Particulate and Colloidal Fouling

Flow rates and good chemistry are the critical factors in successfully

cleaning membranes fouled by colloidal material. The high pH

RoClean formulations contain dispersants that help push particles

away from each other and away from the membrane surface,

allowing a more effective clean.

THE FOLLOWING ARE SYMPTOMS OR INDICATORS OF SILT AND COLLOIDAL FOULING:

High pressure differential on elements.

Surface water feed supply to system.

High turbidity in feedwater.

Element telescoping.

Discoloration of the membranes.

CLEANING APPROACH

Low pH clean with either RoClean P303 or L403 followed by a

high pH clean with RoClean L211.

Consider reverse flow and air sparging during cleaning

procedure.

Heat cleaning solution to maximum allowed by the membrane

manufacturer.

STEPS TO PREVENTING SILT AND COLLOIDAL FOULING: Conduct SDI (Silt Density Index) and turbidity testing to

determine colloidal fouling potential.

Conduct laboratory filtration study to determine fouling

potential, filterability of the water and optimum RoQuest®

coagulant dosage.

Consider a multi media pilot filter study.

Using a microscope, inspect the spent SDI pads from the

MMF feed and effluent to compare filter efficiency.

Use properly designed multi media filtration and consider the

 

Fig 1: Element severely fouled with resin.

Fig 2: Element severely fouled with sand.

Figure 3: Colloidal fouling of a membrane surface.

need for air scour or surface wash.

Consider the use of a coagulant to increase the effectiveness

of the multi media filter.

Use clarifiers or other filtration equipment for high turbidity

waters.

Organics Fouling (non-biological)

THE FOLLOWING ARE SYMPTOMS OF ORGANICS FOULING: Membrane discoloration.

Low flow on individual membrane test data.

Possible high pressure differential on individual membrane test

data.

THE RECOMMENDED CLEANING APPROACH IS: A low pH clean using either RoClean P303 or L403, followed

by a high pH clean using either RoClean P111 or, for severe

cases, RoClean P112 or L212.

The combination of a low pH clean followed by the high pH solution is

extremely effective in removing organics. The low pH cleaner helps

break the bridge between the organics and the membrane. The high

pH solution then lifts the foulant off the membrane surface. This is

why there is sometimes a color discharge only when using the high

pH cleaner. Don’t be misled though, the low pH clean was a vital step

in the cleaning regime.

STEPS TO PREVENTING ORGANIC FOULING INCLUDE: Remove the organics from the RO feedwater with a properly

designed multimedia filter with coagulant addition. Metal salts

blended with a polymer are usually the most effective

coagulant for high organic waters.

NOTES:Figure 3 is an example of a spent cleaning solution. The foulant was

humic acid, dissolved by formulated cleaners.

 

Fig 1: Membrane surface with severe organics fouling.

Fig 2: Membrane fouled with humic acid.

Figure 3: Spent cleaning solution.

Iron and Manganese Fouling

THE FOLLOWING ARE SYMPTOMS OF IRON AND MANGANESE FOULING:

Discoloration of membranes.

Poor salt rejection on individual membrane test data.

Low flow on individual membrane test data.

Possible high pressure differential on individual membrane test

data.

High iron or manganese values reported in feedwater.

High pressure differential reported on first array.

The recommended cleaning approach is a low pH clean (RoClean

P703) followed by a high pH clean (RoClean P111 or L211). Results

can be further improved by heating the cleaning solution to the

maximum allowed by the membrane manufacturer.

STEPS TO PREVENTING IRON AND MANGANESE FOULING INCLUDE:

Conduct a complete and accurate water analysis.

If the water supply contains a high amount of ferrous iron, it is

important to prevent it from oxidizing. Oxidation can be

caused by exposure to chlorine or aeration of the water

(which occurs in the multimedia filter or other similar

equipment).

Iron and manganese can be removed using a greensand filter

regenerated with potassium permanganate. However, if

potassium permanganate is overdose or is not properly

rinsed from the greensand filter, it will oxidize the membrane

surface.

Iron can be removed by chlorinating the water then removing

the oxidized iron or manganese with a multimedia filter.

ADDITIONAL NOTES:Iron in water can be found as ferrous or ferric. Ferrous iron is

 

Fig 1: Iron fouled membrane.

Fig 2: Iron fouled membrane.

Figure 3: Initially, the foulant was a mystery, but it was later determined to be Manganese.

dissolved iron that has not precipitated. Exposure to air turns ferrous

iron into ferric (oxidized) iron, which essentially, has become rust.

Ferrous iron can exist in relatively high levels and not precipitate so

long as it is not oxidized. It becomes a filtration issue if it becomes

oxidized. Ferric iron fouling can be removed using RoClean P703.

Sulfate Scale

SYMPTOMS OF SULFATE SCALE INCLUDE: Material extruding from the downstream end of the last

membranes in the system.

Foulant will not dissolve when introduced to a dilute

hydrochloric acid solution.

Poor salt rejection, low flow, and/or high pressure differential

on individual test data.

8” x 40” element weight exceeds 40 pounds.

Site reports interruption in the antiscalant or acid injection.

Site reports scale in the last vessel or piping of the

concentrate stream.

THE RECOMMENDED CLEANING APPROACH VARIES: If some of the material dissolved in a dilute hydrochloric

solution, begin the clean with a low pH product such as

RoClean L403.

If none of the material dissolved in a dilute hydrochloric

solution, then skip the low pH clean and use only a high pH

solution, RoClean L811.

If possible, heat the cleaning solution to the maximum

temperature allowed by the membrane manufacturer.

ADDITIONAL NOTES:

 

Fig 1: SEM photograph of barium sulfate scale.

Fig 2: SEM photograph of barium sulfate scale.

Figure 3: A magnified view of one of the crystals.

Barium sulfate scale on a membrane, pressure vessel, or pipe feels

like fine grit sandpaper. This abrasiveness can cause damage to a

membrane surface during system operation. When the system is

started and stopped, the vexar (feedspacer) material shifts slightly.

