Microplastics (MPs) pollution has emerged as a major environmental concern due to its widespread presence in aquatic ecosystems. During intense rainfall, combined sewer overflow (CSO), which partially bypasses the wastewater treatment plant (WWTP), could be an important pathway for MPs to enter aquatic ecosystems. Therefore, this study aimed to evaluate the potential impact of CSO on the release of MPs into the environment during extreme rainfall. To address this, monitoring was conducted for more than 1 year at a municipal WWTP in southern Italy. The concentration of MPs in CSO samples was almost twice as high as that of raw wastewater samples collected during dry weather, and 65% higher than the concentration measured during rainy events without CSO activation. Additionally, CSO discharges contained, on average, a concentration of MPs 12 times higher than the treated effluent of the WWTP. This study highlighted that CSO discharges play a major role in increasing the release of MPs into the environment, especially as extreme weather events driven by climate change are expected to increase the frequency of CSO events.

Excess sludge management represents a major challenge in wastewater treatment, requiring effective minimization strategies. Anaerobic side-stream reactors (ASSR) have emerged as a promising approach to reduce sludge production. The effect of anaerobic reactor hydrodynamics on sludge minimization was evaluated under identical anaerobic exposure time (AET). A laboratory-scale activated sludge system was coupled with a compartmentalized ASSR composed of six reactors in series, approximating plug-flow behavior, and compared with a fully mixed configuration. The compartmentalized system achieved significantly lower sludge yield (Yobs), approximately two times lower than the single-compartment reactor at the same AET. This was attributed to the formation of redox gradients, which enhanced microbial stress, endogenous decay, and biomass breakdown. EPS destructuration was observed, with decreasing EPS and increasing SMP, confirming the role of hydrolysis in sludge reduction. Despite a slight decrease in sludge settleability, COD removal remained high. Reactor hydrodynamics was found to play a key role, and compartmentalization proved to be an effective and cost-efficient strategy for sludge minimization.

This study investigated the influence of organic loading rate (OLR) and feeding regime on the production of alginate-like exopolysaccharides (ALEs) from aerobic granular sludge treating citrus-processing wastewater. Two granular sequencing batch reactors were operated under anaerobic and aerobic feast conditions at increasing OLRs (1-2.5 kgCOD/m³d). Results showed that OLR and feeding regime significantly affected extracellular polymeric substances (EPS) yield and composition. Anaerobic feast conditions promoted higher structural EPS fractions, while aerobic conditions favored higher total EPS production. The results highlight the potential of aerobic granular sludge systems for the simultaneous treatment of agro-industrial wastewater and recovery of valuable biopolymers.

This study investigated a compartmentalized anaerobic side-stream reactor (ASSR) as an innovative configuration for excess sludge minimization in activated sludge systems. The core idea behind the study was to better understand the mechanisms driving excess sludge reduction under compartmentalization conditions with the final aim of proposing optimal operating conditions. Four ASSR layouts with increasing compartmentalization (1, 2, 3, and 6 compartments) were operated at anaerobic exposure times (AETs) of 8 and 10 h per day. Increasing the number of compartments, without increasing the overall volume of the anaerobic reactor, significantly reduced the observed sludge yield (Yobs). The 6-compartment configuration achieved a Yobs reduction of about 60% at an AET of 8 h/d and up to 70% at 10 h/d compared with the conventional system. This corresponded to an additional reduction of about 45–50% compared with the single-compartment ASSR configuration. Compartmentalization intensified sludge decay processes along the ASSR. This effect was mainly due to the considerable decrease in extracellular polymeric substances (EPS) and to the threefold increase in the endogenous decay coefficient (bₕ). More reducing conditions developed along the reactor, with oxidation–reduction potential (ORP) values decreasing to −313 mV. Despite the high sludge reduction, the COD removal efficiency remained above 93%, and sludge settling properties were not significantly affected by compartmentalization.The results demonstrated that ASSR compartmentalization intensified sludge minimization mechanisms by promoting stronger anaerobic stress and biomass decay, while maintaining stable treatment performance. This approach represents a promising strategy to reduce excess sludge production in WWTPs, while reducing AET and consequently the size of the anaerobic reactor.

