Development of a Mass and Heat Balance Model for Zero Liquid Discharge (ZLD) Desalination

The freshwater which constitutes about 3% of the world water budget generally remains constant or decreases due to various activities. However, the demand on the freshwater increases every day to cope with the increasing population. As a result, the scarcity of freshwater is becoming significantly high as time passes. To meet the additional water demands for various purposes, different technologies and management practices have been proposed and developed. However, desalination which is a process of separating salt from the saline water can be considered as the main method to augment freshwater. One of the most fundamental theory of separating salt from the saline water is to use heat to boil water until it evaporates and then condenses. To avoid scale formation in the heating unit this paper uses the concept of pressurized water to change the boiling point. The paper presents a mass and energy balance model that can quantify the amount of energy requirement, distilled water production, and the amount of salt production as a byproduct considering no wastewater discharge or zero liquid discharge (ZLD). Given some standard thermal coefficient values and at 3.5% seawater concentration, the model calculates that the energy requirement to produce 50,000 gallons of distilled water is about 485,656 kBtu. At that condition, the salt production is about 6.6 tons. As expected, the energy consumption and salt production are found to be linear with the distilled water production and salt concentration. Model can be improved by considering the reuse of heat from the condensation process.