Optimising system operations and water quality

Abstract

This paper demonstrates how the genetic algorithm technique can be used to optimize the operations of a water distribution system considering both hydraulic and water quality considerations. The methodology developed is demonstrated through application to a simple water distribution system consisting of a reservoir from where water is pumped into a high level tank. It is then gravity fed into a distribution system consisting of 19 pipes and 6 loops. The pump is activated by water levels in the tank called "trigger" levels. When the water level falls below the lower trigger level, the pump switches on and stays on until the water level reaches the upper trigger level. As there is a different electricity tariff for peak and off-peak pumping, different trigger levels may be set for peak and off-peak periods. Hydraulic optimization involves choosing these four trigger levels so as to minimize the pumping cost for the system for a defined daily demand pattern. Water quality is considered in terms of the concentration of free chlorine in the water. Cost penalties are imposed if this concentration falls outside the range of 0.1 mg/L to 1.0 mg/L in the distribution system. The concentration of chlorine after dosing (just downstream of the pump) is an additional decision variable in the problem. The results obtained indicate that different trigger levels are required when water quality is taken into account. The paper demonstrates a methodology that can be used to minimize pumping costs in a distribution system while still ensuring that desirable water quality is achieved.