Desalination – the treatment to turn seawater into tap water – is rapidly gaining ground in the UK. Nicola Martin discovers the crucial role that peristaltic technology is playing in this type of water treatment.
Following trials and small-scale plants in the drought-ridden south-east of England, the UK’s first large-scale desalination plant is due to open in 2010 at Beckton, East London. Treating seawater has historically been considered a high-energy, expensive option – better suited to the oil-rich Gulf States – but technological advances, including implementation of peristaltic technology in water treatment, mean that desalination works are now able to function at a higher efficiency.
“There is a growing market for peristaltic technology in water treatment,” comments Ashley Shepherd, UK sales manager at Watson-Marlow Pumps Group, a world leader in peristaltic pumping. “While peristaltic pumps are already used for treating wastewater, we are now seeing increasing demand for their use in desalination. The drive to make desalination an economical option for the UK means using the most effective equipment for the job.”
The UK’s move towards desalination remains controversial. It’s an energy-intensive process that, when fuelled by oil or gas, is both expensive and polluting. However, the UK is embracing technological advances such as use of renewable energy to power desalination, as well as treatment processes that reduce the energy required. Though a contentious issue, the fact is that desalination has the undeniable potential to create much-needed secure water supplies for the UK.
London and the south-east of England have experienced a rapid population growth over the last century, which seems likely to continue. When conflated with an increasingly unstable climate and the area’s low rainfall rates and high water consumption, it means that the region has current (and potentially escalating) water shortages that are comparable with the Middle East and Australia.
Some of the richest nations in the world, including the UK, are increasingly looking for more ways to supply fresh water for use in homes and industry. According to the Environment Agency, average water use is 148L per person per day in the UK and in the south-east of England it’s as high as 170L (far higher than the government target of 130L). Despite the popular perception of London as an overcast, rain-soaked city, its rainfall rate is, in fact, on a par with Rome, Dallas and Istanbul. Climate change may lead to much drier summers, which could cripple the south east’s already-strained water supplies.
Reverse osmosis
Desalination can be performed in a number of different ways, including distillation, de-ionisation, electrodialysis or freezing. However, due to its comparatively lower energy consumption, reverse osmosis (RO) has become the standard method for treatment of seawater.
In RO desalination the raw seawater is first pre-treated, which changes the consistency of the water and is similar to the filtration in a water treatment plant. The filtered water is then pumped through microfilters before it is pushed through the reverse membrane filters at pressures around 65lb/in². This removes enough salt from the seawater so that it has a sufficiently low salinity to meet drinking water specifications. A post-treatment stage involves adding some minerals, such as calcium, to improve the taste and stability of the soft processed water. In keeping with other treatment methods, chlorine is also added for cleansing and maintenance of the distribution system.
Mr Shepherd comments, “Desalination requires precise dosing and metering of chemicals. The alternative engineering solution for chemical metering is to use gear pumps to achieve the flow rate and pressure required, but this involves a high level of maintenance as well as a complicated pressure regulation system utilising bypass valves. By contrast, peristaltic pumps are simpler and more reliable. Stable flow rates are a must when maintaining a facility that serves the water needs of so many homes and businesses. Furthermore, decreased maintenance requirements mean that not only are costs lower, but engineers’ time is freed up to be spent elsewhere.”
Best practice in desalination has been observed overseas in Israel, where Watson-Marlow’s peristaltic pumps have been specified for a range of metering applications at a desalination plant that opened in 2007 at Palmachim, 30 miles south of Tel Aviv. The Palmachim plant was designed from the outset with peristaltic pumps. With no valves, seals or glands, use of peristaltic technology means that the pumps do not clog, leak, vapour lock or damage when pumping abrasive and corrosive chemicals. Watson-Marlow’s peristaltic pumps were deemed robust enough to facilitate the treatment of the seawater and help to supply 300,000 people with fresh water.
The first large-scale UK desalination plant, located in Beckton, will treat an even larger volume of water. Thames Water is also planning to use Watson-Marlow’s peristaltic pumps at its desalination plant in order to efficiently produce 25 million-140 million litres per day and supply 900,000 people.
A major move to desalination
Though Beckton marks the UK’s major move to desalination, it is not the first plant to be commissioned within the British Isles. Since 1970, Jersey has had a desalination plant that provides an alternative water source for the island. South East Water also conducted a 2005-2007 desalination trial at Newhaven, East Sussex. Plans to build a full-scale plant at Newhaven were deemed too expensive and environmentally-problematic at the end of the pilot scheme. However, a spokesperson for South East Water did not rule out the option of desalination for the region, citing the technological advances that continue to make desalination more efficient.
As the opening of the Beckton plant draws near, Thames Water is seeking to effectively manage the environmental and financial burdens of desalination in London. Because water for the Beckton plant will be taken from the River Thames during the three hours leading up to low tide, the salt content is minimised to less than one-third of seawater. Correspondingly, substantially less energy is required to turn this brackish water into fresh water. Following a failed bid by South East Water to use photovoltaic panels at the Newhaven plant, Thames Water has chosen instead to power the Beckton plant using biodiesel, with wind power an option for the future.
All eyes are on Thames Water to illustrate whether desalination is feasible for the UK. Folkestone & Dover Water and Southern Water have both expressed interest in building desalination plants for their areas, though firm plans have not yet been made. With stable and effective treatment of water an absolute priority when it comes to desalination, the success of UK desalination depends on use of the most specialised equipment available. Peristaltic pumps have a long history and an established reputation in effective treatment of liquids in other industries. Now they seem poised to power the UK’s drive towards desalination in the most efficient way possible.
