• Energy Research

Over the last decades, phytodepuration has been considered an efficient technology to treat wastewaters. The present study reports a bench scale depuration assay of swine wastewater using Lemna minor. The highest observed growth rate obtained in swine wastewater was 3.1 ± 0.3 gDW m−2 day−1 and the highest nitrogen and phosphorus uptake were 140 mg N m−2 day−1 and 3.47 mg P m−2 day−1, respectively. The chemical oxygen demand removal efficiency in the swine wastewater assay was 58.9 ± 2.0%. Furthermore, the biomass valorisation by anaerobic co-digestion with swine wastewater was assessed. Results showed a clear improvement in specific methane production rate (around 40%) when compared to mono-substrate anaerobic digestion. The highest methane specific production, 131.0 ± 0.8 mL CH4 g−1 chemical oxygen demand, was obtained with a mixture containing 100 g of duckweed per liter of pre-treated swine wastewater. The water-nutrients-energy nexus approach showed to be promising for swine waste management.

This work proposes a valorization route for spent coffee grounds (SCG), a widespread lignocellulosic residue, encompassing the production of: biomethane, lignin, and oligosaccharides as value-added products obtained simultaneously during a mild alkaline (NaOH) pretreatment. The studied operational variables were the reaction time (60–240 min), temperature (25–75 °C), and the NaOH concentration (0–2.5 M). The severity factor suitably describes the global process kinetics, with higher severities (log Mo = 5.5) yielding high product yields, 18.02% and 13.25% (on dry SCG basis) for lignin and oligosaccharides (XGMOS), respectively. Solid yield is negatively impacted by all studied variables (at the 95% confidence level). Conversely, XGMOS yield is positively influenced both by time and catalyst concentration, whereas lignin yield is only (positively) influenced by catalyst concentration. Optimal balance between product formation and potential operational costs is putatively achieved when using 0.625 M NaOH, at 50 °C for 60 min. The mild alkaline pretreated biomass (MAP-SCG) was compared to untreated SCG for biomethane production by anaerobic co-digestion with pig slurry (PS), using a ratio of biomass/PS = 1/3 (volatile solids (VS) basis). The proposed valorization route enabled the sequential production of 6.25 kg lignin, 6.36 kg oligosaccharides, and 138.05 kg biomethane per 100 kg of non-extracted SCG (and 287.60 kg pig slurry), in an integrated process that is technically feasible and promotes the circular bioeconomy.

The purpose of this work was to assess the enhancement of the anaerobic digestion process of sewage sludge by the addition of orange peel as co-substrate. In this experimental study, an evaluation of the co-digestion process in a semi-continuous stirred tank reactor was carried out under mesophilic conditions (37 ± 0.2 °C) during an operation time up to 315 days. All trials were performed at a hydraulic retention time of 15 days and the AD reactor was fed daily with a mixture of sewage sludge and orange peel pulp, previously prepared. The organic loading rate was kept at 1.80 ± 0.31 g VS L−1 day−1 in all trials. A reference scenario (T0), mono-digestion, was performed using sewage sludge as substrate (primary sludge and activated sludge, 40:60, v/v). Thereafter, two different anaerobic co-digestion trials were carried out, one with pre-treated orange peel (T1) and the other with reduced d-limonene content (T2). Trials with pre-treated orange peel (T1 and T2) led to two- and threefold improvements in biogas and methane yields, compared with the reference scenario (T0). Furthermore, the reduction of d-Limonene content (T2) increased the methane yield by around 70% compared with T1. Therefore, it was proved that the sieving pre-treatment is effective to avoid the inhibitory effect of d-limonene.

Anaerobic digestion technology can play a significant role in the transition to a low-carbon and circular economy by producing bioenergy (biomethane) and organic fertilizer (digestate). This study proposes a valorization approach for three waste streams widely produced in the Mediterranean area: fish waste (FW), pig slurry (PS), and orange pomace (OP). The FW lipid content can enhance biomethane yield as long as inhibition by long-chain fatty acids is prevented. In this study, the effect of introducing 25% and 50% FW to the anaerobic co-digestion of a reference mixture consisting of 80% PS and 20% OP pulp (OPP) was studied. Co-digestion using 50% FW presented the maximum biomethane yield (669.68 ± 8.32 mL CH4/g VSadded), which corresponds to a 37% increase compared to the reference. No inhibition was detected during the anaerobic digestion assay. The kinetic study showed that the introduction of FW led to a reduction in the degradation rate constant by up to 30%. The lag phase increased as FW content increased, with 50% FW presenting a lag time approximately three times that of the reference mixture. The proposed strategy can encourage sustainable waste management practices and contribute to GHG emissions mitigation.

Animal manure management is a real challenge to minimize environmental impacts and ensure that this valuable material is efficiently used in a circular economy context. One of the main limitations for larger use of animal manure as fertilizer is the availability of land to receive it in an area close to the farm. Indeed, animal manure is traditionally used for cereals and animal feed growth, but the soil area occupied with these crops might not be enough to receive all the manure produced and/or part of this soil might have nutrient contents, namely phosphorous, that do not permit further application of manure. Hence, extra land used for other agricultural activities might be an option. The main objective of the present review was to analyse the constraints and solutions to increase the use of manure in horticulture and orchards. Emphasis was given to the legal framework for manure utilization in the EU that might stimulate or restrain such a solution. The main characteristics of manure that might limit or stimulate manure reuse were also described, and the potential of some treatments to valorise manure was analysed. Several examples of alternative uses of manure in horticulture and orchards were examined, and the society and farmers’ acceptance of the proposed solution was addressed.

The anaerobic co-digestion process in a continuous stirred-tank reactor (CSTR) was carried out under mesophilic conditions (37 ± 0.2 °C). All the trials were performed at a hydraulic retention time (HRT) of 15 days and the AD reactor was daily fed with a mixture of sewage sludge (SS) and elephant grass hydrolyzate (EGH). In this study, three different trials were assessed, with different mixture proportions of SSSS and EGH: F0 (100:0,v/v), F1 (75:25, v/v) and F2 (50:50, v/v), during 90 days each trial, keeping the organic loading rate (OLR) in a range of 0.94-1.16 g VS L(-1) day(-1). The experimental results obtained showed that the soluble chemical oxygen demand (SCOD) removal efficiency was around 77% and 86% for trials F1 and F2, respectively. SS co-digestion with EGH enhanced methane yield, leading to an increment between 23% and 38%, in comparison with the reference scenario (F0).

In this study, co-digestion of mixed sewage sludge from a wastewater treatment plant (WWTP) and partially dephenolised two-phase olive pomace (DOP) as a co-substrate was addressed with the aim of improving the biodigestibility of both substrates. The introduction of DOP into WWTP anaerobic digester facilities could significantly increase biomethane production and enhance the sustainability of both activities. An improvement in the system’s performance was supported by stability parameters: total alkalinity increased and stabilised with the addition of 5% v/v DOP, and the specific energy loading rate was maintained at 0.177 ± 0.03 d−1, which indicated better buffer capacity and stability in the bioreactor, and the possibility of enhancing the organic loading rate. In terms of average daily biogas production rate, an increase of 39% was achieved, up to 0.39 ± 0.11 L L−1d−1. Moreover, there was a 40% and 37% improvement in specific methane production and methane production rate, respectively, up to 0.28 ± 0.02 L CH4 g TVS−1 and 0.26 ± 0.08 L L−1d−1. In addition, the proposed strategy leads to an energy saving of 20,328.6 kWh year−1 at the WWTP as a result of the electric energy production surplus, corresponding to an annual saving of €3293.23.