| Microalgae has great potential applications in biofuel production,carbon reduction,aquaculture and wastewater treatment,but these applications are seriously limited due to its high cost of biomass production.Different from higher plants obtaining CO2 from air,microalgae cultivation requires carbon supply through artificial approach to reach high productivity,and this is the fundamental reason why it has a much higher production cost than plants by far.To solve this problem,the aim of this research is to supply carbon source with sodium bicarbonate solution based on bicarbonate-based integrated carbon capture and algal production system(BICCAPS),to develop a novel floating photobioreactor(PBR)without aeration or an agitation device,which has simple constructure that is easy to be scaled up,and uses wave as sole mixing energy to reduce mixing cost.Then,the feasibility of recycling culture that absorbed CO2 after cultivation was studied.The primary contents and results are shown as below:(1)Development of small scale floating PBRFirstly,alkalihalophilic cyanobacterium Euhalothece sp.ZM001 was used as model microorganism to test the feasibility of using wave energy as sole mixing energy to culture microalgae,where the small scale floating PBRs were deployed on 10 cm high artificial wave.The final biomass concentration was up to 0.91 g L-1 and 1.47 g L-1 for indoor and outdoor cultures,respectively.However,the recorded dissolved oxygen(DO)was occasionally over-saturated(>500%air saturation),indicating the PBR has poor mass transfer;Then,kLa(O2)in these PBRs with different culture depth was measured,and the results showed great variations,from 0.13 h-1 to 4.87 h-1;Also,the growth of at the scale of 1.0 m2 floating PBR was investigated,which was made with low cost membrane and inflatable design and was placed on ocean surface and mixed with natural wave.Biomass concentration of 1.63 g L-1 and productivity of 8.27 g m-2 day-1 was obtained in this culture;Also,other microalgae species of Dunaliella salina,Spirulina platensis and Chlorella sp were both successfully cultured in this PBR.With these results,the feasibility of using wave energy as sole mixing energy to culture microalgae was proved.(2)Scaling-up of floating PBRCultivation of commercial S.platensis was conducted to study the scaling-up of floating PBR from 1.0 m2 to 10 m2.Firstly,the effect of sodium bicarbonate concentration on the growth of S.platensis was studied.The results demonstrated that S.platensis had the highest productivity at 0.3 mol L-1 sodium bicarbonate,but the highest carbon utilization(104 ± 2.6%)was obtained at 0.1 mol L-1 sodium bicarbonate;In cultivation using a floating horizontal PBR at the 1.0 m2 scale,the highest biomass concentration of 2.24 ± 0.05 g L-1 was obtained with a culture depth of 5.0 cm,while the highest biomass productivity of 18.9 g m-2 day-1 was obtained with a depth of 10.0 cm;This PBR was scaled up to 10 m2 with few challenges,where its biomass concentration and productivity during ocean testing were little different than those at the 1.0 m2scale(p<0.05).However,the larger PBR had an apparent carbon utilization efficiency of 45.0 ± 2.8%,significantly higher than the 39.4 ± 0.90%obtained at the 1.0 m2 scale.These results verified the ease of scaling up floating horizontal PBR.(3)Experimental study on hydrodynamics of floating PBRThe liquid mixing in floating PBR was induced by its motion in response to wave,and the motion was affected by wave condition(wave height and wave period),PBR constructure and water depth.Firstly,the effect of wave height(2?10 cm)and wave period(0.8?1.8 s)on the motion was investigated.The results indicated that the motion became more intensive with increasing of wave height,but to be become gentle when the wave period was decreased.However,the square PBR(1.0 m:1.0 m,Length:Width)experienced more intense motion than rectangle PBR(1.7 m:0.6 m,Length:Width),but experienced little mooring force;Then,the effect of water depth(5.0,7.5 and 10.0 cm)was investigated,and the results shown that there was significant difference between floating PBRs' hydrodynamic movements when filled with different depths of water,but the mooring forces are at same level,where the motion of PBR with 10 cm water is most intensive;Finally,the motion and mooring-line force on PBR equipped with different mooring system was studied.The two mooring systems has little effect on PBR motion,but mooring systems with floater can significantly reduce the mooring line force compared with that of system without floater.(4)Alkaline flocculation of Neochloris oleoabundans and its recycling culture with CO2 absorptionN.oleoabundans was created to study the feasibility of alkaline flocculation under high pH condition and its recycling culture after CO2 absorption.The effect of sodium bicarbonate concentration on N.oleoabundans growth showed that the highest productivity was obtained at 0.3 mol L-1 sodium bicarbonate,but the highest apparent carbon utilization efficiency was obtained at 0.1 mol L-1 sodium bicarbonate;The harvest of algal biomass was tested with alkaline flocculation,which is induced by high pH due to bicarbonate consumption.The result showed that the maximum recovery rate of 97.7 ? 0.29%was reached with a supplement of 20 mM Ca2+.Compared with this,alkaline flocculation without Ca2+ also resulted in a high recovery rate of up to 97.4 ± 0.21%in culture with 0.7 mol L-1 sodium bicarbonate;In recycling culture,the spent medium was bubbled with CO2 and re-used for algal culture.After 8 times of recycling,biomass productivity in recycling culture with 0.1 and 0.3 mol L-1 bicarbonate was 0.24 and 0.39 g L-1 day-1,respectively,higher than the 0.20 and 0.30 g L-1 day-1 in the control.The apparent carbon utilization efficiencies achieved in these semi-continuous cultures with 0.1 mol L-1 bicarbonate were 242 ± 3.1%and 266 ± 11%for recycling and control culture,respectively,while those with 0.3 mol L-1 bicarbonate were 98 ± 0.78%and 87 ± 3.6%,respectively.This study proved the feasibility of BICCAPS recycling culture with the first practical example.More importantly,the produced algal biomass can be harvested without any flocculant supplement.Thus,this process can reduce both culturing and harvesting costs in algal biomass production.These results demonstrate the feasibility of using floating PBR driven by wave energy to culture alkalihalophlic microalgae,the scaling-up experiment conducted on ocean verifies that the floating PBR is easy to be scaled up,while the study on hydrodynamic provides basic data for the development of large scale PBR and its safe operation,and the successful recycling culture proves that high alkaline environment can enhance the auto-flocculation of microalgae,and it can absorb carbon dioxide to reculture microalgae without bicarbonate net consumption.The production cost of the floating PBR developed in this study is one order of magnitude lower than that of the traditional PBR,which can greatly reduce the production cost of microalgae.Moreover,using this system to culture microalgae is similar to crop planting,which only cost manpower and energy consumption on inoculation and harvesting process,and the mixing can be realized completely by wave energy during whole cultivation process.Hence,using the floating PBR can not only further reduce the production cost,but also,more importantly,it can achieve the goal that harvesting the higher output energy from microalgae than its input energy,laying a foundation for the production of microalgae biofuels. |
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