More sustainable cooling water systems thanks to softening of makeup water

Need for conditioning of recirculating cooling water

Industry uses open recirculating cooling water systems on a large scale for the dissipation of low-grade, excess heat to the outside air. This is achieved by evaporating some of the recirculating cooling water in the cooling tower (evaporation energy) through intensive contact with air that is introduced (mostly forced but sometimes through natural draught). This results in thickening of the recirculating cooling water (depending on the number of cycles of concentration) which makes it necessary to drain off some of the water to avoid the build-up of excessively high salt concentrations (scaling, corrosion). The system then has to be topped up with fresh makeup water. The traditional method of conditioning these types of cooling water systems is to add chemicals (often in standard formulations) to the recirculating cooling water in order to prevent scaling, corrosion and microbiological growth (bio-film formation) in the system. Examples of these are corrosion inhibitors, anti-scalants, dispersants and biocides. This results in a complex operational system, and high consumption of chemicals. Furthermore, the amount of thickening remains limited. This project studied whether the thickening factor of recirculating cooling water can be increased through full or partial softening/desalination of the makeup water for cooling water systems so as to achieve a saving on water, energy and chemicals.

Softening makeup water as a potential alternative

Model calculations in this study have shown that the softening of the makeup water for cooling water systems can result in a reduction in the consumption of both water and chemicals. This is because a higher thickening factor can be maintained (higher number of cycles of concentration). In addition, the traditional, relatively complex conditioning programmes can be replaced by a conditioning programme that uses only base chemicals such as sodium chloride, sodium hydroxide and sodium carbonate. This substantially reduces the environmental impact caused by discharging the cooling water drain water. The model calculations in this study have also shown that, in particular, managers of cooling water systems which have a relatively low thickening factor (number of cycles of concentration < 5) and high conditioning costs can benefit from an investment in partial softening of makeup water. Moreover, the economic feasibility is strongly influenced by the local availability of water, the costs of water and the costs of energy. Of the three scenarios that were studied for partial softening it appeared that the combination of cation and anion exchange (CIEX/AIEX) is the most beneficial based on operational costs. This is mainly due to the relatively high investment costs (and depreciation costs) of pellet softening, which forms part of the other two scenarios. The model calculations give no definitive answer about the effects of partial softening on the signs of corrosion and microbiological growth in the cooling water system. However, in both cases the specific conditions that are created in the cooling water as a result of softening (high pH, low TOC, low phosphate level) are considered to be beneficial because they suppress corrosion and microbiological growth in a natural manner. Further research under (simulated) practical conditions is still required in order to confirm this. The research partners believe that in any event the option of softening makeup water should be included as a less expensive and more sustainable solution when designing future cooling water systems.