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Wastewater treatment plants (WWTPs) require an urgent transition from a linear to a circular economy operation/design concept with a consequent resource recovery and more sustainable waste management. Natural resources have to be preserved, and wastes have to become an opportunity for recovering resources and materials (water reuse, energy, sludge reuse). However, the transition toward a circular economy is a complex and long process due to the existence of technical, economic, social and regulatory barriers. These existing barriers are critical challenges for a modern and sustainable WWTP concept. The recovery of resources must be considered a strategic target from the earliest process-design phase. In this context, the European Union’s Horizon 2020 project “Achieving wider uptake of water-smart solutions—WIDER UPTAKE” aims to overcome the existing barriers (technological, regulatory, organizational, social and economic) toward the transition from a linear to a circular economy model for WWTPs. This study is aimed at increasing the awareness of the existing barriers to a circular economy and summarizes the key contributions of the WIDER UPTAKE project in terms of water reuse, sludge reuse and nutrient recovery.

The wastewater sector paradigm is shifting from wastewater treatment to resource recovery. In addition, concerns regarding sustainability during the operation have increased. In this sense, there is a need to break barriers (i.e., social, economic, technological, legal, etc.) for moving forward towards water resource recovery facilities and demonstration case studies can be very effective and insightful. This paper presents a new water resource recovery case study which is part of the Horizon 2020 EU Project “Achieving wider uptake of water-smart solutions—Wider Uptake”. The final aim is to demonstrate the importance of a resource recovery system based on the circular economy concept. The recovery facilities at Palermo University (Italy) are first presented. Afterwards, the resource recovery pilot plants are described. Preliminary results have underlined the great potential of the wastewater treatment plant in terms of resources recovery and the central role of the University in fostering the transition towards circular economy. The fermentation batch test highlighted a volatile fatty acids (VFAs) accumulation suitable for polyhydroxyalkanoates (PHAs) production. The results of static adsorption and desorption tests showed that the highest amount of adsorbed NH4+ was recorded for untreated and HCl-Na treated clinoptilolite.

Biochar is a promising material for phosphorus (P) removal from water, but its surface chemistry can limit adsorption efficiency. In this study, biochars produced at 440 °C and 880 °C from the same feedstock were functionalized post-pyrolysis using aqueous solutions of AlCl3, CaCl2, and FeCl3 at two concentrations (0.5 M and 2.0 M). The aim of this work was to assess how both pyrolysis temperature and post-pyrolysis activation with different metals affect the surface charge of biochar and its capacity to retain P from aqueous solution. The treated materials were characterized for pH, point of zero charge (pHpzc), and phosphorus retention from solution. Results showed that Al- and Fe-activation significantly reduced the biochar pH and shifted the pHpzc to more acidic values, enhancing electrostatic attraction toward phosphate species. Phosphorus adsorption was most effective for biochar obtained at 440 °C and treated with AlCl3 and FeCl3, achieving up to 10.2 mg P g−1. These findings highlight the importance of surface charge modulation in tuning biochar performance for phosphate removal from aqueous solution. Based on the obtained results, electrostatic attraction was the main mechanism by which activated biochar adsorbed P from aqueous solution.

The current exploitation of freshwater, as well as the significant increase in sewage sludge production from wastewater treatment plants (WWTPs), represent nowadays a critical issue for the implementation of sustainable development consistent with the circular economy concept. There is an urgent need to rethink the concept of WWTPs from the conventional approach consisting in pollutant removal plants to water resource recovery facilities (WRRFs). The aim of this paper is to provide an overview of the demonstration case studies at the Marineo and Corleone WRRFs in Sicily (IT), with the final aim showing the effectiveness of the resources recovery systems, as well as the importance of plant optimization to reduce greenhouse gas (GHG) emissions from WRRFs. This study is part of the H2020 European Project “Achieving wider uptake of water-smart solutions—Wider-Uptake”, which final aim is to demonstrate the water-smart solution feasibility in the wastewater sector. The main project goal is to overcome the existing barriers that hamper the transition to circularity through the implementation of a governance analysis tool. The preliminary actions in the two demonstration cases are first presented, while, subsequently, the water-smart solutions to be implemented are thoroughly described, highlighting their roles in the transition process. The achieved preliminary results underlined the significant potential of WRRF application, a great chance to demonstrate the feasibility of innovative solutions in the wastewater sector to overcome the existing social, administrative and technical barriers.

