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Ma s s I n t e g r a t i o n 113

Tertiary treatment is especially important in environmentally
sensitive areas where effluent has to have low concentration levels
and loads of nitrogen and phosphorus.

5.3 Introduction to Water Pinch Analysis
Pinch Analysis was first developed for Heat Exchanger Network
synthesis and subsequently extended to yield other energy integration
applications (El-Halwagi, 1997; Klemeš et al., 1997; Smith, 2005). The
analogous characteristics of heat and mass transfer allowed the
application of Pinch Analysis to the synthesis of mass exchange
networks and a series of other mass integration problems (El-Halwagi,
1997). Water Pinch Analysis emerged as a special case of mass
integration following the seminal work of Wang and Smith (1994).
However, that paper’s targeting technique was limited to the fixed
load problem, where water-using processes are modeled as mass
transfer operations. Later work on Water Pinch Analysis has focused
mainly on the fixed flow-rate problem, where flow-rate requirements
are viewed as the important constraints for water-using processes
(Dhole et al., 1996; Hallale, 2002; El-Halwagi, Gabriel, and Harell, 2003;
Manan, Foo, and Tan, 2004; Prakash and Shenoy, 2005).
In the context of Water Pinch Analysis, reuse means that the
effluent from one unit is used in another unit and does not reenter
the unit where it was previously used; in contrast, recycling means
that the effluent will reenter the unit where it was previously used,
usually after certain purification. In addition, one may also use a
regeneration unit (e.g., filter, stripper) to partially purify the water
stream prior to reuse or recycling (Wang and Smith, 1994).
A typical Pinch Analysis study proceeds in two stages. The first is
targeting, whereby minimum freshwater and wastewater flow rates
are set; this is followed by network design to achieve the targeted flow
rates. It is worth emphasizing that the targeting step is the primary
focus in Water Pinch Analysis. The target is needed in order to
determine how well a reuse or recycle system can actually perform in
terms of thermodynamic constraints. Establishing targets in advance
of design provides a clear picture of the mass exchange limitations of
the design problem, indicating the smallest achievable freshwater
intake and wastewater discharge. Once the targets are established, a
water network can be designed using any network design tools.
Wang and Smith (1994) described a methodology for determining
the amount of water required by a set of operations when water is
reused. They showed that significant water savings can be achieved
compared with the case when only freshwater is used. The authors
employ a simple example that makes use of the limiting Composite
Curve (CC) and incorporates four water-using operations. The
problem data is presented in Table 5.2.