Wastewater minimization in batch plants with sequence dependent changeover

The minimization of water in batch plants through process integration has been well established in the literature. While many existing methods have relied on a fixed batch schedule, a few have employed variable production schedules. Nevertheless, methods employing variable schedules in fixed mass load problems have assumed that water-using operations (washing) are sequence independent. This paper presents a mathematical formulation for the simultaneous optimization of batch production and minimization of wastewater in fixed mass load problems. Wastewater minimization is achieved by exploring the sequence of tasks in a unit. The effectiveness of the formulation was demonstrated using three case studies. The results show that even without process integration, water usage can be minimized through the appropriate selection of task sequences in a unit. Reductions in water usage of 33% and 48.8% were observed in case study I and case study II respectively.