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Leach bed reactors (LBRs) are dry anaerobic systems that can handle feedstocks with high solid content, like chicken manure, with minimal water addition. In this study, the chicken manure was mixed with zeolite, a novel addition, and packed in the LBR to improve biogas production. The resulting leachate was then processed in a continuous stirred tank reactor (CSTR), where most of the methane was produced. The supernatant of the CSTR was returned to the LBR. The batch mode operation of the LBR led to a varying methane production rate (MPR) with a peak in the beginning of each batch cycle when the leachate was rich in organic matter. Comparing the MPR in both systems, the peaks in the zeolite system were higher and more acute than in the control system, which was under stress, as indicated by the acetate accumulation at 2328 mg L−1. Moreover, the presence of zeolite in the LBR played a crucial role, increasing the overall methane yield from 0.142 (control experiment) to 0.171 NL CH4 per g of volatile solids of chicken manure entering the system at a solid retention time of 14 d. Zeolite also improved the stability of the system. The ammonia concentration increased gradually due to the little water entering the system and reached 3220 mg L−1 (control system) and 2730 mg L−1 (zeolite system) at the end of the experiment. It seems that zeolite favored the accumulation of the ammonia at a lower rate (14.0 mg L−1 d−1) compared to the control experiment (17.3 mg L−1 d−1). The microbial analysis of the CSTR fed on the leachate from the LBR amended with zeolite showed a higher relative abundance of Methanosaeta (83.6%) compared to the control experiment (69.1%). Both CSTRs established significantly different bacterial profiles from the inoculum after 120 days of operation (p < 0.05). Regarding the archaeal communities, there were no significant statistical differences between the CSTRs and the inoculum (p > 0.05). [ABSTRACT FROM AUTHOR]

Fruit and vegetable waste (FVW) generated locally in open (public or wholesale) markets is a valuable resource and should not be considered as waste. The anaerobic digestion (AD) of FVW can minimize landfill disposal and generate renewable energy, thus decreasing greenhouse gas emissions. Moreover, the digestate after the AD of FVW, devoid of antibiotics and animal fats in manure and food waste, may have a high fertilizing value. In this study, FVW mixtures were composed to mimic the real FVW generated in Mediterranean open markets annually. The first goal was to evaluate the biochemical methane potential (BMP) of different size fractions resulting from FVW grinding. Indeed, the FVW was ground and separated into two size fractions, 0–4 mm and 4–10 mm, respectively. The 0–4 mm fraction exhibited a lower BMP but a higher rate constant than the 4–10 mm fraction. The second goal was to first evaluate the BMP of the lumped fraction of FVW after grinding (0–10 mm) via BMP assays and then feed it to a mesophilic two-stage leaching-bed reactor (LBR)-upflow anaerobic sludge bed (UASB) system for almost one year. The BMP of the FVW ranged between 406 and 429 L kg−1 of volatile solids (VS) independently of the FVW production season. The system received an average organic loading rate (OLR) of 3.1 ± 0.7 g VS L−1 d−1. During operation, the LBR gradually transited from acidogenic to methanogenic, and the overall methane yield of the system increased from 265–278 to 360–375 L kg−1 VS, respectively. The proposed technology does not require water addition or liquid digestate removal. Compared to the continuous stirred tank reactor (CSTR) digester technology, the LBR/UASB system is suitable for the anaerobic digestion of FVW. The results of this study can be further used to upscale the proposed technology and contribute to the societal need for affordable and clean energy included in the Sustainable Development Goals (SDGs). [ABSTRACT FROM AUTHOR]

Agricultural residues are intensively evaluated as potential feedstocks for biogas plants. Vegetable crops generate massive residues during and at the end of their growing seasons. A greenhouse facility in Greece, Thrace Greenhouses S.A., generates 7000–8000 t per year of residual green biomass, resulting from the hydroponic cultivation of tomatoes and cucumbers on 170,000 m2 of land from February to November. The crop residues included leaves, suckers, and stalks. The biochemical methane potential (BMP) estimation was realized on samples taken in March, May, and August, as well as at the end of cultivation (November). Suckers, leaves, and stalks of both plants yielded a range of 221–357, 210–296, and 225–250 NL kg−1 VS, respectively. t-test statistical analysis showed that the BMP of the leaves and suckers were statistically different for tomato and cucumber plants. The BMP of stalks was lower than the other residue types except for the tomato leaves. The diauxic behavior of the specifi'1c methane production curves indicated that the two-phase Gompertz model (TGM) was the most suitable. The model fitting showed that leaves and suckers, in spite of having a higher BMP than the stalks, exhibited a lower maximum specific methane production rate constant than the stalks during the first phase, which may indicate the presence of inhibitory or slowly biodegradable compounds in leaves and suckers in comparison to the stalks. [ABSTRACT FROM AUTHOR]