This shifting can cause the barium sulfate scale to scratch the

membrane surface causing permanent damage and resulting in a

loss of rejection.

Calcium Carbonate Scale

SYMPTOMS OF CALCIUM CARBONATE SCALE INCLUDE: Scale extruding out of the downstream end of the last

membranes in the system.

All of the scale dissolves when introduced to a dilute

hydrochloric acid solution.

Poor salt rejection, low flow, and or, high pressure differential

on individual membrane test data.

8” x 40” Element weight exceeds 45 pounds.

Site reports interruption in the antiscalant or acid injection.

Site reports scale in the last vessel or piping of the

concentrate stream.

THE RECOMMENDED CLEANING APPROACHES ARE: Low pH clean with RoClean L403.

In severe cases, add hydrochloric acid to the cleaning solution

to maintain a pH of 3.0.

TESTING FOR CALCIUM CARBONATE SCALE:

Calcium carbonate scale is not always as white as shown in Figures

1 and 2. But, a quick test can be conducted to see if the foulant is

comprised solely of calcium carbonate.

In a glass beaker, make a 1:1 dilution of HCl and DI water. Drop a

small sample of the foulant into the solution. If the foulant contains

calcium carbonate, it will bubble (see Figure 3). Continue adding acid

 

Fig 1: Element severely fouled with CaCO3 scale.

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Guide Word doc is

missing Figure 1. ]

Fig 2: Element severely fouled with CaCO3 scale.

Figure 3: An acid test is used to determine if a foulant is comprised of

until the bubbling stops or until the scale disappears.

If the beaker contains residual material after the bubbling has

stopped, then the foulant consists of more than just calcium

carbonate.

CaCO3 scale.

Biological Fouling

SYMPTOMS OF BIOLOGICAL FOULING INCLUDE: Visible slime on the feed side of the membrane.

Site reports slime in the cartridge filter housing and piping.

Site reports high pressure differential in the first array.

Odor

Individual membrane test data reports high pressure

differentials.

Membranes are telescoped.

THE RECOMMENDED CLEANING APPROACHES ARE: Add DB20 biocide to a low pH cleaning solution (RoClean

L403 or P303).

Follow the low pH clean with RoClean P111 (for severe cases,

consider P112).

Heat the high pH solution (If P112 is used, temperature range

is 90° – 100°F).

STEPS TO PREVENT BIOLOGICAL FOULING INCLUDE: Properly dose sodium bisulfite.

Clean and sanitize the pretreatment equipment and piping.

Consider an intermittent biocide treatment.

Evaluate the necessity of a continuous injection biocide.

ADDITIONAL NOTES:

 

Fig 1: See “Additional Notes”.

Fig 2: Membrane fouled with biological material.

Figure 3: Membrane fouled with biological material.

Figure 4: MBiological fouling of membrane and

Figure 1 is a membrane sample that was taken from a single 8” x 40”

element. The first 20” of both the membrane and vexar (feedspacer)

were plugged with biological slime while the last 20” were free of the

foulant.

vessel interior.

ackpressure Damage (Delamination)

BACKPRESSURE IS CAUSE FOR CONCERN BECAUSE IT CAN CAUSE DELAMINATION OF REVERSE OSMOSIS MEMBRANES.

Delamination is a separation of the membrane from the backing

material. This type of damage is caused by backpressure from the

permeate side of the membrane and results in a loss of rejection.

Backpressure can be caused by restrictions in the line such as

valves, resin beds, or elevated piping. It can also be caused by

improper relief of product pressure in a product staged (double pass)

RO system.

Figure 1 shows an example of severe backpressure damage where

the pattern of the feedspacer (vexar) is clearly visible.

The bubbling of the membrane shown in Figure 2 is sometimes

subtle and difficult to detect.

Figure 3 shows the membrane envelope, comprised of two sheets of

membrane and one sheet of tricot (permeate carrier) between.

The yellow arrow is the tricot, the red arrow is the backing material

and the blue arrows are the membrane that was pulled away from the

backing material.

Delamination allowed easy separation of the membrane material from

the backing material. This separation would not be possible on an

undamaged element.

 

Figures 1

Fig 2

Figure 3

Membrane Telescoping - High Pressure Differential

TELESCOPING OF A MEMBRANE IS CAUSED BY EXCESSIVE DIFFERENTIALS BETWEEN THE FEED PRESSURE AND THE CONCENTRATE PRESSURE.

The maximum pressure differential for a single 40” long membrane is

10 psi. When this pressure is exceed, damage to the membrane and

its materials of construction can occur.

CONSEQUENCES OF TELESCOPING: Damage to the fiberglass outer wrapping allows water to flow

on the outside of the element. This will reduce crossflow

across the membrane surface and increase the fouling

potential.

If the feed spacer (vexar) moves within the membrane area, it

can scratch the membrane surface and cause permanent

damage.

Glue lines can be stressed and fail.

Flow patterns will be disrupted, resulting in channeling over

the membrane surface and uneven fouling.

The membrane crease will be stressed. Failure can occur at

these points near the permeate tube.

PREVENTION: MONITOR PRESSURE DIFFERENTIALS ACROSS THE

ENTIRE SYSTEM AS WELL AS ACROSS EACH ARRAY. IF THE FIBERGLASS IS DAMAGED, REPAIR WITH VINYL

TAPE OR PLACE THE BRINE SEALS ON THE OPPOSITE END IF THE FIBERGLASS AND ATD ARE NOT DAMAGED ON THAT END.

CLEAN THE SYSTEM BEFORE PRESSURES EXCEED 10 PSI PER MEMBRANE ELEMENT AND CORRECT ANY EXCESSIVE FOULING PROBLEMS.

 

Fig 1: Telescoped element.

Fig 2: Vexar protruding from the element.

Figure 3: Vexar protruding from the element.

Determining Scale Potential for Reverse Osmosis Applications

AN ACCURATE FEEDWATER ANALYSIS IS CRITICAL IN DETERMINING THE SCALING POTENTIAL OF A REVERSE OSMOSIS APPLICATION.