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.

Produced water is a salty and organic-polluted byproduct drilled out during the oil extraction. An annual production of 11.2 million of m3 is estimated, based on the production of at least 3 barrels of PW for every barrel of oil extracted. The composition changes according to the well characteristics and to the addition of chemicals during the extraction process. Generally, a high salinity (up to 300 g L-1), due to the contact with subsurface rocks, is observed together with a large amount of dissolved hydrocarbon compound contaminants. Thus, their treatment could be designed on case-by-case basis through tailored studies and experimental investigations as it is not possible to standardize the characteristics of PW. In this work, an innovative PWs treatment scheme is proposed where an Assisted Reverse Electrodialysis (ARED) unit and a Sequencing Batch Moving Bed Biofilm Reactor (SBMBBR) were coupled: ARED is used to reduce the PWs salinity to values lower than 20 g L-1 of NaCl to let the stream compatible with the downstream biologic treatment, where the organic content is reduced ensuring compliance with legislative disposal limits. The desalination was performed under current-control mode and the ARED required less than 2 kWh·m-3PW for reaching the target outlet concentration. The organic content in SBMBBR successfully decreased, showing a Total Organic Carbon (TOC) abatement of about 80%.

Drinking water treatment plants (DWTPs) are facing emerging challenges affecting raw water quality. In addition, the new regulatory framework (EU 2184/2020) sets stricter limits for turbidity and percentile statistics for continuous compliance, demanding greater robustness of the treatment processes. To achieve this aim, this study proposes a turbidity robustness index (TRI), named TRI95B, to be used as a warning tool for detecting deviations from water quality standards. TRI95B has been compared with the TRIs existing in the literature. Furthermore, the TRI95B validation has been performed by a three-year monitoring dataset of a full-scale DWTP. The proposed TRI95B index has two key novelties compared to the existing indices required for adapting to the new drinking water regulation: i. introduces the 95th percentile as a statistical indicator; ii. considers an additional term that sets an alert when a threshold value is exceeded. The comparison results suggest a better correspondence to the real plant performances of TRI95B than the other TRIs. Indeed, both the sensitivity and specificity of TRI95B were significantly higher than the other TRIs, indicating a better capacity to correctly classify both positive and negative cases. Moreover, while the previous TRIs identify a critical operating condition when the turbidity goal was significantly exceeded, TRI95B highlights a failure condition at a lower discrepancy. Therefore, TRI95B is also able to identify short-duration and low magnitude failures, thus coping with the purpose of the new regulation for drinking water.

Nowadays, natural extreme events are increasing in frequency and intensity due to climate change. These phenomena negatively impact critical infrastructures, such as wastewater treatment plants (WWTPs), resulting in a loss of their functionality and the generation of hazards to the environment and the population. It is important to identify and assess the hazards to which critical infrastructure is subject due to natural extreme events. Based on these considerations, through this work, the vulnerability of the territory and WWTPs of Sicily (south of Italy) was evaluated by using Geographic Information Systems (GIS) tools and a multihazards approach. The multi-hazard approach allowed determining the combined effect on the territory of different hazards (seismic, hydraulic, geomorphological and coastal), starting from the assessment of individual hazards and considering their possible simultaneous action. To this end, a multi-hazard spatial index was defined and subsequently mapped. This index represents an essential starting point for later defining a regional multi-hazard index map. Additionally, potential environmental and health consequences related to the occurrence of extreme events near wastewater treatment plants were assessed. This approach provides valuable guidance to local and regional authorities, aiding in the prevention, management, and mitigation of multi-hazard natural disasters.

