Water & Wastewater Treatment Magazine
Issue link: https://fhpublishing.uberflip.com/i/840541
28 | JULY 2017 | WWT | www.wwtonline.co.uk The recent catastrophic case of Flint Water in Michigan showed the importance of examining other corrosion indices. The water that caused the health problems in Flint had a reasonable LSI (based on the published data) but the big difference was in the chloride and sulphate levels. Con- sideration of the Larson ratio, based on chloride, sulphate and alkalinity, would have highlighted a potential water quality issue. The sole use of the LSI is not suf- ficient to prevent water quality problems, and ensuring a positive LSI does not always prevent corrosion. Balancing water quality needs Balancing all the water quality needs in hard, well-buffered waters tends not to be problematic. Many water companies dose ortho-phosphate for lead minimisation and in hard waters lead pipes become covered in lead carbonate and lead phosphate from phosphate dosing. The LSI tends to be positive as the pHs is lower than in poorly buffered waters. It is expensive to make changes to pH of well-buffered waters and the pH requirements for chlorination ensure that the pH of the treated water at the point of disinfection is closer to what is needed to maintain a positive LSI. With poorly buffered waters it is different. Any chemical addition signifi- cantly impacts the pH and the alkalin- ity, changing the characteristics of the water throughout the works. The final water may require a high pH to achieve a positive LSI with very low alkalinity levels, a pH which is difficult to achieve, maintain and which has the potential to change rapidly in the network. This conflicts with the immediate upstream needs for a relatively low pH for chlorine to achieve effective disinfection. Waters dosed with ortho-phosphate have their own pH requirements and chloraminated waters also need to be within a suitable pH range. Developing a conditioning strategy – it's complicated! Developing a robust conditioning strategy is undoubtedly complex. It requires detailed understanding of water chemistry, pipe materials, potential for corrosion in the network, sampling and assessment of treated and network water quality and the balancing of conflicting water quality needs. Public health remains the primary basis for developing a conditioning policy and rightly so. But sometimes the only way of meeting all the water quality needs and to replace waters where needed is to change the treated water quality more fundamentally than just adjusting the pH. Mindful of the need to manage water quality contacts in a region with both soŠ poorly buffered waters and hard ground- waters, Wessex Water has reviewed the corrosion indices for a range of its sources to inform the development of its future water quality strategy. Training seminars have been run and a design standard developed to complement its existing corrosion control policy. For one of its works it is developing plans for the instal- lation of a carbon dioxide/lime dosing plant at a water treatment works treating soŠ, poorly buffered water to reduce the on-going corrosion in the networks and to use the water interchangeably in its supply zones. It will unquestionably cost a significant amount to install and operate. But replacing corroded mains is expensive too, as is the cost of customer complaints of discoloured water and burst pipes. Improving the water chemistry is an excellent step in the right direction. About the author: Lisa Barrott is a Technical Specialist at MWH (now part of Stantec), and was recently elected Chair of the CIWEM Water Supply and Quality Panel. The author would like to thank Paul Williams, Principal Process Engineer, MWH and Julian Welbank and Martin Gans of Wessex Water who funded the research behind this article. Corrosion of large pipes Corrosion control policies • In a review of six large water companies in the UK, MWH asked how final pH targets are derived for a site, which water conditioning indices are used, how the targets are applied and what chemical methods of corrosion control are employed. Two water companies had specific corrosion control policies; the others did not. Four out of the six had final water requirements for LSI, the other two did not include a corrosion control index in their final water quality requirements. One water company varied its pH target seasonally in response to the change in the alkalinity of its final water and to achieve a positive LSI. What is the LSI? • The Langelier Saturation Index (LSI) is a calculated number used to predict the stability of water with respect to calcium carbonate. It indicates whether the water has potential to precipitate, dissolve, or be in equilibrium with calcium carbonate. The LSI is expressed as the difference between the actual system pH and the saturation pH (pHs). It is calculated from the pH, alkalinity, calcium ions, temperature and dissolved solids. Corrosion control parameters • Three important parameters for corrosion control are alkalinity, buffering capacity and dissolved organic carbon. Alkalinity is a measure of the ability of a water to resist a pH change - poorly buffered waters are those with alkalinity levels less than 50 mgCaCO3/l. Buffering capacity is the ability of a water to resist a change in pH, at a given pH. Finally, dissolved inorganic carbon is the sum of carbonate, bicarbonate and carbon dioxide. All three parameters are linked and overlap and all three must be considered in an effective corrosion control strategy. The Knowledge: pipe corrosion Blockage in a smaller pipe