A Fenton-based approach at neutral and un-conditioned pH for recalcitrant COD removal in tannery wastewater: Experimental test and sludge characterization

The combination of raw animal skin manufacturing processes involves the use of large amounts of chemicals, resulting in the generation of complex and highly polluted tannery wastewater. In this context, the high concentration of chloride in tannery wastewater represents a crucial bottleneck. Indeed, sodium chloride, commonly used in tannery industry to prevent skin rot, increases the concentration of chlorides up to 50 %. At the same time, most of the advanced oxidation processes usually employed in tannery wastewater treatment to remove recalcitrant COD involve the use of conditioning agents, thus increasing the overall concentration of chlorides in the treated effluent. The aim of this study was to evaluate the electrochemical peroxidation process (ECP) efficiency in the treatment of tannery wastewater without changing pH, to improve Fenton technology by avoiding the use of chemicals. The influence of different electric currents on COD and color removal was investigated. The characterization of the produced sludge was conducted through FTIR, SEM and XRD analysis, exploring the morphology and composition of precipitate, depending on the applied current. Although an electrical current of 750 mA yields the highest COD and color removal efficiency (69.7 % and 97.8 %, respectively), 500 mA can be considered the best compromise because of energy consumptions. Iron oxides and hydroxides were generated during the ECP process, playing the role of coagulants through the absorption of organic and inorganic contaminants. The consumption of energy increased as a function of time and applied current; however, cost analysis showed that the electrodes contributed the most to the total cost of the process. In authors' knowledge, the application of ECP process as a tertiary treatment for the removal of recalcitrant COD in tannery wastewater represents a novelty in the literature and the results obtained can be considered as the basis for scaling up the process in future research.

Comparing the effects of Al-based coagulants in waste activated sludge anaerobic digestion: Methane yield, kinetics and sludge implications

Due to its effectiveness and ease of application, the process of flocculation and coagulation is often used for pollution removal in wastewater treatment. Most of these coagulants precipitate and accumulate in waste activated sludge (WAS), and could negatively affect sludge treatments, as observed for anaerobic digestion. Nowadays, wastewater treatment plants (WWTPs) are widely discussed because of the current paradigm shift from linear to circular economy, and the treatments performed at the facility should be planned to avoid or reduce adverse effects on other processes. The aim of this study was to compare the impact of poly aluminum chloride (PAC) and aluminum sulfate (AS) on WAS anaerobic digestion, by feeding replicate serum reactors with different levels of coagulant (5, 10 and 20 mg Al/g TS). Reactors without the addition of any coagulants represented the control group. Results revealed that Al-based coagulants inhibited methane production, which decreased as the coagulant addition increased. The inhibition was much more severe in AS-conditioned reactors, showing average reductions in methane yield from 14.4 to 31.7%, compared to the control (167.76 ± 1.88 mL CH4/g VS). Analytical analysis, FTIR and SEM investigations revealed that the addition of coagulants affected the initial conditions of the anaerobic reactors, penalizing the solubilization, hydrolysis and acidogenesis phases. Furthermore, the massive formation of H2S in AS-conditioned reactors played a key role in the suppression of methane phase. On the other hand, the use of coagulant can promote the accumulation and recovery of nutrient in WAS, especially in terms of phosphorus. Our findings will expand research knowledge in this field and guide stakeholders in the choice of coagulants at full scale plant. Future research should focus on reducing the effect of coagulants on methane production by modifying or testing new types of flocculants.

Removal of heavy metals from dredging marine sediments via electrokinetic hexagonal system: A pilot study in Italy

Among the several treatment options, electrokinetic (EK) remediation is recognized as an effective technique for the removal of heavy metals from low-permeability porous matrices. However, most of the EK decontamination research reported was performed on linear configuration systems at a laboratory scale. In this study, a series of experiments were performed on a pilot-scale system where the electrodes were arranged in a hexagonal configuration, to assess the improvement of the EK process in the removal of inorganic contaminants from sediments dredged in the harbor of Piombino, Italy. HNO3 was used as acid conditioning and both pH effect and treatment duration time were investigated. Sediment characterization and metal fractionation were also presented, in order to understand how the bioavailability of metals affects the process efficiency. The increase in pH due to the buffering capacity of the sediment in the sections close to the cathode favored the precipitation and accumulation of metals. However, the results highlighted that longer treatment times, combined with an efficient pH reduction, can improve treatment performance, resulting in high removal efficiencies for all the target metals considered (a percentage removal greater than 50% was reached for Cd, Ni, Pb, Cu and Zn). Compared to different EK configuration systems, the hexagonal configuration arrangement applied in our study provides better results for the remediation of dredged marine sediment.

