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.

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.

Tuning the Fluorescence and the Intramolecular Charge Transfer of Phenothiazine Dipolar and Quadrupolar Derivatives by Oxygen Functionalization.

A series of new naphthalimide and phenothiazine-based push-pull systems (NPI-PTZ1-5), in which we structurally modulate the oxidation state of the sulfur atom in the thiazine ring, i.e., S(II), S(IV), and S(VI), was designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. The effect of the sulfur oxidation state on the spectral, photophysical, and electrochemical properties was investigated. The steady-state absorption and emission results show that oxygen functionalization greatly improves the optical (absorption coefficient and fluorescence efficiency) and nonlinear optical (hyperpolarizability) features. The cyclic voltammetry experiments and the quantum mechanical calculations suggest that phenothiazine is a stronger electron donor unit relative to phenothiazine-5-oxide and phenothiazine-5,5-dioxide, while the naphthalimide is a strong electron acceptor in all cases. The advanced ultrafast spectroscopic measurements, transient absorption, and broadband fluorescence up conversion give insight into the mechanism of photoinduced intramolecular charge transfer. A planar intramolecular charge transfer (PICT) and highly fluorescent excited state are populated for the oxygen-functionalized molecules NPI-PTZ2,3 and NPI-PTZ5; on the other hand, a twisted intramolecular charge transfer (TICT) state is produced upon photoexcitation of the oxygen-free derivatives NPI-PTZ1 and NPI-PTZ4, with the fluorescence being thus significantly quenched. These results prove oxygen functionalization as a new effective synthetic strategy to tailor the photophysics of phenothiazine-based organic materials for different optoelectronic applications. While oxygen-functionalized compounds are highly fluorescent and promising active materials for current-to-light conversion in organic light-emitting diode devices, oxygen-free systems show very efficient photoinduced ICT and may be employed for light-to-current conversion in organic photovoltaics.