The “ALERT” method, allows to handle management criticalities related to high-turbidity levels in a drinking water treatment plant, is a stepwise procedure based on the 95th percentile-based turbidity robustness index (TRI95B). The method is novel compared to existing literature and identifies the existing relationship between the TRI and the raw water turbidity, allowing setting up critical operational thresholds above which a noticeable plant robustness loss takes place. Additionally, ALERT also proposes two novel turbidity load indicators, the turbidity load index (TLI) and the turbidity event (TE). TLI provides a qualitative indication of the quantity of suspended solids potentially associated with the raw water, whereas TE indicates the occurrence of extreme and short-duration turbidity peaks in the influent. ALERT method has been applied to a full-scale drinking water treatment plant. Results showed that ALERT identified TLI thresholds above which the case study DWTP exhibited a lower robustness and frequent deviations from the regulation turbidity goal. Specifically, sand filter FS3 shows a notable weakness when the TLI exceeds 50, whereas sand filters FS1 and FS2 consistently fall within the very stable class across all TLI levels. Therefore, during events where the TLI surpasses 50, it is advisable to reduce the inlet flow rate to FS3 until its specific TLI drops below 50.The proposed method allows the early detection of plants performance degradation. Indeed, it can support operators at implementing corrective measures to maintain high water quality safeguarding the integrity of the treatment processes under high turbidity load.

The present study has focused on the mainstream integration of polyhydroxyalkanoate (PHA) production with industrial wastewater treatment by exploiting three different technologies all operating in sequencing batch reactors (SBR): conventional activated sludge (AS-SBR), membrane bioreactor (AS-MBR) and aerobic granular sludge (AGS). A full aerobic feast/famine strategy was adopted to obtain enrichment of biomass with PHA-storing bacteria. All the systems were operated at different organic loading (OLR) rate equal to 1-2-3 kgCOD/m3∙d in three respective experimental periods. The AS-MBR showed the better and stable carbon removal performance, whereas the effluent quality of the AS-SBR and AGS deteriorated at high OLR. Biomass enrichment with PHA-storing bacteria was successfully obtained in all the systems. The AS-MBR improved the PHA productivity with increasing OLR (max 35% w/w), whereas the AS-SBR reduced the PHA content (max 20% w/w) above an OLR threshold of 2 kgCOD/m3∙d. In contrast, in the AGS the increase of OLR resulted in a significant decrease in PHA productivity (max 14% w/w) and a concomitant increase of extracellular polymers (EPS) production (max 75% w/w). Results demonstrated that organic carbon was mainly driven towards the intracellular storage pathway in the AS-SBR (max yield 51%) and MBR (max yield 61%), whereas additional stressors in AGS (e.g., hydraulic selection pressure, shear forces) induced bacteria to channel the COD into extracellular storage compounds (max yield 50%) necessary to maintain the granule's structure. The results of the present study indicated that full-aerobic feast/famine strategy was more suitable for flocculent sludge-based technologies, although biofilm-like systems could open new scenarios for other biopolymers recovery (e.g., EPS). Moreover, the AS-MBR resulted the most suitable technology for the integration of PHA production in a mainstream industrial wastewater treatment plant, considering the greater process stability and the potential reclamation of the treated wastewater.

Surface water quality is declining due to climate change, leading to increased concentrations of soluble metals, natural organic matter (NOM), turbidity, and algal blooms. These changes pose challenges to traditional water treatment plants, needing innovative approaches. Ferrate(VI) was explored as a potential solution to address climate change-related water emergencies, either alone or in combination with conventional reagents to producing potable water. Results show that Fe(VI) was more effective to remove soluble manganese compared to a conventional oxidant (permanganate), while requiring a lower stoichiometric dosage (<2mol of Fe(VI) per 3mol of Mn(II)) as the reaction byproducts of Fe(VI) reduction (eg., Fe3+) contributed to manganese removal. A slightly alkaline environment (pH >8.5) was crucial to maximize manganese oxidation since pH closer to neutral caused the reduction of Fe(VI) to Fe(III), thus decreasing its reduction potential. Fe(VI) was also effective toward NOM although its activation was necessary to provide noticeable effects. In combination to conventional coagulant/flocculant agents, Fe(VI) was able to provide noticeable increases in removal of turbidity (<0.30 NTU) while involving a simultaneous decrease in other chemicals requirement (>50%). Besides, Fe(VI) was also capable of providing algae removal of approximately 80% higher than conventional oxidizing agent, through simultaneous oxidation and flocculation.This study demonstrated that employing Fe(VI) as a treatment method for drinking water is very promising as it can serve as an alternative or complement conventional approaches in tackling the challenges presented by climate change and sustaining high-quality standard potable water.

