Storage Design for Maximum Wastewater Reuse in Batch Plants

Overview

In Chapter 4, an approach for wastewater minimisation through exploration of recycle and reuse opportunities was presented with the implicit assumption that the capacity of reusable water storage was fixed. This chapter presents an algorithm for minimisation of reusable water storage in batch plants (Majozi, Ind. Eng. Chem. Res., 45(17): 5936–5943, 2006). This algorithm is applicable to both multipurpose and multiproduct batch plants and is based on a two-stage technique, with each stage focussing on a dedicated objective function. In the first stage, the objective is the minimisation of freshwater, given the maximum reusable water storage capacity. Once the minimum freshwater target has been set, it is then fixed and used as an input parameter in the second stage. In the second stage, central reusable water storage capacity is a variable that needs to be minimised subject to the minimum freshwater target set in the first stage. The overall formulation is based on uneven discretization of the time horizon approach developed by Majozi and Zhu (2001) using a so-called state sequence network, and is an extension of the material presented in Chapter 4 of this textbook. In this chapter, the algorithm is applied to a case study in which it yields more than 45% savings in freshwater demand compared to the case without the exploitation of water recycle and reuse. Moreover, more than 60% reduction in reusable water storage capacity is observed in comparison to the situation in which central reusable water storage capacity is fixed beforehand. Although only a single contaminant case is addressed in this chapter, the presented methodology is also applicable to a multiple contaminant environment. Worthy of mention is that the structure of the resultant mathematical formulation is such that global optimality cannot be guaranteed.