Centralization of wastewater treatment in a tourist area: A comparative LCA considering the impact of seasonal changes

Nowadays, environmental protection has become a topic of primary importance, and the interest in wastewater treatment plants (WWTPs) has increased due to the need for a paradigm shift from linear to circular economy. The centralization level of wastewater infrastructure is the basis for a successful system. The aim of this study was to investigate the environmental impacts generated from the centralized treatment of wastewater in a tourist area in central Italy. The combined use of BioWin 6.2 simulation software and life cycle assessment (LCA) methodology was implemented to evaluate the potential connection of a small decentralized WWTP to a medium-size centralized facility. Two different scenarios (decentralized system, corresponding to the current situation, and centralized) were evaluated in two separate periods: high season (HS), corresponding to the main tourist season, and low season (LS), which is the period before the main tourist season. Two sensitivity analyses were conducted, assuming different N2O emission factors, and considering the period at the end of tourist season, respectively. Although with modest advantages (up to -6 % in pollutant emissions), WWTP connection was the best management option in 10 out of 11 indicators in HS, and 6 out of 11 categories in LS. The study showed that wastewater centralization was promoted by scale factors in HS, as the most impactful consumptions decreased as the degree of centralization increased; on the other hand, the decentralized system was less penalized in LS, as small WWTP was less stressed and energy consuming in this period. Sensitivity analysis confirmed the results obtained. Site-specific conditions can lead to conflicting circumstances, as key parameters may have different behaviors depending on seasonal variations, and the degree of centralization in tourist areas should be addressed by distinguishing separate periods, based on changes in tourist flows and pollution loads.

A comparative LCA of three WWTPs in a tourist area: Effects of seasonal loading rate variations

Although the wastewater treatment is a fundamental utility for the protection of human health and the environment, non-evident drawbacks are associated with it. Wastewater treatment plants (WWTPs) located in tourist areas generally suffer from the seasonal increase in wastewater flow-rate and associated pollution loads. In this study, a Life Cycle Assessment (LCA) of three medium-size urban WWTPs, located in a tourist area in central Italy, was carried out. The study compared the environmental impacts generated by 1 m³ of treated wastewater in low season (LS) and high season (HS). All the material and energy flows involved in the operational phase of wastewater treatment were considered within the system boundaries, including the disposal or recovery treatment of the waste streams generated by the WWTPs, namely screenings, waste from grit removal and wastewater sludge. The analysis was conducted using almost only real data from full-scale plants. In each WWTP, the environmental impacts produced in HS were higher than those generated in LS; therefore, the environmental impacts increased as the loading rate increased. Furthermore, a correlation between WWTP size and environmental performance was observed. Indeed, in 8 out of 11 environmental indicators, the percentage increase in pollutant emissions due to wastewater treatment in HS decreased as the WWTP size increased. Results revealed that larger WWTPs suffered less from seasonal loading rate variations, showing greater flexibility. The existence of a scale factor suggests that the centralization of WWTPs in tourist areas can be an option to enable better treatment performance in terms of environmental impacts. A sensitivity analysis was performed, increasing N₂O emission factors from wastewater treatment in LS: considering a 75 % increase, the outcomes found in default LCA were not confirmed. Future research should investigate the operational factors and biological mechanisms that most affect wastewater treatment when significant seasonal variations are present.