Minimization of excess sludge produced by wastewater treatment plants has become a topical theme nowadays. One of the most used approaches to achieve this aim is the anaerobic side-stream reactor (ASSR) process. This is considered affected by the hydraulic retention time (HRT) of the anaerobic reactor, the anaerobic sludge loading rate (ASLR) and the sludge interchange ratio (SIR), although, studies available in the literature did not reflect a clear relationship with the sludge minimization yields. To overcome this, a novel parameter namely anaerobic exposure time (AET) was defined and related to reduction of the observed yield coefficient (Yobs) in a lab-scale plant implementing the ASSR process. Furthermore, the AET was validated by performing a detailed and thorough review of previous literature. Excess sludge production was successfully reduced (10-60 %) with the increase of the AET (7.9-13 h/d), although maintaining the same HRT in the ASSR and a constant sludge interchange ratio (SIR) (100 %). A strong correlation (Pearson = 0.763) was found between the AET, and the Yobs reduction reported in previous studies, also indicating a linear relationship (R-2 = 0.92) between these parameters. Contrarily, the correlation between the Yobs with the ASLR and the ASSR-HRT resulted moderate (Pearson = 0.186) or weak (Pearson=-0.346), respectively. Overall, while operating at low AET (< 6 h), maintenance and uncoupling metabolism were found the main sludge reduction mechanisms. Increasing the AET (>8 h) favoured the occurrence of extracellular polymeric substances (EPS) hydrolysis and endogenous decay mechanisms, which improved excess sludge reduction. To conclude, the AET could be considered a reliable parameter to be used for design or control purposes for the ASSR-based process.

In the present research, insights about the mechanisms of excess sludge minimization occurring in an oxic-settling-anaerobic (OSA) were provided. The investigation involved two systems operating in parallel. In particular, a conventional activated sludge (CAS) system as control and a system implementing the OSA process both having a pre-denitrification scheme were considered. Five periods (P1-P5) were studied, during which several operating conditions and configurations were tested. Specifically, the hydraulic retention time (HRT) in the anaerobic reactor of the OSA system (P1 8 h, P2-P3 12 h, P4 8 h, P5 12 h) and the return sludge from the anaerobic to the anoxic (scheme A) (P1-P2) or aerobic (scheme B) mainstream reactors (P3-P5) were investigated. The results highlighted that the excess sludge production in the OSA was lower in all the configurations (12-41%). In more detail, the observed yield (Yobs) was reduced from 0.50-0.89 gTSS gCOD-1 (control) to 0.22 -0.34 gTSS gCOD-1 in the OSA process. The highest excess sludge reduction (40%) was achieved when the OSA was operated according to scheme B and HRT of 12 h in the anaerobic reactor (P3). Generally, scheme A enabled the establishment of cell lysis and extracellular polymeric substances (EPS) destructuration, leading to a worsening of process performances when high anaerobic HRT (>8 h) was imposed. In contrast, scheme B enabled the establishment of maintenance metabolism in addition to the uncoupling metabolism, while cell lysis and EPS destruction were minimized. This allowed obtaining higher sludge reduction yield without compromising the effluent quality.

