• Energy Research

Abstract This study evaluated the enhancement of biogas production in semi-continuous anaerobic digestion fed with enzymatically pretreated Chlorella vulgaris . Organic matter in soluble phase increased from 2.5% to 45% after pretreatment with proteases. The soluble COD was easily biodegradable and almost all organic matter available in soluble phase was removed (94.4%) in the anaerobic reactor (CSTR). Methane yield increased 2.6-fold when compared to the CSTR fed with raw biomass. With regard to the nitrogen fate, 77% of the organic nitrogen was mineralized during anaerobic digestion. Slight ammonium inhibition was detected due to the high nitrogen mineralization registered. pHs remained close to neutrality throughout the experimental time. VFAs were accumulated in the last retention time as a consequence of the slight anaerobic digestion inhibition, which revealed an unbalanced equilibrium among the anaerobic microbial population. This fact was corroborated in batch digestion assays. The anaerobic sludge collected from the CSTR exhibited a different profile in terms of methane productivity when compared to the inoculum from WWTP normally used. Further studies are nowadays undertaken to overcome the inhibition and thus increase the methane yield.

Abstract This study evaluated the enhancement of biogas production in semi-continuous anaerobic digestion fed with enzymatically pretreated Chlorella vulgaris . Organic matter in soluble phase increased from 2.5% to 45% after pretreatment with proteases. The soluble COD was easily biodegradable and almost all organic matter available in soluble phase was removed (94.4%) in the anaerobic reactor (CSTR). Methane yield increased 2.6-fold when compared to the CSTR fed with raw biomass. With regard to the nitrogen fate, 77% of the organic nitrogen was mineralized during anaerobic digestion. Slight ammonium inhibition was detected due to the high nitrogen mineralization registered. pHs remained close to neutrality throughout the experimental time. VFAs were accumulated in the last retention time as a consequence of the slight anaerobic digestion inhibition, which revealed an unbalanced equilibrium among the anaerobic microbial population. This fact was corroborated in batch digestion assays. The anaerobic sludge collected from the CSTR exhibited a different profile in terms of methane productivity when compared to the inoculum from WWTP normally used. Further studies are nowadays undertaken to overcome the inhibition and thus increase the methane yield.

[EN] Anaerobic digestion can biotransform the biodegradable fraction of sewage sludge into biogas, while the symbiotic action of algal-bacterial consortia can remove both the CO2 and H2S from biogas and nutrients from digestate. A 100 L anaerobic digester operated at 20 days of retention time coupled with a 180 L high-rate algal pond (HRAP) engineered to upgrade the biogas and treat the liquid fraction of the pilot digester was optimized along four operational stages: (I) operation with a greenhouse during winter; (II) operation without greenhouse; (III) process supplementation with NaHCO3; (IV) process supplementation with Na2CO3. The biogas produced was composed of 63.7 ± 2.9% CH4, 33.7 ± 1.9% CO2, 0.5 ± 0.3% O2 and 1.6 ± 1.1% N2. An average methane productivity of 324.7 ± 75.8 mL CH4 g VSin−1 and total COD removals of 48 ± 20% were recorded in the digester. The CH4 concentration in the biomethane gradually decreased to 87.6 ± 2.0% and 85.1 ± 1.3% at the end of stage I and II, respectively, attributed to the loss of inorganic carbon in the HRAP. The supplementation of NaHCO3 and Na2CO3 mediated an increase in the CH4 content to 90.4 ± 1.5 and 91.2 ± 0.7% in stages III and IV, respectively. Steady state CO2 removals of 90% and 88% in stages I and II, and 95.7 and 97.6% in stages III and IV, respectively, were recorded. A constant biomass productivity of 22 g m−2 d−1, set by daily harvesting 26.5 g dry algal-bacterial biomass from the bottom of the settler, was maintained concomitantly with a complete removal of the N and P supplied via centrate SI The authors thank FUNDACION DOMINGO MARTINEZ, the Regional Government of Castilla y León and the EU-FEDER program (CLU 2017-09, CL-EI-2021-07 and UIC 315). The Spanish Ministry of Science and Innovation (FJC 2018-038402-I) is also acknowledged for funding the Juan de la Cierva-Formación research contract of Lara Mendez. In addition, the authors want to acknowledge the National Autonomous University of Nicaragua (UNAN-Managua) and FUNDACION CAROLINA for funding the research stay of Dimas Garcia

