Cape Town residents queue to collect rationed water, February 2018

Supply and demand: the water pressure on cities worldwide

Many cities are in danger of running out of water. What can they do about it?

“It’s astonishing how people react when they fear they are under threat.” Professor Neil Armitage, an urban water management specialist at the University of Cape Town, explains the lengths to which citizens of South Africa’s second city have gone to protect themselves during a recent three-year drought.

“There have been bumper sales of rainwater tanks that people attached to their roofs in the hope of catching any rain that falls,” he says. “People have been buying up containers of water from supermarkets and stockpiling them in their garages. They have been queueing at springs on the outskirts of the city to collect water, even though it hasn’t been purified. And the rich have been drilling their own private boreholes at huge expense, despite the increased risk of seawater ingress.”

Cape Town’s authorities introduced a public communications initiative this January which ominously counted down to ‘Day Zero’, when the dams supplying the city’s water supply were expected to drop below 13.5 per cent – leaving only a small emergency amount. The first ‘Day Zero’ was expected to arrive in April 2018, but thanks to recent rains the six reservoirs that feed the metropolis are now expected to have supply until 2019.

Cape Town’s experience is just the latest example of a modern city struggling with water scarcity. A combination of climate change, rapidly growing urban populations and ageing infrastructure means that many urban areas are undergoing water stress – a situation that is only expected to worsen. Research from MIT suggests that by 2050, over 50 per cent of the global population will be living in areas with water scarcity.

Many cities that have undergone sustained droughts have demonstrated great improvements in water management. Armitage notes that while the Cape Town authorities have received a lot of flack, their record on water management has in fact been very impressive. “The city has done an outstanding job in terms of water demand management – they have kept total water demand more or less constant since 1999, despite the city’s population nearly doubling in that time, and they have done a lot through the management of water loss and leaks and penalisation of big water users.”

Professor Richard Luthy at Stanford’s department of civil engineering reports similar efforts in California, which has experienced its own fair share of droughts. “It turns out we have actually conserved a lot of water in recent years and are getting better at doing so. In California, people are now much more aware of the water they’re using. The problem is, the number of people keeps on going up.”

So, while water conservation and improved education can certainly help, it seems dry cities are also going to have to start exploring new ways of managing water, and engineering will play a big part in this.

Luthy, who also heads ReNUWIt, a nationwide initiative for improving water infrastructure in the US, explains that “water infrastructure in most cities was developed 50 to 100 years ago, in a very different era. It was created for a time when cities were smaller, when the effects of climate change were not as manifest as they are today, and when energy conservation was not so high up the agenda.”

Our traditional urban water management depends on a fairly limited number of sources. Water is typically drawn from rivers and redirected through storage facilities (such as dams or reservoirs) before entering treatment centres and being sent to homes, farms and businesses through a network of pipes. Finally, it drains into sewers.

Liquid gold

Top 10 water-stressed cities around the world

A 2014 study by conservation organisation The Nature Conservancy ranked the world’s most water-stressed cities:

1. Tokyo   

2. Delhi   

3. Mexico City   

4. Shanghai   

5. Beijing   

6. Kolkata

7. Karachi

8. Los Angeles

9. Rio de Janeiro

10. Moscow

As Cape Town and other cities show, this traditional approach just isn’t up to the demands of modern cities. Fortunately, a lot of interesting research and work is going into creating a ‘new paradigm’ of urban water supply.

Take Arup, the engineering firm. Mark Fletcher, the group’s Global Water Business leader, has called for a “revolution” in how cities manage water. He describes how “the traditional solution was to use grey infrastructure (concrete, pipes and pumps) and hide water away underground”. Instead of hiding this water under tonnes of concrete, Fletcher argues for an enlightened approach that is “more in tune with natural processes where possible”. This would include the use of green infrastructure (vegetation for absorbing and retaining water) and blue infrastructure – typically storing water in ponds and reservoirs in urban spaces within the public realm.

Arup is currently working on the development of the Jurong Lake District Masterplan in Singapore – another water-stressed city. The project aims to use this green and blue infrastructure throughout the construction of the new district.

Another angle is to reassess how we use storm water. The approach has traditionally been to funnel it away from the city as fast as possible – be that into the sea or rivers. But a lot of work is going into capturing that water in the city itself using what’s known as SuDS – ‘sustainable drainage systems’.

