Membrane Filtration

Question 1

Name 4 general membrane’s used in water treatment and their driving forces?

Answer 1

1. Reverse Osmosis - driven by pressure
2. Forward Osmosis - driven by concentration
3. Membrane distillation - driven by temperature
4. Electrodialysis - electric potential driven membranes

Question 2

Name 4 types of pressure-driven membranes used for drinking water treatment with their pore size and what they can remove from the water?

Answer 2

1. Microfiltration: Largest pore size, removes only bacteria and colloid particles - 0.05 Bar
2. Ultrafiltration: 0.01 um pore size, removes viruses and humic acids - 0.1 Bar
3. Nanofiltration: 0.001-0.01 um pore size, removes multi-valent ions - 5 bar
4. Reverse Osmosis: > 0.001 um pore size, removed monovalent ions - 30 Bar

Question 3

Explain the theory of Osmosis and Reverse Osmosis

Answer 3

Osmosis - the movement of water from a low concentration to a higher concentration over a semi-permeable membrane can be reversed by applying the osmotic pressure

RO - The movement of water driven by the reverse osmotic pressure (higher than the osmotic pressure) over a semi-permeable membrane

Question 4

What membrane systemes use cross-flow operation and why?

Answer 4

1. Reverse Omosis
2. Nanofiltration

The ions need to be removed to prevent an accumulation of ions on the membranes

Question 5

Draw a process cross flow scheme for an RO singular element with the flow (Q) and Concentration of salts (C) mass balances?

Answer 5

Qf = Qc + Qp

QfCf = QcCc + Qp*Cp

Question 6

1 RO module has a recovery of around 10% what does this mean?

Answer 6

It means that 10% of the feed is able to pass the membrane to produce permeate

Question 7

What is the formula to RO recovery?

Answer 7

γ = Qp/Qf * 100%

Where γ = recovery %

Qp = permeate flow m3/h

Qf = feed flow m3/h

Question 8

What is the formula for Osmotic Pressure and when can you use this?

Answer 8

Only for lab work and water matrix’s containing one type of salt such as NaCl (Sodium Chloride) or MgSO4 (Magnesium sulphate) - not for sea water

π = (n * R * T * c)/ M

π = ostmoptic pressure N/m2

n = number of ions

R = Universal Gas Constant J/K*mol

T = Temperature K

c = concentration ions g/m3

M = Molecular mass of salt g/mol

Question 9

Sketch a diagram for a RO spiral wound membrane with feed and permeate spacers showing the different pressure losses.

Answer 9

See Slide 21

ΔP hydr = Hydraulic pressure loss along with the feed spacers to the concentration around 0.2 Bar

TMP - Trans-membrane Pressure: Pressure loss over the membrane from Feed to Permeate side (relates to the resistance of the membrane but also the osmotic pressure difference between the feed and permeate)

Question 10

Pressure and Flux in a RO system :

What is the formula for Net Driving Pressure (NDP)?

Answer 10

NDP = TMP - Δπ

NDP = ( Pf - Pp ) - ( πf - πp ) = pressure difference over the membrane -osmotic pressure difference over the membrane

NDP = (( Pf + Pc / 2 ) - Pp ) - (( πf + πc / 2 ) - πp) = the average

Note: in most membrane installations Pp = 0 and πp = 0.

Question 11

Pressure and Flux in a RO system :

1. What will the MTC value tell you about your RO system?

Answer 11

MTC: Mass Transfer Coefficient of water flux m/(s*Pa) is used to describe the resistance of your membrane = 1/membreane resistance

A decrease in MTC (scalling) will result in an increase in your membranes resistance

Question 12

In RO and NF systems what are the two main membrane rejection mechanisms?

Answer 12

1. Uncharged membrane rejection: the size of ions, interaction ions, diffusivity & solubility ions
2. Charged membranes rejection: size of ions, interactions ions, diffusivity & solubility ions, charge & density of charged groups on the membrane and electrostatic interactions ions-membrane

Question 13

Desalination: What type of pre-treatment would you recommend before putting seawater through RO desalination?

