How does Singapore remove salt from seawater to produce drinking water? Singapore’s desalination plants use reverse osmosis: seawater is pretreated to remove suspended material, then forced at high pressure through specialised membranes that let water pass far more readily than dissolved salts. Most dissolved sodium and chloride ions remain in the concentrated reject stream while a much lower-salt stream is collected, treated further and checked to meet drinking-water requirements. PUB identifies desalinated water as Singapore’s fourth National Tap and currently describes five operational desalination plants. Reverse osmosis is energy-intensive, but it makes a source of water available even when rainfall is unreliable. PUB’s official desalinated-water guide explains the national process.
For Hougang students looking for how reverse osmosis works, desalination in Singapore, how salt is removed from water, filtration versus osmosis, desalinated water versus NEWater, Secondary 1 Science separating mixtures, SEC G3 Chemistry or water-treatment Chemistry tuition, the essential distinction is ordinary filter paper removes suitable solid particles, but not dissolved Na⁺ and Cl⁻ ions; reverse-osmosis membranes and applied pressure can separate much of the dissolved salt from water. Evaporation and distillation are different approaches: vapour is produced and condensed, rather than pressure being used to push liquid water through an RO membrane. The salt-water conductivity guide explains why dissolved ions are still present even when the water looks perfectly clear.
Reverse Osmosis Is Not Ordinary Filtration: A Water Chemistry Reference
Start with the chemical species in seawater
Seawater is not one pure chemical. It is a solution containing water, sodium and chloride ions, and smaller amounts of many other dissolved species and suspended materials. Ordinary filter paper can trap suitable undissolved sand or debris but allows dissolved ions to travel through with water. Reverse-osmosis desalination therefore cannot be explained by saying a sieve simply catches visible crystals of salt. The salt is not present as tiny ordinary grains floating in water; much of it is dissolved as mobile ions.
Osmosis and reverse osmosis involve different driving conditions
In a simplified osmosis model, solvent tends to move through a suitable semipermeable membrane towards a side of higher effective dissolved-solute concentration when the conditions permit. In reverse osmosis, sufficient applied hydraulic pressure on the more saline side drives a net flow of water in the opposite direction. The membrane and operating conditions strongly discriminate between water and dissolved salts. A student should not call this ‘boiling under pressure’: the membrane process does not need to create and condense water vapour.
Why pretreatment is part of the science, not a decorative step
Before seawater reaches the main reverse-osmosis membranes, pretreatment can reduce fouling by removing or managing suspended solids, biological material and other substances that might obstruct or damage membrane performance. The exact design depends on the plant and feed-water conditions. For a school answer, the sequence is a system: intake water with multiple impurities → suitable pretreatment → pressurised membrane separation → further product-water conditioning and checks. Each stage addresses a different type of material or operational risk, rather than every stage being a filter with a smaller hole.
A worked salt-rejection calculation
Suppose a hypothetical feed solution contains 35.0 g of dissolved salts per litre, and the product water contains 0.350 g per litre under the same stated measurement convention. The illustrative rejection percentage is (35.0 − 0.350)/35.0 × 100% = 99.0%. The calculation compares concentrations, not total masses of salt removed from the entire plant. It does not imply an actual Singapore facility always achieves precisely that rejection: real desalination performance depends on feed composition, membrane operation and product-quality requirements.
A second calculation: the missing salt goes into another stream
Consider a simplified closed mass-balance example: 100 litres of feed containing 35 g salt per litre represents 3,500 g dissolved salt. Suppose the plant produces 40 litres of low-salt permeate at 0.35 g per litre, carrying 14 g salt. Ignoring density and volume nonadditivity for this illustrative exercise, the remaining brine stream contains about 3,486 g salt in approximately 60 litres of stream, or around 58.1 g per litre. The crucial conservation principle is that reverse osmosis does not destroy salt; it partitions most of the salt into a more concentrated reject stream.
