Water & Wastewater Treatment Magazine
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The Knowledge: pipe corrosion www.wwtonline.co.uk | WWT | JULY 2017 | 27 W ater companies now place their customers' experience of their water supplies at the very heart of their business. The use of Outcome Delivery Incentives (ODIs) has incentivised improvements to water quality both at the treatment stage and at the customers' taps. Bluntly put, it costs money if water quality degrades in the network. The UK water industry has resolved to minimise water quality complaints, particularly the 'red-water' complaints linked to corrosion of pipes. Water companies routinely assess the impacts of ‚ ow and pressure changes in the networks on water quality, and invest large sums to reduce these impacts. But what about the eƒ ect of the treated water quality on the pipes that make up the networks and cause the discolouration in the „ rst place? Could optimising „ nal water chemistry help reduce complaints more e‡ ciently through maintaining pipe Corrosion of water pipes: out of sight, out of mind? Pipe corrosion is a major cause of water quality complaints by customers, but how o en is the water itself responsible for causing it? MWH's Lisa Barrott explores the tests and conditioning for ensuring drinking water is non-corrosive LISA BARROTT SENIOR TECHNICAL SPECIALIST, MWH integrity, reducing troublesome blockages and preventing expensive pipe failures? For years the Langelier Saturation Index (LSI) has been used widely to determine whether a water is corrosive. But just how useful is it? Turbidity rules? Turbidity, the scattering of light caused by large numbers of individual particles oŒ en invisible to the naked eye, is a key indicator of water quality. Historically, water companies have carried out „ nal water conditioning using lime-based dosing systems. More recently, a focus on disinfection and stringent „ nal water quality targets have driven a shiŒ towards using caustic or pure limewa- ter. Water companies which dosed lime slurry struggled to achieve tight turbidity targets due to the introduction of particu- lates from lime slurry, and „ nal water pH would not be optimised for corrosion control purposes. Conditioning of „ nal water was eƒ ectively sacri„ ced for „ nal turbidity compliance. This was perhaps understandable given the importance of turbidity as a surrogate for monitoring Cryptosporidium – something which is still very important today. The impact of corrosive water on the distribution side of the supply chain was to all intents and purposes 'unseen'. What do water companies do now? In 2016 MWH reviewed the conditioning policies and strategies for conditioning of six large UK water companies (see box). All the water companies had a strategy to reduce lead, and compliance with the demanding lead standard of 10 µg/l has improved dramatically. There was a diversity of approaches with respect to other parameters, partly re‚ ecting the quality of the source waters treated by the companies, but in some cases taking quite fundamentally diƒ erent approaches to corrosion indices and pH. In one case „ nal pH targets are changed seasonally, but for some companies there is no target pH. However of particular interest was the inconsistent use of the well-known corrosion index, the Langelier Saturation Index (LSI). Is the Langelier Saturation Index still useful? The use of the well-known LSI is very widespread, but has limitations, and some water companies are moving away from setting „ nal water quality targets solely based on this index. The fundamental question has to be asked: why use an index based on calcium carbonate chemistry to control the corrosion of iron pipes? For many years it was thought that the precipitation of a thin layer of calcium carbonate would form a protective layer on the surface of the pipes and prevent corrosion of the pipes and the production of discoloured water. However, it is now well-established that the layer of calcium carbonate does not prevent corrosion. Where the LSI is undoubtedly eƒ ective is to predict the ability of the water to corrode structures such as cement-lined pipes, which contain calcium as an essential part of their structure and to prevent blockages in small pipes and mixers from calcium carbonate precipitation. Buƒ ering of the water, as measured through the buƒ ering intensity, and other mitigation measures to reduce dissolution of inner pipe surfaces and scale also need to be considered. The buƒ ering capacity can be increased by raising the dissolved inorganic carbon through the addition of alkalinity in the form of bicarbonate/ carbonate. Disturbed A er 3 hours A er 24 hours 'Red-water' samples before and a er settlement.