Answer 13

Seawater must be pre-treated before RO to prevent membrane fouling

1. Removal of NOM, particles and colloids: Coagulation, Sedimentation & Filtration

2 Cartlidge Filtration to remove ion-exchange particles & corrosion pieces

3. Chemical dosing: Chlorine -> kills bacteria and prevents biofouling but you must remove it again (Bisulphite deactivates Chlorine).
4. pH adjustment: Acid -> Scaling tendency

Question 14

What in RO desalination is it not possible to achieve a recovery of more than 50%.

Answer 14

The osmotic pressure increases too much after 50% recovery resulting in the need for very high feed pressures for very little extra permeate production - too expensive to implement

Question 15

What are the main differences of RO treatment with Brackish Water vs Sea Water?

Answer 15

1. Bw uses Inland installation pumped from freshwater aquifers which have been affected by salt intrusion (choose an aquifer with a low salt concentration as possible): brine cannot be recharged into waterbodies. SW uses on shore installations for extraction and brine can be deposited back in the ocean (50m from the extraction zone).
2. Bw: Low TDS therefore low osmotic pressure, SW: High TDS and high osmotic pressure
3. Higher recovery (90% 2 to 3 stages) in BW due to low osmotic pressure (1-20 Bar) and Lower recovery (50% 1 stage) in SW due to higher osmotic pressure (30 Bar)
4. BW requires less energy, SW more energy
5. BW requires little pre-treatment whereas SW always must be pretreated

Question 16

What is the applications of Microfiltration and Ultrafiltration and what flow conditions can be operated and why?

Answer 16

MF & UF act as a good pretreatment for RO/NF as they remove particles which can cause fouling of the RO/NF systems.

MF removes: Bactera, NOM and humic acids, UF same but also viruses - does not work for removing ions because pores are too big…there is equal osmotic pressure on both sides of the membrane therefore you can operate dead end design as cross flow is not required to flush ions.

Question 17

What are the main differnces between NF/RO and MF/UF?

Answer 17

NF/RO spiral wound; MF/UF Cappillary

NF/RO cross flow; MF/UF dead end (mostly)

NF/RO no BW; MF/UF with backwash

NF/RO can not deal with particles; MF/UF especially for particle removal

NF/RO removes salts; MF/UF 100% salt passage

NF/RO scaling; MF/UF no scaling

NF/RO Biofouling; MF/UF no biofouling

Question 18

Describe the type of fouling which can occur in a MF/UF system? How can these types of fouling be controled?

Answer 18

1. Cake layer
2. Pore blocking
3. Adsoption in the pore

Cleaning methods:

• Backflush or Backwash
• Forward flush or cross flow
• Airflush or air/water flush
• Chemical enhanced flush or Chemical backwash (CEB) - automatic every 12 hours
• Inseive chemical cleaning or cleaning in place (CIP) - manual once a month or every 5 months

Question 19

In MF/UF Cross-flow filtration can be applied to treat raw water with high suspended soilds content to prevent fouling but what are the main drawbacks of using cross-flow filtration over dead-end filtration?

Answer 19

Cross-flow is more energy inteivse and therfore will be more expensive compared to dead-end.

Question 20

Why is softening usually added as a pre-treatment step before RO

Answer 20

Scaling on the RO membrane is a comon form of fouling therefore to prevent this by removing Ca and Mg

Question 21

How can you test for CaCO3 - scalling of an RO system ?

Answer 21

Use the Langelier SI:n

SI = pHa - pHs = actual pH of water - pH in equilibrium (saturation)after precipitation or dissolving CaCO3

SI = 0 => equilibrium
SI > 0 => scale forming
SI < 0 => CaCO3- dissolving (corrosive)

Question 22

Scaling of RO memebranes is a challange for DWTP’s how can you prevent scaling?

Answer 22

Scalling usually occurs in the end modules of a RO treatment scheme due to the recovery; higher recovery will have more highly concerted ions in the concentrate which can scale.

