Open recirculating cooling water systems are employed on a large scale in industry for the dissipation of low-value, surplus heat into the ambient air. This involves the evaporation of part of the water, which recirculates over the cooling towers (evaporation energy), caused by intensive contact with forced air (usually by means of mechanical and sometimes natural draught). This leads to the thickening of the recirculating cooling water (thickening factor), whereby some of the water in the system needs to be discharged to prevent rapidly increasing concentrations of salt (scaling, corrosion). Fresh makeup water is introduced to the system to maintain water volume.
The traditional conditioning of this kind of cooling water system involves the addition of chemicals (usually in standard formulation) to the recirculating cooling water to prevent scaling, corrosion and microbial growth (biofilm formation) in the system. This contains for instance corrosion inhibitors, anti-scalants, dispersants and biocides. The process involves complex operational management and high use of chemicals. Moreover the degree of thickening remains limited.
This project studied whether the complete or partial softening/desalination of the cooling system’s makeup water can increase the thickening factor of the recirculating cooling water, so that savings can be made in water, energy and chemical use.
The model calculations of this study demonstrated that the softening of the makeup water of cooling water systems can lead to a reduction in the use of both water and chemicals. The reason is that a higher thickening factor can be managed (higher number of cycles). In addition, the traditional, relatively complex conditioning programmes can be replaced by a conditioning programme using only base chemicals like sodium chloride, sodium hydroxide and sodium carbonate. This considerably diminishes the environmental impact of cooling water discharges.
The study’s model calculations also showed that managers of cooling water systems with a relatively low thickening factor (number of concentration cycles <5) and high conditioning costs could particularly benefit from an investment in the partial softening of the makeup water. The economic feasibility depends strongly on the local availability of water, and on the costs of water and energy.
Of the three scenarios of partial softening studied, the combination of cation and anion exchange (CIEX/AIEX) was the most favourable on the basis of operational costs. This is primarily a result of the relatively high investment (and depreciation) costs of pellet softening, which was a feature of the other two scenarios.
The model calculations provide no definitive answer regarding the effects of partial softening on corrosion phenomena and microbial growth in the cooling water system. But in both regards the specific conditions that arise in cooling water as a result of softening (higher pH, low TOC, low phosphate content) are considered beneficial, because they suppress corrosion and microbial growth in a natural manner. It is felt that more detailed research under (simulated) practical conditions would be needed for confirmation in this context.
The research partners believe that the option of softening the makeup water should, in any event, be included in the design of future cooling water systems as a more economical and sustainable solution.