Yeo-Myeong Yun

Optimization of dark fermentative H2 production from microalgal biomass by combined (acid + ultrasonic) pretreatment

In this study, individual pretreatments (acid and ultrasonic) and a combination of these pretreatments were optimized to enhance the efficiency of dark fermentative hydrogen production (DFHP) from microalgal biomass. The experimental results show that the maximum H₂ production performance of 42.1 mL H₂/g dry cell weight (dcw) was predicted at 0.79% (v/w) HCl and at a specific energy input (SEI) of 49,600 kJ/kg dcw in the combined pretreatment, while it was limited in both individual pretreatments. Repeated batch testing of the predicted optimal conditions revealed that the combined pretreatment conditions for DFHP from microalgal biomass were successfully optimized by increasing the solubilization of the feedstock and by reducing the formation of the toxic 5-hydroxymethylfurfural (HMF).

Effect of the accuracy of pH control on hydrogen fermentation.

pH, known as the most important parameter in H2 fermentation, cannot be precisely controlled in a scaled-up fermenter as in a lab fermenter. In the preset work, to assess the effect of pH control accuracy on H2 fermentation, the pH was controlled at 6.0±0.1, 6.0±0.3, 6.0±0.5, 6.0±0.7, and 6.0±0.9 during batch fermentation of food waste. Up to deviation of ±0.3, a high H2 yield of 1.67-1.73 mol H2/mol hexose(added) was attained with producing butyrate as a major metabolite (>70% of total organic acids produced). A huge drop of H2 production, however, was observed at deviation >±0.5 with lowered substrate utilization and increased production of lactate. Next generation sequencing results showed that Clostridium was found to be the dominant genus (76.4% of total number of sequences) at deviation of ±0.1, whereas the dominant genus was changed to lactic acid bacteria such as Streptococcus and Lactobacillus with increase of deviation value.

Hydrogen fermentation of food waste by alkali-shock pretreatment: microbial community analysis and limitation of continuous operation.

In the study, at first, batch tests were performed to investigate the effect of alkali-shock on H2 production from food waste (FW). After alkali-pretreatment of FW at pH 9.0-13.0, the FW was cultivated under mesophilic condition at pH 6.0 for 30 h without external inoculum addition. The amount of H2 production from FW pretreated at pH 11.0 and 12.0 was higher than that achieved in other pretreatment pH. The main metabolite was butyrate, and Clostridium were dominant at pH 11.0 and 12.0. Meanwhile, lactate was the main metabolite with Enterococcus and Streptococcus being the dominant genus at other pretreatment pH. When the batch process was switched to a continuous mode, H2 production was significantly dropped due to the increased activity of H2-consumers. The reliability of alkali-pretreatment at pH 11.0 was proven by repeating the scale-up batch process, recording 1.57±0.11 mol H2/mol hexose(added) (17±2LH2/kg FW) and 4.39±0.32LH2/L/d.

Biohydrogen production from food waste: current status, limitations, and future perspectives.

Among the various biological routes for H2 production, dark fermentation is considered the most practically applicable owing to its capability to degrade organic wastes and high H2 production rate. Food waste (FW) has high carbohydrate content and easily hydrolysable in nature, exhibiting higher H2 production potential than that of other organic wastes. In this review article, first, the current status of H2 production from FW by dark fermentation and the strategies applied for enhanced performance are briefly summarized. Then, the technical and economic limitations of dark fermentation of FW are thoroughly discussed. Economic assessment revealed that the economic feasibility of H2 production from FW by dark fermentation is questionable. Current efforts to further increase H2 yield and waste removal efficiency are also introduced. Finally, future perspectives along with possible routes converting dark fermentation effluent to valuable fuels and chemicals are discussed.

Scenedesmus-based treatment of nitrogen and phosphorus from effluent of anaerobic digester and bio-oil production

In this study, a microalgae-based technology was employed to treat wastewater and produce biodiesel at the same time. A local isolate Scenedesmus sp. was found to be a well suited species, particularly for an effluent from anaerobic digester (AD) containing low carbon but high nutrients (NH3-N=273mgL(-1), total P=58.75mgL(-1)). This algae-based treatment was quite effective: nutrient removal efficiencies were over 99.19% for nitrogen and 98.01% for phosphorus. Regarding the biodiesel production, FAME contents of Scenedesmus sp. were found to be relatively low (8.74% (w/w)), but overall FAME productivity was comparatively high (0.03gL(-1)d(-1)) due to its high biomass productivity (0.37gL(-1)d(-1)). FAMEs were satisfactory to the several standards for the biodiesel quality. The Scenedesmus-based technology may serve as a promising option for the treatment of nutrient-rich wastewater and especially so for the AD effluent.

Application of a novel enzymatic pretreatment using crude hydrolytic extracellular enzyme solution to microalgal biomass for dark fermentative hydrogen production.

