Abstract: A simple denitrification bioreactor for nitrate-containing wastewater without organic compounds was developed. This bioreactor consisted of packed gel envelopes in a single tank. Each envelope comprised two plates of gels containing Paracoccus denitrificans cells with an internal space between the plates. As an electron donor for denitrification, ethanol was injected into the internal space and not directly into the wastewater. P. denitrificans cells in the gel reduced nitrate to nitrogen gas by using the injected ethanol. Nitrate-containing desulfurization wastewater derived from a coal-fired thermal power plant was continuously treated with 20 packed gel envelopes (size, 1,000 x 900 x 12 mm; surface area, 1.44 m(2)) in a reactor tank (volume 1.5 m(3)). When the total nitrogen concentration in the inflow was around 150 mg-N.L(-1), the envelopes removed approximately 60-80% of the total nitrogen, and the maximum nitrogen removal rate was 5.0 g-N.day(-1) per square meter of the gel surface. This value corresponded to the volumetric nitrogen removal performance of 0.109 kg-N.m(-3).day(-1). In each envelope, a high utilization efficiency of the electron donor was attained, although more than the double amount of the electron donor was empirically injected in the present activated sludge system to achieve denitrification when compared with the theoretical value. The bioreactor using the envelopes would be extremely effective as an additional denitrification system because these envelopes can be easily installed in the vacant spaces of preinstalled water treatment systems, without requiring additional facilities for removing surplus ethanol and sludge. Biotechnol. Bioeng. 2007;97:1439-1447. (c) 2007 Wiley Periodicals, Inc.

Abstract: The photosynthetic performance of a conical, helical tubular photobioreactor (HTP) incorporating Chlorella sorokiniana was investigated under conditions of high temperature and light intensity during midsummer in an outdoor environment. Although the culture medium temperature exceeded 40 degrees C for approximately 5 h each day, peaking at 47.5 degrees C under sunny conditions, a photosynthetic productivity of 30.0 g x m(-2) (installation area) x day(-1) and a photosynthetic efficiency of 8.66% [photosynthetically active radiation (PAR), 400-700 nm] were achieved. A maximum photosynthetic productivity of 33.2 g x m(-2) x day(-1) was achieved on a sunny day, when solar energy input was also maximal (11.5 MJ x m(-2) x day(-1) [PAR]). On the other hand, a maximum photosynthetic efficiency of 9.54% was obtained on a day that was rainy in the morning and cloudy in the afternoon, and there was relatively little solar energy input. The average daily photosynthetic efficiency over the two culture periods (August 4 to 7 and August 10 to 13, 1999) was 7.25%. Thus, a high level of photosynthetic performance was achieved in the conical HTP incorporating Chlorella sorokiniana despite the fact that culture medium temperature was not controlled. The use of Chlorella sorokiniana in the conical HTP should be a good choice to produce microalgal biomass during the summer under field conditions.

Abstract: The characteristics of the flow of culture medium significantly affects the photosynthetic productivity of bioreactors incorporating microalgae. Therefore, in order to optimize the performance of a conical helical tubular photobioreactor (CHTP) designed to be useful in practical applications, we characterized the flow pattern of the culture medium through the reactor. The effects of medium flow conditions on the photosynthetic productivity of Chlorella sp. were investigated using several different CHTP units with 0.50-m2 installation areas which were designed to vary the direction and rate of flow driven by airlift. In addition, the performance of two- and four-unit systems constructed by combining individual CHTP units was evaluated. We found that when medium flowed from the bottom to the top of the photostage, it exhibited smoother flow of culture medium than when flowing from top to bottom, which led to higher photosynthetic productivity by the former. Consistent with theoretical calculations, varying the lengths of vertical flow passages caused flow rates to vary, and higher flow rates meant smoother circulation of medium and better photosynthetic performance. Flow of medium through a four-unit CHTP system was similar to that in single units, enabling a photosynthetic productivity of 31.0 g-dry biomass per m2-installation area per day to be achieved, which corresponded to a photosynthetic efficiency of 7.50% (photosynthetically active radiation (PAR; 400-700 nm)). This high photosynthetic performance was possible because smoother medium flow attained in single units was also attained in the four-unit system.

Abstract: Microalgal photosynthesis requires appropriate culture medium temperatures to achieve high photosynthetic performance and to maintain production of a high-quality biomass product. Enclosed systems, such as our conical, helical tubular photobioreactor (HTP), can accomplish high photosynthetic efficiency and the small amount of culture medium used by these systems means that the culture medium temperature may be effectively controlled. On the other hand, because a high ratio of surface area to culture medium volume leads to rapid heating under the illumination condition and substantial heat loss at night, maintaining a suitable culture medium temperature is necessary to achieve efficient, commercially practical biomass production. In order to predict changes in the culture medium temperature caused by changes in solar irradiance and ambient temperature, it is necessary to understand the heat balance within the photobioreactor. We therefore investigated the heat balance in three major parts (photostage, degasser, and helical heat exchanger) of our conical HTP, analyzed the time-dependent changes in medium temperature at various room temperatures and radiant energy inputs, and predicted changes in the culture medium temperature based on the characteristics of heat transfer among the three parts. Using this model, the predicted changes in culture medium temperature were very similar to the changes observed experimentally in the laboratory and under field conditions. This means that by calculating the time-dependent changes in the culture medium temperature, based on measurements of solar energy input and ambient temperature, we should be able to estimate the energy required to maintain the culture medium temperature within a range where photosynthetic performance of microalgae is high.

Abstract: As photosynthetic efficiencies are relatively high at irradiation levels of <500 micromol m(-2) s(-1), photosynthetic productivity could be increased by redistributing strong light over a larger photo-receiving area using conical, helical, tubular photobioreactors (HTP). When Chlorella were exposed to light irradiation of 980 micromol m(-2) s(-1), the ratio of productivities was 1.00:1.13:1.23:1.66 for conical HTPs with cone angles of 180 degrees (flat type), 120 degrees, 90 degrees, and 60 degrees, respectively. This suggests that photo-redistribution technology is a highly effective and convenient approach for increasing the photosynthetic productivity of microalgae.

Abstract: The batch culture of a newly isolated strain of a green microalga, Chlorella sorokiniana, was carried out using a conical helical tubular photobioreactor. The isolate was capable of good growth at 40 degrees C under an airstream enriched with 10% CO2. The maximum photosynthetic productivity was 34.4 g of dry biomass/(m2 of installation area x d) (12-h light/12-h dark cycle) when the cells were illuminated with an average photosynthetic photon flux density (photosynthetically active radiation ([PAR] 400-700 nm) simulating the outdoors in central Japan (0.980 mmol photons/[m2 x s]). This corresponded to a photosynthetic efficiency of 8.67% (PAR), which was defined as the percentage of the light energy recovered as biomass (394 kJ/[reactor x d]) to the total light energy received (4545 kJ/[reactor x d]). A similarly high photosynthetic efficiency (8.12% [PAR]) was also attained in the combined presence of 10% CO2, 100 ppm of NO, and 25 ppm of SO2. Moreover, good photosynthetic productivity was also obtained under high temperature and high light intensity conditions (maximum temperature, 46.5 degrees C; 1.737 mmol photons/[m2 x s]), when simulating the strong irradiance of the midday summer sun. This strain thus appears well suited for practical application for converting CO2 present in the stack gases emitted by thermal power plants and should be feasible even during the hot summer weather.