 Layer                     -12 to 18 fine sand               -0.45 to 0.6 mm e.s.

 Layer                     -6 coarse sand                      -0.8 to 1.2 mm e.s.

 Bottom Layer          -4 fine gravel                        -1/4 to 1/8 diam.

 

C. MANGANESE ZEOLITE FILTERS

Top Layer                -0 to 6 No. 1 anthrafilt                -0.65 to 0.76 mm e.s.

Layer                      -12 to 18 manganese zeolite       -0.27 to 0.37 mm e.s.

Layer                      -4 to 6 fine sand                       -0.45 to 0.6 mm e.s.

Layer                      -0 to 6 coarse sand                     -0.8 to 1.2 mm e.s.

Bottom Layer           -4 fine gravel                             -1/4 to 1/8 diam

NOTE: Bed depths will depend on available height of pressure filter and at least 50% of bed depth should be allowed as free board for bed expansion.

 

Deep Bed Filtration - Design Recommendations

DUAL MEDIA

Recommended Depth:

        Coal                          18 - 24

        Sand                         15 - 18

FILTRATION RATE

5 Gallons/Minute/Square Foot                              Dual Media

15 Gallons/Minute/Square Foot                             Multi Media

                  

 
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Applied Membranes, Inc. 2007