[EN] Anaerobic digestion can biotransform the biodegradable fraction of sewage sludge into biogas, while the symbiotic action of algal-bacterial consortia can remove both the CO2 and H2S from biogas and nutrients from digestate. A 100 L anaerobic digester operated at 20 days of retention time coupled with a 180 L high-rate algal pond (HRAP) engineered to upgrade the biogas and treat the liquid fraction of the pilot digester was optimized along four operational stages: (I) operation with a greenhouse during winter; (II) operation without greenhouse; (III) process supplementation with NaHCO3; (IV) process supplementation with Na2CO3. The biogas produced was composed of 63.7 ± 2.9% CH4, 33.7 ± 1.9% CO2, 0.5 ± 0.3% O2 and 1.6 ± 1.1% N2. An average methane productivity of 324.7 ± 75.8 mL CH4 g VSin−1 and total COD removals of 48 ± 20% were recorded in the digester. The CH4 concentration in the biomethane gradually decreased to 87.6 ± 2.0% and 85.1 ± 1.3% at the end of stage I and II, respectively, attributed to the loss of inorganic carbon in the HRAP. The supplementation of NaHCO3 and Na2CO3 mediated an increase in the CH4 content to 90.4 ± 1.5 and 91.2 ± 0.7% in stages III and IV, respectively. Steady state CO2 removals of 90% and 88% in stages I and II, and 95.7 and 97.6% in stages III and IV, respectively, were recorded. A constant biomass productivity of 22 g m−2 d−1, set by daily harvesting 26.5 g dry algal-bacterial biomass from the bottom of the settler, was maintained concomitantly with a complete removal of the N and P supplied via centrate SI The authors thank FUNDACION DOMINGO MARTINEZ, the Regional Government of Castilla y León and the EU-FEDER program (CLU 2017-09, CL-EI-2021-07 and UIC 315). The Spanish Ministry of Science and Innovation (FJC 2018-038402-I) is also acknowledged for funding the Juan de la Cierva-Formación research contract of Lara Mendez. In addition, the authors want to acknowledge the National Autonomous University of Nicaragua (UNAN-Managua) and FUNDACION CAROLINA for funding the research stay of Dimas Garcia

Abstract Anaerobic digestion of microalgae has been enhanced by several pretreatments; however the reported net energy ratio was negative. In order to cope with this issue, this investigation focused on thermal pretreatment (120 °C for 40 min) at increasing biomass loads of Chlorella vulgaris and Scenedesmus sp. During that thermal pretreatment, carbohydrates solubilisation prevailed over proteins for both strains. Regardless the biomass load pretreated, anaerobic biodegradability of C. vulgaris was enhanced by 50% and therefore, pretreatments of high biomass loads was suggested to counterbalance the energy input. On the other hand, thermally pretreated Scenedesmus sp. biomass supported an enhancement of 21–27%. The specific cell wall composition was suggested as a potential reason for the differences registered on their anaerobic biodegradabilities.

Abstract Anaerobic digestion of microalgae has been enhanced by several pretreatments; however the reported net energy ratio was negative. In order to cope with this issue, this investigation focused on thermal pretreatment (120 °C for 40 min) at increasing biomass loads of Chlorella vulgaris and Scenedesmus sp. During that thermal pretreatment, carbohydrates solubilisation prevailed over proteins for both strains. Regardless the biomass load pretreated, anaerobic biodegradability of C. vulgaris was enhanced by 50% and therefore, pretreatments of high biomass loads was suggested to counterbalance the energy input. On the other hand, thermally pretreated Scenedesmus sp. biomass supported an enhancement of 21–27%. The specific cell wall composition was suggested as a potential reason for the differences registered on their anaerobic biodegradabilities.