BACKGROUND Biogas can be upgraded to biomethane, which can be used as vehicle fuel and natural gas substitute. Bioelectrochemical biogas upgrade is an innovative alternative to energy‐consuming physicochemical processes and bio‐upgrade methods which require H2 supply. Bioelectrochemical biogas upgrade is conducted by methanogenic microorganisms that convert CO2 into CH4, in the biocathode of a bioelectrochemical system (BES), using electric current as energy source. The aim of the present work was to study the efficiency of an H‐type BES in the conversion of CO2 into CH4, by applying different potentials at the electromethanogenic biocathode. RESULTS: The H‐type BES was operated in a three‐electrode configuration (working: graphite rod; counter: Pt/Ti; reference: Ag/AgCl) with a potentiostat, which set the biocathode's potential initially at −0.7 V versus a standard hydrogen electrode (SHE) and monitored the current demand. Based on cyclic voltammetry runs, a highly electroactive methanogenic biocathode was developed in a short time. The methane production rate (MPR) at a cathode potential of −0.7 V versus SHE was 31.1 mmol m−2 d−1, with an electron capture efficiency of 77.6%. The efficiency of the BES was reduced by applying a potential of −0.5 V versus SHE at the biocathode, resulting in negligible CH4 production. The BES achieved its maximum performance at a potential of −0.9 V versus SHE with a MPR of 53.8 mmol m−2 d−1 and an electron capture efficiency of 86%. The CO2 consumption rate achieved was 0.8 mmol d−1. CONCLUSIONS: The H‐type BES achieved an effective biolectrochemically driven methane production, while the biocathode electroactive behavior was evaluated during the whole operation of the system. © 2021 Society of Chemical Industry (SCI). [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons (PAHs) are highly persistent compounds in the environment, recalcitrant to biodegradation and highly lipophilic, while some of them cause acute carcinogenic and mutagenic toxicity. Due to the risk of soil contamination whenever sewage sludge is disposed to agricultural land, the investigation of their presence in sewage sludge is a priority need. This work concerns the occurrence of five PAHs, selected from the priority list of the US Environmental Protection Agency (phenanthrene, fluoranthene, pyrene, benzo[b]flouranthene, and benzo[a]pyrene), in municipal sewage sludge and to assess their fate in a sewage treatment plant. An analytical method based on microwave- assisted extraction (MAE), followed by HPLC-UV coupled with a fluorescent detector (FD) was used for their measurement. The method efficiency was evaluated as to its linearity, repeatability, accuracy and sensitivity. The contamination levels of the PARs were determined in sludge from the sewage treatment plants of three main Greek cities: Athens, Patras and Heraklion. PAHs ranged from 0.2 to 4 mg kg-1 d.m for each PAH in mixtures of primary and secondary sludge, while they were significantly lower in secondary sludge (less than 0.1 mg kg-1 d.m). [ABSTRACT FROM AUTHOR]

Biofuels produced from photosynthetic microorganisms such as microalgae and cyanobacteria could potentially replace fossil fuels as they offer several advantages over fuels produced from lignocellulosic biomass. In this study, energy production potential in the form of bioethanol was examined using different biomasses derived from the growth of a cyanobacteria-based microbial consortium on a chemical medium and on agro-industrial wastewaters (i.e., dairy wastewater, winery wastewater and mixed winery–raisin effluent) supplemented with a raisin residue extract. The possibility of recovering fermentable sugars from a microbial biomass dominated by the filamentous cyanobacterium Leptolynbgya sp. was demonstrated. Of the different acid hydrolysis conditions tested, the best results were obtained with sulfuric acid 2.5 N for 120 min using dried biomass from dairy wastewater and mixed winery–raisin wastewaters. After optimizing sugar release from the microbial biomass by applying acid hydrolysis, alcoholic fermentation was performed using the yeast Saccharomyces cerevisiae. Raisin residue extract was added to the treated biomass broth in all experiments to enhance ethanol production. Results showed that up to 85.9% of the theoretical ethanol yield was achieved, indicating the potential use of cyanobacteria-based biomass in combination with a raisin residue extract as feedstock for bioethanol production. [ABSTRACT FROM AUTHOR]

Sunflower residues are considered a prominent renewable source for biogas production during anaerobic digestion (AD). However; the recalcitrant structure of this lignocellulosic substrate requires a pretreatment step for efficient biomass transformation and increased bioenergy output. The aim of the present study was to assess the effect of alkaline pretreatment of various parts of the sunflower residues (e.g., heads and stalks) on their methane yield. Experimental data showed that pretreatment at mild conditions (55 °C; 24 h; 4 g NaOH 100 g−1 total solids) caused an increase in the biochemical methane potential (BMP) of both heads and stalks of the sunflower residues as determined in batch tests. The highest methane production (268.35 ± 0.11 mL CH4 g−1 volatile solids) was achieved from the pretreated sunflower head residues. Thereafter; the effect of alkaline pretreatment of sunflower head residues was assessed in continuous mode; using continuous stirred-tank reactors (CSTRs) under two operational phases. During the first phase; the CSTRs were fed with the liquid fraction produced from the pretreatment of sunflower heads. During the second phase; the CSTRs were fed with the whole slurry resulting from the pretreatment of sunflower heads (i.e., both liquid and solid fractions). In both operating phases; it was observed that the alkaline pretreatment of the sunflower head residues had a negligible (phase I) or even a negative effect on biogas production; which was contradictory to the results of the BMP tests. It seems that; during alkaline pretreatment; this part of the sunflower residues (heads) may release inhibitory compounds; which induce a negative effect on biogas production in the long term (e.g., during continuously run digesters such as CSTR) but not in the short-term (e.g., batch tests) where the effect of the inoculum may not permit the inhibition to be established. [ABSTRACT FROM AUTHOR]