Methane oxidation of residual landfill gas in a full-scale biofilter: human health risk assessment of volatile and malodours compound emissions

A human health risk assessment was performed to evaluate if a biofilter for the biological methane oxidation reduces the risk from exposure to landfill gas emissions and improves the air quality mitigating odour emissions from an aftercare landfill. Accordingly, three different scenarios of landfill gas management were defined, 9 volatile organic compounds (VOCs) (cyclohexane, n-hexane, 2-methylpentane, 3-methylpentane, benzene, xylenes, toluene, dichlorodifluoromethane, vinyl chloride) were identified and using the CALPUFF dispersion model; the pollutant concentration at eleven sensitive receptors was determined. Consequently, the risk (for cancer and non-cancer compounds) was assessed applying the methodology proposed by USEPA 2009. From one hand, to determine concentration and emission rates of VOCs and hydrogen sulphide, a sample of raw landfill gas and three air samples from the biofilter surface were collected with dynamic flux chamber method and analysed in accordance with US EPA, 1986 and USEPA TO-15, 1999. To the other hand, odour emissions were assessed based both on chemical and dynamic olfactometric measurements (EN 13725:2003). The field surveys results showed a reduction of the cancer risk on average by 79% and of the hazard quotient on average by 92%. In contrast, the results of olfactometry measurements showed a lower efficiency on odour reduction than the target value of 70%. Nonetheless, the odour concentration was always far below 300 uo_E m^-3 at the biofilter surface and odour concentration never exceed 1 uo_E m^-3 at the sensitive receptors.

Modeling of electrokinetic remediation combining local chemical equilibrium and chemical reaction kinetics

A mathematical model for reactive-transport processes in porous media is presented. The modeled system includes diffusion, electromigration and electroosmosis as the most relevant transport mechanisms and water electrolysis at the electrodes, aqueous species complexation, precipitation and dissolution as the chemical reactions taken place during the treatment time. The model is based on the local chemical equilibrium for most of the reversible chemical reactions occurring in the process. As a novel enhancement of previous models, the local chemical equilibrium reactive-transport model is combined with the solution of the transient equations for the kinetics of those chemical reactions that have representative rates in the same order than the transport mechanisms. The model is validated by comparison of simulation and experimental results for an acid-enhanced electrokinetic treatment of a real Pb-contaminated calcareous soil. The kinetics of the main pH buffering process, the calcite dissolution, was defined by a simplified empirical kinetic law. Results show that the evaluation of kinetic rate entails a significant improvement of the model prediction capability.

Influence of the pH control strategy and reactor volume on batch fermentative hydrogen production from the organic fraction of municipal solid waste

Three different experimental sets of runs involving batch fermentation assays were performed to evaluate the influence of the experimental conditions on biological hydrogen production from the source-separated organic fraction of municipal solid waste collected through a door-to-door system. The fermentation process was operated with and without automatic pH control, at a pH of 5.5 and 6.5, food-to-microorganism ratios of 1/3 and 1/1 (wet weight basis) and with different working volumes (0.5 and 3 L). The experimental results showed that the pH control strategy and the reactor volume did not affect the final hydrogen production yield but played an important role in determining the time evolution of the process. Indeed, although the different experimental conditions tested yielded comparable hydrogen productions (with maximum average values ranging from 68.5 to 88.5 NLH₂ (kgTVS_OF)^-1), the automatic pH control strategy improved the process from the kinetic viewpoint resulting in a t₉₅ reduction from an average of 34.9 h without automatic pH control to an average of 19.5 h.

Multispecies reactive transport modelling of electrokinetic remediation of harbour sediments

We implemented a numerical model to simulate transport of multiple species and geochemical reactions occurring during electrokinetic remediation of metal-contaminated porous media. The main phenomena described by the model were: (1) species transport by diffusion, electromigration and electroosmosis, (2) pH-dependent buffering of H⁺, (3) adsorption of metals onto particle surfaces, (4) aqueous speciation, (5) formation and dissolution of solid precipitates. The model was applied to simulate the electrokinetic extraction of heavy metals (Pb, Zn and Ni) from marine harbour sediments, characterized by a heterogeneous solid matrix, high buffering capacity and aged pollution. A good agreement was found between simulations of pH, electroosmotic flow and experimental results. The predicted residual metal concentrations in the sediment were also close to experimental profiles for all of the investigated metals. Some removal overestimation was observed in the regions close to the anode, possibly due to the significant metal content bound to residual fraction.

