7.
(a) Describe, with the aid of a sketch, a reverse osmosis plant, from feed water inlet, to product tank, labelling ALL components and showing the position in the system of the monitoring instruments.(7)
(b) Describe the feed water pre-treatment process before the water enters the spirally wound membrane modules.(3)
(a) Reverse Osmosis Plant – Description with Sketch (7 marks)
A reverse osmosis (RO) plant converts seawater into fresh potable water by forcing it through semi-permeable membranes. The process flow and components are:
- Seawater intake & coarse strainer – removes large debris (seaweed, shells).
- Feed pump – provides flow from the intake to the pre-treatment stage.
- Pre-treatment system – includes coagulation/dosing, multi-media filters, and cartridge filters to remove fine particles.
- Chemical dosing system –
- Antiscalant dosing (prevents scaling).
- Sodium bisulphite dosing (neutralises chlorine to protect membranes).
- High-pressure pump – raises the pressure (typically 55–70 bar for seawater) to overcome the osmotic pressure.
- Membrane modules (spirally wound) – housed in pressure vessels; salt and impurities are rejected, producing:
- Permeate (fresh water) – passes to the product line.
- Brine/reject – discharged overboard.
- Post-treatment – permeate water may be re-mineralised, sterilised (UV or chlorination), and stored in the product tank.
Monitoring instruments in the system:
- Pressure gauges – at feed pump outlet, before/after high-pressure pump, across membranes (to check pressure drop).
- Flow meters – on feed, brine reject, and permeate lines.
- Conductivity/TDS meters – on permeate line to ensure quality (salt rejection).
- pH meters – at chemical dosing stage.
✏️ Sketch (exam-style) – A block flow diagram with labelled arrows:
- Seawater Inlet → Strainer → Feed Pump → Sand Filter → Cartridge Filter → Dosing (antiscalant + dechlorination) → High-Pressure Pump → RO Membranes → Product Tank (permeate) & Brine Reject (overboard).
- Mark instruments: pressure gauges (P), flow meters (F), conductivity meter (C), pH meter.
(b) Feed Water Pre-treatment Process (3 marks)
Before seawater reaches the spirally wound membrane modules, it undergoes treatment to protect the membranes:
- Screening/filtration – coarse strainers and sand/multi-media filters remove suspended solids.
- Chemical dosing – antiscalants prevent scaling; sodium bisulphite neutralises chlorine to avoid membrane damage.
- Cartridge filtration – fine filtration (typically 5 µm) ensures no fine particles or silt enter the membranes.