How is water purified on a large scale?
Community water supplies rely on a multiple-barrier approach. WHO lists the control measures as pretreatment, coagulation, flocculation, sedimentation, filtration and disinfection. The textbook trio for exams is storage → filtration → chlorination, and each step removes a different share of the microbial and particulate load.
| Stage | What happens | Key point |
|---|---|---|
| Storage (off-stream reservoirs) | Natural purification of the raw water before treatment | WHO lists off-stream storage as pretreatment that reduces the microbial, organic and particulate load |
| Coagulation and flocculation | A coagulant (classically alum) is added; fine particles clump into flocs | WHO: chemical coagulation is the most important step deciding how well later clarification and filtration work |
| Sedimentation | Flocs settle in a clarifier | Removes most turbidity before the filters |
| Filtration | Slow sand (biological) or rapid gravity sand (physical) filters | Removes remaining particles, bacteria and protozoal cysts |
| Disinfection | Usually chlorination | Kills remaining pathogens and leaves a residual to protect the distribution system |
What is the difference between slow sand and rapid sand filters?
This comparison is the single most-tested area of the topic. WHO's treatment annex sums up the core difference: slow sand filtration is essentially a biological process, whereas rapid gravity filters are physical treatment processes.
| Feature | Slow sand filter | Rapid sand (gravity) filter |
|---|---|---|
| Main mechanism | Biological — the schmutzdecke | Physical — straining and trapping of floc |
| Sand size (effective) | Fine: 0.15–0.3 mm | Coarser: 0.5–1.0 mm |
| Bed depth | 0.5–1.5 m | 0.6–2.0 m |
| Flow rate | 0.1–0.3 m3/m2·h | 4–20 m3/m2·h (many times faster) |
| Pretreatment | Best for low-turbidity or pre-filtered water | Mostly used to remove floc from coagulated water |
| Where impurities collect | Top few centimetres of sand | Upper layers of the bed |
| Cleaning | Scraping — the top few centimetres of sand are removed and replaced periodically | Backwashing with treated water, sometimes preceded by air scouring |
What is the schmutzdecke (vital layer)?
The schmutzdecke (German for 'dirt layer'), also called the vital layer or biological layer, is a slimy film of microbes and their exopolymers that forms on the surface of a slow sand filter. WHO describes it as a biologically active layer that retains microbes and often leads to their inactivation and biodegradation. Turbidity and microorganisms are removed mainly in the top few centimetres of sand.
- Removal improves as the filter matures and its biological layer develops (WHO).
- Performance depends on the presence of the schmutzdecke, grain size, flow rate and operating conditions such as temperature and pH (WHO Table 7.7).
- Scraping removes the clogged top few centimetres of sand, which are replaced periodically.
- The household biosand filter is a small, intermittently dosed version of the same idea.
How does chlorination work — chlorine demand and breakpoint?
Chlorine — whether as gas, sodium hypochlorite or calcium hypochlorite — dissolves in water to form hypochlorous acid (HOCl) and hypochlorite ion (OCl−). Before any chlorine is left over to disinfect, some of it is used up by organic and inorganic matter.
- Chlorine demand — the amount of chlorine consumed by chlorine-reactive substances in the water. It must be satisfied first.
- Combined chlorine — chlorine bound to nitrogen compounds such as ammonia (chloramines); a weak disinfectant.
- Free (residual) chlorine — chlorine left available for disinfection; it also protects water from recontamination.
- Total chlorine = combined + free chlorine.
Breakpoint chlorination means adding enough chlorine to rapidly oxidise all the ammonia nitrogen and still leave a suitable free residual to protect water from the point of chlorination to the point of use. Chlorine that reacts with ammonia forms chloramines (combined chlorine), a weak disinfectant; only once that demand is met does added chlorine stay as free chlorine.
| Technique | What it means | When used |
|---|---|---|
| Marginal chlorination | Simple dosing to a desired free residual | High-quality water with very low chlorine demand (a breakpoint may not even occur) |
| Breakpoint chlorination | Dose enough to destroy ammonia and leave free residual | Standard approach when ammonia or organic matter is present |
| Superchlorination–dechlorination | A large dose for rapid disinfection, then excess chlorine removed | Variable bacterial load or too little contact time |
How much residual chlorine should drinking water have?
WHO's guideline for chlorine reads: for effective disinfection there should be a residual concentration of free chlorine of ≥ 0.5 mg/L after at least 30 minutes' contact time at pH < 8.0, a residual should be maintained throughout the distribution system, and at the point of delivery the minimum free chlorine should be 0.2 mg/L. The health-based upper guideline value for free chlorine is 5 mg/L.
| Point | Value | Source |
|---|---|---|
| At the treatment plant after contact | ≥ 0.5 mg/L free chlorine after ≥ 30 min, pH < 8 | WHO GDWQ |
| At the consumer's tap | Minimum 0.2 mg/L free chlorine | WHO GDWQ; BIS IS 10500:2012 |
| When protection against viral infection is needed | Minimum 0.5 mg/L at the consumer end | BIS IS 10500:2012 remark |
| Aesthetic upper comfort level | Above about 1.5 mg/L water may be rejected for taste | WHO SEARO chlorination guide |
| Health-based guideline maximum | 5 mg/L | WHO GDWQ |
How is residual chlorine tested — OT test and Horrock's apparatus?
