RESOURCE RECOVERY FROM WASTEWATERAND THEIR REUSE FOR AGRICULTURAL PURPOSES

Wastewater treatment technologies have historically been designed with the primary goal of reducing pollutant and nutrient levels, such as nitrogen (N) and phosphorus (P), to ensure that the treated water is safe for humans, animals, and the environment. This approach is supported by research and guidelines that recommend keeping pollutant and nutrient levels within permitted limits to avoid ecological and public health risks (Mo and Zhang, 2013; Yadav et al., 2021). However, the increasing demand for clean water, coupled with population growth and the depletion of non-renewable natural resources, has led to a significant change in the way wastewater is managed. The scientific and regulatory communities now highlight the need for sustainable and circular resource use, supporting resource recovery from wastewater instead of simply focusing on pollutant removal (Svardal and Kroiss, 2011). The transformation of wastewater treatment plants (WWTPs) into integrated resource recovery plants is the key point to modify. These plants not only aim to remove pollutants, but also to recover nutrients, like N and P, which are crucial for agriculture. This change is driven by a wider comprehension of wastewater as a valuable resource rather than a waste stream, bringing wastewater management practices in line with national and European sustainable development goals. The redesign of wastewater treatment plants into water resource recovery facilities (WRRFs) allows for the recovery of water, energy, nutrients, and other products, contributing to a more circular economy and addressing obsolete concepts of wastewater treatment that have prevailed since the early 20th century (Faragò et al., 2021). The importance of N and P in agriculture cannot be underestimated. These nutrients are essential for crop growth and yield. However, their overuse in agriculture has led to significant environmental challenges, including eutrophication, anoxia in water bodies and disruption of natural nutrient cycles (Kehrein et al., 2020). Furthermore, the recovery of N and P from wastewater offers a dual benefit: to mitigate these environmental impacts and to provide a sustainable source of fertiliser for agriculture (Saliu and Oladoja, 2021). This is particularly critical in the context of P, which is derived from phosphate rock, a finite resource at risk of depletion. The recovery and reuse of phosphorus from wastewater not only addresses environmental concerns but also the economic and supply risks associated with phosphate rock, which is critical to global food production (Cordell and White, 2013; Günther et al., 2018; Robles et al., 2020). In summary, the evolving approach to wastewater management recognises the value of N and P not only as contaminants that need to be removed but as essential resources that can be recovered and reused. This transition to resource recovery and the introduction of circular economy practices 2 in wastewater treatment reflects a more sustainable and efficient use of natural resources, bringing it in line with wider environmental sustainability goals and ensuring the long-term availability of critical nutrients for agriculture (Mannina et al., 2021). In the context of global sustainability, soil, and water are increasingly recognised as critical non-renewable resources for the future of agriculture and environmental health (FAO, 2021). The United Nations has set ambitious sustainable development goals (SDGs), emphasising the need to end hunger, achieve food security, improve nutrition, promote sustainable agriculture and ensure universal access to water and sanitation. To achieve these goals, a sustainable food production system must integrate agricultural practices that improve soil productivity and food production, thereby addressing the twin challenges of water quality and soil fertility (El Wali et al., 2021; Desavathu et al., 2018). Soil management practices are crucial in this regard. However, challenges such as nutrient depletion, over-reliance on inorganic and organic fertiliser, lack of soil pH regulation, inadequate soil moisture management, and poor tillage practices have significant impacts on soil fertility and production (Paramesh et al., 2023). Furthermore, the imminent threat of depletion of phosphate rock reserves highlights the urgent need for efficient P use, recovery, and reuse (Walsh et al., 2022). Nitrogen and phosphorous pollution from excessive fertiliser application worsens the degradation of natural water bodies and soil health by promoting nutrient leaching and soil acidification (Guaya et al., 2016). Addressing these problems requires not only making potable water safe and accessible but also implementing strategies for the safe reuse of wastewater to mitigate food security risks associated with inadequate water quality and sanitation. Recovery and reuse of P from urban wastewater and other secondary streams is a viable solution to reduce dependence on limited phosphate rock reserves and mitigate food security vulnerabilities (Marshner and Rengel, 2023; Nättorp et al., 2019). Innovative recovery techniques, such as selective adsorption and chemical precipitation, allow the efficient extraction of inorganic phosphorus (PO43-) from waste streams for use as soil fertiliser (Hermassi et al., 2020). The investigation of nutrient recovery technologies in wastewater treatment, including chemical, biological, membrane systems and advanced adsorption methods, highlights the potential for mitigating environmental impacts and supporting agricultural needs (Ye et al., 2020; Wu et al., 2022). Adsorption technology, in particular, offers an effective and operationally simple solution for phosphate removal, even at low concentrations, and facilitates adsorbent regeneration with minimal by-product generation. This approach not only meets the immediate challenges of water 3 and soil quality, but also contributes to the overall goals of sustainable agriculture and environmental management by conserving resources for future generations. Zeolite and biochar have emerged as effective materials for the adsorption of ammonium and phosphate from wastewater due to their properties as beneficial soil amendments. Once enriched with these nutrients, they can be applied directly to the soil, increasing its fertility (Guaya et al., 2020). Zeolites, with their three-dimensional porous structure formed by Al3+ and Si4+ tetrahedra connected by oxygen bridges, offer a negatively charged lattice ideal for ion