Sun-Jin Hwang

Effects of acid pre-treatment on bio-hydrogen production and microbial communities during dark fermentation.

Optimal conditions for acid pre-treatment were investigated for the enrichment of hydrogen-producing bacteria (HPB) in a mixed culture using three strong acids: HCl, HNO(3), and H2SO4 x HCl was selected as a suitable acid for the enrichment of HPB in the fermentation process. The volume of bio-hydrogen produced when the mixed culture was pre-treated using HCl at pH 2 was 3.2 times higher than that obtained without acid pre-treatment. Changes in the microbial community during acid pre-treatment were monitored using images obtained by the fluorescent in situ hybridization (FISH) method and the Live/Dead cell viability test. The tests clearly indicated that the Clostridium species of cluster I were the predominant strains involved in bio-H(2) fermentation, and could be selectively enriched by HCl pre-treatment.

Factors affecting nitrous oxide production: a comparison of biological nitrogen removal processes with partial and complete nitrification.

Nitrous oxide (N2O) emission from biological nitrogen removal (BNR) processes has recently received more research attention. In this study, two lab-scale BNR systems were used to investigate the effects of various operating parameters including the carbon to nitrogen (C/N) ratio, ammonia loading, and the hydraulic retention time on N2O production. The first system was operated in a conventional BNR mode known as the Ludzack–Ettinger (LE) process, consisting of complete denitrification and nitrification reactors, while the second one was operated in a shortcut BNR (SBNR) mode employing partial nitrification and shortcut denitrification, which requires less oxygen and carbon sources. As the C/N ratio was decreased, a significant increase in N2O production was observed only in the anoxic reactor of the LE process, indicating that N2O was released as an intermediate of the denitrification reaction under the carbon-limited condition. However, the SBNR process did not produce significant N2O even at the lowest C/N ratio of 0.5. When the SBNR process was subjected to increasing concentrations of ammonia, N2O production from the aerobic reactor was rapidly increased. Furthermore, the increasing production of N2O was observed mostly in the aerobic reactor of the SBNR process with a decline in hydraulic retention time. These experimental findings indicated that the increase in N2O production was closely related to the accumulation of free ammonia, which was caused by an abrupt increase of the ammonium loading. Consequently, the partial nitrification was more susceptible to shock loading conditions, resulting in a high production of N2O, although the SBNR process was more efficient with respect to nitrogen removals as well as carbon and oxygen requirements.

Sodium (Na+) concentration effects on metabolic pathway and estimation of ATP use in dark fermentation hydrogen production through stoichiometric analysis

Batch experiments were conducted to investigate the effects of Na(+) concentration on hydrogen production with dark fermentation. The Na(+) concentration was varied from 0 to 8 g/L in the mixed culture using an anaerobic sludge treated by acid. The maximum hydrogen production was achieved with 1 g-Na(+)/L, whereas the hydrogen production was decreased over 2 g-Na(+)/L due to the inhibitory of Na(+). The mechanisms of Na(+) inhibition to the hydrogen production are studied using pure culture of Clostridium butyricum by calculating the energy balance. At a high sodium concentration, C. butyricum used a greater proportion of the ATP generated via fermentation for cell maintenance rather than for cell synthesis. Additionally, higher Na(+) concentrations shifted the fermentation process toward the acetate synthesis pathway instead of the butyrate pathway, and the value of Y(X/ATP) decreased. With high Na(+) concentrations, a greater ratio of hydrogen was produced via the oxidation of NADH. Excess hydrogen production decreased as the Na(+) concentration increased.

Three-dimensional, highly porous N-doped carbon foam as microorganism propitious, efficient anode for high performance microbial fuel cell

Three-dimensional (3D) N-doped open-porous carbon foam was fabricated using the simple procedure of calcining a melamine sponge. The properties of the fabricated carbon foam and its performance in microbial fuel cells (MFCs) using Shewanella oneidensis MR-1 (S. oneidensis) were compared with those of commercial graphite felt. The MFC with the carbon foam anode produced approximately 2 times higher power density than the commercial graphite felt. The superior performance of the as-prepared carbon foam in MFC was attributed to the higher surface area (687.19 m2 g−1) and open-porous scaffold structure. Moreover, the appearance of the hydrophilic functional groups such as CN–C, N–CO on the surface of the as-prepared carbon foam facilitated extracellular electron transfer, resulting in a decrease in charge transfer resistance and an increase in biocompatibility. Owing to the excellent biocompatibility, a large amount of microbial biomass colonized both the surface and inside the carbon foam, which helped enhance the performance of the MFC.

