Model based optimization of fertilization with treated wastewater reuse

Abstract

Treated wastewater contains vital crop nutrients and offers the possibility for not only irrigation but also fertilization from a renewable source. The feasibility of replacing conventional chemical fertilizers with wastewater reuse is investigated, focusing on the implications of coupling irrigation and fertilization. We consider a multi-objective dynamic optimization problem of crop irrigation and fertigation with treated wastewater, with the objectives of maximizing crop production and minimizing environmental costs. A double modeling method is proposed that relies on both a modern complex crop model – the simulation model – and a low-order dynamical systems model – the control model – to solve efficiently the optimization problem, whilst maintaining a detailed representation of the system of study. Through a case study, we show that it is possible to achieve high levels of crop production. The maximum nutrient concentration of the reclaimed waters CNmax is a key parameter of the system performance: when CNmax is high enough, it is possible to substitute entirely conventional fertilization for irrigation with wastewater. However, when CNmax is small, our results highlight that meeting crop needs would require excessive irrigation, with the added risk of dilution of the nutrients already present in soil. We find that there is a maximum amount of nutrients that can be delivered efficiently to the soil-crop system and identify the conditions when wastewater irrigation is only be capable of providing a portion of the crop needs and should be complimented with other sources of fertilization. The double modeling method and the control model provide a means to estimate the key quantities to determine the best strategy to adopt for wastewater fertilization, as well as to obtain efficient and simple controls, that could be applied in practice.