A heuristic revamp strategy to improve operational flexibility of water networks based on active constraints

A novel heuristic revamp strategy is presented in this paper to improve the operational flexibility of existing water networks. The well-established concept of flexibility index (Swaney and Grossmann, 1985a, b) is adopted for quantitatively characterizing the ability of a given water network to cope with uncertain disturbances. Since it is necessary to solve a mixed-integer nonlinear program (MINLP) for this purpose, the convergence of corresponding numerical optimization process is not guaranteed. Two solution techniques are developed to promote efficiency, namely (1) generating the initial guesses by minimizing freshwater consumption rate of the nominal network structure, and (2) incorporating the smoothing functions to eliminate the binary variables in the MINLP model. A set of heuristics are also suggested to identify possible measures for relaxing the active constraints in the resulting optimal solution. Other than increasing the upper limit of freshwater supply rate, additional flexibility enhancement options concerning structural changes (which have never been systematically applied before) are considered thoroughly in the present study. These revamp methods include: (1) inserting/deleting pipeline connections and (2) adding/replacing treatment units. The implementation results of several case studies are provided at the end of this paper to demonstrate the effectiveness of proposed strategy.