HydrionisGlobal Water Intelligence
Water treatment

How tap water is made

From a river or borehole to your tap, through a sequence of barriers designed so that no single one has to work perfectly.

introductoryUpdated 23 August 2026

Public water supply is built on a principle called the multiple barrier approach. No single process is trusted to handle everything, because every process has a failure mode. The barriers are arranged so that the failure of one is caught by another.

Conventional drinking water treatment

The classic surface water treatment train. Each stage is a barrier, and the design principle is that no single stage is relied upon alone.

Conventional drinking water treatmentRaw water passes through screening, then coagulation and flocculation where a chemical dose makes fine particles clump, then sedimentation where flocs settle out, then rapid gravity filtration through sand, then activated carbon for taste, odour and micropollutants, then disinfection by chlorine or ultraviolet light, then pH adjustment and the addition of a chlorine residual before entering the distribution network.Raw waterriver or reservoirScreeningdebris removalCoagulation+ flocculationSedimentationflocs settleFiltrationsand / membraneFiltration: The critical barrier against Cryptosporidium, which chlorine cannot kill.Carbontaste, pesticidesDisinfectionchlorine / UVConditioningpH and residualSludgeto disposalDistributionto customersEach stage is a separate barrier. No single one is relied on alone.Groundwater sources are often far simpler — the aquifer has already done the filtering.
Multiple barriers
Each stage removes different things. Filtration handles Cryptosporidium; chlorination handles bacteria and viruses; carbon handles organics. No single barrier covers everything.
Order matters
Organic matter is removed before chlorination, because chlorine reacting with organics forms disinfection by-products.
Residual
A small chlorine concentration is deliberately maintained in the network to guard against contamination between the works and the tap.

Groundwater sources are often much simpler — sometimes disinfection alone — because the aquifer has already provided filtration.

The barriers, and what each is for

  1. Catchment protection
    The first and cheapest barrier: keeping contamination out of the source in the first place, through abstraction protection zones, land management agreements and pollution control.
    Every contaminant excluded here is one that does not need removing later.
  2. Screening
    Coarse and fine screens remove debris, leaves and larger material that would damage downstream plant.
  3. Coagulation and flocculation
    A metal salt neutralises the charge keeping fine particles suspended so they clump into settleable flocs. Also removes much of the natural organic matter that would otherwise form disinfection by-products.
  4. Sedimentation
    Flocs settle out under gravity in large tanks, removing the bulk of the solids load before filtration.
  5. Filtration
    Sand, multimedia or membrane filtration removes what remains. This is the critical barrier against Cryptosporidium, which chlorine cannot kill.
    Filter performance is monitored continuously by turbidity, because a microbiological result takes at least a day and a filter can fail in minutes.
  6. Activated carbon
    Adsorbs pesticides, taste and odour compounds and other organics that pass through the earlier stages.
  7. Disinfection
    Chlorine, ultraviolet light or ozone inactivates bacteria and viruses. UV is used specifically because it works against the chlorine-resistant protozoa.
  8. Conditioning
    pH adjustment for corrosion control, orthophosphate dosing where lead pipes remain, and a chlorine residual for the journey through the network.

Why groundwater is different

A borehole in a confined chalk or sandstone aquifer may need almost no treatment. The aquifer has already provided filtration over years or decades of percolation, and the water arrives clear, cool and microbiologically clean. Many groundwater sources go to supply with disinfection alone, sometimes with iron and manganese removal.

The trade-off is that groundwater carries whatever the geology and past land use put into it — nitrate from decades-old fertiliser, naturally occurring arsenic or fluoride — and those are much harder to remove than particles.

What happens after the works

Treated water goes to a service reservoir, which balances the steady output of the works against the peaky demand of a city. From there it enters the distribution network — hundreds of thousands of kilometres of it nationally, much of it a century old.

The network is where a surprising share of water quality problems originate: iron and manganese deposits accumulated over decades and released by a change in flow, ingress through a defect during a pressure loss, or growth in a low-turnover section. This is why a chlorine residual is maintained and why a supplier’s response to a discoloration complaint is usually flushing rather than anything at the works.

A municipal water system end to end

From catchment to tap and back again. Every stage has its own regulator, its own failure modes and its own cost.

A municipal water system end to endRain falls on a catchment. Water is abstracted from a reservoir, river or borehole under licence, treated at a water treatment works, stored in service reservoirs, and distributed through a network of mains to properties. After use it enters the foul sewer, is treated at a wastewater treatment works, and is discharged back to a river, where it may be abstracted again downstream.CatchmentSourcereservoir, river, boreholeWater treatment worksService reservoirbalancing storageDistribution networkCustomersSewer networkWastewater worksRiverreused downstreamLeakagelost from the network
Indirect reuse
Treated effluent discharged upstream is abstracted again downstream. On heavily used rivers the same water may be used several times before reaching the sea.
Different regulators
In England, drinking water quality is the DWI, the environment is the Environment Agency, and economics is Ofwat. A failure can be one, two or all three of their concerns.
Where the cost is
Treatment is a minority of the cost. Most of it is in the pipes: laying, maintaining, repairing and eventually replacing hundreds of thousands of kilometres of network.

Sources

More on water treatment

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