The conversion chart in Table 1 outlines the ions that must be known

in order to complete an antiscalant projection. A complete analysis

should also include the pH of the water at the time of sampling.

For ions such as Aluminum, Barium, Iron, and Strontium, it is

important to ensure that the testing methods can detect ion levels in

the range of 0.01 ppm.

The Avista Advisor™ computer program allows the user to enter site-

specific feedwater and system data to:

Recommend an antiscalant and dosage for a specific

application.

Determine the scaling potential of a feedwater.

Calculate chemical injection rates.

Determine maximum system recovery based on scaling

potential.

It can also be a very useful system troubleshooting tool.

 

Fig 1: Conversion chart.

Fig 2: Avista Advisor™ dialog.

Antiscalant Dosing Calculations

Errors in chemical dosing are common, but also very preventable.

They typically occur when chemical feed pumps are not calibrated

correctly or when dosage calculation errors are made. The purpose

of the information below is to provide a quick and easy dosage

calculation.

 

Fig 1: Avista Advisor™

STEPS TO DETERMINE DOSAGE PARAMETERS:1. Identify the optimum ppm dosage of the appropriate Vitec®

antiscalant. This can be determined using the Avista

Advisor™ Dosing Report (Example shown in Figure 1).

2. Determine the desirable dilution in the chemical day tank (see

Figure 2). 10% is the maximum recommended dilution.

3. Identify the feedflow rate to the RO system.

4. Calculate the dosage using the values determined in Steps 1-

3 and the calculation below.

DOSAGE CONFIRMATION

Using a drawdown assembly (as shown in Figure 3), verify that the

proper ml/minute dosage is being delivered to the system.

If there is no drawdown tube, calibrate the pump by placing the pump

suction line into a graduated cylinder filled with the solution to be

pumped. Measure the drop in solution over a one-minute period to

verify the ml per minute injection rate. Adjust the pump accordingly

until it is correct.

The Avista Advisor computer program is available to recommend and

dose Vitec antiscalants based on site specific feedwaters.

Dosing Report.

Table 1: Quick Reference Dilution Chart.

Fig 3: Drawdown assembly diagram.

System flow in gallons per minute

Xppm Dosage of

AntiscalantX

% Dilution Constant(“M” value in Table

1)=

ml/minute injection rate

Multi Media Filter Operation

Membrane manufactures typically specify that RO feedwater should

have a SDI (Silt Density Index) of < 5.0 and a turbidity value <0.20

NTU. Low SDI and turbidity values are believed to mean that the

potential for colloidal fouling is reduced. With proper application,

design, and operation, a multi media filter (MMF) can achieve these

goals.

 

Fig 1: Turbidity vs. Time.

TURBIDITY

When a MMF is put on-line following a backwash, the turbidity value

can sometimes be similar to the feedwater turbidity. Over a brief

period of time, the turbidity should drop and stabilize to an acceptable

level. As the system operates, the turbidity rises. When the turbidity

increases by 10%, a backwash should be initiated. If the MMF is not

backwashed, the turbidity in the effluent may eventually exceed the

feedwater turbidity.

PRESSURE DROP (DELTA–P)It is important to monitor the delta-P of the MMF. Increases in delta-P

signal the filter is working and removing particles. The MMF should

be backwashed at or before the delta-P reaches ten (10).

DELTA-P AND TURBIDITY ARE INDEPENDENT

It is important to understand that delta-P and turbidity breakthrough

can be independent of each other. The MMF should be backwashed

based on whichever occurs first.

TURBIDITY VS. DOSAGE

When dosing a coagulant, high turbidity does not always indicate an

inadequate dosage. It is important to understand that both

underdosing and overdosing can cause turbidity to rise. Additionally,

significant overdosing can cause the effluent turbidity to exceed the

feedwater NTU. The optimal dosing range at the bottom of the curve

usually has a spread of 1 to 2 ppm.

IMPORTANT OPERATING TIPS

Frequently test MMF feed and effluent turbidity.

Use a drawdown tube to verify coagulant dosages.

Use reliable pressure gauges to measure MMF delta-P.

Initiate MMF backwash based on turbidity, delta-P or time.

Fig 2: Delta-P vs. Time.

Fig 3: Turbidity vs. Dosage.

Fig 4: Photo shows the interior of a failed MMF bed. The garnet, anthracite and sand are intermixed and the surface of the bed is cratered.

Silt Density Index (SDI)

Although the values do not directly correlate to the fouling potential of

a specific water, the Silt Density Index, or SDI, test is considered to

be an industry standard for measuring the colloidal fouling potential of

spiral wound membranes.

For SDI test results to be accurate, the feed line to the kit must be

connected to the raw water line representative of the feedwater to the

RO system. Ensure that all of the air is purged from the apparatus

and that the feed pressure is adjusted to 30 psig.

SDI TEST PROCEDURE:The initial time required to fill a 500 ml graduated cylinder is

measured and recorded as t0. A measure of the time required to

collect 500 ml volumes is noted again at 5, 10 and 15 minutes after

the initial start. These times are recorded as t5, t10 and t15

respectively.

CALCULATION OF SILT DENSITY INDEX (SDI):The SDI value is then calculated using the following equation:

SDI =

( 1 - t0 / t15 ) 100

T

t0 = Initial time in seconds required to collect a 500 ml sample.

t15 = Time in seconds required to collect a 500 ml sample after fifteen

min.

T = Total test time in minutes.

RECOMMENDED SDI VALUE:The major membrane manufacturers typically recommend

maintaining an SDI value of 3.0 to 5.0 for feedwater to a reverse

osmosis system.

EVALUATING SPENT FILTERS:Figures 1 – 3 are photos of actual SDI pads taken at a single

customer site. The benefit of injecting coagulant ahead of the

multimedia filter is apparent by comparing Figure 2 and Figure 3. The

SDI test in Figure 3 was taken on the MMF effluent of an on-site pilot

filter that was drawing feedwater from the same source as Figure 2.

 

Fig 1: SDI pad of the Feedwater.

Fig 2: SDI pad of the downstream MMF effluent, no coagulant addition.

