Modulating nodal outflows to guarantee sufficient disinfectant residuals in water distribution networks

The paper aims to propose modulation of nodal outflows in water distribution networks (WDNs), to solve the problem of low disinfectant concentrations at critical dead-end nodes, in which low flow velocities and long residence times cause excessive disinfectant decay. In fact, the slight increase in nodal outflows at these sites, which can be obtained through the opening of a blowoff at the hydrant site, can help in tackling this problem with no need of increasing disinfectant doses at the source(s) and of installing additional disinfectant booster stations. The methodology is based on the combined use of optimization and of flow routing/water quality modelling of WDNs. The concentration of disinfectant at the source(s) and the values of nodal emitter coefficients at the critical dead-end nodes are the decisional variables to be optimized. Two objective functions are considered in the optimization, namely the total volume of water delivered in the network (inclusive of supply, leakage and additional nodal outflow considered for fixing chlorine residuals) and the total mass of disinfectant injected into the network. The effectiveness of the methodology is proven on a real WDN, yielding an insight into the economic feasibility of the solution.