Optimal rehabilitation model for water-distribution systems

This paper describes a new methodology that can select the pipes to be rehabilitated and/or replaced in an existing water-distribution system and determine the increase in pumping capacities so that the water demand and pressure requirements at all demand nodes are satisfied while the total rehabilitation and energy cost is minimized. Four cost functions are considered: Pipe replacement cost, pipe rehabilitation cost, pipe repair cost, and pumping cost. The methodology considers the trade-offs among decisions regarding each pipe: Replace pipes, reline pipes, or leave as is. The mathematical formulation is a mixed-integer nonlinear programming problem. The solution procedure combines an implicit enumeration scheme using a branch and bound algorithm along with a generalized reduced gradient procedure to solve a nonlinear subproblem. The nonlinear subproblem is solved by interfacing a hydraulic simulator (KYPIPE) with the GRG2 optimizer in which the pipe diameter and pump horsepower are the decision (control) variables.