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As the wide variety of compounds that are solubilised and generated during hydrothermal treatments of sewage sludge cannot be defined individually, this study proposes a classification based on chemical characteristics and biodegradability. Moreover, it has deepened understanding of the role of oxygen in the reaction mechanisms and the evolution of chemical oxygen demand (COD) fractions and their components during hydrothermal treatments (180 °C, 80 bar). This information is necessary before separating specific compounds or using the hydrolysates directly as fertiliser or fermentation media. Results revealed the existence of an initial stage of sludge solubilisation (30 – 60 min), mainly produced by hydrolysis reactions and without mineralisation, followed by a second stage where oxygen promoted the conversion of solubilised compounds, mineralisation and the minimisation of the solid fraction. Total COD and total suspended solids (TSS) were reduced by 48% and 75% after 210 min of wet oxidation, compared to 28% and 50% after thermal hydrolysis. Oxygen led to a final liquid hydrolysate with fewer easily assimilable compounds (38% of the initial total COD converted into readily biodegradable), inorganic elements (17 g/kg TSSo) and refractory, inhibitory and less assimilable compounds (1.1 g O2/L of slowly and non-biodegradable COD) compared to that obtained in its absence (61%, 24 g/kg TSSo and 2.3 g O2/L, respectively). Oxidation mechanisms favoured the conversion of non-volatile hydroxy acids (NVHA) and intermediate compounds to volatile fatty acids (VFA), with VFA concentration being almost 3 times higher under an oxidising atmosphere, but with NHVFA 57% lower.

Science, Innovation and University Office of the Principality of Asturias (Spain) through the Severo Ochoa pre-doctoral aid programme [BP19-041] & project GRUPIN AYUD/2021/51041

Spanish Ministry of Science, Innovation and Universities (MCIU) [RTI2018-094218-B-I00]; FEDER funds from European Union; FEDER funds from Employment, Industry and Tourism Office of the Principality of Asturias (Spain) [AYUD/2021/51041]; Principality of Asturias, Spain [BP19-093]

Two-dimensional (2D) transition metal nanosheets are promising catalysts because of the enhanced exposure of the active species compared to their 3D counterparts. Here, we report a simple, scalable, and reproducible strategy to prepare 2D phosphate nanosheets by forming a layered structure in situ from phytic acid (PTA) and transition metal precursors. Controlled combustion of the organic groups of PTA results in interlayer carbon, which keeps the layers apart during the formation of phosphate, and the removal of this carbon results in ultrathin nanosheets with controllable layers. Applying this concept to vanadyl phosphate synthesis, we show that the method yields 2D ultrathin nanosheets of the orthorhombic β-form, exposing abundant V4+/V5+ redox sites and oxygen vacancies. We demonstrate the high catalytic activity of this material in the vapor-phase aerobic oxidation of ethyl lactate to ethyl pyruvate. Importantly, these β-VOPO4 compounds do not get hydrated, thereby reducing the competing hydrolysis reaction by water byproducts. The result has superior selectivity to ethyl pyruvate compared to analogous vanadyl phosphates. The catalysts are highly stable, maintaining a steady-state conversion of ∼90% (with >80% selectivity) for at least 80 h on stream. This “self-exfoliated” synthesis protocol opens opportunities for preparing structurally diverse metal phosphates for catalysis and other applications.

In this work, six different types of protic ionic liquids (PILs) have been studied. For all these PILs, the bacteria toxicity and environmental biodegradability have been investigated, as well as their corrosive behavior and miscibility in different types of lubricating base oils. The PILs studied showed poor to moderate biodegradability (BOD5/COD from 0.072 ± 0.002 to 0.18 ± 0.01) and slight toxicity (TU values between from 1.18 ± 0.04 and 2.7 ± 0.1). However, these environmental properties are much better than those obtained for the reference lubricant additive used in industry (ZDDP). In addition, five of the PILs did not show corrosive activity on steel. Only one of the PILs showed some miscibility in an ester as a base oil and the rest of the mixtures were highly unstable.