In this study, a novel enzymatic pretreatment of Chlorella vulgaris for dark fermentative hydrogen production (DFHP) was performed using crude hydrolytic extracellular enzyme solution (CHEES) extracted from the H2 fermented effluent of food waste. It was found that the enzyme extracted at 52 h had the highest hydrolysis efficiency of microalgal biomass, resulting in the highest H2 yield of 43.1 mL H2/g dry cell weight along with shorter lag periods. Even though a high amount of VFAs was accumulated in CHEES, especially butyrate, the fermentative bacteria on the DFHP was not affected from product inhibition. It also appears that the presence of organic acids, especially lactate and acetate, contained in the CHEES facilitated enhancement of H2 production acted as a co-substrate. Therefore, all of the experimental results suggest that the enhancement of DFHP performance caused by CHEES has a dual role as the hydrolysis enhancer and the co-substrate supplier.

Effect of hydraulic retention time on lactic acid production and granulation in an up-flow anaerobic sludge blanket reactor

In the present work, lactic acid (LA) production performance with granulation was investigated at various hydraulic retention times (HRTs), 8-0.5h. Glucose was used as a feedstock, and anaerobic mixed cultures were inoculated in an up-flow anaerobic sludge blanket reactor. As HRT decreased, the average diameter and hydrophobicity of the granules increased from 0.31 to 3.4mm and from 17.5% to 38.3%, respectively, suggesting the successful formation of granules. With decreasing HRT, LA productivity increased up to 16.7gLA/L-fermenter/h at HRT 0.5h. The existence of rod-shaped organisms with pores and internal channels at granule surface was observed by scanning electron microscope. Next generation sequencing revealed that Lactobacillus was the dominant microorganism, accounting for 96.7% of total sequences, comprising LA-producing granules.

Elucidating a synergistic effect of food waste addition on the enhanced anaerobic digestion of waste activated sludge

Waste activated sludge (WAS) has often been anaerobically digested with other types of organic waste, expecting a synergistic enhancement. In the present work, a slight amount of food waste (FW) such that the change of the substrate concentration and the C/N ratio could be neglected was added to WAS and biological methane potential tests were performed. As the amount of FW input increased, the total amount of CH4 produced and CH4 yield increased. The calculation proved that at least 30% of the increased amount of CH4 produced was derived from WAS, clearly signalling a synergistic enhancement. Measurements of the hydrolytic extracellular protease activity and ammonia concentration support the finding of synergism in that the addition of easily biodegradable organics to WAS facilitated the degradation of protein, a major constituent of WAS. This is the first report clearly revealing a synergistic effect of FW addition on the enhanced digestion of WAS.

Enhanced anaerobic digestion of livestock waste by ultrasonication: A tool for ammonia removal and solubilization

Ultrasonication was applied to lower the ammonia level in livestock waste to enhance the anaerobic digestion performance. In simulated waste tests, in spite of an identical temperature increase, a higher ammonia removal rate was observed at lower frequency. This could be explained by the existence of athermal effects, accounting for 64% of the total ammonia removal rate. These effects originated from various convections (micro-streaming, micro-convection, shock-waves, and micro-jets), possibly caused by stable bubbles, and this indigenous mixing ability led to a negligible effect of aeration in the ultrasound assisted ammonia stripping process. In actual waste tests, an ammonia removal rate of up to 55% was achieved with a 0.77 h−1 mass transfer rate coefficient. After ultrasonication (28 kHz, pH 11, 15 min) of livestock waste, 58% higher CH4 yield was achieved due to the decrease of ammonia concentration (28%) and enhanced solubilization (51%).

Producing desulfurized biogas through removal of sulfate in the first‐stage of a two‐stage anaerobic digestion

In the present work, a two‐stage anaerobic digestion system (TSADS) was newly designed to produce biogas with a greatly reduced H2S content. The role of first (sulfidogenic)‐stage digester was not only acidogenesis but also sulfidogenesis (sulfate reduction to H2S), which would minimize the input of H2S‐producing source in the followed second (methanogenic)‐stage digester. For the coexistence of acidogens and sulfate reducing bacteria (SRB) in the sulfidogenic‐stage digester, it was found that pH played a crucial role. The acidogenic activity was not affected within a pH range of 4.5–6.0, while it was important to maintain a pH at 5.5 to achieve a sulfate removal efficiency over 70%. The highest sulfate removal attained was 78% at a hydraulic retention time (HRT) of 5 h at pH 5.5 ± 0.1. The H2S content in the biogas produced in the conventional single‐stage digester (SSAD), used as a control, reached 1,650 ± 25 ppmv. In contrast, the biogas produced in the methanogenic‐stage digester of the developed process had an H2S content of 200 ± 15 ppmv. Microbial analysis, done by the next generation sequencing technique, clearly showed the changes in community under different operating conditions. Desulfovibrio bastinii (4.9%) played a key role in sulfate removal in the sulfidogenic‐stage of the TSADS owing to its characteristics of a short doubling time and growth in an acidic environment. Biotechnol. Bioeng. 2017;114: 970–979.