Ligand-enhanced electrokinetic remediation of metal-contaminated marine sediments with high acid buffering capacity

The suitability of electrokinetic remediation for removing heavy metals from dredged marine sediments with high acid buffering capacity was investigated. Laboratory-scale electrokinetic remediation experiments were carried out by applying two different voltage gradients to the sediment (0.5 and 0.8 V/cm) while circulating water or two different chelating agents at the electrode compartments. Tap water, 0.1 M citric acid and 0.1 M ethylenediaminetetraacetic acid (EDTA) solutions were used respectively. The investigated metals were Zn, Pb, V, Ni and Cu. In the unenhanced experiment, the acid front could not propagate due to the high acid buffering capacity of the sediments; the production of OH(-) ions at the cathode resulted in a high-pH environment causing the precipitation of CaCO3 and metal hydroxides. The use of citric acid prevented the formation of precipitates, but solubilisation and mobilisation of metal species were not sufficiently achieved. Metal removal was relevant when EDTA was used as the conditioning agent, and the electric potential was raised up to 0.8 V/cm. EDTA led to the formation of negatively charged complexes with metals which migrated towards the anode compartment by electromigration. This result shows that metal removal from sediments with high acid buffering capacity may be achieved by enhancing the electrokinetic process by EDTA addition when the acidification of the medium is not economically and/or environmentally sustainable.

Laboratory tests for the phytoextraction of heavy metals from polluted harbor sediments using aquatic plants

The aim of this study was to investigate the concentrations and pollution levels of heavy metals, organochlorine pesticides, and polycyclic aromatic hydrocarbons in marine sediments from the Leghorn Harbor (Italy) on the Mediterranean Sea. The phytoextraction capacity of three aquatic plants Salvinia natans, Vallisneria spiralis, and Cabomba aquatica was also tested in the removal of lead and copper, present in high concentration in these sediments. The average detectable concentrations of metals accumulated by the plants in the studied area were as follows: >3.328 ± 0.032 mg/kg dry weight (DW) of Pb and 2.641 ± 0.014 mg/kg DW of Cu for S. natans, >3.107 ± 0.034 g/kg DW for V. spiralis, and >2.400 ± 0.029 mg/kg DW for C. aquatica. The occurrence of pesticides was also analyzed in the sediment sample by gas chromatography coupled with mass spectrometry (GC/MS). Due to its metal and organic compound accumulation patterns, S. natans is a potential candidate in phytoextraction strategies.

Ornamental plants for micropollutant removal in wetland systems

The objective of this paper was to evaluate the efficiency of micropollutant removal, such as Cu, Zn, carbamazepine, and linear alkylbenzene sulfonates (LAS), through the use of a subsurface vertical flow constructed wetland system with ornamental plants. Zantedeschia aethiopica, Canna indica, Carex hirta, Miscanthus sinensis, and Phragmites australis were selected and planted in lysimeters filled up with gravel. The lysimeters were completely saturated with synthetic wastewater (N 280 mg L(-1), P 30 mg L(-1), Cu 3.6 mg L(-1), Zn 9 mg L(-1), carbamazepine 5 μg L(-1), linear alkylbenzene sulfonates 14 mg L(-1)), and the leaching water was collected for analysis after 15, 30, and 60 days in winter-spring and spring-summer periods. Nutrients (N and P) and heavy metals decreased greatly due to both plant activity and adsorption. C. indica and P. australis showed the highest metal content in their tissues and also the greatest carbamazepine and LAS removal. In these plants, the adsorption/degradation processes led to particularly high oxidative stress, as evidenced by the significantly high levels of ascorbate peroxidase activity detected. Conversely, Z. aethiopica was the less efficient plant in metal and organic compound removal and was also less stressed in terms of ascorbate peroxidase activity.

Reed bed systems for sludge treatment: case studies in Italy

In recent years, reed bed systems (RBSs) have been widely considered as a valid technology for sludge treatment. In this study are presented results about sludge stabilization occurring within beds in four RBSs, situated in Tuscany (Italy). The results showed that stabilization of the sludge over time occurred in all RBSs, as shown by the low content of water-soluble carbon and dehydrogenase activity, which measures indirectly the overall microbial metabolism, and by the re-synthesis of humic-like matter highlighted by the pyrolytic indices of mineralization and humification. Results about heavy metal fractionation, an appropriate technique to estimate the heavy metal bioavailability and sludge biotoxicity, showed that the process of sludge stabilization occurring in RBSs retains metals in fractions related to the stabilized organic matter, making metals less bioavailable. Moreover, the concentrations of various toxic organic compounds were below the limit of concentration suggested by the European Union's Working Document on Sludge, for land application. The effectiveness of the stabilization processes in RBs was hence clearly proven by the results that measured mineralization and humification processes, and by the low levels of bioavailable heavy metals and toxic organic compounds in stabilized sludges.

