CELPIP Reading Part 3 Practice Test: Fresh Water Through Reverse Osmosis

Paragraph A

Reverse osmosis separates water from dissolved salts by forcing feedwater against a semi-permeable membrane at pressure. Water molecules pass through more readily than most ions, producing a lower-salinity permeate and a concentrated brine. The process reverses the direction that water would naturally move under osmotic pressure, so applied pressure must exceed that tendency and overcome flow resistance. A membrane is selective rather than perfectly sealed. Small neutral compounds may pass more easily than salts, and defects can reduce rejection. Product quality therefore depends on membrane condition and the substances present, not only on total salinity. Recovery is limited because some feedwater must remain with the rejected salts to carry them away from the membrane surface.

Paragraph B

Pretreatment protects narrow membrane channels from suspended particles, mineral scale, and biological growth. Screens and filters remove solids, while chemical adjustment may limit precipitation or microbial activity. The correct sequence depends on seawater, brackish groundwater, or wastewater quality. Stronger pretreatment can reduce membrane cleaning, but it adds equipment, chemicals, and residuals of its own. Operators monitor pressure difference, flow, salt passage, and cleaning history to distinguish fouling from normal change. Cleaning can restore some performance, yet repeated harsh treatment may age the membrane, making prevention and timely diagnosis preferable to indiscriminate chemical use.

Paragraph C

Pressure is the largest operating energy demand in many desalination systems. Energy-recovery devices capture pressure from the outgoing brine and transfer much of it to incoming feed, reducing but not eliminating electricity use. Warmer water, membrane choice, recovery ratio, and target quality all affect consumption. Pushing a greater fraction of feed into product reduces the volume of brine but raises concentration and scaling risk. The lowest energy setting is not automatically the lowest-cost or safest operating point. Plants also track specific energy per unit of product because total electricity can rise simply when more water is delivered to meet demand.

Paragraph D

Coastal plants may use diffusers and mixing zones, while inland facilities face evaporation, deep disposal, or further concentration. Ecological effects depend on discharge salinity, temperature, chemicals, currents, and organisms at the site. Desalinated water may also require minerals to be added back for stability and taste before distribution. Reverse osmosis can provide a drought-resistant supply, but output still depends on electricity, maintenance, intake access, and disposal permission. It diversifies a water system rather than making that system independent of infrastructure or environmental limits. Intake design may protect marine organisms with lower approach velocity and screening, adding another environmental boundary beyond the chemistry of the final discharge.

Paragraph E

Not given in any of the above paragraphs.