Fig 3: SDI pad of the downstream MMF effluent, with coagulant addition.

Fig 4: Typical SDI apparatus.

urbidity

Turbidity is an important water quality indicator for almost any

treatment application. Turbidity represents the presence of dispersed,

suspended solids-particles not in true solution and often includes silt,

clay, algae and other microorganisms, organic matter and minute

particles.

Suspended solids obstruct the transmittance of light through a water

sample and impart a qualitative characteristic, known as turbidity, to

water. Turbidity is not a direct measure of suspended particles in

water. Instead, it is a measure of the scattering effect such particles

have on light.

The way in which suspended particles scatter light is very complex.

Particle size, shape, and nature all effect scattering. In addition,

colored solutions adsorb light and can affect turbidity. Very small

particles (0.2 microns) scatter light equally in the forward and

backward direction. Larger particles (1 micron) scatter light primarily

in the forward direction. Forward scattering is intensified as the

concentration of suspended solids increases.

Turbidity is determined electronically by the apparatus shown in

Figure 1. Light from a tungsten filament (usually) passes through the

sample to be measured. Light scattered at an angle of 90 degrees is

measured. Ninety degrees is chosen because it is very sensitive to

light scatter.

Dividing scattered light intensity by that transmitted in a forward

direction compensates for the affect of sample color. An instrument of

this type is known as a ratio turbidimeter.

 

Fig 1: Hach Turbidimeter.

Fig 2: Turbidity vs Particle Count.

Troubleshooting Guide

Product Flow

Salt Rejection

Delta Pressure

Direct Cause

Indirect Cause Corrective Action

Decreasing

Decreasing

Increasing in Second

stage

Scale Fouling

No antiscalant Antiscalant underfed

Carry out mass balance, confirm that antiscalant is present in the concentrate stream and ensure shutdown flush operating to determine source of problemClean the system

Increasing in First stage

Colloidal Fouling

Poor pretreatment

Clean the systemImprove the pre-treatment performance

Compaction

Poor Pretreatment

System operation outside limits

Stop train and clean before the DP increases beyond 52 psi (3.6 bar)Improve pretreatment as colloidal fouling on the front end membrane normally accompanies first element compaction

Incompatible Chemicals

Use of non-approved products

Carryover from pretreatment processes

Review chemical compatiblyClean the system

BiofoulingIneffective or absent biocide program

Carry out a bacterial surveyDisinfect/clean the entire system and pipingAdd or increase biocide injection and continue to monitor until the system proves to be under control

No ChangeNo

ChangeOrganic Fouling

Oil and/or Polyecltroclye residual in RO feedwater

Investigate source water and improve pretreatmentClean the system

Increasing No Change

Compaction High pressureUneven flux

Compacted elements are irreparably damaged. Replace effected membranes and

determine how to reduce pressure and re-balance flow rates

Increasing DecreasingNo

Change

Oxidation Damage

Free chlorine or use of an incompatible cleaner

Attempt to use Resize to restore performance temporarilyReplace affected elements

Membrane Failure

Product backpressure or vacuum , abrasion, installation damage to permeate tube

Remove source of backpressure/vacuumImprove pretreatment

O-Ring leakMovement or improper installation

If possible, probe elements to determine which membrane(s) in the vessel are effectedReplace o-rings, shim vesselDetermine how to avoid "ramming" the membranes during system start-up and shut down

Water MathDetermining Water Equivalents:

One U.S. Gallon Water =3.785 Liters3785 milliliters (mL)8.34 pounds

One Cubic Foot Water = 7.48 US Gallons

2.3 ft. of water = 1.0 PSI

1 grain hardness = 17.1 ppm

CALCULATING PARTS PER MILLION TO GRAINS PER US GALLON:A. To convert ppm of hardness to grains per US Gallon, divide by 17.1:

Calculation:  

ppm hardness

17.1

= grains/US Gallon

Example:  

250 ppm hardness

17.1

= 14.61 grains per gallon

B. To convert grains per U.S. Gallon to parts per million of hardness, multiply by 17.1:

Calculation:   (Grains hardness) * (17.1) = ppm hardness

Example:   (14.61 grains) * (17.1) = 249.83 ppm hardness

C. Softener Capacity: Estimate 25,000 grains removal capacity per cubic foot of resin:

Calculation:  

(Cubic feet of resin) (25,000)= Softener capacity in gallons

(Feed hardness in grains)

Example:  

(35 ft3) (25,000 grains/ft3)

(25 grains/gal)

= 35,000 gallons

CALCULATING REVERSE OSMOSIS SYSTEM RECOVERY:Calculation:   (Permeate GPM/Feed GPM) (100) = % Recovery

Example:   (75 GPM/100 GPM) (100) = 75% Recovery

CALCULATING REVERSE OSMOSIS SALT REJECTION:Calculation:   (1 - (permeate TDS/feed TDS)) x 100 = % rejection

Example:   (1- (20/1000)) 100 = 98%

CALCULATING THE CONCENTRATION FACTOR:A. Recovery Based Concentration Factor:

Calculation:   1 / (1-recovery) = concentration factor

Example:   1 / (1-0.75) = 4

B. Conductivity Based Concentration Factor:

Calculation:  

(Concentrate Conductivity – Permeate Conductivity)

(Feed Conductivity – Permeate Conductivity)

=concentration factor

Example:  

(4000 us – 3 uS)

(1000 us – 3 uS)

= 4

CONVERTING TEMPERATURE

A. Degree Fahrenheit to Degree Celsius:

Calculation:   (°F – 32) (0.556) = °C

Example:   (77°F – 32) (.556) = 25.02°C

B. Degree Celsius to Degree Fahrenheit:

Calculation:   °C / (0.556 + 32) * 100 = °F

Example:   25°C / (.556 + 32) * 100 = 76.8°F

CALCULATING CHEMICAL INJECTION RATES IN MILLILITERS PER MINUTE:

Calculation:  Feed GPM * Chem PPM * (8.34)

(chemical lb per gallon) * 1,000,000= GPM/minute * 3785 = ml/mn

Example:  Feed GPM = 200Chemical dosage PPM = 2.0Chemical # per gallon = 10.48

   

(200 * 2 * (8.34) * 3785)

10.48 * 1,000,000

=1.20 milliliters per minute of neat chemical

   1.20 milliliters per minute

Percent dilution=

milliliters per minute of a diluted solution

Note:   Note: Express percentages as decimals.

