Pretreatment

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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 µ 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".

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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

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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