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TECH 0530 - Reverse Osmosis

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1. How can systems improve recovery while maintaining high flow across membranes?
Remove feed pre‑filters Heat membranes above 120°F Recirculate concentrate back to feed with a recirculation pump Eliminate cross‑flow entirely Lower pressure below osmotic pressure
2. Which membrane type is the focus of this training module?
Nanofiltration (NF) Microfiltration (MF) Reverse osmosis (RO) Ultrafiltration (UF) Distillation
3. At which end of the filtration spectrum does RO operate?
Ionic/atomic scale (smallest) Sediment only Bacterial size range only Macro‑particle scale (>1 micron) Colloidal range only
4. One micron (1 µm) equals:
One thousandth of a meter One centimeter One millionth of a micron One millionth of a meter One billionth of a meter
5. A thin‑film composite RO membrane is:
A hollow glass capillary only Pure polyester without support A paper filter sheet A thick drilled metal sieve An ultra‑thin selective layer on a polysulfone support with polyester fabric
6. Why isn’t an RO membrane simply a ‘uniform sieve’?
It relies only on heat It has no pores at all Charge interactions and complex pores reject ions smaller than apparent pores It uses UV light It dissolves contaminants
7. What role does water’s bipolar nature play in RO transport?
It widens the pores mechanically It ‘keys’ into the membrane’s charged pathways, aiding passage It blocks all flow through the membrane It heats the membrane by friction It neutralizes all ions
8. What typically causes ions like chloride to be rejected by RO membranes?
Magnetic attraction Electrostatic repulsion/charge effects and size exclusion Freezing Boiling Gravity separation
9. Natural osmosis moves water:
By moving solute through the membrane Only if both sides are pure water Only through metal membranes From dilute side to concentrated side to equalize concentration From concentrated to dilute without energy
10. Reverse osmosis requires:
Zero pressure change Applied pressure exceeding the osmotic pressure Only heat input Very low pressure below osmotic pressure Negative pressure on permeate
11. Why is a dead‑end (deadhead) RO configuration problematic?
It requires no pressure It produces infinite recovery It sterilizes water too much Fouling/concentration build‑up drives passage and clogs pores It cannot be monitored
12. What risk is associated with back‑pulsing thin RO membranes?
Cooling the membrane to safe levels Automatic sterilization Instant scaling removal with no risk Delamination/tearing of the active layer Guaranteed improved rejection
13. What is the preferred RO configuration to manage fouling?
Only periodic back‑pulse Dead‑end only Counter‑osmosis with no pressure Cross‑flow across the membrane surface No flow, just soak
14. ‘Recovery’ in RO is defined as:
Reject TDS divided by feed TDS Feed flow divided by permeate flow Permeate volume divided by total water used (permeate + reject) Permeate TDS divided by feed TDS Permeate pressure divided by feed pressure
15. If feed TDS is 100 mg/L and permeate TDS is 3 mg/L, what are rejection and passage?
Rejection ≈ 90%, passage ≈ 10% Rejection ≈ 3%, passage ≈ 97% Rejection ≈ 100%, passage ≈ 0% Rejection ≈ 50%, passage ≈ 50% Rejection ≈ 97%, passage ≈ 3%
16. As a rule of thumb, membranes are candidates for change when overall rejection falls:
Only when recovery is 0% Below ~80% Below ~99% Only when feed pressure is 0 psi Below ~30%
17. Terminology: RO product water is called ______, while UF/MF/NF product is often called ______.
Reject; product Brine; filtrate Filtrate; permeate Permeate; filtrate Feed; brine
18. Cross‑flow velocity is based on ______; flux is based on ______.
Reject flow; feed TDS Permeate flow; reject pressure Membrane area; feed pressure only Salt passage; salt rejection Feed flow per area; permeate flow per area
19. Typical single‑membrane residential RO without pump may see:
~1 gallon permeate per up to ~10 gallons reject ~10 gallons permeate per 1 gallon reject Only recirculation, no drain Guaranteed 75% recovery without pretreatment Zero reject water
20. Which is a common RO/UF membrane format described?
Spiral‑wound elements Magnetic sheets Concrete slabs Rubber balloons Lead foil rolls
21. Hollow fiber ‘straw’ membranes are most typical of:
Sand filters Distillation units Ultrafiltration (UF) modules Reverse osmosis Ion exchange resins
22. How does pressure affect RO volume and quality?
Pressure only changes temperature More pressure → more permeate (to a limit) and better rejection Less pressure → more permeate Pressure has no effect on salt passage More pressure always linearly increases permeate forever
23. How does temperature affect RO?
Temperature has no effect Hotter water → tighter membrane and better salt rejection Higher temperature → more permeate but poorer rejection Colder water → more permeate Higher temperature → less permeate and better rejection
24. Why does a POU RO system include a pressurized storage tank?
To increase osmotic pressure of feed To remove scale from the membrane To add chlorine automatically RO product exits with very low pressure; tank provides delivery pressure To eliminate the need for prefilters
25. What is the purpose of a post‑treatment polishing cartridge after the RO tank?
Increase TDS for flavor Neutralize osmotic pressure Improve taste/odor (e.g., remove any ‘rubbery’ note from storage) Raise permeate pressure Add hardness to prevent scaling