Recovery and rejection describe different things
Salt rejection addresses how strongly the membrane reduces dissolved salt concentration in the product compared with the feed. Water recovery concerns the fraction of feed water volume recovered as useful product under the given approximation. In the illustrative previous problem, 40 litres of product from 100 litres of feed corresponds to 40% recovery, even while the concentration rejection is 99%. These numbers are not interchangeable; students can calculate each and explain why improving one operating objective may create other practical constraints.
Distillation makes vapour; reverse osmosis moves liquid water
To obtain fresh water by distillation, a suitable liquid mixture is heated so that more volatile water is vaporised and then condensed and collected. The energy, apparatus and separation mechanism differ from pressurising seawater through an RO membrane. Solar evaporation and ordinary filtration are again separate physical processes. The answer to ‘Which process should I use?’ depends on what is dissolved, the desired product, available energy, equipment and scale. A student should not declare one technology always best for every chemical mixture or real-world plant.
Singapore water security gives this Chemistry topic a real purpose
PUB describes desalinated water as Singapore’s fourth National Tap and identified five operational desalination plants in its 26 December 2025 statement. The number is dated rather than asserted to be immutable. Other National Taps include local catchments, imported water and NEWater. Unlike desalination of seawater, NEWater starts from treated used water and involves an advanced reclamation process; both technologies can use membranes but the original feed source and larger treatment chains differ. This is a helpful example of Chemistry supporting a real infrastructure system rather than a standalone examination definition.
Energy, brine and responsible environmental explanation
Reverse osmosis requires energy to maintain pressure and operate supporting equipment; desalination can be energy intensive, especially compared with some other water sources. The concentrated brine stream also needs responsible management because returning a higher-salinity solution to the environment can have ecological consequences depending on treatment, dispersion and location. A Chemistry question about sustainability should discuss these trade-offs rather than claim that seawater conversion is free or that the salt vanishes. A student can distinguish scientific necessity—separating dissolved ions—from engineering choices and environmental safeguards.
A three-student tutorial can use one diagram at three depths
For a lower-secondary Science learner, show a salt solution, ordinary filter paper and a reverse-osmosis membrane, then ask where the dissolved salt goes. For a Secondary 4 learner, ask for a concentration calculation and a mass-balance explanation. For a suitably prepared JC student, ask how changes to pressure, membrane performance or recovery influence the streams conceptually, without treating an advanced membrane-transport model as a compulsory H2 syllabus requirement. In any three-pax session, all students must explain the particle basis and solve an unfamiliar example independently.
Evidence and interpretation are not the same thing
A diagram showing cleaner-looking product water is not enough to prove a drinking-water supply meets health standards. Quality assurance depends on measured relevant chemical, microbiological and regulatory parameters; clarity alone cannot establish potability. Similarly, a statement that water passed through a membrane does not prove complete absence of all solutes. Use PUB’s official published source for plant context and current guidance for any real drinking-water questions. The classroom example illustrates separation principles, not an instruction to produce potable water at home.
Where this reference belongs without competing with its neighbours
This article owns how reverse osmosis removes dissolved salts from seawater, including pressure, recovery and brine. The Singapore tap-water treatment article owns treatment stages and disinfection, while Why Salt Dissolves owns the beginner’s particle model. For the local educational framing see How Chemistry Works in Hougang, and return to the Hougang Chemistry hub for the precise Secondary or JC tutor route. It is not a claim that a desalination plant or a Chemistry classroom operates in Hougang.
A familiar glass of water has a surprising journey
Picture a Hougang student looking from a classroom window at a cloudless afternoon. Where does the family’s next glass of drinking water come from if the rain stays away? Singapore’s water supply combines sources and technologies so that it is not wholly dependent on one weather pattern. Seawater is a plentiful local resource, but it comes with a stubborn problem: it contains considerable dissolved salts and cannot simply be supplied as ordinary drinking water.
Now make the problem precise. The goal is not only to remove visible particles. Salt dissolved in seawater behaves as ions dispersed through liquid water. A clear seawater sample can contain a great deal of dissolved sodium chloride and other salts. Its clarity does not mean it is safe or suitable for drinking. A separation method must address the scale and interactions of those dissolved species.
