December 04, 2014
London is classed by the United Kingdom’s Environment Agency as ‘seriously water stressed’. It receives less rainfall per person than Rome, Istanbul, Dallas or Sydney and pressure on water resources will continue to grow in the future. Another 700,000 people are forecast to be living in London by 2021 and climate change will increase the likelihood of longer, hotter summers.
To help tackle water shortages, the UK’s first water desalination plant, the Thames Gateway Water Treatment Works, opened in 2010.
The plant treats brackish water from the River Thames, and can produce up to 150 million litres of drinking water daily—enough to supply one million people. Within the plant, saline river water passes through lamella clarifiers to remove solid particles. The clarifiers are large, open tanks containing a coarse filter supported by a grillage of 78 I-beams made of Type 2205 (UNS S32205) nickel-containing duplex stainless steel.
Salt is removed from the water by a reverse osmosis process where water is forced at high pressure through very fine membranes. The treated water is re-mineralised so that it has similar properties to other local potable water supplies and then disinfected.
The lamella clarifiers are divided into three cells, which are split in two. Each half is supported by 13 stainless steel beams of 17.5m span, with supporting steel bracing at approximately 3m intervals. The grid of beams carries a load of 2.5 kN/m2 during full working conditions. The most common profile for the stainless steel I beams was 512mm x 132mm. All the steelwork supporting the main beams was also fabricated in Type 2205 duplex stainless steel, to avoid bimetallic corrosion.
Drinking river water in London
Drinking Water Inspectorate (DWI) approval is needed for all materials that come into contact with drinking water. The main beams for the Thames Gateway Water Treatment Works were initially specified to be carbon steel with an epoxy coating. However, there was a high risk of damage to the epoxy coating by follow-on operations and maintenance, which would have resulted in rusting and subsequent damage to the £7 million (US $11 million) desalination membranes.
Duplex for desalination
Duplex stainless steel grade Type 2205 was specified instead. This grade is DWI approved, requires little maintenance and is durable in brackish water without any applied coating. The 2205 alloy is considerably stronger than the standard stainless steels such as 304L and 316L—close to the strength level of the carbon steel beams. The higher material cost of stainless steel is offset by the low maintenance requirements and higher plant availability, as well as greater assurance of water quality throughout the plant’s design life of at least 60 years. The stainless steel will retain its value and be recovered and recycled when the plant is eventually replaced.