Characteristics Of Transfer And Biochemical Reactions In Microalgal Biofilm Photobioreactors

Microalgae has been regarded as one of the most potential renewable biomass energy owing to its advantages of high photosynthetic efficiency, carbon neutrality, ease to cultivate, short growth cycle, high lipid content, strong adaptability and so on. The microalgae attached cultivation method can improve the biomass production per unit volume of photobioreactor. Compared with conventional suspended cultivation mode, the attached mode offers many benefits, such as high operation stability, convenience of harvesting and low energy consumption of subsequent processing. In this study, agar gels were coupled with mixed cellulose membrane to build a photosynthetic microalgal biofilm reactor containing a twin-layer source/substrate structure. The agar gel layer was used for supply of nutrient sources during microalgal growth, while the filtration membrane layer was used as the framework for biofilm. Chlorella vulgaris was cultured both in a conventional column photobioreactor and a twin-layer microalgal biofilm reactor. The effect of light penetration and CO₂ concentration on C. vulgaris cells growth were explored to demonstrate the reasons why attached cultivation could achieve higher productivity and higher photosynthetic efficiency than suspended cultivation methods. Then, the effects of nutrient solution contents on absorbtion of nutritent solution, biofilm configuration, microalgal growth and lipid accumulation were investigated in a twin-layer microalgal biofilm reactor. In addition, to realize high-density cultivation of microalgae, save water source investment and reduce dehydration energy comsumption of harvesting, we proposed a biofilm photobioreactor that is based on the following rules:?1? the biofilm is inoculated onto permanent hydrophilic PVDF/PTFE composite membrane of good strength and biocompatibility,?2? the biofilm is sealed in a moisture saturated chamber. To verify the effectiveness of the proposed tactic, a serial of experiments were carried out to optimize the operational conditions and estimate the minimum water footprint required by C. vulgaris.The main conclusions of this work are as follows.?1? C. vulgaris was both cultured in biofilm and suspended cultivation systems. After seven days cultivation, areal density of C. vulgaris cultivated in attached system achieved a stable value of 99.44 g/m2, which is 35.35% higher than that in suspended system. The results showed that the upper layer of biofilm with 41.31?m?40 g/m2 of areal density? was effectively illuminated in the attached cultivation system under light intensity of 120 ?mol/?m2·s?. Attached cultivation facilitated relatively high biomass density of 100 g/m2 with 40% of microalgal cells in the effectively illuminated area. In contrast, only lower than 2.5% of microalgal cells were effectively illuminated withthe areal biomass density of 100 g/m2. When the initial inoculum density was 12.5 g/m2, which was considered as the optical inoculum density, biofilm growth rate of 27.85g/?m2·day? was obtained.?2? Furthermore, the microalgae that attached cultivated seem to be more amiable to CO₂ than that suspended cultivated. Microalgal biofilm grew better than suspension when air was used as the carbon source. The CO₂ saturation point was 1.5% CO₂ in attached cultivation systems and 4.5% CO₂ in suspended cultivation systems, respectively.?3? As the agar content increased from 0.125% to 8%, the nutrient contents decreased, microalgal cells shrunk, the thickness of microalgal biofilm decreased from 91.23 to 62.33?m, and the water content per unit biofilm thickness of day two reduced from 18.12 to 9.19mg/?m. Consequently, the CO₂ and light transfer resistance of internal biofilm decreased, whereas the areal density of microalgal biofilm incresased from 40.56 to 59.16 g/m2?45.86% increase?, and the lipid production increased from 25.21% by 63.39% to 28.24%.?4? According to the effects of CO₂ and light transmission and nutrient solution content of biofilm on microalgae C. vulgaris biofilm growth. A biofilm membrane photobioreactor with sealed chamber was designed and constructed. The optimum operational condition for high-density cultivation of microalgae and water source investment saving was 5 cm of chamber height, 200?mol/?m2·s? of illumination intensity, 0.1N-BG11×20 for medium stratage, 0.018 VVM aeration rate with 10%?v/v? CO₂ and 5L/m2 lowest medium colum. The biofilm membrane photobioreator realized high performance: biomass production of biofilm areal density 178.25g/m2, lipid content of 20.25%, lipid production of 36.10g/m2 were achived. Consequently, the water footprint for producing 1 kg C. vulgaris dry mass?which contains more than 20.25 % of lipid? and 1 kg pure lipid were 28.05 and 138.50 L, respectively, which were mostly decreased by 2828.95L/kg?dry mass? compared with that cultivated with suspended cultivation method.