(426b) On the Appropriate Modeling of Process Plant Water Systems for Zero Liquid Discharge

The Water Allocation Problem (WAP) has been extensively studied and several approaches to solve it have been presented. A comprehensive review of methods presented up to 2000 is given by Bagajewicz (2000); additional overviews can be found in a few books (Mann and Liu, 1999; Sikdar and El-Halwagi, 2001).

When Takama et al. (1980) discussed the architecture of this problem, they made sure to include a wastewater treatment system and discharge concentration limits. Moreover, their model considers that a recycle of the water treated by these treatment units can be used to feed the water-using units. Later, many works (Wang and Smith, 1994; Doyle and Smith, 1997; Polley and Polley, 2000; Bagajewicz et al., 2000; Hallale, 2002; Koppol et al., 2003; Prakotpol and Srinophakun, 2004; Teles et al., 2008) assumed an implicit end-of-pipe treatment (it is not part of the model) omitting the disposal concentration limits. Much in the same way as it was suggested by Takama et al. (1980), we argue that if an end-of-pipe treatment has to be part of the water system, it's effluent should also be available as an option for reuse/recycle. In fact, there is no water system without any kind of regeneration process (even those that were classified as ?end-of-pipe?). Thus, all water allocation problems must at least include one treatment unit in which its treated stream can be reused/recycled.

We also argue that water pre-treatment system should be part of the models. Most of the papers, including Takama et al. (1980), have assumed that one source of freshwater was available, usually with zero contaminant concentration, and have not included the pre-treatments used to bring the freshwater to such quality. Occasionally, multiple sources of different contaminant concentration are mentioned, but rarely their use is discussed in detail, much less modeled.

Finally, we argue that in addition to allowing water from the wastewater treatments (regeneration and/or EoPT) to be recycled to the water-using units, one could additionally include interaction with the pre-treatment units. Ultimately, we argue that only when complete decentralization of the system is allowed, one is sure that the global optimum of the system is achieved, although such global optimum may feature centralized solutions. We also argue that when seeking zero liquid discharge cycles, this is the appropriate route to adopt. Indeed, we will show in our examples that some consumption targets presented in the literature are not valid anymore if pre-treatment units are included. Even if only one pre-treatment is considered, and its output is a stream free of contaminants, water from any water-using unit could be recycled back to the pre-treatment to reduce the amount of freshwater needed.

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