How Does a Water Supply Work? From a River to a Tap Under Pressure
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Water arriving at a tap has been collected, stored, filtered through several stages, disinfected, tested continuously and pushed through kilometres of pipe at a pressure high enough to reach an upstairs bathroom. Every one of those stages exists because of a specific failure in the past, and the system is judged by a standard no other utility meets: it must work every time.
From source to reservoir
Supplies come from surface water, meaning rivers, lakes and impounding reservoirs in upland catchments, or from groundwater abstracted through boreholes, and the choice determines the treatment required. Groundwater has been filtered through rock over years and is generally cleaner microbiologically, needing less treatment, while being vulnerable to contamination that is very slow to reverse and to over-abstraction. Surface water is more variable, carrying sediment, organic matter, agricultural runoff and whatever the catchment contributes, and requires full treatment. Protecting the catchment is the cheapest treatment available, which is why some cities pay upstream landowners to change farming practice rather than building larger plants, New York's arrangement with its watershed being the most cited example. Raw water is stored before treatment, which allows sediment to settle, gives resilience against a pollution incident by providing time to stop abstraction, and evens out seasonal variation in flow.
Treatment
Conventional treatment runs through a sequence, each stage removing what the previous one could not:
- •Screening, removing debris, leaves and fish before anything else
- •Coagulation and flocculation, dosing a chemical that neutralises the charge keeping fine particles suspended so they clump into larger flocs
- •Sedimentation, letting those flocs settle out in large tanks
- •Filtration through sand or membranes, removing what remains in suspension including most protozoan parasites, which resist chlorine and are therefore a filtration problem rather than a disinfection one
- •Disinfection, historically and still mostly with chlorine, sometimes with ozone or ultraviolet light, which kills bacteria and viruses
- •A maintained chlorine residual in the distribution network, which is the crucial detail, since it continues protecting water against contamination entering through a pipe defect on the way to the customer
- •pH adjustment and corrosion control, which prevents the water dissolving metals from the pipes it travels through
Getting it there
Distribution is a pressure problem. Water must arrive at every property with enough pressure to reach upper floors and to supply firefighting, and pressure is generated either by pumping or by gravity from a service reservoir or water tower placed above the area it serves, which is why towers exist and why they are the height they are. Maintaining pressure everywhere is also a safety requirement, because a pipe at positive pressure leaks outward while a pipe that loses pressure can draw contaminated groundwater in through the same defect, which is how several waterborne outbreaks have occurred. The network is looped rather than branched so that supply can be maintained during repairs. Leakage is the persistent difficulty, with losses running from a modest percentage in the best-run systems to enormous proportions elsewhere, driven by ageing pipes, pressure and the difficulty of finding leaks underground, and the economic level of leakage is a genuine calculation rather than an excuse, since finding the last leak costs more than the water is worth.
What goes wrong
Failures follow recognisable patterns. Microbiological contamination causes the most acute harm, and the Walkerton outbreak in Canada in 2000, where contaminated groundwater reached a supply through failures of chlorination and of oversight, killed seven people and sickened thousands. Chemical contamination from the pipes themselves caused the Flint crisis in the United States from 2014, when a change of source without adequate corrosion control caused lead to dissolve from service lines into drinking water, exposing a population for months while officials disputed the evidence. Disinfection by-products form when chlorine reacts with organic matter, which is a regulated trade-off against the far larger risk of not disinfecting. Emerging contaminants including persistent fluorinated compounds are now being regulated and are expensive to remove. Ageing infrastructure is the underlying problem in most wealthy countries, where pipes laid over a century ago are reaching the end of their lives faster than they are being replaced, which is a slow, unglamorous and very large bill.
The takeaway
Water is stored, coagulated, settled, filtered and disinfected, with filtration removing chlorine-resistant parasites and a maintained chlorine residual protecting the water on its way through the pipes. Distribution depends on keeping pressure everywhere, since a depressurised pipe can draw contamination in through the same defect that would otherwise leak. Catchment protection is the cheapest treatment. Walkerton and Flint illustrate microbiological and chemical failure, and ageing pipes are the underlying long-term problem.