Cost versus network length criteria in water network optimal design

Abstract A cost-based optimization criterion is used to find the best water network topology which reduces both the investment, and operating costs, when water sources with or without multiple contaminants are available. When data regarding the costs related to pipes, energy and pumps are not available, unreliable or could undergo large fluctuations, another optimization criterion could be the minimum active network length, including both the internal topology and the supply and discharge piping systems. The mathematical model of the wastewater network, assembling all the unit operations, is based upon total and contaminant species mass balances, together with the input and output constraints for each and every unit. The cost-based criterion includes the piping network cost, based upon the pipes', diameter, and the pumping cost, while the network length criterion is simply the sum of all the pipes through which contaminated water flows with a flow-rate higher than a threshold value. The optimal topologies found using these two criteria are compared against each other and also with the best topology acquired using supply water savings as criterion. In all cases, the optimization is carried out via an improved Genetic Algorithm variant, which uses one of the two aforementioned criteria, and observes, in the same time, all restrictions.