SDS Ltd is a British firm that fits a wide range of these SuDS to help its customers control water drainage more effectively. Richard Averley, head of sales and marketing at SDS, explains: “Most urban drainage systems prioritise getting the water to the closest river as fast as possible – but at times of heavy rain, this can actually result in more flooding.” So, the company has engineered a range of SuDS which help customers retain water and only gradually let it seep out. This might be through the use of semi-porous surfaces on car parks that help limit flooding or plant-based drainage systems, which retain water for longer.  

While in rainy Britain SuDS are mainly used for controlling the speed of runoff into rivers to limit flooding, in drier places SuDS could be used to capture and store that water.

In South Africa, Armitage researches how storm water runoff can be collected in ponds and other structures and then be filtered through into the aquifers for later extraction as groundwater.

Storm water runoff is usually filthy – contaminated with pollution, heavy metals and even sewage, and cleaning it would be very expensive. But, if you pass it through SuDS and allow it to filter into a city’s aquifers, this water gradually becomes clean: “The magic number seems to be one year,” Armitage explains. “If you can store this runoff underground for 12 months, it seems to kill most pathogens and filter out other harmful substances.” It then becomes much easier to purify for general consumption.

Desalination would seem to be the obvious solution to water scarcity – when some 70 per cent of the planet’s surface is covered in salt water, why not use it? The fact is desalination is expensive, energy intensive and the highly concentrated brine it produces can damage ecosystems if it is pumped straight back into the sea.

Desalination requires huge amounts of pressure to force water through filters to separate potable water from the salty stuff. And these filters tend to be expensive and need to be replaced regularly.

However, Manchester-based G2O Water Technologies proposes an intriguing solution with the wonder-material graphene. Its product helps separate pure water from brine and can also be used for waste water treatment. Tim Harper, the firm’s CEO, explains the benefit of the technology: “You can apply a very thin layer of our graphene filter to the existing filters in a desalination plant – or any other water cleaning facility – and cut the energy required to filter by as much as 50 per cent. And because it’s so easy to apply, you can just retrofit it to your existing infrastructure.”

So how will all these innovations be introduced? Stanford’s Luthy expects urban water management to become more decentralised, and describes a future where neighbourhoods have their own waste water treatment and recycling plants. At Stanford, ReNUWIt’s industry partners have helped fund the construction of such a decentralised water treatment plant to test ideas on how to clean water in a more energy-efficient manner. This could be used to help meet non-potable water demands such as irrigation.

While the potential for all these new approaches to water management is exciting, there are a number of barriers to innovation. Arup’s Fletcher explains “the water sector is historically conservative” – board rooms are risk-averse due to high public expectation of clean water, which makes innovation tricky.

Luthy also points to the problem of ownership when water is recycled and stored underground. “I was recently attending a workshop with attendees from 50 different cities and utilities firms about the potential for aquifers. One thing that came out was the question of how to collaborate over groundwater storage – if an aquifer is sitting below the boundaries of two different authorities, who owns that water?” The kind of political in-fighting this kind of question could cause seems depressingly inevitable.

Yet despite the challenges, water scarcity does seem to be encouraging a lot of exciting engineering innovations and discoveries. And back in Cape Town, the countdown to ‘Day Zero’ also helped consumers understand the value of the water they use too. As Armitage says: “It’s been an educational exercise; people now understand where their water is coming from, and they have also learnt how to live on less.”

Water in a dry climate

Israel: a global leader in water management

With its growing population and dry climate, Israel has experience dealing with the water scarcity issues that are increasingly affecting other countries.

For potable water, Israel has invested heavily in desalination. Opened in 2013, Sorek, near Tel Aviv, is the world’s largest seawater desalination plant, and has the capacity to process 624,000m³ per day. Five more major plants should be up and running by 2020. Israel already exports water to neighbouring countries and hopes to expand this market in future.

Israel’s success with water management goes beyond desalination. The country is the world leader in waste water recycling: 90 per cent of water is reused (Spain comes in second, recycling around 20 per cent). Most of Israel’s waste water is used for irrigation – but of course few farmers would be happy spreading sewage on their crops. Israeli engineers got round this problem decades ago by using sand dunes as a natural filter for dirty water. Once it has worked its way down into aquifers, it can be safely used for watering crops within six months.

Israel treats water differently for different uses. For instance, water that is intended for agriculture gets fed into aquifers, water for industry is less thoroughly cleaned, while water for reuse on urban parks is treated more intensively at local filtration centres (but less so than water intended to be drunk by humans). This is different from most other countries where all water is treated to the point that you could drink it.

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