This is where Chemistry becomes the story of engineering. A pupil who understands mixtures, solutes, particle size, pressure and membrane selectivity has the conceptual equipment to understand why desalination works. The surrounding pumps, energy supply, intake treatment and monitoring then turn the principle into a useful public system.
What is dissolved in seawater?
Seawater is a mixture containing water, dissolved ions, gases, organic substances and suspended material. Sodium, Na⁺, and chloride, Cl⁻, are especially abundant, alongside ions such as magnesium, Mg²⁺, calcium, Ca²⁺, sulfate, SO₄²⁻, and others. The concentrations vary by location and conditions. For school learning it is adequate to recognise that NaCl dissolves by producing hydrated Na⁺ and Cl⁻ ions.
A common mistake is to imagine tiny solid grains of table salt floating through seawater. In a well-dissolved solution, the sodium and chloride ions are distributed among water molecules; they are not ordinarily retained by a coarse sieve or filter paper. That is why filtering a salt solution with an ordinary school funnel does not remove the dissolved sodium chloride.
| Component | What it does in seawater | Separation implication |
|---|---|---|
| Suspended grit and organic particles | May cause turbidity or membrane fouling | Pretreatment and suitable filtration |
| Dissolved Na⁺ and Cl⁻ | Contribute strongly to salinity and conductivity | Need desalination suited to dissolved ions |
| Other dissolved salts | Contribute to water chemistry and membrane loading | Also substantially rejected by appropriate RO membranes |
| Water molecules | The desired product stream | Crosses a suitable membrane under pressure |
| Microorganisms | May pose water-quality risks | Requires appropriate barriers, disinfection and monitoring |
Osmosis first: the idea before the machine
Osmosis is the net movement of water through a selectively permeable barrier from a region of higher water chemical potential to a region of lower water chemical potential. In a simple school comparison this is often described as water moving from a more dilute solution toward a more concentrated solution through a suitable membrane. The underlying driving factor is the difference in water’s chemical potential, rather than the dissolved salt being pushed through in the opposite direction.
Real water-treatment membranes are engineered materials, not the skin of a plant cell. Their transport properties depend on design, pressure, dissolved compounds and operating conditions. The idea of selective movement is shared, but the details are not identical to every textbook osmosis diagram involving living tissues.
Reverse osmosis: why pressure changes the direction
In reverse osmosis, pumps supply enough pressure on the saline side to overcome the osmotic driving pressure and force a net flow of water across the membrane toward the lower-salinity product side. The membrane preferentially transports water while rejecting most dissolved salts. It does not operate because a mysterious chemical reagent consumes every chloride ion, and the salts do not vanish.
The two water streams after membrane separation have different compositions: a lower-salt permeate and a more concentrated brine or concentrate. A student should draw both in a simple diagram. If the only product shown is pure water and no other stream, the diagram fails to account for where the separated salts went.
| Osmosis in a simple model | Reverse osmosis in desalination |
|---|---|
| Net water flow tends toward the more concentrated solution when separated by a suitable membrane | Applied pressure drives water out of the saline side toward the treated-water side |
| Natural chemical-potential difference supplies the main driving effect | External pumps supply energy |
| Illustrates water transport without assuming complete salt permeability | Engineering membranes provide substantial salt rejection |
| Does not automatically produce drinking water | Requires pretreatment, further treatment and water-quality compliance |
What happens before the reverse-osmosis membrane?
Seawater intake can carry fine particles, microorganisms and organic substances. Without protection, these may foul membranes, increase pressure losses and shorten equipment life. That is why industrial plants use appropriate pretreatment. Technologies can include screening, dissolved-air flotation, microfiltration, ultrafiltration and other methods depending on the plant.
PUB describes Singapore’s Tuas Desalination Plant as using dissolved-air flotation and ultrafiltration for pretreatment, while Tuas South incorporates ultrafiltration and reverse osmosis. The Keppel Marina East plant has a distinctive dual-mode design able to treat either seawater or reservoir water, according to prevailing conditions. These are national case studies, not a claim that each plant has an identical sequence.
