Evolutionary multi-objective optimization of the design and operation of water distribution 
network: total cost vs. reliability vs. water quality

Abstract

An expanded rehabilitation of the hypothetical water distribution network of Anytown, USA is 
considered. AS well as pipe rehabilitation decisions, tank sizing, tank siting and pump operation 
schedules are considered as design variables. inclusion of pump operation schedules requires 
consideration of water system operation over the demand pattern period. Design of distribution 
storage facilities involves solving numerous issues and trade-offs such as locations, levels and 
volume. This paper investigates the application of multi-objective evolutionary algorithms in 
the identification of the pay-off characteristic between total cost, reliability and water quality 
of Anytown's water distribution system. A new approach is presented for formulation of the model. 
To provide flexibility, the network must be designed and operated under multiple loading conditions. 
The cost of the solution includes the capital costs of pipes and tanks as well as the present value 
of the energy consumed during a specified period. Optimization tends to reduce costs by reducing; 
the diameter of, or completely eliminating, pipes, thus leaving the system with insufficient capacity 
to respond to pipe breaks or demands that exceed design values without violating required performance 
levels. Here a resilience index is considered as a second objective to increase the hydraulic reliability 
and the availability of water during pipe failures. Considering reliability as one of the objectives 
in the optimization process will decrease the level of vulnerability for the solutions and therefore 
will result in robust networks. However, oversized distribution mains and storage tanks will have adverse 
effects on water age with negative effects on water quality due to low flow velocity and little 
turnover, respectively. Therefore, another objective in the design and operation of distribution systems 
with storage facilities is the minimization of residence time, thus minimizing deterioration in water 
quality, which is directly associated with the age of water. Residence time must include not only the 
time in tanks but also the travel time before and after the water's entry into the storage facilities. 
The residence time of the water in the network is considered as a surrogate measure of water quality. 
Results are presented for the pay-off characteristics between total cost, reliability and water quality, 
for 24 h design and five loading conditions.