Assimilative capacity and flow dilution for water quality protection in rivers

Industrial and urban development is a common cause of increased pollution. Pollutants are in many instances discharged untreated to rivers due to lack of adequate treatment facilities and high treatment cost. In many cases, the detriment of pollution discharge to a river exceeds its self-purification capacity, and it may cause irreparable damages to the riverine environment. In this regard, water flow in a river is an effective characteristic behind its assimilative capacity that can be used to decrease pollution damages. Determining a river's assimilation capacity and the flow necessary for dilution of pollutants are important tasks. In this paper, pollution damage to a riverine environment is a function of the pollutant's concentration and the contact duration with river water. Pollutant transport in a river is simulated based on mathematical equations of pollutant advection-dispersion. The optimum values of a river's assimilation capacity and the dilution flow required in a river to mitigate pollution are determined using a nonlinear programming (NLP) method and the nondominated sorting genetic algorithms II (NSGA-II). The optimum assimilation capacity of a river was calculated in an application example for different reservoir releases. The results show that the magnitude of river flow can improve the total riverine assimilation capacity by up to 80%. Optimal Pareto boundaries were obtained for pollutant concentration and the duration of pollutant contact by means of river flow adjustment.