When students compare pretreatment with RO, the key question is what each barrier targets. A filter for suspended material serves a different purpose from a selective membrane for dissolved salts. A waterworks can need both. The Hougang tap-water treatment guide describes how those different barriers fit together.
Singapore’s five desalination plants: useful context
PUB currently lists the five operational plants as SingSpring, Tuas South, Tuas, Keppel Marina East and Jurong Island. Its December 2025 release also describes a feasibility study for another possible plant; a feasibility study does not mean that the proposed facility is already operating. The exact production and distribution arrangements can change, so parents should use PUB’s current information for plant-specific claims.
There is no reason to claim that a tap in Hougang is fed exclusively by Marina East or another particular plant. National drinking-water networks and source combinations are managed systems; tracing a named household to one upstream source would require operational information that this educational article does not possess.
What happens after reverse osmosis?
The lower-salinity permeate still needs appropriate conditioning and verification before distribution. Depending on the system, post-treatment can adjust mineral content and pH, provide disinfection and ensure that water meets the relevant drinking-water standards. Industrial and municipal requirements go far beyond observing that a sample no longer tastes salty.
This is why a school diagram labelled “seawater → membrane → drinking water” is helpful but incomplete. Real systems have pretreatment, membrane separation, post-treatment, monitoring and distribution. Chemistry supports a designed sequence, and the standards define what the final product must achieve.
Worked question 1: why not just filter seawater?
Question: A student pours seawater through ordinary filter paper and predicts that the dissolved sodium chloride will remain on the paper. Explain why the prediction is wrong.
Model answer: Dissolved NaCl exists as Na⁺ and Cl⁻ ions spread through the water. Ordinary filter paper is meant to retain suitable suspended solids, not dissolved ions. The ions can pass through with water, so a different separation method such as reverse osmosis or distillation is needed to reduce dissolved salinity.
Worked question 2: calculate illustrative salt rejection
Question: An illustrative feed sample has total dissolved salt concentration 35,000 mg/L. The RO permeate is measured at 350 mg/L on the same reporting basis. Calculate the percentage reduction in concentration.
- Start with the feed concentration: 35,000 mg/L.
- Subtract the permeate concentration: 35,000 − 350 = 34,650 mg/L.
- Percentage concentration reduction = (34,650 ÷ 35,000) × 100%.
- Answer: 99.0%.
These are invented illustrative concentrations, not measurements from a particular PUB plant. Also note that concentration reduction alone does not tell us the total mass of salt removed or the water-recovery fraction. Those require the volumes or flow rates of feed, permeate and concentrate. A membrane rejects salt into a separate stream rather than destroying matter.
Worked question 3: where did the salt go?
Question: If a desalination plant receives 100 units of seawater, is it reasonable to say that all 100 units can always emerge as fresh drinking water with no other liquid output?
Answer: No. Reverse osmosis produces a lower-salt permeate and a more concentrated reject or brine stream. The amounts depend on the system’s recovery fraction and operating conditions. Salt is conserved across the separation, apart from any other clearly specified transfers or processes.
This is a powerful materials-balance lesson. The membrane does not make Na⁺ and Cl⁻ disappear. Keeping track of both streams connects Chemistry to engineering and prevents a student from assuming that a separation process is a chemical destruction process.
Distillation versus reverse osmosis
Another way to remove salt is to produce water vapour and condense it, leaving many nonvolatile salts behind. That is distillation. In RO, the product is water that passes as a liquid through a selective membrane under pressure. Both can produce low-salt water under appropriate conditions, but their driving energies and equipment differ.
| Property | Distillation | Reverse osmosis |
|---|---|---|
| Main separation idea | Vaporise and condense water | Pressure-driven selective membrane transport |
| What happens to dissolved NaCl? | Remains primarily in the nonvolatile residue/concentrate | Remains primarily in brine/concentrate |
| Main energy pathway | Thermal input plus operating needs | High-pressure pumping plus associated treatment |
| Is it simply a school filter paper? | No | No |
| Typical learning role | A prescribed separation method under SEC Chemistry | A useful real-world membrane application/enrichment |
The published SEAB K324 Chemistry syllabus includes distillation and fractional distillation under methods of separation and purification. Reverse osmosis is a valuable industrial application, but this article does not claim that the design and operating calculations of RO are separately examinable in K324. The student’s relevant subject and year determine the right depth.
