An MILP model for cost - Effective water treatment synthesis

Abstract

Water is a precious and scarce resource, essential for sustaining nature, human life and industry. Efficient water treatment is recognised as the only method to sustain safe supplies of water in the future. So far scientific work has focused solely on the performance optimization of individual water purification units due to overall process complexity. A whole - systems approach in the area will be of a significant benefit to industry by increasing overall process efficiency and decreasing plant costs. This work addresses the current gap by considering synthesis of water treatment trains using mixed integer linear programming (MILP). The model accounts for the most common contaminants found in water, secondary treated wastewater, seawater or brackish water. Such major pollution indicators are chemical oxygen demand (COD), biochemical oxygen demand (BOD), total dissolved solids (TDS), total suspended solids (TSS), turbidity and coliforms. The water source is treated to meet potable, process or reclaimed water standards. The set of candidate steps is selected to reflect the most extensively utilised industrial processes such as coagulation-flocculation, membrane filtration and UV disinfection at various operating conditions. The overall number of trains is minimised based on efficiency removal factors and final water purity specifications. The former takes into account the physicochemical properties of the contaminants and the respective regression models for rejection or retention of an addressed impurity in a certain candidate. A particular case of desalination for drinking water supply is studied. The model is tested for the standard level of contaminants in seawater, TDS and TSS, to be removed by a set of up to 34 candidate trains. For production of ca. 600 m3/h water the model identifies an optimum solution of overall 6 trains consisting of ultrafiltration (UF), nanofiltration(NF) and reverse osmosis (RO). The objective function minimised is the annual operating cost as a function of pumps electricity consumption, membrane cleaning and replacement practices. Overall, the results obtained agree with the recent trends in industrial desalination process synthesis and hence, the model can provide a valuable guidance in water purification processes design.