Abstract Among biofuel production processes using microalgae biomass, biogas generation seems to be the least complex. Nevertheless, its efficiency is hampered due to the hard cell wall. In order to enhance its anaerobic biodegradability, the present investigation evaluated the effect of two pretreatments (low temperature autohydrolysis at 50 °C for 24 and 48 h incubation and alkaline (0.5, 2 and 5% w/w NaOH dosages)) on Chlorella vulgaris and Scenedesmus sp. The autohydrolysis resulted in 16 and 6% chemical oxygen demand (COD) solubilisation for Chlorella and Scenedesmus , respectively. During thermoalkaline pretreatment, COD in soluble phase (CODsol) was increased up to 19% for Chlorella and 17% for Scenedesmus sp. The highest carbohydrates solubilisation corresponded to 2 and 5% w/w NaOH dosage for 48 h at 50 °C for Chlorella (20%) and Scenedesmus (40–43%). When compared to Chlorella , Scenedesmus biomass exhibited higher carbohydrates solubilisation, although methane yield enhancement was low for both substrates. Best case scenario for Scenedesmus sp. (20% increase) was attained with 5% NaOH at 50 °C for 24 h. Despite the lower carbohydrates solubilisation observed for Chlorella , similar methane yields were similar to Scenedesmus sp. The low methane production enhancement was ascribed to the fact that the organic matter solubilised were exopolymers released during pretreatments rather than intracellular material.

Abstract Among biofuel production processes using microalgae biomass, biogas generation seems to be the least complex. Nevertheless, its efficiency is hampered due to the hard cell wall. In order to enhance its anaerobic biodegradability, the present investigation evaluated the effect of two pretreatments (low temperature autohydrolysis at 50 °C for 24 and 48 h incubation and alkaline (0.5, 2 and 5% w/w NaOH dosages)) on Chlorella vulgaris and Scenedesmus sp. The autohydrolysis resulted in 16 and 6% chemical oxygen demand (COD) solubilisation for Chlorella and Scenedesmus , respectively. During thermoalkaline pretreatment, COD in soluble phase (CODsol) was increased up to 19% for Chlorella and 17% for Scenedesmus sp. The highest carbohydrates solubilisation corresponded to 2 and 5% w/w NaOH dosage for 48 h at 50 °C for Chlorella (20%) and Scenedesmus (40–43%). When compared to Chlorella , Scenedesmus biomass exhibited higher carbohydrates solubilisation, although methane yield enhancement was low for both substrates. Best case scenario for Scenedesmus sp. (20% increase) was attained with 5% NaOH at 50 °C for 24 h. Despite the lower carbohydrates solubilisation observed for Chlorella , similar methane yields were similar to Scenedesmus sp. The low methane production enhancement was ascribed to the fact that the organic matter solubilised were exopolymers released during pretreatments rather than intracellular material.

Abstract The aim of the present study was to compare cyanobacteria strains ( Aphanizomenon ovalisporum , Anabaena planctonica , Borzia trilocularis and Synechocystis sp.) and microalgae ( Chlorella vulgaris ) in terms of growth rate, biochemical profile and methane production. Cyanobacteria growth rate ranged 0.5–0.6 day −1 for A . planctonica , A . ovalisporum and Synecochystis sp. and 0.4 day −1 for B . tricularis . Opposite, C . vulgaris maximum growth rate was double (1.2 day −1 ) than that of cyanobacteria. Regarding the methane yield, microalgae C . vulgaris averaged 120 mL CH 4 g COD in −1 due to the presence of a strong cell wall. On the other hand, anaerobic digestion of cyanobacteria supported higher methane yields. B . trilocularis and A . planctonica presented 1.42-fold higher methane yield than microalgae while this value was raised to approximately 1.85-fold for A . ovalisporum and Synechochystis sp. In the biogas production context, this study showed that the low growth rates of cyanobacteria can be overcome by their increased anaerobic digestibility when compared to their microalgae counterpartners, such is the case of C. vulgaris .