Demineralization Flashcards
(26 cards)
Mineral rejection is
The ability of the membrane to reject minerals
Reverse osmosis facts (4)
1.Defined as passage of water from weaker to stronger solutions
2. Membranes only allow solvent/water not solutes/solids
3. Water flux is dependent on applied pressure while mineral flux is not
4. Water through this process is called permeate
Why is chlorine added to the feed water of an RO unit?
Prevent biological fouling
If iron in the ferric form is found in the water
the unit will act as a filter
To remove inorganic particles in RO MEMBRANE use
And acid flush or citric acid
Optimal water characteristics for ion exchange (6)
- Ph 6.5-9.0
- NO3(nitrates) < 5mg/l
- SO4(sulfate) < 50 mg/l
- TDS total dissolved solids <500mg/l
- Turbidity < 0.3 NTU
- Total hardness < 350 mg/l
(2) RO treatments
Cellulose acetate- subject to biological attack, hydrolysis
thin film composite- not subject to biological attack, hydrolysis, compaction
Hydrolysis rate for cellulose acetates membrane, in order to slow hydrolysis an acid is added
As pretreatment before demineralization
Iron and manganese particles can plug
The filter media in stage 1
Rinse stage, if rinse isnt sufficient
Salty taste will be noticed
3 water supply classification
Fresh water < 1000 mgl tds
Brackish water 1,000-10,000 mgl tds
Seawater 35,000 mgl tds
Demineralization
RO reverse osmosis- NF uf mf
NF nano filtration
UF ultra filtration
MF macro filtration
ED electrodialysis
iO ion exchange
(5) constituents affecting ro process
PH
Temp
Solids
Minerals
Microbes
Most frequently used scale inhibitor
Hexametaphosphate
PH of what is standard for most feed water
5.5
Bacterial film covering the entire filtration area of a membrane is known as
Confluent growth
Polarization (2)
Build up of mineral deposits along edge of membrane
Most common and serious problem increasing precipitation of salts and matter on membrane surface
When should ro elements be cleaned (4)
- when pressure increases 15%
- flow decreases 15%
- membrane fouling lower flow rates
- membrane fouling lower salt rejection
Membranes are cleaned
About 45 minutes
(5) reasons not to operate feed water
- CL residual
- Hydrogen sulfide residual
- Calgon/hexametphospate over 10mgl
- Manganese over 0.1 mgl
- Iron over 0.3 mgl
Ion exchange (zeolite) defined as
Exchanging hardness causing ions for sodium ions
7 parts of ion exchanger
Vessel
Distributor
Backwash space
Resin
Resin support
Underdrain
Piping & valves
Service life for ion exchange resins
Up to 10 years but typically 3-5. Usually replaced when capacity is reduced by 25%
Ion exchange consideration, normal cL dosage is ok but
High dosage could damage resin and reduce life span