CALCULATING CHEMICAL DILUTIONS (APPLICABLE TO VERY DILUTE SOLUTIONS*):

Calculation:   8.34 * Vd * Cd / 100 * Dn = Vn

Where:  

Vn = Volume of neat chemical required, gallonsVd = Dilution volume, gallonsCd = Percent dilution by weightDn = Neat solution density, lb/gallon

Example:  Vd = 200 gallonsCd = 5%Dn = 10.48 lb/gallon

    8.34 * 200 * 5 / 100 * 10.48 == 7.96 gallons

Note:  *To calculate more concentrated solutions, use Avista’s Chemdose program.

Water Math

Question: The water temperature is 80°F. What is the temperature in °C?

Answer: 26.68°C?

Question: The water temperature is 15°C. What is the temperature in °F

Answer: 59°F

Question:You’ve been advised to inject 3 ppm of Vitec 3000 into a system feed flow of 100 gallons per minute. The Vitec 3000 weighs 10.48 pounds per gallon. What ml/min dose rate is required?

Answer: 0.902 mL/min

Question: A storage tank contains 10 feet of water. What is the head pressure at the bottom?

Answer: 4.34 PSI

Question: Convert 275 PPM of hardness into grains per US gallons.

Answer: 16.08

Question: Calculate the volume of neat chemical required to make 500 gallons of a 3% solution of chemical. Density of neat chemical is 9.6 pounds per gallon.

Answer: 13 gallons

Scaling and Antiscalants

Scaling means the deposition of particles on a membrane, causing it to plug. Without some means of scale inhibition, reverse osmosis (RO) membranes and flow passages within membrane elements will scale due to precipitation of sparingly soluble gas, such as calcium carbonate, calcium sulfate, barium sulfate and strontium sulfate. Most

natural waters contain relatively high concentrations of calcium, sulfate and bicarbonate ions.

In membrane desalination operations at high recovery ratios, the solubility limits of gypsum and calcite exceed saturation levels leading to crystallization on membrane surfaces. The surface blockage of the scale results in permeate flux decline, reducing the efficiency of the process and increasing of operation costs. The effects of scale on the permeation rate of RO systems is illustrated in the following figure. Following an induction period, plant flow decrease rapidly. The length of this period varies with the type of scale and the degree of super saturation of the sparingly soluble salt.

As it is evident from the graph, the induction period for calcium carbonate is much shorter than that for sulfate scales, such as calcium sulfate. It is economically preferable to prevent scaling formation, even if there are effective cleaners for scale. Scale often plugs RO element feed passages, making cleaning difficult and very time consuming. There is also the risk that scaling will damage membrane surface.

There are three methods of scale control commonly employed:

acidification ion exchange  softening

antiscalant addiction.

Acidification: acid addiction destroys carbonate ions, removing one of the reactants necessary for calcium carbonate precipitation. This is very effective in preventing the precipitation of calcium carbonate, but ineffective in preventing other types of scale. Additional disadvantages include the corrosivity of the acid, the cost of tanks and monitoring equipment and the fact that acid lowers the pH of the RO permeate.

Ion exchange softening: this method utilizes the sodium which is exchanged for

magnesium and calcium ions that are concentrated in the RO feed water, following the chemical equations:

Ca2+ + 2NaZ => 2Na+ + CaZ2

Mg2+ + 2NaZ => 2Na+ + MgZ2

(NaZ represents the sodium exchange resin).When all the sodium ions have been replaces by calcium and magnesium, the resin must be regenerated with a brine solution. Ion exchange softening eliminates the need for continuous feed of either acid or antiscalant.

Antiscalants: they are surface active materials that interfere with precipitation reactions in three primary ways:

Threshold inhibition : it is the ability of an antisclant to keep supersaturated solutions of springly soluble salts.

Crystal modification : it is the property of an antiscalants to distort crystal shapes, resulting in soft non adherent scale. As a crystal begin to form at the submicroscopic level, negative groups located on the antiscalant molecule attack the positive charges on scale nuclei interrupting the electronic balance necessary to propagate the crystal growth. When treated with crystal modifiers, scale crystals appear distorted, generally more oval in shape, and less compact.

Dispersion : dispersancy is the ability of some antiscalants to adsorb on crystals or colloidal particles and impart a high anionic charge, which tends to keep the crystals separated. The high anionic charge also separates particles from fixed anionic charges present on the membrane surface.

Threshold Mechanism Dispersancy

During the past two decades new generations of antiscalants have emerged commercially, in which the active ingredients are mostly proprietary mixtures of various molecular weight polycarboxylates and polyacrylates.Calculation procedures exist for predicting the likelihood of scale formation. Use of these predictors depends upon an up-to-date water analysis and a knowledge of system design parameters. The ions contained in the feed water concentrate though the RO system, the point of maximum scale potential is the concentrate stream. Antiscalant type and dosage is therefore based upon the mineral analysis at this point. It is important to find the optimization of antiscalant treatment with respect to type and

dosage, identifying the proper antiscalant to use and the dosage-induction type relationship for the extended level of super saturation.

Lenntech can help you in the selection of the best antiscalant for your particular application.

Economical analysis

Acid addition is not very cost effective because of the cost of acid, tanks and monitoring equipment. Unless removed by degasification, excess of carbon dioxide contained in the permeate of acid-fed systems increases the cost of ion exchange regeneration.Antiscalants are relatively cheap products and have no additional costs.When compared to either acid or antiscalant addition, the main disadvantage to softening is cost factoring in equipment costs. Through a present worth analysis there is no level of hardness in which softening competes economically with antiscalants addition. The following table gives a cost comparison between softening and antiscalant treatment options for different levels of hardness, on a basis of a RO system designed to produce 75 gpm (17 m3/h) of permeate at 75% recovery.

