Thursday, 15 October 2015

Pretreatment

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 ยต 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 25°C, 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 25°C
Temperature (°C)
Multiply the 25°C 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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