Water recycling, the process of treating wastewater so it can safely be used again, is also expected to be a key part of the future water system. In fact, it may play a bigger role in maintaining water security than desalination, Bateman suggests.

There are two main water recycling approaches. The first is indirect potable recycling, where treated effluent is released into a river, reservoir or groundwater system to undergo further natural and conventional treatment before becoming drinking water.

The second is direct potable recycling, in which highly treated recycled water, with multiple treatment and monitoring barriers, is put directly into the drinking water system.

Indirect potable recycling has the advantage of an additional environmental buffer. By returning highly treated water to an aquifer, reservoir or river source, it is treated again to become drinking water. This comes at an additional cost because the water effectively undergoes treatment twice.

Direct potable recycling removes that additional step, taking highly treated wastewater through advanced treatment and directly into the drinking-water supply, offering significant operational and cost benefits – provided the appropriate monitoring and safeguards are in place.

Water recycling can be more energy and cost-efficient than desalination in some locations because the water is already relatively close to where it is needed. The salt content of seawater doesn’t need to be removed. Recycling is also attractive because it can increase water availability without relying on new reservoirs or taking additional water from rivers or sensitive chalk stream environments.

There is significant international experience in water recycling. Much of this is in the agricultural industry, where recycled water is often used for irrigation. Australia also provides useful lessons in indirect potable recycling. In western Australia, for example, treated effluent undergoes advanced water treatment, extensive sampling and monitoring, before being reintroduced into a large aquifer. The water can then be abstracted and treated again at a conventional water treatment works before entering supply. Sydney has also experimented with indirect potable recycling.

When it comes to water recycling, a major consideration is customer perception. People understandably want to know where their water comes from, particularly when it originates from treated wastewater.

However, public support for water recycling in the UK is growing, believes Bateman. “Surveys generally indicate a preference for water recycling over desalination, although concerns remain about water quality and the need for robust safeguards.” With the right treatment processes, monitoring and multiple safety barriers, recycled water can be converted into a safe drinking-water supply, he says. “The UK has arguably been complacent about the availability of suitable sources for drinking water.

“As water stress increases, however, there are important lessons to learn from countries where water recycling and desalination have been established for many years.”

In the US, particularly in California, there is extensive experience of water recycling, including the technology, monitoring and controls required to ensure water quality. There is also a move towards direct potable recycling in states such as Arizona. Australia is also now moving towards purified recycled water (PRW) for drinking. For water recycling to take off in Britain, public confidence and demonstrating that health risks can be effectively managed will be critical.

But ultimately water recycling will be just one part of a portfolio of rainfall-independent sources of water supply. A future of extreme heat and drought conditions will require the problem to be tackled on several fronts, including not just recycling but also desalination and those new reservoirs such as Havant Thicket.

It’s this combination of solutions that will help prevent that nightmare scenario of falling billions of litres short in supply.