The classic Indian field test is the orthotoluidine (OT) test. Residual chlorine oxidises the orthotoluidine (orthotolidine) reagent in acid conditions, producing a yellow colour whose depth is matched against colour standards in a comparator. Because it reacts with chlorine, hypochlorous acid and chloramines alike, it measures total (free plus combined) chlorine. Indian dialysis-water guidance still uses an orthotoluidine reagent and comparator disc for routine chlorine checks.
- OT test — yellow colour, total residual chlorine.
- OTA (orthotoluidine-arsenite) test — a modification of the OT test listed in Indian emergency water guidance alongside the OT, thiosulphate and neutral red tests.
- DPD test — the method WHO's chlorination guide now describes for most commercial kits; the sample turns pink if chlorine is present.
Horrock's apparatus (Horrock's water-testing case) is an old army field kit for working out how much bleaching powder a given water needs. A weak solution of bleaching powder is added in graded amounts to samples of the water, which are then tested for free chlorine. The result guides how much bleaching powder to add to the whole supply — in effect a field estimate of the water's chlorine demand.
What chlorine compounds are used, and how strong is bleaching powder?
| Compound | Approximate active chlorine | Notes |
|---|---|---|
| Bleaching powder (chlorinated lime) | About one-third — WHO SEARO quotes ~35%; textbooks quote 33% | Strength falls with age and storage conditions (WHO SEARO) |
| High-test hypochlorite (calcium hypochlorite) | ~70% | Concentrated powder; also degrades on storage |
| Sodium hypochlorite solution | Commonly 5%, 10% or 15% | Liquid; 0.5–1% solutions are marketed for household water treatment |
| Sodium dichloroisocyanurate (NaDCC) tablets | Releases free chlorine on dissolving | Household and emergency use |
| Chlorine gas | Pure chlorine | Large treatment plants |
How is water purified at household level?
| Method | How | Strengths and limits |
|---|---|---|
| Boiling | Bring to a rolling boil and let cool without adding ice | WHO: simplest and most effective way to kill all pathogens, even in turbid water or at altitude; leaves no residual and does not remove turbidity |
| Chlorination | Household bleach, NaDCC tablets or calcium hypochlorite | At about 25 °C allow at least 30 minutes contact; double the time for each 10 °C colder; leaves a residual |
| Filtration | Ceramic, cloth/fibre, membrane or biosand filters | Biosand relies on a biological layer; ceramic and cloth filters are poor at removing viruses |
| Solar disinfection (SODIS) | Clear plastic bottles exposed to sunlight | Works through UV plus heat; depends on sunlight, turbidity and container depth |
What are the key drinking-water quality standards?
Both WHO and the Bureau of Indian Standards set the same bacteriological rule: E. coli or thermotolerant coliform bacteria must not be detectable in any 100 mL sample of water intended for drinking. For treated water entering or in the distribution system, total coliforms must also be undetectable in any 100 mL sample.
| Parameter | Acceptable limit | Permissible limit |
|---|---|---|
| E. coli / thermotolerant coliforms | Not detectable in any 100 mL | No relaxation |
| pH | 6.5–8.5 | No relaxation |
| Turbidity | 1 NTU | 5 NTU |
| Total dissolved solids | 500 mg/L | 2,000 mg/L |
| Fluoride | 1.0 mg/L | 1.5 mg/L |
| Nitrate (as NO3) | 45 mg/L | No relaxation |
| Free residual chlorine (minimum, at consumer end) | 0.2 mg/L | 1 mg/L |
What are the common exam traps in water purification?
- Schmutzdecke / vital layer belongs to the slow sand filter, not the rapid one.
- Rapid sand filters need coagulation (alum) first and are cleaned by backwashing.
- Slow sand filtration is biological; rapid sand filtration is physical.
- Free residual chlorine: ≥ 0.5 mg/L after 30 min (WHO) at the plant; ≥ 0.2 mg/L at the tap.
- Chlorine demand must be satisfied before any free residual appears — that point is the breakpoint.
- OT test gives a yellow colour and measures total chlorine; DPD gives pink.
- Bleaching powder is about one-third available chlorine and is unstable on storage.
- Chlorine is unreliable against *Cryptosporidium* — filtration is needed.
- Drinking water: zero E. coli or thermotolerant coliforms per 100 mL.