Centrifugation of anaerobically digested sewage sludge gives rise to a solid phase, which could be employed as a fertilizer, and a liquid fraction (ADL), which should be treated before being spilled out. This is not an easy task because this liquor is characterized for presenting high COD (~16000 mg O2/L), high ammonium content (~4000 mg/L) and low biodegradability (BOD5/COD ~0.2). With the objective to pre-treat this aqueous waste before its treatment by means of more traditional aerobic processes, different physico-chemical methods (ultrasound, ozonation, hydrolysis and wet air oxidation) were assessed in this work. Ultrasound and thermal hydrolysis gave solubilizations around 47% and 68% respectively. The best results in terms of total COD removal were obtained when wet air oxidation (8 h, 160 C-200 °C and 6.0 MPa) and ozonation (8 h, 25 °C, 12 g/h O3) techniques were employed achieving COD degradations of 71% and 38%, respectively. The pre-treatment of ADL with the four assayed techniques improved considerably the biodegradability (BOD5/COD) of the effluent, with values around 0.3–0.4, depending on the treatment. The experimental data were successfully fitted by kinetic models and the kinetic constants for the solubilization and degradation steps were obtained. Application of the proposed models can be of interest for the optimization and selection of the most suitable techniques and operational conditions, in each particular case.

In this study, the effect of ageing on the performance and fouling mechanisms in the ultrafiltration of landfill leachates using titania-zirconia (ZrO2-TiO2) tubular membrane was thoroughly studied. Results revealed that the maturation of the leachate has a positive effect on its ultrafiltration, with a twofold higher final permeability compared with the young one. This is the result of the higher organic load, particularly that corresponding to proteins and carbohydrates, of the young leachate. Resistance-in-series analysis demonstrated that the loss of permeability was mainly due to reversible fouling, caused by cake filtration. Either irrecoverable or irreversible fouling were scarce and not conditioned by the stage of maturation of the leachate. Chemical oxygen demand (COD) rejection for the mature leachate varied with volume concentration ratio (VCR) following an approximately sigmoidal shape, from an initial value of 18.5% to a final one of 49.6%, with the faster increase at VCR ranging from 1.2 to 1.7, due to the permeation of fatty acids by the ultrafiltration (UF) membrane. On the other hand, COD rejection remained approximately constant at 48% during the ultrafiltration of the young leachate, which can be attributed to the presence of higher molecular weight compounds in its composition.

The direct oxidative dehydrogenation of lactates with molecular oxygen is a “greener” alternative for producing pyruvates. Here we report a one-pot synthesis of mesoporous vanadia–titania (VTN), acting as highly efficient and recyclable catalysts for the conversion of ethyl lactate to ethyl pyruvate. These VTN materials feature high surface areas, large pore volumes, and high densities of isolated vanadium species, which can expose the active sites and facilitate the mass transport. In comparison to homogeneous vanadium complexes and VOx/TiO2 prepared by impregnation, the meso-VTN catalysts showed superior activity, selectivity, and stability in the aerobic oxidation of ethyl lactate to ethyl pyruvate. We also studied the effect of various vanadium precursors, which revealed that the vanadium-induced phase transition of meso-VTN from anatase to rutile depends strongly on the vanadium precursor. NH4VO3 was found to be the optimal vanadium precursor, forming more monomeric vanadium species. V4+ as the major valence state was incorporated into the lattice of the NH4VO3-derived VTN material, yielding more V4+–O–Ti bonds in the anatase-dominant structure. In situ DRIFT spectroscopy and density functional theory calculations show that V4+–O–Ti bonds are responsible for the dissociation of ethyl lactate over VTN catalysts and for further activation of the deprotonation of β-hydrogen. Molecular oxygen can replenish the surface oxygen to regenerate the V4+–O–Ti bonds.

In this work, the role of an oxidising atmosphere during the hydrothermal treatment of an activated sludge at 160 °C and 40 bar, was determined. The composition and molecular weight sizes of the soluble biopolymers generated during the sludge treatment in presence (wet oxidation “WO”) or absence (thermal hydrolysis “TH”) of oxygen were compared. Likewise, the characteristics of organic material, settleability, colour and pH of the treated sludge during both treatments were analysed. The thermal treatment in presence of oxygen provided better results in terms of solubilisation, settleability and mineralisation. WO initially favoured a more intense cellular lysis, causing a higher degree of solubilisation than that achieved by TH. Either in presence or absence of oxygen, thermal treatments caused a marked worsening of the settleability of the sludge. However, the degradation of biopolymers during WO led subsequently to an improvement of the settleability properties for longer reaction times. Both treatments caused a fast solubilisation of biopolymers at the beginning by effect of the release of extracellular and intracellular material from sludge. Subsequently, the presence of oxygen produced a significant decrease in the concentration of those biopolymers. In contrast, the proteins were the only one biopolymer that was degraded during TH.