Phytoremediation of dredged marine sediment: monitoring of chemical and biochemical processes contributing to sediment reclamation

In this study, a pilot phytoremediation experiment was performed to treat about 80 m(3) of silty saline sediments contaminated by heavy metals and organic compounds. After preliminary mixing with a sandy soil and green compost application, three different plant treatments [Paspalum vaginatum (P); P. vaginatum + Spartium junceum (P + S); P. vaginatum + Tamarix gallica (P + T)] were compared to each other and to an unplanted control (C) in order to evaluate the plant efficiency in remediating and ameliorating agronomical and functional sediment properties. The experiment was monitored for one year after planting by taking sediment samples at two depths and performing several chemical and biochemical analyses. After one year, the increase in hydrolytic enzyme and dehydrogenase activities indicated the stimulation of sediment functionality. Additionally, the availability of energy sources derived from organic matter application and plant-root activity promoted the formation of a stable organic matter fraction. Finally, P + S and P + T were also effective in decontaminating polluted marine sediments from both organic (total petroleum hydrocarbons, TPH) and inorganic (heavy metal) pollutants.

Short-term performance analysis of sludge treatment reed beds

Sludge treatment reed beds (STRBs) represent an established technology for managing sludge produced by wastewater treatment plants (WWTPs). In this study, an analysis conducted on sludge stratigraphy in a STRB serving the municipal WWTPs of Helsinge (42,000 person equivalent, Denmark) is presented. The STRB has been in operation since 1996. The analysis was conducted for four months including two consecutive loadings of one of the 10 basins. The amount of dewatering during the short initial phase was significant, and the rehydration of deeper layers was negligible. A clear reduction trend was observed for volatile solids and labile organic matter, which is the first step towards organic matter humification. This hypothesis was also confirmed by water soluble carbon, which decreased with the increasing depth and the time of resting. About the N cycle, ammonification and nitrification showed a reciprocal balancing effect, so as to cause an equilibrium between the stable trends of NH(4)(+) and NO(3)(-) which were similar along the depth and the time of monitoring. The highest values of both parameters were found at the surface, where the combined effect of plant action, direct diffusive oxygen transfer from the atmosphere and high nutrient contents of the freshly loaded sludge was likely more intense.

Organic matter stabilization in reed bed systems: Danish and Italian examples

In this study, results about sludge stabilization in reed bed systems (RBSs) after the entire period of operation in two different systems situated in Denmark (Helsinge 42,000 population equivalent (p.e.) - 10 years) and in Italy (La Fontina, 30,000 p.e. - 6 years) were presented. In order to evaluate the process of sludge stabilization, parameters that highlighted the biochemical and chemico-structural properties of organic sludge matter have been determined. The level of total and soluble nutrients, and enzyme activities, parameters related to overall microbial activity, showed that stabilization of the sludge similarly occurred in both RBSs, even though in different landscape ecosystems. The chemical-structural characterization of sludge organic matter highlighted how the processes of stabilization have occurred satisfactorily in both RBSs; in fact, significant levels of pyrolytic indices for mineralization and humification were reached. The successful stabilization of organic matter occurred in both RBSs and was confirmed by the absence of Escherichia coli, and also by the results of organic compounds (polycyclic aromatic hydrocarbons, linear alkyl benzene sulfonates, nonylphenol ethoxylates, di-2-ethylhexyl-phthalate) and heavy metals.

Assessment of pollution impact on biological activity and structure of seabed bacterial communities in the Port of Livorno (Italy)

The main objective of this study was to assess the impact of pollution on seabed bacterial diversity, structure and activity in the Port of Livorno. Samples of seabed sediments taken from five selected sites within the port were subjected to chemical analyses, enzymatic activity detection, bacterial count and biomolecular analysis. Five different statistics were used to correlate the level of contamination with the detected biological indicators. The results showed that the port is mainly contaminated by variable levels of petroleum hydrocarbons and heavy metals, which affect the structure and activity of the bacterial population. Irrespective of pollution levels, the bacterial diversity did not diverge significantly among the assessed sites and samples, and no dominance was observed. The type of impact of hydrocarbons and heavy metals was controversial, thus enforcing the supposition that the structure of the bacterial community is mainly driven by the levels of nutrients. The combined use of chemical and biological essays resulted in an in-depth observation and analysis of the existing links between pollution macro-indicators and biological response of seabed bacterial communities.

