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How Reverse Osmosis Works — Complete Technical Explanation
A complete, accurate explanation of the reverse osmosis process: membranes, osmotic pressure, permeate, reject water, recovery rate and what RO removes from Pakistani bore water. No marketing language.
🔧 Written from direct field experience: Aqua Prima is based in Ichhra, Lahore — near Shama Chowk, Baba Azam Chowk, Hamza Hospital and FMH College. This guide reflects direct experience testing and treating Lahore bore water.
Direct Answer
What Is Reverse Osmosis?
Osmosis is the natural movement of water through a semi-permeable membrane from lower dissolved-solids concentration to higher concentration. Reverse osmosis runs this in reverse: pressure applied to the high-concentration side forces water molecules through the membrane against the natural osmotic gradient, leaving dissolved contaminants behind.
| RO Term | Definition | Why It Matters |
|---|---|---|
| Feed water | Raw water entering the RO system | Quality determines all downstream design decisions |
| Permeate | Purified water that passes through the membrane | The product: typically 95–99% lower TDS than feed water |
| Reject / Concentrate | Concentrated contaminants that did not pass | 25–40% of feed water; must be managed or repurposed |
| Recovery rate | % of feed water that becomes permeate | Higher source TDS = lower achievable recovery |
| TDS rejection | % of dissolved solids rejected by the membrane | Typically 95–99%; declines as membrane ages or fouls |
| Operating pressure | Pressure applied to push water through the membrane | Typically 8–20 bar for Pakistani bore water |
Contaminant Removal
What Reverse Osmosis Removes from Pakistani Bore Water
| Contaminant | RO Effectiveness | Relevant to Pakistan? | Notes |
|---|---|---|---|
| TDS (dissolved salts) | 95–99% rejection | Critical — Lahore bore water 700–2000+ ppm | Primary function of RO |
| Hardness (Ca, Mg) | 95–99% rejection | Yes — very high hardness in Lahore bore water | Pretreatment protects membrane from scaling |
| Arsenic | 85–95% rejection | Yes — present in some Punjab bore water | Confirm with water test; RO is effective treatment |
| Lead, Chromium | 90–99% rejection | Yes — possible in urban areas | Divalent ions rejected at high rate |
| Bacteria | 99%+ (physical exclusion) | Yes — microbiological risk in Lahore bore water | UV post-treatment adds second safety layer |
| Viruses | 99%+ (high-grade membranes) | Yes | UV post-treatment recommended |
| Fluoride | 85–95% rejection | Relevant in some Punjab areas | Less effective than for divalent ions but significant |
| Nitrates | 60–90% rejection | Yes in agricultural areas | Confirm removal rate is adequate for your level |
| Chlorine | Not removed by RO membrane | WASA supply & some bore water | Carbon prefilter must handle chlorine BEFORE membrane |
| Iron | Partially — prefilter recommended | Yes — some Lahore bore water | Iron removal prefiltration essential to protect membrane |
Key limitation: RO does not remove chlorine — activated carbon prefiltration handles this. Iron and hardness should be removed before the membrane via pretreatment. Skipping pretreatment for these substances causes premature membrane failure, which is the most common RO problem in Pakistan.
Lahore Context
Reverse Osmosis for Lahore Bore Water
| Lahore Area | Typical TDS Range | Key Water Quality Concerns | RO Appropriate? |
|---|---|---|---|
| Ichhra | 900–1800 ppm | Very high TDS, very high hardness, microbiological | Yes — with full pretreatment |
| Shama Chowk & surrounds | 800–1600 ppm | High TDS, high hardness | Yes — with softener pretreatment |
| Samanabad | 700–1600 ppm | High TDS, hardness, occasional iron | Yes — with pretreatment |
| Rehmanpura | 800–1700 ppm | High TDS, very high hardness | Yes — softener essential |
| Baba Azam Chowk / Ferozepur Rd | 800–1700 ppm | High TDS, hardness | Yes — with pretreatment |
| WASA municipal supply | 200–500 ppm (variable) | Chlorine, turbidity during monsoon | If TDS elevated; carbon filter if TDS acceptable |
People Also Ask
FAQs — How Reverse Osmosis Works
Is RO water safe to drink?
Yes. RO permeate is one of the most thoroughly purified forms of drinking water available. For typical Pakistani bore water, the removal of high TDS, hardness, arsenic and microbiological contamination far outweighs any concern about mineral removal. A mineralisation stage can be added after the membrane if desired.
Does RO waste a lot of water?
A typical RO system has a recovery rate of 60–75%, meaning 25–40% of feed water becomes reject concentrate. For a domestic system producing 200 litres of permeate per day at 70% recovery, approximately 86 litres goes to reject. This reject water can often be repurposed for garden watering or cleaning.
What TDS should RO output be?
Most well-functioning RO systems on Pakistani bore water produce permeate at 30–150 ppm TDS. Pakistan drinking water standards allow up to 1000 ppm. WHO guidelines suggest below 600 ppm is acceptable and below 300 ppm is good quality. Record your baseline TDS at installation and monitor monthly.
How is RO different from a regular water filter?
A conventional water filter removes particles and some chemicals but cannot remove dissolved salts, most heavy metals or dissolved microorganisms. RO operates at the molecular level, rejecting dissolved substances that pass through conventional filters entirely. For Lahore bore water with TDS 700–2000 ppm, conventional filtration alone is not a solution.
Does RO remove fluoride from Pakistani bore water?
RO membranes reject fluoride at approximately 85–95% efficiency. For Pakistani bore water with elevated fluoride, RO provides significant reduction. A water test confirms whether fluoride is present and whether the RO rejection rate will bring levels within acceptable limits.
Get a Free RO Consultation
Aqua Prima tests your source water before recommending any system. Serving Lahore from Ichhra.
📍 Ichhra, Lahore | 0327 457 5527