1. Pre-treatment : removal of divalent ions (Ca2+, Mg 2+ etc) from feed water by IEX Resins or does anticalents or acids
2. Optimisation process operation & design:

• Higher cross-flow velocity => lower concentration polarisation
• Lower recovery = > Lower salts concentration in last element
• Design: Staging!! increases cross flow velocities in the 2nd stage

3. Chemical cleaning: limited to only once a year (avoided by most DW companies as a last resort as it also damages the membranes) -

Question 23

An engineer decides to pre-treat raw water before an RO installation to prevent scaling. What would you recommend and what are the advantages of pre-treament?

Answer 23

1. Removal of divalent ions (Ca2+ & Mg 2+) from feed flow with IEX Resin
2. Antiscsalants or acid dosing

Main advantages:

• Higher recovery
• Higher Flux => less membrane surface are => lower investment costs
• Less concentrate produced

Question 24

What is the purpose of a christmas tree configuration “staging” of an RO system?

Answer 24

Increasing the cross flow velocities: Cross flow velocy decreases in every element because of the permeate production - by combing two concentrates in the 2nd stage you will increase the cross flow velocity - prevents scaling

Question 25

Explain why Biofouling in RO filtration is a problem?

Answer 25

It is possible to kill the microgoranmisms by cleaning to reduce the pressure drop but it is not possible to remove the dead biomass from the spacers and therefore new biofouling can grow rapidly using the nutrients from the dead cells.

Question 26

Define concentration polarization in SW and BW desalination and name three parameters which are affected by this phenouma.

Answer 26

A phenomua where there is a salt concentration gradient at the feed side of an RO membrane is increasing on the staganat layer on top of the membrane. 1. Redcues the Flux on the membrane: CP increases the osmotic pressure but for the same driving force and TMP this will cause the flux to decrease. 2. Salt Passage will be higher: more ions will pass through to the permeate side as the membrane becomes saturated - lower permate quality 3. Increase in SI because your Ca2+ concetration and carbonate concentrations will be higher near the membrane which will increase the risk of scaling

Question 27

Decribe the phenomenon of Concentration Polerization?

Answer 27

Feed flows from left to right but the water is pushed towards the membrane (driving force) where it will pass through (Flux) to the permate but the salts are rejected. These rejected ions congrigate in a thin layer above the membrane called the stagnant boundary layer (1nm) where there is no movement of water. The ions then diffuse though the bulk solution (due to the concetration difference) where the cross flow will wash them away. There is an equilibrium between salts moving towards the membrane and diffusion away from the membrane to the bulk solution known as Cm. This creates a concentration gradient of low concentration of ions at the top (Cf) and a high concentration of salts at the membrane (Cm) known as concentration polerisation.

Question 28

What is the formula for concetration polerisation? Give an estimate for β?

Answer 28

β = Cm / Cf = exp ( J * δ / Di ) ``` β = Concentration polarisation factor (-) Cm = Concetration at the membrance surface mg/L Cf = Concentration in the feed mg/L ``` β = 1.2 means the conentration at the membrance wall is 20% higher than the concetration of the feed. ``` J = permate flux (m3/ m2*s) δ = thickness of the boundary layer (m) D = Diffusion coefficient (m2/s) ```

Question 29

What increases Concetration Polerization? How it be prevented?

Answer 29

Higher flux's and/or a thicker boundary layer will increase CP. - Higher cross flow velocities will reduce the thickness of the stanagt zone but proper feed spacers must be implemented to maximise diffusion. - Reducing the flux will allow for more diffusion back to the bulk solution but it will reduce the permate production and quality

Question 30

Name three retention mechanisms of membranes for micropolluatn removal?

Answer 30

1. Steric hindrance - size exlusion: moleucles larger than the pore size are retained and only small molecules can pass 2. Electrostatic interactions - charge exclusion: If the membrane is charged (i.e negativly charged) all negativly charged molecules (& too large molecules) are rejected - only positively charged molecules will pass 3. Hydrophobic interactions - The average hydrophobic molecules do not like the water but will stick to the membrane which is a polymer and pass through. The Hydrophilic molecules are not attracted to the membrane so will not interact with the membrane and be rejected.