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Exergy cost assesment for water-related technologies

Abstract:

Fresh water withdrawal increases yearly twice faster than the world population. The trend towards more urbanized societies and the growing number of people (increase of 40-50% in the next 50 years) living in large cities have very large implications for freshwater use and wastewater management. In addition to the regular fresh water catchment directly taken from the hydrological cycle, water reclamation and reuse, and desalination of seawater are nowadays new available options for water supply. Exergy analysis has proved to be an appropriate methodology to study the performance of technological processes, including those regarding water management. In this paper, the unit exergy cost (k) of different technologies involved in the water treatment and supply chain are obtained: from cleaning and pumping to desalination techniques when available fresh water is not enough. These coefficients are very important to measure in energy units the required exergy in a watershed to restore the current damaged state of their rivers up to any environmental objective for the water bodies. Obtained values range from a minimum of about 1.5 for the unit exergy cost of pumping, to about 5 for the unit exergy cost of desalination, but varying in a wide range. Different processes that take place in waste water treatment plants were analyzed, including those with a complete tertiary treatment. Once the unit exergy cost of the different water-related processes are obtained, they are used to translate into real exergy costs of the plan of technical measures required to fulfil the environmental objectives established by the competent water authority. The results clearly demonstrate that the exergy needed to restore the ancient status of water bodies is considerably higher than the theoretical minimum stablished by Thermodynamics.