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Thanks to multiple recirculation of process water, the German paper industry has succeeded in decreasing the specific fresh water demand from an average of 50 m3/t thirty years ago to 13 m3/t today. Although the increasing closure of white water loops creates many problems, it is bound to be part of the German paper industry's ongoing development. For a few years, in the production of packaging paper, two paper mills have been running with a totally closed water system including different process water treatment plants as 'kidneys'. This paper presents a comprehensive survey of the pros and cons of closed process water systems followed by significant examples of effluent-free production of corrugating medium and test liner. Additionally, operation experiences and economic aspects are discussed.

Calcium carbonate provides good optical properties to graphic paper and is relatively inexpensive. Therefore, the use of calcium carbonate for the production of paper has steadily increased. But the high amount of calcium carbonate in paper for recycling and the closure of water circuits leads to a high concentration of calcium ions in the process water of paper mills. The consequence can be undesired precipitation and deposition of calcium carbonate in machines and plant components of the mills. Enhanced knowledge about the precipitation potential of calcium carbonate is needed to avoid or reduce undesired precipitation and deposition. The potential can be described using the Langelier saturation index (LSI). Our investigations showed that a deposition of calcium carbonate only happens if the LSI value is higher than 0.5. An LSI value above 0.5 often exists in process water that is treated in a biological effluent treatment plant. Reasons for such values are the increase in hydrogen carbonate concentration (m-alkalinity) by carbon dioxide production and the increase of the pH value by reduction of fatty acids. With the knowledge about the precipitation potential of calcium carbonate in a process water line, suitable measures could be implemented.

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