exchange, which has been particularly effective in ammonium adsorption (Liu et al., 2018; Han et al., 2021). The efficiency of zeolites in nutrient adsorption can vary depending on their physicochemical properties and modifications such as acid/base treatments, physical alterations to increase surface areas have been investigated to improve this efficiency (Wang and Peng, 2010; Zielinski et al., 2016; Zhang et al., 2016; Putra and Lee, 2020; Muscarella et al., 2021). Biochar, a carbon-rich material produced from biomass under oxygen-limited conditions, offers a sustainable and cost-effective alternative for nutrient adsorption and soil fertility enhancement. Its high surface area, porosity and abundance of oxygen functional groups make it particularly useful for removing organic and inorganic pollutants (i.e. phosphate) from water and improving soil conditions (Almanassra et al., 2021). The reuse of treated wastewater for agricultural irrigation represents another way for resource recovery, harnessing its nutrient content for the benefit of soil, plant health and overall ecosystem productivity. Studies have examined the impact of wastewater reuse on soil chemistry, plant nutrient uptake and public health, highlighting its potential in contributing to sustainable agriculture and water management practices (Ofori et al., 2021). Sewage sludge, as a by-product of wastewater treatment, offers an additional source of nutrients for agriculture, if composted to meet regulatory standards for safe application (Khan et al., 2022). The composting of sewage sludge transforms organic matter into stable humic substances, reducing pathogens and enriching the soil with beneficial nutrients. The choice of bulking agents in the composting process, such as zeolite or biochar, can further enhance environmental benefits by absorbing greenhouse gases and increasing the mineralization of organic matter (Awasthi et al., 2017; Collivignarelli et al., 2019). This comprehensive approach to nutrient recovery from wastewater and sewage sludge, combined with innovative composting techniques, aims to reduce greenhouse gas emissions, and improve agricultural productivity. It emphasises the importance of integrating various waste streams into soil conservation strategies, thereby contributing to the sustainability of agricultural ecosystems and the wider goal of preserving soil fertility, particularly in regions vulnerable to desertification. 4 The primary objective of the thesis was to explore the recovery of nutrients from the wastewater treatment process and their subsequent reuse in agriculture. This effort is in line with the objectives of the European Horizon 2020 project 'Wider Uptake', which was approved and funded by the European Community. The project aims to synergise different research groups to improve the wastewater treatment process and overcome social barriers through the implementation of smart water solutions, thus advancing the circular economy goals set by the EU. The structure of the thesis is as follows: CHAPTER I: provides an in-depth examination of the Wider Uptake project, analyzing its contributions to the circular economy within the water sector. CHAPTER II: reports a review of the current state of the art regarding zeolites as a tool for nutrient recovery, assessing their efficacy and applicability in various contexts, including agriculture. CHAPTER III: explores composting techniques for sewage sludge and their reuse as soil amendments. CHAPTER IV: features the characterization of zeolite and biochar through batch test-scale experiments, aiming to evaluate these materials nutrient adsorption capacity. This chapter includes three scientific articles on the adsorption of ammonium and desorption using natural or chemically modified zeolites, and the assessment of how a mineralogical mixture can affect ammonium adsorption and desorption. Additionally, it contains a manuscript on the study of phosphorus recovery capabilities from a P mono-component solution using chloride-activated biochar. CHAPTER V: discusses the transition from batch tests to pilot plant scale. Initially, it details the Wastewater Resource Recovery Facility constructed within the Wider Uptake project at the Department of Engineering of the University of Palermo. It then presents two manuscripts describing experimental adsorption tests conducted in columns, first with zeolite and subsequently with biochar, using real treated wastewater. These experiments focused on evaluating the adsorption mechanisms of ammonium and phosphate in columns, varying zeolite particle size and pyrolysis temperature for biochar, respectively, and at different flow rates. CHAPTER VI: concentrates on the project final phase regarding the agricultural reuse of the recovered products. Initially, it discusses the reuse of treated wastewater on tomato cultivation, examining its effects on the plant-soil system and the microbial community within a greenhouse located at the Department of Agricultural, Food, and Forest Sciences of Palermo University. The second part analyzes the composting process of sewage sludge from the Palermo WRRF using three different mixtures of bulking agents to determine if the composting process can be improved 5 by selecting the best bulking agent. Furthermore, it evaluates the effect of the obtained compost on the growth of sunflower plants in a pot trial conducted in a greenhouse. These last two studies were carried out during a 10-month period abroad at the Escuela Tecnica Superior de Ingenieria Agronomica (ETSIA) of the University of Seville. They focused on the application of ammonium-enriched zeolite during column trials as a fertilizer for N release using wheat as the test plant and the application of enriched biochar during column trials to assess their phosphorus release capacity, using sunflower as test plant. Both studies were conducted in pot trials in a growth chamber using two soils with different characteristics. The final part of the thesis contains two articles published as a result of additional research carried out with the Agricultural Chemistry research group of Department of Agricultural, Food, and Forest Sciences of Palermo University during the doctoral years. Finally, conclusions and perspectives derived from the thesis are provided. This comprehensive structure reflects the thesis ambitious scope, aiming to contribute significantly to the field of sustainable agriculture and environmental management by addressing critical aspects of nutrient recovery and reuse, in a circular economy perspective.