关键词: wastewater minimization;water-using system;reuse;regeneration;mass exchange network

Abstract: A sequential three-step programming method is proposed for determining the minimum flowrate
of freshwater and corresponding regenerated water in water-using system of single
contaminant with regeneration reuse.In step 1, a programming with the objective of min fws
is used to determine the minimum flowrate of fresh water,in which the mathematical
representation is a mixed integer nonlinear programming (MINLP1). Then under thesame
constraints with step 1, a programming with the objective of min freg in step 2 and a
programming withthe objective of min Cr in step 3 are subsequently used to determine the
minimum flowrate of regenerated waterand the minimum inlet concentration to regeneration
process corresponding to the minimum flowrate of fresh waterbased on step 1. The method is
easy to apply because we only need to change the objective function but keep theconstraints
constant to go along the following steps after step 1. In addition, the relationship
between the freshwater flowrate required, fws, and inlet concentration to regeneration
process, Cr, is investigated. It is found thatthere exist three relationships between fws
and Cr, which indicate three possibilities for Cb: below the pinch, abovethe pinch or at
the pinch. Therefore, a new conclusion is drawn, which differs from that "regeneration of
water atpinch minimizes fresh water flowrate" derived in literature and indicates that in
some cases, regeneration at otherpoint also minimizes fresh water flowrate.

Key words: wastewater minimization, water-using system, reuse, regeneration, mass exchange network