Performance, 5-aminolevulinic acid (ALA) yield and microbial population dynamics in a photobioreactor system treating soybean wastewater: Effect of hydraulic retention time (HRT) and organic loading rate (OLR)
Introduction
Purple non-sulfur bacteria (PNSB) have been applied to treat various nutrient wastewaters such as soybean wastewater, molasses wastewater, fermented starch wastewater, citric acid wastewater (Liu et al., 2015, Prachanurak et al., 2014, Yetis et al., 2000, Zhi et al., 2010). PNSB can utilize organic substances in wastewaters to produce valuable biomass resources including bacteriochlorophylls, biopolymers, 5-aminolevulinic acid (ALA), carotenoids, CoQ10, etc (Kang et al., 2012, Kien et al., 2010, Kuo et al., 2012). Among these valuable products, ALA has drawn increasing interests as a photodynamic chemical, herbicide or insecticide, which has been widely applied in medical, agricultural biotechnological fields (Carmichael, 1992, Kang et al., 2012, Sasikala et al., 1994, Sasaki et al., 2002). ALA production by PNSB has great potential because of simple biosynthetic process, low cost and widely substances (Choorit et al., 2011, Sasaki et al., 1998). Since ALA is of increasing demand in many fields, more and more attention are devoted to increasing ALA production by PNSB (Choorit et al., 2011, Chung et al., 2005).
PNSB wastewater treatment is usually carried out in a photobioreactor. Controlling operation parameters is critical to the treatment performance of and products yields in a photobioreactor system. Previous studies focused on enhancing PNSB biomass and ALA yield by regulating conditions including pH (Chung et al., 2005), metal ions addition (Tangprasittipap et al., 2007), synthetic substrates (Ano et al., 1999), ALA dehydratase inhibitor (Ano et al., 2000) and metabolic engineering (Kang et al., 2012). Moreover, soybean wastewater was proved to be a popular culture for PNSB growth and biomass accumulation (Lu et al., 2011, Wu et al., 2012). Hence, soybean wastewater was selected for PNSB biomass and ALA production in this study.
Two functional microorganisms exist in the photobioreactor system, one is chemoheterotrophic bacteria, the other is photosynthetic microorganisms including microalgae or PNSB. Photosynthetic microorganisms have longer growth period than chemoheterotrophic bacteria (Park and Craggs, 2011, Chu et al., 2015). The specific growth rate of PNSB is generally slower than that of chemoheterotrophic bacteria, so PNSB requires relatively longer hydraulic retention time (HRT) than chemoheterotrophic bacteria in the photobioreactor system (Eroglu et al., 1999, Koku et al., 2003). Moreover, it was reported that HRT and organic loading rate (OLR) had obvious impacts on biomass and carotenoids productions in the photobioreactor for treating acid wastewater in our previous study (Liu et al., 2016). Hence, regulating HRT was critical to the pollutant removal efficiency, PNSB biomass and valuable materials production of PNSB-chemoheterotrophic bacteria wastewater treatment system. Few studies are reported about the effects of HRT on PNSB biomass, ALA production and functional microorganism population dynamics. Different levels of PNSB biomass, ALA yield and pollutant removal were influenced by varying HRTs. Similarly, as another key parameter, OLR has significant impacts on the efficiency of photobioreactor system. Soybean wastewater was proved to be a good culture medium for PNSB growth and ALA production (Liu et al., 2015), so it was used for biomass and ALA production of PNSB in this study.
The purposes of this study were to assess the efficiency of soybean wastewater treatment under different HRTs and OLRs in a photobioreactor; to investigate the impacts of different HRTs and OLRs on PNSB biomass and ALA yield in the continuous culture photobioreactor; to analyze the functional microorganism population dynamics depending on different HRTs and OLRs in the photobioreactor.
Section snippets
Inoculum
A purple non-sulfur bacteria (PNSB) strain (Rhodobacter sphaeroides ATCC17023) was obtained from China General Microbiological Culture Collection Center (CGMCC) in this study. It was cultured in a thermostat shaker (static, 30 °C) under light-microaerobic conditions with PYG medium for 36 h. PYG medium comprised 10 g/L polypepton, 5 g/L yeast extract and 1 g/L glucose, pH of PYG medium was adjusted to 6.8–7.0. The density of R. sphaeroides at logarithmic growth phase in the initial inoculum was 6.8 ×
HRT and OLR conditions set
R. sphaeroides growth and its ALA production kinetics were investigated and the results were displayed in Fig. 1A and B. The dynamics simulations revealed that R. sphaeroides growth and ALA biosynthesis followed Logistic model and Luedeking Piret model, respectively. R. sphaeroides growth period was about 96 h and ALA biosynthesis period was 24–96 h. Hence, to increase R. sphaeroides biomass accumulation and ALA production by optimizing HRT condition, HRT ranged between 24 and 96 h. Three HRT
Conclusions
Optimal HRT and OLR improved the pollutant removal efficiency, biomass accumulation and ALA production of the photobioreactor system. The highest biomass and ALA productions were got under the HRT of 60 h and 2.48 g/L/d. Functional microorganisms including PNSB and total bacteria dynamics of the photobioreactor were obviously influenced by HRT and OLR, which determined the photobioreactor performance. At the optimal operation phase, PNSB were the dominant microorganism in the system, and PNSB/
Acknowledgements
This study was financially supported by the National Natural Science Foundation of China (51278489).
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