Multi-scenario Optimization of Groundwater-Sourced Water Production Networks With Daily Well Shutdown Requirements

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Abstract
Water supply in the countryside of São Paulo state, Brazil, is based on groundwater resources that can be contaminated with substances such as heavy metals or fluoride, requiring the usage of water treatment technologies such as Reverse Osmosis (RO); however, RO systems create a stream of high-salinity brine, with negative environmental consequences. Besides, regulatory constraints demand that well operations must be interrupted for a daily contiguous period. In this work, a Mixed-Integer Linear program (MILP) was implemented to define water network topologies and well exploitation schedules, under these downtime constraints, aiming the minimization of RO plant capacity (and, therefore, of brine discharges). This model was then applied to the water supply of a small city in the São Paulo countryside, with around 8000 inhabitants, where high fluoride concentrations warranted the implementation of an RO system. Demand variations between weekdays and weekends (with demands 52.7% higher) were modeled using a multi-scenario formulation. This model was optimized under different conditions of tank capacity and membrane technologies. Different membrane systems (with salt retention rates varying from 85% to 99.5%) associated with different storage tanks (with volumes between 50 and 1000 m³) led to different system topologies, and a trade-off was observed between plant capacity, tank capacity, and operating pressure: while a low pressure system with a small tank capacity required larger plant capacities (up to 44.02 m³/h), high pressure systems associated with large storage tanks required smaller plants for treatment (with a capacity as small as 11.15 m³/h).