The present study evaluated different sludge-reduction mechanisms in the oxic-settling-anaerobic (OSA) process in terms of their effects on methane productivity by anaerobic digestion of sewage sludge. Two different layouts were investigated for the sludge return from an anaerobic side-stream reactor (ASSR) to the anoxic (scheme A) or the aerobic (scheme B) reactor of a pre-denitrification plant. Biochemical methane-potential (BMP) assays performed on the excess sludge revealed that scheme A promoted an overall increase of methane production in the OSA (20 mLCH4 gVSS−1d−1, +19%), although compared with a control CAS plant a significant decrease in the excess sludge production (31%) was obtained. Operating conditions in scheme A caused the occurrence of cell lysis and EPS hydrolysis, thereby increasing the biodegradability of sludge. In contrast, scheme B favoured the occurrence of uncoupling and a maintenance metabolism that did not involve sludge hydrolysis. Consequently, despite a higher reduction of excess sludge (82%), a significant decrease in methane productivity in the OSA (4 mLCH4 gVSS−1d−1, −41%) was observed. Based on the results, implementing the OSA process may allow high levels of methane production by anaerobic digestion to be maintained if specific sludge-reduction mechanisms are triggered in the waterline, also raising the possibility of co-digestion with other feedstocks.

The recovery of biopolymers from sewage sludge could be a crucial step in implementing circular economy principles in wastewater treatment plants (WWTP). In this frame, the present study was aimed at evaluating the simultaneous production of polyhydroxyalkanoates (PHA) and extracellular polymeric substances (EPS) obtainable from the treatment of agro-industrial wastewater. Two biological enrichment systems, aerobic granular sludge (AGS) and a conventional activated sludge operating as a sequencing batch reactor (SBR), were monitored for 204 and 186 days, respectively. The maximum biopolymers accumulation capacity was close to 0.60 mgPHA-EPS gVSS−1 in the AGS when operating at 3 kgCODm−3d−1, whereas in the SBR, it was about half (0.35 mgPHA-EPS gVSS−1). Biopolymers extracted from the AGS were mainly constituted by EPS (>70%), whose percentage increased up to 95% with the OLR applied in the enrichment reactor. In contrast, SBR enabled obtaining a higher PHA production (50% of the biopolymers). Results suggested that organic carbon was mainly channeled toward metabolic pathways for extracellular storing in AGS, likely due to metabolic stressors (e.g., hydraulic selection pressure, shear forces) applied for promoting aerobic granulation.

The present study investigated the combined production of reclaimed water for reuse purposes and polyhydroxyalkanoates (PHA) from an agro-food industrial wastewater. A pilot plant implementing a two-stage process for PHA production was studied. It consisted of a mainstream sequencing batch membrane bioreactor (SBMBR) in which selection of PHA-accumulating organisms and wastewater treatment were carried out in, and a side-stream fed-batch reactor (FBR) where the excess sludge from the SBMBR was used for PHA accumulation. The performance of the SBMBR was compared with that of a conventional sequencing batch reactor (SBR) treating the same wastewater under different food to microorganisms' ratios (F/M) ranging between 0.125 and 0.650 kgCOD kgTSS-3 d-1. The SBMBR enabled to obtain very high-quality effluent in compliance with the relevant national (Italy) and European regulations (Italian DM 185/03 and EU, 2020/741) in the field of wastewater reclamation, whereas the performances in the SBR collapsed at F/M higher than 0.50 kgCOD kgTSS-1d-1.A maximum intracellular storage of 45% (w/w) and a production yield of 0.63 gPHA L - 1h- 1were achieved when the SBMBR system was operated with a F/M ratio close to 0.50 kgCOD kgTSS-1d-1. This resulted approximately 35% higher than those observed in the SBR, since the ultrafiltration membrane avoided the washout of dispersed and filamentous bacteria capable of storing PHA. Furthermore, while maximizing PHA productivity in conventional SBR systems led to process dysfunctions, in the SBMBR system it helped mitigate these issues by reducing membrane fouling behaviour. The results of this study supported the possibility to achieve combined recovery of reclaimed water and high-value added bioproducts using membrane technology, leading the way for agro-food industrial wastewater valorization in the frame of a circular economy model.