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Preventing Scale Formation in Reverse Osmosis SystemsPosted on June 18, 2012 by admin

By Wes Byrne

Reverse osmosis (RO) systems behave similar to a boiler or a cooling tower in that they cause the dissolved salts in their makeup water to become more concentrated.  As pure water permeates the RO membrane, the salts are left behind in a concentrated stream.  If the solubility of any particular salt is exceeded, the potential exists

that the salt will form scale directly on the membrane surface or possibly within the flow channels through the membrane element.

This scale formation will occur first in the tail end of the RO system, within the last elements through which the water flows before exiting the system in its concentrate stream.  The scale will cause the RO system to require increased pressure to achieve the same permeate flow rate, and the permeate conductivity may increase in the tail end of the system.  Restoring performance will require cleaning with an acidic solution, although membrane cleaning is only going to be effective with certain types of scale.  It is unlikely to be fully effective with silica scale, or with most of the sulfate salts.  If cleaning cannot restore performance, membrane replacement becomes the only option for restoring the lost RO performance.

For most water sources, preventing scale formation is not a necessarily difficult challenge.  It can usually be accomplished by applying one or more of the following three options to the RO feed stream:

1. Acid injection2. Water softening

3. Injection of a scale inhibitor

The specific requirements for the water source can be predicted using U.S. Water Services’ Scale Inhibitor Projection Program (SIPP), which takes into account the particular makeup of the water source, the water temperature and pH, and the salt concentration factor relative to the desired permeate recovery of the RO system.  The right method will often depend on the priorities of the application.

Acid injection is effective at preventing calcium carbonate scale, but it is only marginally effective at preventing sulfate or silica scale.  A major disadvantage of acid injection for many applications is it results in the formation of carbon dioxide that will readily permeate the RO membrane.  It may then pose a removal burden for downstream ion exchange or deaeration equipment.

Lime softening will reduce the water hardness, alkalinity, and dissolved silica to a point that an RO unit might be able to operate at a low permeate recovery with reduced potential for scale formation.  However, increased permeate recovery that takes full advantage of the RO feed pressure and minimizes water waste will require that the water hardness be reduced to a much greater extent.  This then would require zeolite softening of the RO feed water, which uses a strong-acid cation exchange resin that is regenerated using sodium chloride.  An advantage of this approach is it enables caustic (sodium hydroxide) to be injected upstream of the RO system to raise its feed water pH, which will result in increased

removal of alkalinity, dissolved silica, and boron.  Operation at elevated pH will also reduce the potential for the formation of silica scale.

The most cost effective method for preventing scale formation (where applicable) is injecting a scale inhibitor.  Using similar chemistry to the antiscalants used in boilers and cooling towers, the formation of scale crystals can be slowed sufficiently to allow those crystals to exit the RO system in its concentrate stream.  In this manner, saturation points may be safely exceeded for calcium carbonate and sulfate salts as long as the RO system is operating.  When the RO unit shuts down, it is critical that the system be flushed of supersaturated salts using permeate water, or by allowing low pressure feed water to displace the water held up within the system.

Some of the scale inhibition products used in the early years of the RO industry consisted of a single polymer that tended to lose its solubility when injected into certain water sources, such as ones that contained iron.  This would result in heavy fouling of RO prefilter cartridges, as well as of the membrane elements.

Products such as US Water’s RO 503 and 504 are blend products that contain both polymers and phosphonates can be utilized in the RO systems.  They work synergistically to improve their solubilities, while providing superior prevention against scale formation.  In fact, they will also assist in keeping iron in suspension to reduce its potential for fouling the RO system.  The appropriate dosage can be determined using the SIPP program.

With the correct application of the chosen scale prevention method, most RO systems should never have to experience scale formation.

Membrane autopsy results from the Genesys Membrane laboratories in Madrid indicate that 35% of the foulants identified on reverse osmosis (RO) membrane surfaces during autopsy are organic in nature containing significant levels of biomass.

Biofouling in RO systems can be defined as the growth of biomass on a membrane surface which is sufficient to cause operational problems. The effects of biofouling will show as an increase in differential pressure (dP), with a consequent reduction in flux, and increase in pumping costs. Severe biofouling may result in membrane degradation and physical damage. During operation these symptoms (dP) are usually observed in the lead elements of the system, closest to the source of contamination, however telescoping and compaction

may occur in the rear elements

During off-line periods biofouling can occur throughout the entire plant if shutdown procedures are not followed correctly. Consequently on start up problems of reduced flux, increased dP and salt passage may occur across the entire system. At the RO design stage analysis of feed water, SDI tests and ionic water analyses indicate the colloidal fouling and scaling potential of the water. Less thought is given to the microbiological content of the water and the consequent biofouling potential of the operational system. This is usually only given consideration when membrane performance is affected.

Every RO system has a different biofouling potential. The feed water source, pre-treatment (both chemical and physical), maintenance, cleaning regime and system downtime all contribute to the biofouling potential of a system.

Site specific financial and operational factors must be considered when selecting a suitable method of control. It is therefore difficult to recommend a "single" best treatment regime for all biofouling issues. Taking this into account the objective of this application guide is to give an overview of the most common methods of control and also the available methods for determining the presence of biofouling within a membrane system.

Identification:

The definitive method for identifying the presence and nature of biofoulants on a membrane is via autopsy. Care must be taken to select the correct element for autopsy with biofouling usually occurring in the first elements of a system. In order to enable representative samples to be taken the membrane must be packaged correctly (free of SBS and/or biocides) and despatched within 24 hours of removal from the plant. SEM-EDAX (Scanning Electron Microscopy - Dispersive X-ray Analysis) and microbiological identification and counting techniques are used to examine the membrane surface for the presence of microorganisms.

