Membrane Definition (RO/UF/NF/MF)

Hello subscribers! I'm K in H&K Environment.

 

I'm going to post about membrane process used in waste water treatment. 

 

Membrane is a kind of selective film that is able to separate specific species in a fluid by seiving and diffusion mechanisms. 

- Semipermeable and synthetic material

- High permeability for certain components, Low permeability for other components

- To separate constituents in fluid, water has to be pumped to surface to a membrane.

 

1. Classfication Criteria

- Membrane pore size

- Molecular Weight Cut Off (High molecular substance which separates 90% solute by membrane)

- Membrane material and geomerty

- Target material to be separted

- Treated water quality

 

2. Membrane type

- RO&NF: Used for water softening and desalination (Removal for ions in water)

- RO(Reverse Osmosis): Mostly used for desalination

- NF(Nano Filtration): Removal for hardness and disinfection byproducts

- MF(Micro Filtration)&UF(Ultra Filtration): Separate suspended solids from water (colloid, virus, tiny organic material), Used in traditional water treatment system, Used as pre-treatment for RO/NF, Removal for iron and manganese (after oxidation)

Membrane type, pore size and components to be removed (right side)

 

3. Membrane material

<Requirements>

- High surface porosity

- Wide total surface pore cross-sectional area

- High throughtput, selectivity

- Mechanical strength

- Thermal & Chemical resistance

 

<Material (mostly used)>

- Polymer and Ceramic

 

4. Consideration for membrane installment

- Membrane area-volume ratio → Packing Density

- Strong turbulence for mass transfer promotion of feed water

- Low energy expenditure

- Design for easy cleaning

- Modular design: design method which is independent and detailed in each steps.

 

5. Operation method

가. Dead-end

- Filtration for all feed water (High flux)

- Resistance is proportional to cake thickness, filtrated volume

- Rapid permeability decrease

- Need to clean periodically membrane surface 

 

나. Cross-flow

- Filtration while feed water crosses membrane surface

- High liquid removal rate

- Feed water remains in the form of a slurry (Relatively less fouling of membrane surface)

- Possible to fractionate particles of different sizes

Short Review on Predicting Fouling in RO Desalination, Alejandro Ruiz-Garcia, 2017

 

 

6. Recovery rate

- The ratio of feed flow rate to permeate flow rate. Index to express removal efficiency

- R = 100% (Qp / (Qp + Qc))

- Qp: Permeate flow rate 

- Qc: Concentrate flow rate

- R: Recovery rate %

PURETEC Industrial Water (puretecwater.com)

 

7. Rejection rate

- The rate that the permeation of particular solute is blocked by a membrane. Practical removal rate

- R = (Cfeed – Cpermeate) / Cfeed × 100%

- Cfeed: Solute concentration of influent

- Cpermeate: Solute concentration of effluent(filtrated, treated) water

 

8. Cleaning

가. Physical cleaning

- Back washing

- Application for temporary fouling of surfcae (Reversible)

 

나. Chemical cleaning

- Application for permanent fouling of surface

- Chemicals: Acid, caustic soda, hypochlorite

 

Thank you.