Biofouling of reverse osmosis membranes used in river water purification for drinking purposes: analysis of microbial populations

Biofouling in water treatment processes represents one of the most frequent causes of plant performance decline. Investigation of clogged membranes (reverse osmosis membranes, microfiltration membranes and ultrafiltration membranes) is generally performed on fresh membranes. In the present study, a multidisciplinary autopsy of a reverse osmosis membrane (ROM) was conducted. The membrane, which was used in sulfate-rich river water purification for drinking purposes, had become inoperative after 6 months because of biofouling and was later stored for 18 months in dry conditions before analysis. SSU rRNA gene library construction, clone sequencing, T-RFLP, light microscope, and scanning electron microscope (SEM) observations were used to identify the microorganisms present on the membrane and possibly responsible for biofouling at the time of removal. The microorganisms were mainly represented by bacteria belonging to the phylum Actinobacteria and by a single protozoan species belonging to the Lobosea group. The microbiological analysis was interpreted in the context of the treatment plant operations to hypothesize as to the possible mechanisms used by microorganisms to enter the plant and colonize the ROM surface.

Phytotreatment of sludges (Phragmites australis) for their reuse in the environment

The aim of this study is the evaluation of the agronomic characteristics acquired by a phytotreated sludge coming from a wastewater treatment plant (WWTP) located in Tuscany (central Italy). The chemical characterization showed values which are within the Italian legislation limits for mixed composts. From an agronomic point of view, the sludge did not present a level of phytotoxicity, as shown by the germination index (GI% = 77). Furthermore, pathogen compounds are inexistent (Escherichia coli < 1,000 CFU/g). Different substrates (obtained by mixing the sludge with sandy agronomic soil - 0.5% w/w, 1% w/w, 2.5% w/w and 5% w/w) were prepared in order to evaluate the best mixture performance in terms of water retention capacity and plant growth. No significant differences were observed for all sludge mixtures. Different plants were tested in plots (Lepidium sativum, Cucumis sativus and Avena sativa). The best plant adaptation, measured as dry biomass production, was observed for Avena sativa. The results obtained underlined that the phytotreatment of sludge can bring about the transformation of sewage sludges into organic products that are reusable in agriculture, if previously mixed with other appropriate materials and taking into account their heavy metal content.

Hydraulic and biochemical analyses on full-scale sludge consolidation reed beds in Tuscany (Italy)

The management of sewage sludge has recently become one of the most significant challenges in wastewater management. Reed bed systems appear to be an efficient and economical solution for sludge management in small wastewater treatment plants. Four years ago, one of the holding companies for water and wastewater in central Italy adopted this technology in 6 wastewater treatment plants. Hydraulic and biochemical analyses were performed on the most representative site to asses the behaviour of reed beds with regard to dewatering, mineralization and humification of disposed sludge. Moreover, daily water content analysis were performed in the interval between subsequent sludge loadings. Results indicated a decrease of sludge volume by about 93% on a yearly basis. Biochemical analysis highlighted that mineralization processes decrease over time due to a rapid decrease of microbial activity and labile substrates, such as DHase enzyme and water-soluble carbon and ammonium, respectively. Moreover, a significant interrelationship between the parameters linked with mineralization was found: after two years of operation, the process of mineralization of organic matter is still predominant in the humification of organic matter. Daily water content data were used to define a semi empirical equation describing the dynamics of the dewatering process. Overall, the use of sludge reed beds resulted feasible, ecologically sustainable and cost-effective.

Nitrogen, phosphorus and organics removal in two vertical flow experimental wetlands receiving pretreated municipal wastewater

Pollutants removal in two vertical flow experimental wetlands receiving pre-treated municipal wastewater was investigated. Laboratory investigations using microcosms were also carried out in order to identify key processes regulating pollutants removal. Results obtained from experimental wetlands seem to confirm the ability of a single stage installation, where nitrification and denitrification processes are jointly optimised using a gravel medium, always wet but not saturated with water. Results obtained from microcosm investigations show the prominent role of O2 supply, of microbial activity and of bio-available C from vegetal metabolism in the nitrogen removal.