Contamination source:

Similar procedures applied to pre-treatment cartridge filters or 0.45 μm SDI papers can be used to evaluate the source of the contamination. This is a suitable method for determining the presence and type of fouling (by colloidal mater and some metal oxides) in the feed water, but does not give an indication of the degree of fouling affecting the membrane elements. Also bacteria counts cannot be 100% reliable as in some cases there may not be bacteria present in the feed water, but there are nutrients which will assist the bacteria growing on the membranes.

Biofouling Control - Chemical pre-treatment On-Line Chlorination & Halogens (NaClO+Cl2

gas+ClO2, chloramines, Bromine, etc) Use of halogens is limited due to the potential oxidation of polyamide membranes. The effectiveness of chlorine depends mainly on feed water pH and exposure time (usually 20-30 minutes of contact time is required). The oxidation reaction is catalysed by the presence of trivalent cations (Fe3+/Al3+) common to RO feed waters.

Typically on surface waters chlorine is added continuously to the intake to give a free residual chlorine concentration of 0.5-1.0mg/l. In order to protect polyamide membranes from oxidation de-chlorination is required upstream of the membranes using sodium bisulphite solution (Genesys RED) or activated carbon filtration.

It is recognised that chlorine oxidizes and breaks down natural organic matter (NOM) present in the feed water to more easily biodegradable by-products providing a nutrient source to micro-organisms. In addition as no chlorine is present on the membrane surface biofilm growth can occur requiring more frequent sanitization

Off line chlorination (system sanitization) This process is designed to limit the amount of bacterial ingress into the membrane system from contaminated pre-treatment systems.

In order to sanitize the pre-treatment and distribution system chlorine is applied at regular intervals determined by the degree of fouling. Feed water is sent to drain before reaching the membranes and ORP/Redox is used to measure chlorine levels before returning to normal service.

This method is aimed at sanitizing the pre-treatment system reducing bacterial contamination into the RO and as such has no direct effect on existing biofilm growth on the membrane surface. This process should therefore be used in conjunction with approved nonoxidising biocides such as Genesol 30. While oxidising biocides are unable to control bacteria on the membrane surface Genesol 30 has the advantage that it can reach the membrane surface killing the bacteria with no detrimental effect on the polyamide structure. The dose and frequency of application is determined by the degree of bacterial ingress to the system and the consequent effects on operating parameters; flux, dP and salt rejection.

Hydrogen Peroxide & Peracetic Acid and formaldehyde Hydrogen peroxide or a combinationof hydrogen peroxide/peracetic acid and also formaldehyde are sometimes used as a method of disinfecting RO membranes. However there is a danger of oxidation of polyamide membranes which can be further catalysed by the presence of iron and manganese which limits the practical application of this method.

Non-oxidising biocides

Due to the potential damage to polyamide RO membranes caused by oxidising biocides it is

often preferable to use non-oxidising biocides to control biofouling.

These biocides are applied through normal dosing pump or introduced via CIP tank and do not require additional "neutralising" treatment or carbon filtration

Genesol 30: fast acting biocide:

Off-line Sanitization: dose at 300mg/l for periods up to 60 minutes. On-line application: in non-potable applications Genesol 30 dose at 400mg/l for 30 minutes.

Genesol 32: membrane preservative:

Off line Sanitization: dose at 1,000-1,500mg/l (0.1-0.15%) for a period of 6-8 hours followed by an alkaline clean.

On-line Dosage: dose at 700-1000mg/l (0.07-0.1%) for 8-12 hours once every 5-21 days depending on the severity of the fouling.

In cases of severe fouling or for seasonal operation at higher ambient temperatures it can also dosed at 5-50ppm continuously.

Genefloc ABF is a flocculant with biocidal and algaecidal properties. A dosage of 2-10mg/l can reduce the silt density Index, replace iron and aluminium coagulants and inhibit algae and bacteria growth in prefilters, cartridges and membranes.

Preventative treatment - membrane cleaning:

In reality biofouling in an RO system is successfully controlled using a combination of preventative biocide treatment in conjunction with a well designed and effective membrane cleaning programme.

The extremes of pH combined with surfactants used in cleaning will kill the majority of bacterial cells, However inefficient cleans may not remove the remaining nutrient rich biomass from the system giving any remaining viable cells the opportunity to rapidly multiply into viable populations. A well designed and applied cleaning regime will remove the biomass from the system preventing rapid re-growth. Using different chemicals will help to avoid resistance to a particular biocide developing within the microbial population.

Efficient removal of biofilm is achieved using alkaline cleaning chemicals and depends on temperature, pH, contact time and good cleaning practice. In addition combining alkaline detergents and biocide cleaning steps will improve santitisation.

Off-line preservation:

All membrane environments contain a viable population of microorganisms which will grow and degrade the membranes if the system is not treated correctly on shut down. It is recommended by membrane manufacturers that the system must firstly be flushed with permeate to remove highly concentrated water and avoid mineral deposition. Any plant that will be stopped for over 24 hours should be treated to limit potential microbiological fouling.

Non-oxidising biocide:

Genesol 32 for membrane preservation and storage:

24-36 hours 300mg/l = 0.03%

36-168 hours 500mg/l = 0.05%

1-4 weeks 800mg/l = 0.08%

1-6 months 1000mg/l = 0.1%

>6 months drain and refill every 6 months

Sodium Bisulfite (SBS) - Genesys RED:

Used as a method for inhibiting growth of aerobic bacteria at a concentration of 1%. Care must be taken to avoid the ingress of air into the system if using this method.

It must be noted that SBS is not a biocide it works by removing oxygen from the water, so aerobic bacteria will not survive. It has no effect on anaerobic bacteria and the solution must be refreshed regularly when it becomes exhausted.

Conclusions

All aquatic environments have a bacterial population and if not controlled in an RO plant biofouling may cause significant operational problems with a consequent impact on costs and performance. Increased cleaning frequency, increased pumping costs, reduced permeate quality and decreased membrane life make good bacterial control essential.All systems are different and it is therefore difficult to assess the fouling potential. A combination of biocide application with thorough cleaning practice is an efficient way to control microbial activity and ensure efficient plant operation.

Always follow membrane manufacturers' guidelines with respect to application of oxidising

biocides.

