Water, water everywhere…

Even rainy summers won’t meet south-east England’s demand for water, but desalination plants planned by two utility companies could tap the sea’s vast resources. George Coupe reports.

The old cliche about the UK being a little, damp island somewhere off the tip of northern Europe has, it seems, officially had its day.

While we remain a small island, our climate is, in fact, not damp enough – to meet the needs of the UK’s water companies, at least. Despite recent downpours, utilities companies are so pushed to find enough of the wet stuff to go around that Thames Water and South East Water are about to use desalination to manage in times of peak demand – a first for the UK industry.

Desalination is not a new technology. It is already used in the Middle East and on the Mediterranean coast to purify sea water for drinking. The system of reverse osmosis, the most common method of removing salt, has been applied and refined over the years in these warmer climates, conditions in which the process works most efficiently.

Although the benefits of reverse osmosis are proven, it is costly and energy intensive, and the UK’s colder waters will present a challenge to the R&D arms of Thames Water and South East Water, which are planning to build industrial scale desalination plants.

In the past few years, both companies have been forced to accelerate the development of new water sources as more people move into their regions. Space to build new reservoirs is running out, while the amount of ground water is diminishing each year and usage per person increasing.

In east London, where large-scale housing developments are planned, Thames Water has already agreed to make permanent use of the boreholes dug to remove water from the Channel Tunnel Rail Link. But this water source will not be enough.

The company is awaiting planning permission for a £150m desalination plant it plans to build on the Thames estuary at Barking, where it already has a trial plant. The new facility will be capable of producing a maximum of 150 million litres of fresh water a day under ideal conditions.

‘We are having to look for more innovative alternatives and we accept that they are going to have a higher energy cost,’ a spokesman for Thames Water said. He was unable to say precisely how much energy the desalination plant will use, but said the company was working to limit energy use.

The process requires a lot of energy to reverse the phenomenon of osmosis, in which fresh water passes naturally through a semi-permeable membrane, such as in a cell wall, to mingle with salt water on the other side. Sea water is unable to pass through the membrane because the salt blocks its pores. In reverse osmosis, the sea water is pressurised to force it through the membrane and leave the salt behind.

A desalination plant can recover some of the energy it uses and both Thames Water and South East Water are planning to do this. When fresh water is produced, a stream of salty water is discharged under pressure, and this can be put through a turbine to generate electricity. Using a lot of power produces a greater amount of fresh water from a set volume of sea water but gives less opportunity to recover energy, so it is important to strike a balance.

Alternatively, more sea water can be used at minimal pressure to produce less fresh water, while allowing some energy to be recovered. But the problem here is that all the sea water taken into the plant must be filtered and purified prior to osmosis. This is costly, especially when up to two thirds of the treated water is dumped back into the sea. The most efficient balance will depend on the conditions at each plant.

Thames Water hopes to achieve a uniquely high recovery ratio by taking in water for about three hours at every low tide, when the water will be at its least salty, having already mingled with the fresh estuary waters. In a four-stage reverse osmosis process, which will include some energy recovery, it hopes to produce 80 litres of water from every 100 litres of treated sea water.

Warm water is more easily forced through the membrane than cold water. Peak demand occurs at times of the year when UK water temperatures are at their highest, but Thames Water believes the plant will be at its most efficient operating all year round at a rate of 20 million litres per day – well below its maximum capacity.

An entirely different set of circumstances applies to South East Water, which is planning to build a desalination plant at Newhaven, on the south coast, where the water will always have a higher salt content – somewhere in the region of 35gm per litre.

Gael Lehimas, South East Water’s senior engineer on the project, said his team aims for a 45 to 50 per cent recovery ratio. Because the company’s plant will return a larger amount of salty water to the sea than the Thames plant, South East Water will concentrate on optimising pre-osmosis water treatment technologies.

The development and testing of this part of the process will be the central focus of the company’s trial plant, which is now being assembled at Newhaven harbour, according to Lehimas. ‘We are not really focusing on the reverse osmosis aspect because it is difficult to scale down and we know that it works,’ he said. ‘We will be using a well established design (for the reverse osmosis process itself) but we will be testing the coagulation and sand filtration processes in parallel.’

First the raw sea water will be dosed with a flocculent, or coagulant, to bring particles and impurities together for the purpose of extraction. Then it will go through sand filtration to remove other particles. ‘We have to get to a level of purity before reverse osmosis, otherwise the particles could block the pores in the membrane,’ said Lehimas.

By improving the purification process before the water reaches the reverse osmosis stage, South East Water will keep the operation efficient and extend the life of the membrane. The pilot project will run for a year and if it is a success, the company will go ahead with plans for a full-scale desalination plant.

This would have a relatively small capacity of eight million litres per day – enough water to meet the needs of 50,000 people. Another question the pilot project will hope to answer is how much energy the large-scale plant will use. This will be an important factor in making desalination plants acceptable to customers on environmental and cost grounds.

South East Water will conduct an environmental impact study, but there seem to be few alternatives if desalination proves expensive. Demand for water will continue to grow, particularly in the south east, where large-scale housing development has been sanctioned. Many are already worried about the lack of infrastructure to support this housing growth, and provision of water is rocketing to the top of their agenda.