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)
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
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 %
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.