This work reports the results obtained with an innovative configuration of a closed-static solar greenhouse for sludge drying. The novelty of the solar greenhouse configuration consisted in using a forced ventilation system to provide hot air for sludge drying and the utilization of solar irradiation for energy supply. Wet sewage sludge (97% humidity) was successfully dried up to a residual humidity close to 5% after 25 days during wintertime. The increase of the airflow rate supplied under the sludge bed improved the sludge drying rate. Moreover, the fraction of volatile suspended solids decreased from 70% to 41% after 13 days, indicating that air supply promoted the simultaneous stabilization of the sludge as a side-effect to the drying process. Overall, the specific energy consumption per ton of evaporated water was estimated to approximately 450 kWh/t, resulting in about 55% of energy demand lower than a conventional thermal drying system, while using only free solar energy. The achieved high weight reduction of up to 99% implies a noticeable reduction of the excess sludge handling costs, indicating that solar greenhouse drying is a highly interesting opportunity for sludge drying in medium-small sized WWTPs.

In this study, the presence of microplastics in the sludge of three wastewater treatment plants (WWTPs) was examined. The investigated WWTPs operated based on a conventional activated sludge (CAS) process, with (W1) or without (W2) primary clarification, and a membrane bioreactor process (MBR) (W3). The microplastics (MPs) concentration in the samples of W3 was approximately 81.1 ± 4.2 × 103 particles/kg dry sludge, whereas MPs concentrations in W1 and W2 were 46.0 ± 14.8 × 103 particles/kg dry sludge and 36.0 ± 5.2 × 103 particles/kg dry sludge, respectively. Moreover, MPs mainly consisted of fragments (66–68%) in the CAS plants, whereas the fractions of MPs shapes in the MBR sludge were more evenly distributed, although fiber (47%) was the most abundant fraction. Furthermore, samples from the MBR showed a greater diversity in MPs composition. Indeed, all the main polyesters (i.e., textile fibers and polyethylene terephthalate), polyolefins (i.e., polyethylene and polypropylene) and rubber (i.e., polybutadiene) were observed, whereas only polybutadiene, cellulose acetate and polyester were detected in the CAS plants. These findings confirmed that MPs from wastewater are transferred and concentrated in the waste sludge. This is a critical finding since sludge disposal could become a new pathway for microplastic release into the environment and because MPs might affect the fouling behavior of the membrane.

The production of polyhydroxyalkanoates (PHA) using industrial wastewaters as feedstocks is a current and challenging topic. This study investigated the production of biopolymers by a mixed microbial culture under different OLRs equal to 1 kgCOD m(-3)d(-1) (Period 1), 2 kgCOD m(-3)d(-1) (Period 2) and 3 kgCOD m(-3)d(-1) (Period 3). The maximum PHA content was achieved in Period 2 (0.38 gPHA gTSS(-1)), whereas lower values were obtained in Period 1 (0.13 gPHA gTSS(-1)) and Period 3 (0.26 gPHA gTSS(-1)). Overall, the maximum PHA productivity resulted equal to 0.08 gPHA L-1 h(-1) (P2), 0.05 gPHA L-1 h(-1) (P1) and 0.04 gPHA L-1 h(-1) (P3), respectively. The molecular weight of the PHA increased from Period 1 (250 kDa) to Period 2 (417 KDa) and Period 3 (463 KDa), although resulting in a slight decrease of crystallinity degree. Microbial community analysis revealed a reduction in bacterial diversity and a progressive shift of the microbial community with the increasing OLR. Alpha-diversity indexes based on Operational Taxonomic Units (OTUs) at 99% identity revealed higher species richness (Taxa (S) 280) and diversity (Shannon (H) 4,06) in Period 1, whereas Period 3 was characterized by reduced richness and diversity and higher dominance (Taxa (S) 133, Shannon (H) 2,40). Based on the results obtained, it was pointed out that the OLR variation determined significant effects on the process performances, as well as on the productivity and quality of the biopolymers. This means that OLR is a key control parameter to maximize the PHA production and control the physical-chemical characteristics of the polymers.