Moving water against gravity

Pumps, tanks, pipes, network leaks, pressure and electricity. Theory, formulas, worked examples and applications.

Water weighs a tonne per cubic meter

Why moving water always costs energy

A cubic meter of water weighs exactly one tonne — a fact that usually goes unnoticed until it has to be moved. Lifting it against gravity to a tank at a higher elevation, transporting it over considerable distances, or simply maintaining enough pressure throughout an entire distribution network always requires mechanical work, and that work is paid for in electricity. Pumps convert that electricity into movement and pressure; elevated tanks store both the water and the gravitational potential energy it has been given, releasing it gradually as the water flows back down toward points of consumption; and sectorization valves divide the network into independent zones, preventing a neighborhood at a lower elevation from receiving excessive pressure that could damage its own household pipes.

A network made of very different materials, all with an expiry date

An urban distribution network combines cast-iron, steel, reinforced-concrete or various types of polymer pipes, depending on the era of construction and the specific stretch, joined by fittings and valves, fitted with individual meters, and backed by pumping stations distributed throughout the city. Manufacturing and burying all that infrastructure requires considerable materials and years of civil works. Over time, internal and external corrosion appears in metal pipes, visible bursts occur that flood the street, and — much harder to detect — invisible leaks can waste water for months or years before anyone notices. Every liter that is pumped and never actually reaches a user wastes, in cascade, all the capture, all the chemical treatment and all the pumping electricity already invested in it.

More pressure is not always better pressure

Keeping higher pressure throughout the network ensures robust service even at peak demand hours, and provides the flow needed for firefighting in an emergency. But that same extra pressure also increases the flow escaping through any existing leak — a leak under higher pressure loses more water per unit of time — and raises the overall electricity consumption of the whole pumping system. Reducing pressure too much, conversely, harms service quality in the network's farthest or highest zones, and can leave upper floors of a building without enough pressure. Modern network operators use distributed pressure sensors, active network sectorization into independent zones, acoustic leak-detection techniques — which identify the characteristic sound of water escaping underground without needing to dig — and selective renewal programs for the oldest or most problematic pipe stretches, instead of having to dig up an entire city to locate every loss.

Two different leaks, two different responsible parties

It is worth clearly distinguishing between two types of leak often confused in public conversation. A leak occurring in the municipal distribution network itself — a pipe buried under the street — is the service operator's responsibility, and repairing it depends on the investment and management of that public infrastructure. A leak occurring inside a home or building — a continuously dripping cistern, a tap that does not fully close — is the responsibility of whoever lives in that space. Both types of leak add up their real cost over many continuous hours, day and night, including every hour of the night when no one is consciously using water. Repairing them avoids wasting water and all the energy invested upstream to capture and treat it, and if the water lost had already been heated, the repair also avoids that additional domestic energy cost — developed in more detail in this section's domestic hot water leaf.

More pressure is not always better pressure

In favor of robust pressure: ensures reliable service at peak demand hours and enough flow for firefighting in emergencies.

To watch: every pressure increase also raises the flow lost through any existing leak and the overall electricity consumption of pumping — it is not a cost-free decision in either direction.

Before digging up an entire street to find a leak: distributed pressure sensors, active sectorization and acoustic detection locate losses without needing to dig up the whole infrastructure — always compare the cost of that technology with the cost of continuing to lose water and energy indefinitely.

Know who is responsible for which leak. The municipal network is the service operator's responsibility; a dripping cistern or tap inside a home is the responsibility of whoever lives there. Both add up real cost over many continuous hours, including all night.

The numbers, with their source

Urban networks lose, on average, more than a third of pumped water

Recent estimates put average losses in urban water systems at around 35% of water supplied as "non-revenue water" — physical leaks plus metering errors —, equivalent to more than one trillion cubic meters lost per year worldwide, and about $14 billion in associated economic damage. The World Bank recommends keeping this figure below 25%; in regions with severe water stress, such as parts of the Middle East, the rate can exceed 50%.

Sources: We Are Water — Water waste: Ending water losses, a global priority; World Bank — The Challenge of Reducing Non-Revenue Water.

Why every percentage point of leakage is also lost electricity

Every cubic meter lost in the network already went through capture, chemical treatment and pumping before escaping through an invisible crack under the asphalt. A network losing 35% of pumped water is, in practice, spending pumping electricity to move water that will never provide any service — reducing those losses is often one of the cheapest ways available for a network operator to save energy, compared with building new generation or treatment capacity.

The comparative table by region

What this table is missing

These figures mix real physical leaks — which require investment in pipes and detection — with "commercial" losses from unbilled connections or poorly calibrated meters, which require completely different management solutions; separating the two causes requires data not every network operator publishes with the same detail.

Sources

• The IEA analyzes the energy cost of water pumping in Water-Energy Nexus.
• The World Bank documents the challenge of reducing non-revenue water in The Challenge of Reducing Non-Revenue Water.
• Aquatech summarizes leak-detection techniques in Essential Guide: Leakage & Non-Revenue Water.