Resource recovery from wastewater is very important in view of a circular economy approach in the water field. Among the different technologies applied to realize circular economy, an attractive option is the use of nutrient-enriched media that can be utilized as slow-release fertilizers. Zeolites have been re-discovered for their key role in ammonium (NH4+) adsorption from treated wastewater. Although many studies have been carried out to assess the ability of zeolites to adsorb NH4+, only few papers concerning NH4+ desorption from zeolites are available in the literature. Therefore, this study investigated NH4+ desorption from mineralogically different zeolites, before (ZNS and ZNC) and after (ZSS and ZSC) their treatment with sodium chloride. The zeolites differed in mordenite content. The amount of the desorbed NH4+ varied from 78 to 84% of the total NH4+ adsorbed. In particular, the NaCl-treated materials showed the largest desorption (27.6 ± 0.2 mg L−1, and 27.9 ± 0.7 mg L−1, ZSS, and ZSC, respectively) as compared to the untreated zeolites (22.9 ± 0.3 mg L−1, and 24.2 ± 0.3 mg L−1, ZNS, and ZNC, respectively) because of the different affinity of the cations for the zeolite surface. A monomodal pseudo-first-order model best approximated the desorption kinetics, suggesting only one mechanism of NH4+ desorption from zeolites. Such a mechanism is based on the ion exchange between dissolved Na+ and adsorbed NH4+. The desorption kinetics also showed that NH4+ desorbed slower from the NaCl-treated zeolites than the untreated ones. This effect was explained by the different affinity of Na+ and NH4+ for the zeolite surfaces as due to the diverse sizes of the Na+ and NH4+ hydration spheres. By revealing the effect of zeolite mineralogy and surface treatments in the desorption of NH4+, this study can suggest new and effective synthetic strategies for the achievement of cheap new materials to be applied in environmental remediation within a circular economy perspective.

Cover crop management in vineyards under a semiarid Mediterranean environment needs strategies that enhance soil C and N status and microbial functioning without increasing disturbance. This study compared cover crops biomass incorporation (harrowing, HR; rotary tillage; RT) and non-incorporation (NI, residues left on the topsoil) into the soil in a 12-year Grecanico dorato vineyard. Traditional vineyard soil management (continuously tilled for weeds control) was also used as a control. Soil samples from 0 to 20 and 20 to 40 cm were analyzed for total organic carbon (TOC), total nitrogen (TN), microbial biomass carbon (MBC) and nitrogen (MBN), and enzyme activities. NI and HR raised TOC and TN in the topsoil versus TR, with NI frequently maintaining advantages at depth. NI also maximized MBC/MBN and reduced the metabolic quotient (qCO2), indicating improved microbial C-use efficiency; RT showed intermediate chemistry but depressed subsoil MBC and altered MBC/MBN. Enzyme profiles reflected contrasting mechanisms: RT boosted β-glucosidase in the topsoil, TR peaked for urease and arylsulfatase but alongside lower biomass and higher specific enzyme activities, while NI supported greater overall functioning via larger biomass and lower per-C enzyme demand. The calculated geometric mean enzyme (GMea) index emphasized transient TR flush versus steadier conservation functioning. Strong vertical stratification occurred for all indices, yet NI transmitted some benefits to 20–40 cm. We conclude that residue retention or moderate incorporation promotes larger, more efficient microbial population and more balanced nutrient cycling, whereas repeated rotary tillage risks subsoil inefficiencies. In semi-arid Mediterranean vineyards, low-disturbance cover-crop incorporation (HR) or, preferably, residue retention at the topsoil (NI) offer a simple, scalable route to sustain soil quality and long-term fertility.

The need for more sustainable agriculture less dependent on mineral fertilizers has intensified the interest in the reuse of agro-industrial by-products as alternative nutrient sources. This study investigates the agronomic potential of citrus sewage sludge (CSS), derived from citrus wastewater treatment, as a nitrogen (N) source for wheat cultivation. An experiment was conducted using two Mediterranean soils with contrasting physicochemical properties, comparing a non-fertilized control (CTR), inorganic N fertilization (NH4NO3) (CTR + N), and CSS; fertilizers were applied once at 30 mg of N per plant. Differences in soil organic carbon availability and C/N ratio, together with carbonate-related properties, influenced N dynamics in the soil–plant system. In the soil with higher oxidizable organic C and a more favorable C/N ratio (S1), CSS increased soil ammonium concentrations by about 70% compared with the control and by nearly 50% compared with the soil characterized by lower organic C availability (S2). In S2, the lower concentrations of both NH4+ and NO3− indicate reduced microbial mineralization and nitrification, consistent with its lower availability of readily degradable organic carbon. Moreover, wheat grown with CSS exhibited a total biomass about 40% higher than that of the CTR. The Mineral Fertilizer Replacement Value (MFRV) reached 73% in S1 and 46% in S2, confirming the potential of CSS as a sustainable N source, particularly in soils where organic C availability supports microbial activity and N transformations. Future strategies should focus on improving CSS use through specific soil management practices.

