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Foulants Possible Location

Metal Oxide

Colloidal Oxidant(Cl )2

Abrasion

Last Stage

Any Stage

Probing with ¼ 'plastic tube and by measuring how far it has been inserted.

Failure can lead to increase salt passage, increase permeate flow. Decrease pressure drop.

Brine valve Should not be closed fully.

If fully closed, 100%

recovery will result and cause membrane damage due to precipitation of inorganic salt.

Cleaning of RO Membrane

Symptom of fouling

Indications that the system requires cleaning

Types of Foulants

Types of Membrane Cleaning Solutions

RO membranes get fouled with suspended solids contained in the feedwater or with sparingly soluble salts, as minerals are concentrated. Pretreatment is done to reduce the fouling potential of feedwater but inspite of that fouling occurs over a period of time.

1. Decrease in Product flow.

2. Increase in salt passage.

3. Increase in differential pressure 4. Deterioration in permeate quality

5. Increase in the differential pressure across the RO stage.

1. A 10 to 15 % decline in normalized Product flow.

2. A 10 % increase in salt passage.

3. 15 % increase in differential pressure.

1. Inorganic fouling – Like Calcium Scales or Metal Oxides 2. Organic Fouling – Example Humic Acid

3. Particulate Deposition or colloidal fouling –Particulate matter 4. Biofouling

The number of formulation for cleaning solutions is varied but we are mentioning only the common type of cleaners used for most common fouling problems.

Foulant

Inorganic Salts

Metal Oxides (Iron)

Inorganic Colloids (silt)

Biofilms

Organics

Cleaning Chemicals 0.2 % HCl

0.5 % Phosphoric Acid 2.0 % Citric Acid 0.5 % Phosphoric Acid 1.0 % Sodium Hydrosulphite 0.1%Sodium Hydroxide,30 Co

0.025 % Sodium Dodecylsulphate 0.1 % NaOH, 30 Co

0.1 % NaOH, 30 Co

1 % Sodium salt of ETDA and 0.1 % NaOH

0.025 % Sodium Dodecylsulphate 0.1 % NaOH 30 Co

0.1% sodium triphosphate 1.0 %

Remarks

Flux

Number of Elements:

Osmotic pressure

Selection of Feed pumps

Scaling of Membrane Process

The throughput of a pressure-driven membrane filtration system expressed as flow per unit of membrane area (e.g., gallons per square foot per day (gfd) or liters per hour per square meter (Lmh).

If the water quality is better, higher flux that can be used without causing excessive fouling.

When the flux has been set and the element area (a function of the specific membrane selected) is known, the required number of elements can be calculated:

Number of elements =Permeate Flow (LPD)/(LMH)*Active Membrane area (M )2

Recovery Rate = (Permeate Flow rate / Feed flow rate)*100

Osmotic pressure can be defined as the pressure and potential energy difference that exists between two solutions on either side of a semipermeable membrane.

A rule of thumb for osmosis is that 1 psi of osmotic pressure is caused by every 100 ppm (mg/l) difference in total dissolved solids concentration (TDS).

Feed pumps should be selected on the basis of high efficiency. Variable frequency drives now are commonplace in brackish water RO Plants. These frequency drives should also be selected on similar basis. Typical feed pump energy requirements for brackish water RO plants range from 0.5 to 2 kWh/M3 and for seawater it is less than 3 kWh/M3 with the use of energy recovery device.

Scaling is predicted by Langelier Saturation Index (LSI) or at a higher ionic strength the Stiff & Davis Index predicts the scaling tendency more accurately.

Type of Water System Operating Water

2 3 2

Flux (gpd/ft ) & (M /M .d) Municipal wastewater (sewerage)

Treated River or Canal water Surface Water (lakes/Reservoir) Deep Wells (low turbidity) RO Permeate Water Surface seawater Beach well seawater

8-12 - or 0.33-0.49 8-14 – or 0.33-0.57 8-14 – or 0.33-0.57 14-18- or 0.33-0.73 20-30 –or 0.81-1.22 7-10 - or 0.29 –0.40 7-10 - or 0.29 –0.40

uIf pH >pHs (or pHsd) then water is saturated with calcium carbonate.

uIf pH <pHs (or pHsd) then water is unsaturated.

uA positive value of index indicates tendency towards scaling.

uWith the scale inhibitors available nowadays an LSI <+2.4 can be easily controlled.

uCirculating a muriatic acid solution can easily redissolve carbonate scale. Lowering the pH during operation can also dissolve it.

uIn predicting the solubility limits of sulphate two points are important.-ua) Modern RO membranes reject divalent ions very well. Therefore it is

reasonable to assume a zero percent salt passage when calculating the concentrating factor CF.

ub) Compounds are more soluble in the concentrate than in feed water.

The solubility product constant Ksp of each compound increases with ionic strength.

uAs a rule of thumb, the scale inhibitor dosages for RO systems are calculated as concentrations in the concentrate of 12 –18 Mg/liter. This value is then converted to a feed water dosage using the CF for design recovery and assuming zero percent salt passage.

uThere are four important pieces of information needed to predict the product and concentrate composition and volume:

uRecovery rate, (Ret): -The recovery rate is limited by the concentration of sparingly soluble salts in the feed water. Lowering the pH and adding anti-scalants can increase the potential recovery rate.

The other determining factor is the configuration of the membrane system. Each element can recover approximately 10 percent of the feed flow as product. Generally, 50 percent recovery is assumed for a 6-element vessel.

uRejection rate: -Manufacturers lists a rejection rate for chloride and one for sulfate or other divalent ions for NF membranes. For greater accuracy, use a weighted average based on the feed water composition. For instance, if the feed water has a ratio of 3: 1 mono-valent to multi-mono-valent ions and the rejection rates are 90 percent for chloride and 99.5 percent for sulfate, the weighted average rejection rate would be Rejection = (0.75*0.9)+(0.25*0.995) / (0.75+0.25)

=0.924 If the goal is to minimize concentrate volume, choose a membrane with a very high rejection. However, if the goal is to minimize concentrate TDS, choose a membrane that will produce the

To predict the product and concentrate composition and