Sewage sludge reduction and system optimization in a catalytic ozonation process.

The main objective of this study was to suggest a feasible, effective process for the reduction of sewage sludge using ozone oxidation catalysed by metal ion. A series of lab‐scale experiments was conducted to select a suitable catalyst and its proper dose to achieve optimum sludge reduction. Using a central composite design under response surface methodology (RSM), system optimization with respect to sludge reduction and cost‐effectiveness was performed by varying the independent parameters: dosages of ozone and ions. Five metal ions, Mn2+, Fe2+, Zn2+, Cu2+, and Al3+, were tested, and the manganese ion showed the highest sludge reduction, as measured by a decrease in total suspended solids. The ozone/Mn combination achieved approximately twice as much sludge reduction as the ozonation alone. Furthermore, the Mn dose of 10 mg/g‐TS (total solids) resulted in the highest sludge reduction efficiency among the different doses, which ranged from 0 to 20 mg‐Mn/g‐TS. The predicted efficiency of sewage sludge reduction using the RSM was found to agree well with the experimental results, and the statistical analyses predicted optimum ranges for the doses of ozone and Mn ions, taking into account the overall cost for sewage sludge treatment.

Enhanced bio-energy recovery in a two-stage hydrogen/methane fermentation process

A two-stage hydrogen/methane fermentation process has emerged as a feasible engineering system to recover bio-energy from wastewater. Hydrogen-producing bacteria (HPB) generate hydrogen from readily available carbohydrates, and organic acids produced during the hydrogen fermentation step can be degraded to generate methane in the following step. Three strong acids, HCl, H(2)SO(4), and HNO(3), were tested to determine the appropriate pre-treatment method for enhanced hydrogen production. The hydrogen production rates of 230, 290, and 20 L/kg(-glucose)/day was observed for the sludge treated with HCl, H(2)SO(4), and HNO(3), respectively, indicating that the acid pre-treatment using either HCl or H(2)SO(4) resulted in a significant increase in hydrogen production. The fluorescent in situ hybridization method indicated that the acid pre-treatment selectively enriched HPB including Clostridium sp. of cluster I from inoculum sludge. After hydrogen fermentation was terminated, the sludge was introduced to a methane fermentation reactor. This experiment showed methane production rates of 100, 30, and 13 L/kg(-glucose)/day for the sludge pre-treated with HCl, H(2)SO(4), and HNO(3), respectively, implying that both sulfate and nitrate inhibited the activity of methane-producing bacteria. Consequently, the acid pre-treatment might be a feasible option to enhance biogas recovery in the two-stage fermentation process, and HCl was selected as the optimal strong acid for the enrichment of HPB and the continuous production of methane.

Influence of light conditions of a mixture of red and blue light sources on nitrogen and phosphorus removal in advanced wastewater treatment using Scenedesmus dimorphus

The effects of several light conditions (various light/dark cycles, light intensities, and light frequencies) of a mixture of intermittent red and blue light on nitrogen and phosphorus removal from wastewater using Scenedesmus dimorphus were evaluated. The nitrogen removal rates were almost the same (~ 10 mg/L/day) during light/dark cycles of 16:8, 20:4, and 24:0, and were two-fold higher than those observed during light/dark cycles of 12:12 and 8:16. The phosphorous removal rate also reached its highest value (~ 1.5 mg/L/day) during the light/dark cycles of 20:4 and 24:0. These results suggest that microalgae should be illuminated for at least 16 and 20 h for the efficient removal of nitrogen and phosphorous, respectively. Moreover, both the nitrogen and phosphorous removal rates were significantly enhanced when the light intensity was raised from 50 to 400 µmol/m2/sec. Increasing the light intensity above 400 µmol/m2/sec caused photo-inhibition, such that microalgae production decreased. Besides the light/dark cycle and light intensity, flashing frequency also controls the growth and nutrient removal rate of Scenedesmus dimorphus. It was found that the most appropriate flashing frequency was 2,500 Hz.