Bicarbonate (HCO₃^- ) can be used as an inorganic carbon source for hydrogen-based denitrification (HD). Since HCO₃^- is considered to accelerate the NO₂^- reduction rate, this study is attempted to minimize the hydraulic retention time (HRT) of HD systems using varied amounts of HCO₃^- : deficient, moderate, and abundant amounts. The results implied that a low NO₂^- amount was removed at unsuitably short HRTs, resulting in poor HD efficiency despite being supplemented with abundant HCO₃^- amounts. HCO₃^- assisted in rapidly acclimatizing the bacteria having nirS gene, causing higher NO₂^- reduction rate and aided in changing the bacterial communities. Thauera spp. were the most dominant bacteria in abundant HCO₃^- conditions, achieving high HD efficiency at 8 to 24 h HRT whereas satisfactory efficiency was achieved in the deficient and moderate HCO₃^- amount-systems through the collaboration of Rhodocyclaceae, Alcaligenaceae, and Xanthomonadaceae as predominant bacteria in the community. A strong correlation between the abundance of nirS gene and Thauera spp. was also found. The findings in this study revealed the importance of using HCO₃^- for the enrichment of H₂-oxidizing denitrifiers containing nirS gene in order to reduce NO₂^- accumulation to enhance the HD efficiency.

 Musty odor is an unavoidable problem for drinking water. 2－Methylisoborneol (2－MIB) is one of the major musty odorants that has been set as a quality standard for drinking water in Japan since 2004. Between 1988 and 1994, 2－MIB-producing cyanobacteria, like Phormidium tenue and Oscillatoria tenuis, were observed at the intake basin of Keage Water Purification Plant (KWPP), which draws water from the southern basin of Lake Biwa (SBLB). However, 2－MIB occurred in the intake basin of KWPP and SBLB surface water between 2007 and 2017 (except 2013 and 2014) in the absence of 2－MIB-producing cyanobacteria. On average, 92% of total 2－MIB was detected as dissolved between 2007 and 2017, but only 60% was found as dissolved between 1988 and 1994. It is believed that the bottom water of SBLB was more contaminated with 2－MIB than the surface water, and P. tenue was found in the submerged plants at the bottom of the lake. The culture strain of P. tenue PTG, isolated in SBLB, effectively produced dissolved 2－MIB (D－2－MIB) under the light with the same photon flux density at the bottom of the lake. In conclusion, the D－2－MIB that occurred in SBLB between 2007 and 2017 was attributed to P. tenue attached to the plants that sank at the bottom of the lake.

 Microalgae are recognized as a renewable feedstock for biofuel production. However, low lipid productivity and high cultivation costs are the major bottlenecks for the practical application of microalgal biofuel production. This study aimed to investigate the benefits of a mixotrophic cultivation of Chlamydomonas reinhardtii using organic wastes and wastewater effluent for enhanced biomass and lipid production and reducing cultivation cost. C. reinhardtii was cultured under mixotrophic conditions in a synthetic C medium or sewage effluent supplemented with a commercial organic carbon source (glucose, fructose, sucrose, or acetate) or organic wastes (molasses or corn steep liquor). C. reinhardtii grew and accumulated lipids in the all cultivation conditions using C medium or sewage effluent. Sewage effluent could be an alternative culture medium for C. reinhardtii. Supplementing a commercial organic carbon or organic wastes enhanced C. reinhardtii’s biomass production. The highest biomass (86 mg–dry cell/L/d) and lipid (34 mg/L/d) production by C. reinhardtii was observed in the C medium with 1 g/L molasses. Cultivation using sewage effluent with 1 g/L of molasses also provided a relatively high biomass (73 mg–dry cell/L/d) and lipid (27 mg/L/d) production; these rates were significantly higher than those under photoautotrophic cultivation and mixotrophic cultivation in C medium supplemented with other commercial carbon sources except for acetate. Molasses was thus found to enhance growth and lipid synthesis of C. reinhardtii. These results indicate that mixotrophic cultivation using sewage effluent and molasses can be a promising strategy for a cost-effective and highly efficient biofuel production by C. reinhardtii.