Energy matters: where does the electricity go?
Pressurising seawater takes electricity, and other stages also consume energy. PUB’s desalinated-water reference describes an energy consumption of about 3.5 kWh per cubic metre of produced desalinated water for its current process discussion. That is a broad PUB figure rather than a universal measured value for every plant, day or water source. It helps explain why desalination is an energy-intensive National Tap.
A hypothetical simple estimate for two cubic metres at that indicative intensity is 2 × 3.5 = 7.0 kWh. That is an order-of-magnitude teaching calculation, not a quotation for a household bill or an operating record from any named PUB facility. It also does not include every potential upstream electricity-system impact.
As desalination expands, engineers try to reduce energy use, improve recovery and manage membrane and brine impacts. The PUB feasibility-study announcement from December 2025 describes efforts to explore flexible plant designs and land use. A useful sustainability response acknowledges both water security and the costs of achieving it.
Brine is an environmental question, not a missing line on a worksheet
The membrane’s reject stream contains more concentrated salts and can require careful marine-discharge design or other management. Potential local ecological impacts depend on dilution, mixing, temperature, discharge location and receiving-water conditions. It would be wrong to claim that every brine discharge necessarily damages a reef, and just as wrong to claim that concentrated reject streams have no environmental considerations.
The larger principle is mass conservation. Solutes separated from the product water must end up somewhere. This is the same reason a student keeps the residue in a separation diagram and records it in a calculation. Environmental systems thinking starts with the same habit as a good Chemistry equation: nothing simply vanishes.
What is the difference between NEWater and desalinated water?
NEWater is high-grade reclaimed water obtained from treated used water using advanced treatment such as membranes and UV disinfection. Desalinated water is produced from seawater through a process designed to remove its high dissolved salt content. Reverse osmosis can appear in both treatment routes, but the starting water, other barriers, target applications and operating design differ.
PUB describes NEWater as primarily serving non-potable high-grade industrial demands, with some use in indirect potable supply by blending into reservoirs under appropriate conditions. Desalinated water is a source of drinking-water supply. The comparison teaches students to distinguish the feedstock from one common technology; identical equipment labels do not make the whole processes identical.
A concept test using three mixtures
| Mixture | Suitable school-level first thought | Important qualification |
|---|---|---|
| Sand and water | Filtration | Filter must retain the suspended particles |
| Salt and water | Distillation if collecting relatively pure water | Evaporation if recovering solid salt; RO is an industrial option |
| Oil and water | Separating funnel for suitable immiscible liquids | Emulsions and density conditions can complicate separation |
These three examples are more useful than one memorised list of apparatus. Ask what property is different: particle size, volatility or immiscibility. Then select the method whose mechanism addresses that difference. The Hougang dissolving and mixtures guide supplies the lower-secondary starting point.
Six misconceptions that make a desalination question confusing
| Incorrect idea | Why it fails | Better connection |
|---|---|---|
| “Clear seawater contains no salt.” | Dissolved ions can be invisible | Use salinity or conductivity evidence |
| “Filter paper removes Na⁺ ions.” | Confuses suspended solids with dissolved ions | Explain the scale and nature of the solute |
| “Reverse osmosis works without any energy.” | Pressure requires power | Track pump input |
| “The salt is destroyed by the membrane.” | Violates conservation | Account for the brine stream |
| “Every Singapore waterworks uses the same process.” | Source-specific designs differ | Use appropriate PUB treatment information |
| “NEWater and desalination are exactly the same.” | Starting sources and full systems differ | Compare treated used water with seawater |
A five-question check for Secondary Science and Chemistry
- Explain why a clear salt solution can conduct electricity while a sugar solution behaves differently.
