Evaluation and validation of tools and technologies for the sustainable management of on-farm irrigation with desalinated seawater

Abstract

This doctoral dissertation has been presented in the form of thesis by publication. In arid and semi-arid regions, the scarcity and variable availability of water resources led irrigated agriculture to evaluate and apply, at an on-farm level, different techniques aimed at the efficient use and saving of water. Among them, the use of covers to reduce evaporation losses, remote control of irrigation systems, deficit irrigation strategies and even the hydroponic cultivation have been consolidated as effective methods to guarantee water savings. However, although these techniques allow a more efficient and sustainable use of resources, in no case do they allow increasing their availability. This situation has led to implement new solutions that ensure the maintenance and sustainability of irrigated agriculture, among which the use of unconventional water resources stands out. Currently, the most relevant non-conventional resources are (i) reclaimed wáter from urban usage, (ii) brackish or groundwater and (iii) desalinated seawater (DSW), which provide additional volumes of water to those traditionally used in irrigation. However, the unsteady supply of resources and their unique characteristics give rise to irrigation water with a variable and sometimes problematic chemical composition. Concretely, the use of DSW in agricultural irrigation may be limited due to its low electrical conductivity, its low nutritional content and the high concentration of certain elements, such as boron, chloride or sodium, which may produce phytotoxic effects in sensitive species; this last is the most notable aspect. The phytotoxicity threshold for sensitive crops is around 0.5 mg/L, while on a regular basis the average DSW available in Spain can double that convenient solution, farmers are compelled to blend different waters to adapt their singular composition to the requirements of the different crops, which also hinders the correct nutritional adjustment, and, therefore, the fertigation process The implementation of certain systems and technologies at an on-farm level is a truly novel aspect, and the technical and economic analysis of its implementation have barely been studied. At the same time, the optimization of techniques and tools already present entails a turning point in the sustainable use of water resources, which is a fundamental pillar in the development of agricultural activities in this region. In this context, achieving a sustainable management of available water resources and facing the difficulties presented, among others, by the use of AMD in agricultural irrigation, are the main object of this doctoral thesis The thesis is presented as a compendium of three publications, and is divided into two main research lines: (i) the implementation and evaluation of on-farm systems to reduce boron in DSW (articles 1 and 2), and (ii) the development, analysis and application of a technical irrigation head and a smart system for optimal management of irrigation water and fertilizers (article 3). Firstly, the article Boron Removal from Desalinated Seawater for Irrigation with an On-Farm Reverse Osmosis System in Southeastern Spain section 4.1), presents the technical and economic evaluation of a reverse osmosis (RO) membrane system. The study analyzes the effect of the temperature, pH and pressure of the feed water on the ability of the membranes to reduce the boron concentration in DSW with levels around 1.0 mg/L. The results show that, regardless of the temperature range considered, the increase in pH up to values of 9.5 and pressure up to 9 bar, allows reducing the boron concentration below the phytotoxicity threshold of sensitive crops, but also decreasing the presence of the rest of the ions, due to the null specificity of the membranes for said element. The economic analysis shows that the scaling of the experimental prototype to systems with a production of 20 40 m3/h, reduces the operating costs to competitive values ( 0.30 /m3). The second article (section 4.2 Ion Exchange Resins to Reduce Boron in Desalinated Seawater for Irrigation in Southeastern Spain an ion exchange resins equipment. In this case, the resin exhibits a total selectivity for boron and an obvious independence of the feed water temperature. In addition, neither does it require pH or pressure variations to improve the boron retention percentage, and it shows lower specific costs than the RO system ( 0.23 /m3). This system has also shown other advantages, such as the easy maintenance of the equipment or the negligible volume of brine produced compared to RO. Finally, the third article (section 4.3), "Evaluation of an autonomous smart system for optimal management of fertigation with variable sources of irrigation water", analyzes the development and implementation of a technical irrigation head, which manages to improve the use of different irrigation waters available, in addition to reducing the supply of fertilizers. The system is connected to a web application and a user database, having the elements necessary for the control and supply of water and fertilizers. It is capable of developing an optimal fertigation plan and controlling the volume and time of fertigation, overcoming the drawbacks of using irrigation water with variable composition that necessitate continuous blending to prevent deficiencies or excesses of certain elements, and it is shown to be technically and economically feasible for on-farm resource management. Generally speaking, the technologies and tools analyzed are technically feasible and economically promising, especially when refereeing to larger scales than the prototypes. Their implementation was able to address several irrigation concerns that arose with DSW, and allowed for the management of water resources and fertilizer without the risk of causing harmful effects on the crop. However, the study shows the presence of two major drawbacks: (i) the high price of AMD ( 0.60 /m3) which makes it difficult to implement boron reduction systems at on-farm scale and increase the price of final irrigation water up to unaffordable values for most crops; in other words, it causes a considerable loss of profitability; and (ii) the limited availability of easily accessible and user-friendly technologies for fertigation management, and the substantial price of some of them, hinder the farmers to adopt these tools. Nevertheless, the results are highly promising, and promotes the optimization of previously evaluated systems as well as the implementation of alternative and innovative technologies to face their main drawbacks and improve resource management at on-farm level.