Correct preservation of off-line systems is vital to ensure that membranes do not degrade during prolonged shut down and that the system is not repopulated on restart.

Introduction to Reverse Osmosis

A technique used in processes requiring high-quality, purified water, such in semiconductor processing or biochemical applications, is reverse osmosis. It can be used to treat boiler feedwater, industrial wastewater, or process water. Reverse Osmosis is a water purification technique that reduces the quantity of dissolved solids in solution (Kucera, 54). It was first developed in the 1950's by the US government to provide fresh drinking water for the Navy, and since then, advances have made it much more feasible for obtaining purified water from wastewaters produced in many industrial applications. RO uses waterline pressure to push raw wastewater against a special semipermeable membrane. It is essentially a molecular squeezing process which causes H20 molecules to separate from the contaminants. The separated water molecules then pass thru to the inside of the membrane on to a holding reservoir. The contaminants are washed from the membrane and disposed of. Recently, RO has been used in treating boiler feedwater, in addition to industrial and process wastewaters. Boilers are found throughout the chemical processing industry and the primary method to treat boiler wastewater is an ion-exchange based demineralization. However, RO has been demonstrated to be more cost effective than this demineralization process (Kucera, 54).

Example of RO system

Problems With Reverse Osmosis:

It is necessary to establish feedwater quality guidelines to optimize system performance and prevent the three main problems associated with RO: scaling, fouling, and degradation of ROmembranes (Kucera 55) These problems tend to decrease system productivity because they reduce wastewater purity. Scaling occurs on RO membranes when the concentration of scale-forming species exceeds saturation, producing additional solids within the RO feedwater. Scalants include such chemical species as calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate, and reactive silica (Kucera 55). Since these species have very low solubilities, they are difficult to remove from RO membranes. Scaling decreases the effectiveness of the membranes in reducing the solids and causes more frequent cleanings. A scale on a membrane provides nucleation sites that increase the rate of formation of additional scale (Kucera 55).

Methods to minimize scaling

In order to minimize scaling, pretreatment methods involving chemical or ion exchange techniques are used. Ion exchange methods remove scale-forming species from the RO feedwater, while chemical techniques change the characteristics of the RO feedwater so that crystal formation is not favored. An example of a chemical technique to prevent fouling is lime softening, which involves chemical processes that reduce the hardness of the wastewater, essentially preventing material from precipitating out. (Kucera 56) Lime, soda, ash, and NaOH are used to convert soluble calcium and magnesium to insoluble calcium carbonate and magnesium hydroxide. Magnesium hydroxide tends to absorb silica, another scalant. These solids are then collected as sludge from the bottom of the "softener". Another softening procedure involves zeolite in an ion exchange process. A strong acid cation resin in the sodium is used to remove scale-forming cations, suchas calcium, magnesium, barium, and iron. (Kucera, 56) These cations are exchanged with the sodium to yield "soft water", that is, water of low hardness. Another pretreatment technique to prevent scaling is acidification, which specifically reduces the crystallization of calcium carbonate. Sulfuric acid is most commonly used in this process, but can often increase the formation of sulfate scales. Therefore, where sulfuric acid cannot be used, hydrochloric acid is substituted. (Kucera, 57). Often used with acidification, or by itself, are antiscalants. Antiscalants are chemicals added to wastewater to minimize scale carbonate or sulfate based scale (57). They consist of acrylates and phosphonates which inhibit the precipitation of carbonate or sulfanates.

Methods to prevent fouling

The second problem with reverse osmosis is with the fouling of membranes. Fouling occurs when suspended solids, microbes and organic material deposit on the surface of the membrane. Soluble heavy metals, such as iron, can be oxidized within membrane modules and foul the membranes. Another problem is from colloidal sulfur, which when oxidized from H2S can foul RO membranes. (Kucera 55). Colloidal

sulcar tends to be very sticky and therefore can attach easily to the surface of RO membranes Hydrogen sulfide would be found most commonly in well-water. The primary methods used to combat fouling are mechanical processes that physically remove the suspended solids or chemical treatments the deactivate the foulant. Coagulation is one technique that neutralizes the negative surface of the suspended solids, allowing the particles to cometogether. (Kucera 57) These large particles are then easy to remove from the water using filtration. The most common coagulants used are cationic polymers, inorganic salts, and aluminum and iron salts. Inorganic solvents tend to form large particles, while catonionic polymers require much less product for coagulation. Similar to coagulation is the clarification method, which destablizes suspended particles through charge neutralization (58). These particles conglomerate and are removed using sedimentation or filtration techniques. One particular type of filtration uses manganese greensand as a filter to remove soluble iron and manganese from the water source. This is generally done by oxidizing iron and manganese and physically removing the precipitates in the manganese greensand bed. Chlorination is the primary technique to minimize microbiological foulants, as it is very effective against a wide variety of microbes and can be easily deactivated using sodium metabisulfite. (59). After chlorination, activated carbon filters can be used to remove chlorine and reduce organics. However, activated carbon tends to foster microbial growth by providing nutrients for microbes, so it is not a very effective filtration technique. Finally, to treat H2S containing feedwater, which can form colloidal sulfur, a combination the above techniques is used. First, the water is oxidized to precipitate the sulfur, which is then coagulated and filtered. Any colloidal sulfur that may have formed is converted to thiosulfates with the addition of sulfite. Finally, chlorination is done to convert the thiosulfates to sulfates.(60)

 Methods to Minimize Membrane Degradation

The final problem with reverse osmosis is membrane degradation. It occurs when the membranes are exposed to conditions that destroy the polymers used to

create the membranes. Some membranes are susceptible to hydrolysis at high and low pH, while others are degraded by exposure to oxidizers such as chlorine.(Kocera 56) To prevent membrane degradation by acidic or alkaline waters, a corrective amount of acid of base should be added to the feedwater to make the pH approximately neutral. To prevent oxidation reactions, dechlorination is used. (57)

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References "Kucera, Jane". Properly Apply Reverse Osmosis Chemical Engineering Progress. February 1997. Pgs 54-61.