Weed control in urban and peri-urban orange orchards is challenging due to operational and legislative restrictions. Tillage, besides from negatively affecting soil fertility and microorganisms, is demanding for humans. On the other hand, herbicides are advised against due to the possibility to reach waterbodies from the soil surface. Therefore, in urban and peri-urban areas, instead of tillage and herbicides, mulching with black plastic geotextile fabric is often used. This study aimed at assessing the impact of long-term soil mulching with black plastic geotextile fabric on soil fertility, microbial community and yield of an orange orchard in comparison to conventional tillage. To this aim, four soil management systems were set up: rotary tillage for the last 15 years, mulching with black plastic geotextiles for the last 15 years, rotary tillage for 7 years followed by mulching for the last 8 years, mulching for 7 years followed by rotary tillage for the last 8 years. Soil samples were analyzed to determine the chemical and biochemical parameters related to soil fertility. In addition, the abundances of the main microbial groups were investigated. Mulching increased soil total organic C at least by 65%. The greater soil organic C in mulched soil in turn contributed to increase the cation exchange capacity (+62% on average) and microbial biomass C (+120% on average). Additionally, the microbial quotient exhibited higher values in mulched soils compared to tilled ones, suggesting a greater soil organic matter accessibility by soil microorganisms. Moreover, mulching favored fungi over bacteria, and Gram-positive bacteria over Gram-negative bacteria, thus contributing to the establishment of a microbial community more efficient in utilizing C sources. The latter result was confirmed by the lower values of the metabolic quotient in mulched soil compared to tilled one. Overall, the black plastic geotextile fabric improved chemical and biochemical soil fertility that, in turn, lead to a higher orange yield in mulched soil.

Sewage sludge, a by-product of wastewater treatment, is a potential source of energy and resources. Its use as raw material presents a promising perspective for waste management within the framework of the circular economy. Despite its potential benefits, the excessive production of sewage sludge poses environmental and socio-economic challenges, including the threat of contamination by heavy metals, pathogens and organic micro pollutants. Moreover, the European Directive regulating the agricultural use of sewage sludge (86/278/EEC) does not fully reflect current scientific knowledge and technological advancements, particularly in regard to emerging contaminants and harmonised reuse strategies among Member States. To date, only a limited number of comprehensive reviews have addressed the main impacts of sewage sludge in the agricultural sector, particularly regarding its effects on soil physical, chemical, and biological properties. Sewage sludge valorisation, including soil fertilization, plays a pivotal role in improving soil quality and long-term productivity. This is proven by an increased supply of organic matter and nutrients, as well as improvements in soil ecosystem health that promote crop growth. In addition, the application of sewage sludge makes the soil structure more stable and less vulnerable to erosion. Furthermore, through different physical-chemical processes, it is possible to recover materials and energy to be used as end products or included in other production processes, thus contributing to promote a sustainable and circular economy approach. The implications of this review point to the need for suitable EU laws and regulations, greater social acceptance, and continued research to exploit the full potential of sewage sludge for agricultural purposes.

Rapid population and economic growth have increased the demand for depleting resources. Nitrogen (N) and phosphorus (P) are mineral elements that perform important functions in plants, but their extraction is not sustainable. In addition, these elements contribute significantly to the eutrophication of water bodies. The recovery of these nutrients from wastewater by adsorption techniques offers a promising solution. Previous studies have demonstrated the adsorption capabilities of materials such as zeolite for ammonium (NH4+) and biochar for P. In addition, these materials can serve as a source of N and P for plants in a circular economy context. In this regard, this study aims to evaluate the recovery of N and P by the adsorption capacities of zeolite and biochar through a column test with treated wastewater. Two columns positioned in series, one filled with 2.7 kg of zeolite and the other with 397 g of biochar, were placed at the outlet of the full-scale sewage treatment plant of Marineo (Italy). The zeolite adsorbed 3.6 g of NH4+ accumulated during the test with a rate of adsorption of 44% and adsorption of 1.33 mg g−1 of NH4+. The biochar adsorbed about 11 g of P accumulated during the test, with an adsorption percentage of 13% and an adsorption of 26.75 mg g−1 of P. Despite some problems related to the effluent used during the test, the tested materials showed good adsorption properties.