Fresh drinking water with the Carix® process

The town of Rottenburg in the Southwest of Germany not only enjoys an idyllic location on the River Neckar - it is very attached to the water in other respects too. For the public utility company operates top-quality wells which provide 60 per cent of the drinking water to the town’s inhabitants via 400 kilometres of supply mains.

17 districts, more than 11,000 households and 42,000 people in this medium-sized Swabian town have to be supplied with water. They consume almost two million cubic metres per year, and their supply is of very good quality indeed, because the public utility company, Stadtwerke Rottenburg am Neckar GmbH (SWR), makes every effort to keep the wells, waterworks and supply grid in tiptop condition. They also test the quality of the water in Rottenburg on a daily basis in order to make sure that it always meets the high standards stipulated in the drinking water regulations.

In order to comply with these stipulations and achieve excellent water quality, the untreated water has to be treated in several stages. It is filtered, disinfected with ozone and softened with the Carix^® process¹. This involves removing salts dissolved in the water with what is known as an ion exchange process. Around 700,000 kilograms of carbon dioxide a year is needed for this. SWR has decided to work with Messer as the supply partner for the required gases.

The Carix^® process is based on the combination of two ion exchangers. The mildly acidic cation exchanger removes amongst others, calcium and magnesium and thus lowers the hardness of the water.

The anion exchanger extracts sulphate, chloride and nitrate. Both ion exchangers are located in a reactor vessel. Once they are fully laden with these ions and have thus reached their capacity limits, they are jointly and simultaneously regenerated by the introduction of carbon dioxide. Unlike in other processes, no additional salts are required for regeneration of the ion exchangers. Carbon dioxide suffices in order to dissolve the removed ions from the exchangers again. The flushed water from the regeneration process therefore only contains the salts separated from the untreated water. Most operators of Carix^® installations in Germany can therefore discharge their flushed water into surface waters. The fact that the majority of the carbon dioxide that is used can be recovered and reused in the process also contributes to the environmental friendliness of the process.

The most important advantage of the process is that the degree of hardness and the sulphate, chloride and nitrate content are simultaneously lowered to the desired level in one step. This makes the process particularly simple and increases its efficiency. Partial desalination with the Carix^® process also has less of an impact on the materials of which the drinking water plant is constructed. Since not only are bicarbonates reduced, as with rapid decarbonisation, but sulphates and chlorides as well, the water reaches a better value in terms of the corrosion index and therefore causes much less corrosion to the equipment and pipes. The mixture ratio between anion and cation exchangers can even be adjusted as required so that the emphasis of partial softening is shifted towards an anti-corrosive effect.

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¹ Carix^® is a registered trademark of VA TECH WABAG