Water & Wastewater Treatment Magazine
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www.wwtonline.co.uk | WWT | NOVEMBER 2018 | 15 • CHLORINE FACTS • Chlorine is a highly reactive chemical element which appears as a yellow-green gas at room temperature. In this form, it is poisonous to humans and has been used in chemical weapons. However, it easily forms compounds that are both plentiful and useful, the most notable being sodium chloride (salt). • Chlorine has been used in water treatment since the late 19th century, both to eliminate pathogens at the treatment works and as a residual disinfectant which can help maintain water quality through the distribution network. Water may remain in the distribution network for a week before reaching the customer's tap and, without any residual disinfectant, there would be a risk of microbiological contamination. • Alternative disinfection processes such as UV and ozone are well established, but do not offer this same residual effect. Water is supplied without chlorination in many European countries, such as the Netherlands, Germany, Denmark, Switzerland and Austria, but these countries have less need of residual disinfection as their pipe networks are newer, less lengthy or in better condition. • The distinctive 'chlorine aroma' associated with swimming pools is not the result of chlorine itself, but of chloramine, a chemical compound produced by the reaction of free dissolved chlorine with amines in organic substances. When present in drinking water this taste and odour is a leading cause of complaints to water companies by consumers, even though it is almost always harmless. • Disinfection by-products caused by chlorine can include trihalomethanes (THMs) which when present in sufficient quantities are adverse to human health. THMs are monitored and the subject of strict water quantity limits in the EU and elsewhere; however, other disinfection by- products are less well researched and understood. • CHLORINE OPTIMISATION B ut if removing chlorine entirely is a distant prospect, can the amounts used be reduced or optimised, to make the best use of resources and help achieve better tasting water? The challenge in residual network disinfectant dosing is getting the optimal amount that balances safety with palatability. Companies will oen seek to keep free chlorine to no more than 0.5mg/l, so that it does not impact on taste and smell, but they must still ensure there is enough to provide protection. Within that target range, companies avoid using excessive quantities of the chemical and keep Opex costs under control. However, this is no easy task: the chlorine residual decreases over time and is difficult to predict across a complex network. Water is routinely tested at service reservoirs and customers' taps and, should the chlorine residual regularly fall beneath the target, secondary doses are oen applied at service reservoirs or pumping stations. Atkins, a member of the SNC-Lavalin Group, has recently carried out water quality modelling work that predicts changes in concentrations much more accurately, giving companies the knowledge on how much to dose and how to optimise secondary dosing systems. "While chlorination has been practiced for over 100 years, people's understanding of it and the way it acts on different pathogens is still developing," Hugh Thomas, technical authority for process at SNC-Lavalin's Atkins business, says. "Our specialist teams have been focusing on how effective disinfection with chlorine is best achieved at the treatment works, and the different operating conditions it works best under." The difficulty in modelling the chlorine residual in distribution systems is that the decay rate is influenced by water chemistry and also the condition of the network. Atkins has undertaken lab-based water quality analysis to establish bulk decay characteristics, and deployed field instruments to establish wall decay. "If you've got a network in poor condition, with a lot of corrosion by- products and biofilm, the chlorine decays much faster than if it's passing through pipes in good condition, that convey the water without any changes," Thomas adds. "It's this information that allows us to predict accurately how to optimise chlorine dosing over a large water quality zone. We can input the bulk and wall decay parameters into a hydraulic model and say, for example, that if the water le the treatment plant with a chlorine residual of 0.5, it will have a residual of 0.4 when it's 10 hours old. You can predict what the chlorine is at any point in the network, so it's moving from a reactive to a proactive model." It has already been implemented successfully in a section of United Utilities' supply area and Atkins is now working on other projects around the country. "Customer acceptability is a key measure for the water companies and maintaining the chlorine in that target range is something they've been trying to do using experience and a trial-and-error basis," he says. "With the model, you can experiment with different primary and secondary dosing strategies without having to test it with the customer." If a pipe network is in poor condition, chlorine will decay faster