- Name the key property that ordinary filtration exploits and why it will not remove dissolved salt.
- State what an RO membrane principally passes and which stream retains most salt.
- Calculate the concentration reduction from 20,000 mg/L to 200 mg/L on the same basis.
- Give one benefit of desalination and one environmental or energy consideration.
Answers
1. Salt dissolves into mobile ions; sugar remains as uncharged molecules under ordinary conditions. 2. Suitable suspended particles are retained; dissolved ions are not. 3. Water passes preferentially; most salt is rejected into a concentrated stream. 4. 99% reduction. 5. A weather-resilient local water source; energy and brine-management challenges.
Five short revision sessions
| Session | Concept | One transfer question |
|---|---|---|
| 1 | Mixture versus dissolved compound | Why can salt water look clear? |
| 2 | Ordinary filtration versus RO | What passes through which barrier? |
| 3 | Pressure and energy | Why does the plant need pumps? |
| 4 | Product and reject streams | Where does the salt go? |
| 5 | Singapore’s National Taps | What is different between NEWater and desalination? |
This sequence gives the student a usable explanation without asking a Secondary 1 child to derive osmotic-pressure equations. More advanced Chemistry and engineering can explore the chemical potential and membrane transport in depth; the younger student’s first goal is to recognise what is being separated and how the separation actually works.
How a three-pax tutor can diagnose the confusion
One child may confuse ions with small grains of salt. Another can draw the water molecule but thinks every membrane is ordinary filter paper. A third knows the membrane but forgets the brine mass balance. Each requires a different short explanation. Good tutoring starts from the first broken concept, asks for a changed example and only then adds another practice question.
The eduKate small-group three-pax reference describes tuition near Sixth Avenue MRT. This article serves Hougang families as a local learning resource; it is not a claim that eduKate operates a physical Chemistry centre within Hougang. The Hougang Chemistry tuition guide is a starting point for judging actual learning needs and travel fit.
Frequently asked questions
Does Singapore use reverse osmosis for desalination?
Yes. PUB describes reverse osmosis as Singapore’s current desalination technology, alongside plant-specific pretreatment and post-treatment.
How many desalination plants does Singapore have?
PUB currently lists five operational desalination plants. A later facility under study would not be operational merely because a feasibility study was announced.
Why not simply boil all Singapore’s seawater?
Thermal separation is scientifically possible, but desalination methods have different energy, equipment and operating characteristics. Singapore currently uses RO for its seawater desalination supply.
Is salt removed because seawater has a different pH?
No. RO primarily uses pressure-driven selective transport through membranes. pH affects water chemistry but is not a substitute for removing the dissolved salt ions.
Can I make drinking water by passing seawater through a home filter?
An ordinary household filter is not a reliable desalination system. Drinking-water safety requires appropriate treatment and testing. Do not assume a demonstration removes microbial, chemical and salt-related hazards.
Is reverse osmosis part of every SEC G3 Chemistry calculation?
No. The published K324 syllabus includes core separation and purification techniques. RO is an applied example; detailed industrial membrane calculations should be treated as enrichment unless the assessment explicitly provides the necessary context and data.
Does desalination mean Singapore no longer needs reservoirs?
No. Singapore’s Four National Taps are complementary sources. A resilient national supply involves infrastructure, resource management and multiple treatment routes.
The question to keep from this lesson
“What exactly are we removing, what allows that separation, and where does the removed material go?” That is the same strong Chemistry question for seawater, pigments, crude oil fractions and laboratory mixtures. Once a child can answer it consistently, the subject stops feeling like a separate procedure for every substance.
Continue through Hougang Science Learning Hub · Singapore Tap-Water Treatment · Dissolving and Mixtures · Salt Water versus Sugar Water · Chemistry Topic Library · PUB Desalinated Water · SEAB 2027 SEC G3 Syllabuses.