top of page

London’s tap water: where does it come from, where does it go, and why do we need to understand both

Updated: Nov 23, 2020

The water that we drink in London comes from a range of places and ultimately leaves London through the River Thames before going out to sea. Along the way, many processes ensure the water won’t harm us or the environment. In this post, we will briefly explore what happens to our water in London, from its abstraction to ultimate release back into the river, and why it’s essential to look at these stages as a whole.

Water is taken from the River Thames and River Lee from eleven pumping stations, as long as the rivers are not too dry. These pumps send the water to reservoirs, and freshwater treatment works. The reservoirs store water in case the rivers become too dry. The oldest reservoirs on the Thames were built in 1903, while on the Lee, they date to 1866! The freshwater treatment works ensure the water is safe for consumption and send it to our houses for use via an extensive distribution network. In our houses, we use water for a variety of purposes though Londoners use around 35% of their tap water for bathing and 20% for flushing the loo.

Once we’ve used our water, an extensive sewer network collects it. This network was famously created by Joseph Bazalgette in the 1860s to prevent cholera occurring due to the River Thames being treated as an open sewer. The wastewater travels along the sewer network to wastewater treatment works to be made safe for the environment and then is released into the Thames. If you live in central London, rainwater also enters the same network. This sharing can be problematic if it rains a lot, which causes the sewer to overflow into the River Thames (this is ‘Combined Sewer Overflow’ or CSO). Solutions such as the Thames Tideway Tunnel are under construction to collect all this overflowing sewage.

We summarise the process below.

Traditionally each process here is planned for separately, representing adjoining processes as boundaries. For example, when we plan to expand a reservoir in the supply system, we consider how much water we need at freshwater treatment and how much water is in the river. However, we do not consider that these lowered river levels might interact with the output of wastewater treatment.

In our work, we aim to make the river, and particularly the quality of water in the river, central to the planning process. We hope that by doing this, we can minimise the unforeseen consequences of infrastructure projects on the environment.

We have also revealed opportunities for protecting river quality by adopting a holistic view. For example, it is theoretically possible to strategically reduce river abstractions to dilute sewer overflow events. This ‘Abstraction Effluent Dilution’ (AED) technique is a small change to implement, but the increase in river quality it provides would otherwise require £200 million of built infrastructure to achieve. It highlights how, by expanding the boundaries of what we consider in the planning process, we can leverage broader system information to make smarter decisions that can benefit the environment.

You can read more about modelling work on integrated water management of London in our preprint at


Written by Barney Dobson

Research Associate




bottom of page