The pollution of water caused by the excessive presence of organic and inorganic compounds, such as nitrates, phosphates, heavy metals, antibiotics, agrochemicals, etc., is one of the major environmental problems in many countries. Various approaches to remediate wastewater are available, and this review mainly provides the state of the art about the possible adoption of microalgae-based treatments (phycoremediation), which may represent a good alternative to conventional purification methods. Because of its composition, wastewater can provide several nutritional compounds (e.g., carbon, nitrogen, and phosphorus), which represent the essential nutrients for microalgae growth. Microalgae are also attracting the interest of worldwide researchers due to their multipurpose applications; in particular, microalgae cells can represent a useful feedstock for various sectors, among these, the agricultural sector. This review proposes a detailed description of the possible application of microalgae in the process of remediation of wastewaters of different sources, highlighting their possible advantages. Moreover, the review aims to report the application of the microalgae biomasses and their extracts in agriculture, as microalgae-based products can represent a valid alternative to traditional agrochemicals, offering sustainable solutions to improve agricultural technologies. Therefore, since the recently developed wastewater depuration technology based on phycoremediation may directly provide valuable microalgae biomasses, it can be used as a powerful starting means to produce agricultural products able to improve yield and quality of crops (biostimulants, biofertilizers), as well as induce pest and disease resistance (biopesticides).

The cultivation of microalgae using urban wastewater as a nutrient substrate represents a promising bio-refinery concept that can serve multiple purposes; indeed, it allows for the generation of biomass, which can be used for various applications while meanwhile removing nutrients from wastewater. In this study, the potential of urban wastewater collected at two different time periods in a farmhouse as a nutrient substrate for microalgal growth was assessed. Wastewater samples were treated on a laboratory scale, inoculating reactors with two common species, Chlorella vulgaris (CV) and Scenedesmus quadricauda (SQ), and with an autochthonous strain of Klebsormidium sp. K39 (Kleb), directly isolated from effluents of the same system. The main aim of the study was to compare the microalgae’s performances in terms of wastewater re-mediation and biomass productivity. In the first case study, which involved an effluent with a lower pollutant level, microalgal cultivation showed removal efficiencies in the range of 57–63% for total nitrogen, 65–92% for total phosphorous, 94–95% for COD, and 100% for E. coli. In the second case study, involving an effluent with a higher pollutant level, the remediation performances of the three microalgae strains ranged from 93 to 96% for total nitrogen, from 62 to 74% for total phosphorous, from 96 to 97% for COD, and 100% for E. coli. At the end of the experimental trials, treated waters showed values of pollutants suitable for irrigation use, in accordance with environmental and national legislation, which established specific thresholds for irrigation purposes.

Municipal waste biomass could be valorized as an alternative feedstock to produce compounds beneficial for agricultural applications. The foliar spray application of biostimulants emerges as a promising and innovative technique due to its environmental safety and ability to enhance crop yields. In recent years, the exploitation of biopolymers obtained through alkaline hydrolysis of the solid anaerobic digestate from municipal biowastes has attracted researchers’ interest. The aim of this study is to investigate the effects on lettuce growth of a product obtained through alkaline hydrolysis from municipal biowaste, Biopolymers (BPs), and of a derivate subjected to a further oxidation process, Biopolymers Oxidate (BPs OX). The effects of the treatments at various concentrations were evaluated by monitoring plant growth and observing the trends in the activities of the main enzymes involved in the nitrogen metabolic pathway of lettuce. Results suggest that the best treatments in terms of fresh weight were achieved by using BPs at 10 mg/L and BPs OX at 100 mg/L, increasing yield by around 28% and 34%, respectively. The innovative aspect of this work was to make easier for farmers the biopolymers application by testing a foliar spray methodology for BPs and BPs OX, which has never been tested before in any crop.

This study investigates bio-acidification as a method to decrease the pH of animal manure in dairy farms through the application of coffee silverskin (i.e., a coffee torrefaction waste) and acetic acid. The aim was to focus on the preliminary analysis needed to assess the suitability of using this mitigation strategy. This analysis was carried out by developing a three-step methodology. The first step included the identification of the appropriate proportions of coffee silverskin and acetic acid at the laboratory scale; in the second step, the best treated proportions were analysed in field conditions to compare the statistical differences among the pH of the control and treated samples. In the third step, territorial evaluation was carried out to verify the availability of the coffee waste in the territory based on the use of a Geographic Information System (GIS). Based on the results, a reduction of 38% and 31% in pH was observed in samples treated with acetic acid and coffee silverskin at the laboratory scale and in field conditions, respectively. The territorial analysis showed that it is possible to valorise this agro-industrial waste while minimising environmental impacts due to transportation if the coffee industry is located within a 75 km distance.

The water sector is undergoing rapid transformation in response to population growth and climate change. The growing frequency and severity of droughts highlight the need for effective adaptation and mitigation strategies that are supported by sustainable technologies. In many arid and semi-arid regions, water scarcity has led to a surge in interest in reusing treated wastewater for agricultural irrigation. However, wastewater treatment plant effluents may introduce contaminants of emerging concern (CECs) into agroecosystems, posing potential risks to food safety and environmental health. The aim of this study was threefold: (i) to assess the suitability of using synthetic solution containing high concentration of three CECs (carbamazepine, sulphapyridine and caffeine) for irrigating lettuce seedlings grown in pots, and the potential contamination risk; (ii) to evaluate the CEC removal using innovative materials as alternative substrates in lab-scale vertical flow constructed wetland (lab-scale VF-CW) for urban wastewater treatment; and (iii) to verify the safety of using treated wastewater for irrigating lettuces. The results showed that carbamazepine and caffeine accumulated in lettuce tissues in a concentration-dependent manner, with no visible symptoms of morphological stress observed, while sulphapyridine was not detected. All of the tested substrates demonstrated a high level of contaminant removal from wastewater. Furthermore, treated wastewater could be successfully reused for irrigation with no evidence of contaminant accumulation in plant tissues, suggesting a reduced risk of contaminant transfer to the food chain under these experimental conditions. These findings provide new insights into the removal of CECs from wastewater and offer valuable options for the reuse of treated wastewater for irrigation in agriculture.

One of the main objectives of agriculture is to improve crop production and mitigate oxidative damage caused by climate change, such as water stress. This study evaluated the potential use of Klebsormidium sp. K39 extract (Kleb), obtained by phycoremediation treatment, to address these agricultural challenges. The experimental trials involved the application of Kleb at 1 mg C_org L⁻¹ through foliar spray, under standard or water stress (WS) conditions. The effect of Kleb was evaluated by monitoring growth parameters, antioxidant activities, and lipid peroxidation. The quality parameters of the tomato fruits were also evaluated. The results demonstrated that the application of Klebsormidium extract enhanced tomato growth while maintaining root development and canopy cover under water stress conditions. These findings suggest that Kleb may act as a biostimulant to improve crop resilience. Moreover, treated plants under water stress conditions had an increase in yield of around 35% with respect to untreated stressed plants. Although fruit quality parameters were not significantly affected, in Kleb-treated plants lycopene content increased only under standard conditions, while polyphenol content significantly increased under both WS and standard conditions. In treated plants, lipid peroxidation decreased by 58% in the leaves and 19% in the fruits under WS conditions, suggesting a significant reduction in oxidative damage. In the fruits, correlation analysis revealed positive relationships among key stress markers. These findings suggest that Kleb extract enhances resilience to water stress in tomato plants by modulating antioxidant responses and secondary metabolite production, making it an eco-friendly approach to sustainable crop management under climate-related stressors.