CPD PROGRAMME | WATER TREATMENT Figure 1: A 40m filter basket from a reaction tank (as in Figure 3) that was removed shortly after installation into a new-build system all relevant parameters and implementing a customised treatment plan based on a specific system and water characteristics is key to minimising corrosion risk and ensuring the longevity of a closed-loop system. There are several methods that are used to treat water for closed-loop systems either for continuous use, or to treat fill or makeup water, that may be applied individually or in conjunction with others, depending on the specific system requirements. Physical methods are often used to remove impurities from the water. This may be through some form of filtration to remove particulate matter, by deaeration methods that reduce dissolved and partially dissolved gases; and employing techniques such as reverse osmosis, which forces water through a semipermeable membrane that has microscopic pores to allow water molecules but not most contaminants to pass through. Resin-based demineralisation units (or cartridges), strainers, softeners and clarifiers are employed to remove impurities from the water. In the UK, chemical water treatment is currently the most common type of water treatment for closed systems, where chemicals are added to the water to inhibit corrosion, prevent scale formation, and kill bacteria. However, chemical-free is becoming increasingly popular a method of water treatment for closed-loop hydronic systems that does not use inhibitors or biocides. The method employs an initial fill of demineralised, clean water, and then electrochemistry is used to control the causal elements of scale and corrosion minerals, salts, oxygen and other gasses. In recent years, this method has increased in popularity; it also meets the requirements of Germanys influential guideline VDI 20352 (see VDI 2035 boxout). Chemical-free water treatment systems typically consist of a demineralisation unit and a reaction tank, and although this employs chemical and electrochemical processes, it does not add any chemicals to the system water. As shown in the example system in Figure 2, mains water passes through the demineralisation tank, which contains a mix of cation and anion exchange resins. Cation resins exchange positive ions such as calcium and magnesium ions with sodium ions, and anion resins exchange negative ions and remove dissolved solids, salts and other ions from water, so that the fill water has no, or low, conductivity. The mixed resin also removes carbon dioxide. Over time, the resin becomes saturated with contaminants, and this will lead to decreasing water quality. Resin capacity is dependent on the hardness of the local water and the total flow through the unit over time. As the resin becomes saturated with mineral ions it needs regeneration or replacement. Regular monitoring of conductivity or specific ion levels helps determine when resins can no longer perform their function. Resin typically has a useful life of three years in a system, when it will then require changing even if it has not become saturated. The reaction tank (such as that shown in Figure 3) employs sacrificial magnesium anodes surrounded by a filter and a strainer, which is then contained in a cylindrical stainless-steel housing. The tank is positioned in the system where the water is hottest (for example, near the boiler output as in Figure 2 or the input to a chiller), as this is where there will be the greatest number of bubbles of entrained air and gases. In the illustrated reaction tank, the water enters tangentially, creating a swirling motion, and as a result of centrifugal forces, heavier particulate matter is thrown outwards, which then falls and collects at the base of the unit. The outer cylindrical strainer captures any remaining larger particles while a finer (micron) inner filter removes smaller particles. These particles will be held in the strainer and micron filter, and will drop to the lower chamber when the unit is being blown down. (During scheduled maintenance, the blow-down valve is briefly opened to force a controlled flow of water back through the filter and strainer screen, in order to force out accumulated debris through the blow-down valve into an appropriate wastedisposal vessel. The valve is then closed.) This filter also traps micro air and gas bubbles, which then amalgamate and buoyantly rise, to be removed from the system by an automatic air vent. This degassing process is continuous, and when the treated, degassed water leaves the tank, it cools and passes around the distribution system absorbing trapped system air that will, in turn, be removed to atmosphere as the water passes back through the reaction tank. The magnesium anodes in the tank have significantly more negative Figure 2: Simplified example of a heating system schematic employing chemical-free water treatment 50 March 2024 www.cibsejournal.com CIBSE March 24 pp49-52 CPD Module 230.indd 50 23/02/2024 13:55