Water resources are under pressure worldwide, resulting in scarcity and deterioration of freshwater quality. According to European directives, we could reduce the pressure on water resources in urban areas by increasing the reuse of treated wastewater, reducing the impact on water bodies, and promoting water recycling through multiple uses of urban wastewater. Besides the need to address water supply challenges, wastewater treatment systems show environmental stewardship and innovative practices. Using reclaimed water for agricultural irrigation is gaining interest because of the drought conditions experienced in Europe over the past few years. Furthermore, using treated wastewater for agricultural irrigation may help to restore nutrients (N and P) to natural biogeochemical cycles. This review highlights the importance of water reuse, current legislation, and existing technologies to implement in wastewater treatment systems to meet the minimum requirements to produce reclaimed water to reuse in agricultural irrigation.

Phosphorus (P) is a critical element for life, and wastewater treatment systems can be strategic points for its recovery, thereby avoiding eutrophication pollution in nature. The aim of this research was to investigate P recovery via struvite, namely in terms of the influence of operating parameters, coexisting interfering ions, and seeding. This paper focuses on synthetic solutions, although an assessment was performed on wastewater. The results of the assessment indicated that, in the synthetic solution, the minimum concentration for struvite precipitation is about 30 mg P/L, and that the Mg/P molar ratio of 1 promotes P removal efficiency with less contribution from other minerals. In order to assess the results in terms of real-world scenarios, the influence of coexisting ions (calcium and sodium) was investigated. Calcium was shown to have the greatest impact on the process, as 80% was removed for an initial concentration of 200 mg Ca/L. Indeed, these experiments generated an amorphous precipitate that did not contain struvite. The utilization of biomass ash (size < 63 µm) as seeding in crystallization increased the P removal efficiency compared to the sample without seed and helped to control the pH. The precipitation experiments with wastewater demonstrated good P removal efficiencies (over 90%) but indicated a reduction in the purity of the final product (struvite was a minor crystalline phase identified in XRD—15%wt).