| Advanced wastewater purification with microalgae possess unique advantages including low energy consumption, fixing carbon dioxide(CO2), generating oxygen(O2) and production of useful biomass, etc. However, the poor settleability of microalgae and the difficulty to separate the microalgae cells from the treated water have largely hindered its industrial application. Therefore, achieving effective separation through high settleability is one of the keys to promote the industrial application of microalgae based wastewater treatment technologies.To address these issues, this study attempted to enrich settleable microalgae species form mixed culture by adopting certain selective pressures. The selective pressures were created by controlling the settling time(ST) and volume exchange ratio(VER, filled volume/total reactor volume) in batch microalgae cultivation systems. The principal is that the ST and VER together will result in a critical settling velocity. Any microalgae species with a settling velocity lower than that critical velocity will be washed out from the cultivation system, and the settleable species become dominant.The whole experimental study consisted of two stages: Stage 1, exploratory experiments, aiming at validating the above hypothesis. According to different VER values, the experiments were divided into two groups:(1) VER=0.2; and(2) VER=0.7. Within each group, there settling times – 10 min, 30 min and 60 min was set respectively. Stage 2, extended experiments, focusing on physiological & biochemical characteristics of the selected settleable microalgae species, wastewater purification performance and biomass utilization potential. According to the different VER values, the experiments were divided into three groups:(1) low VER=0.2;(2) moderate VER=0.5 and(3) high VER=0.7. Within each group, two settling times – 10 min and 60 min was set respectively. All the experiments were inoculated with the same microalgae suspension, which was wastewater(secondary effluent) born mixed culture. The inoculation was dominated by Chlorella and Scenedesmus, with poor settleability of around 30%.The results from the exploratory experiments validated that applying selective pressures(ST and VER) could indeed wash out the slowly settleable species and thus enrich the fast settleable species. Among them, the systems with a VER of 0.2 finally enriched Oscillatoria sp. as the dominant species, with a highest biomass density up to 2.745 g·L-1; while the systems with a VER of 0.7 finally enriched Melosira as the dominant species, with a highest biomass density up to 4.515 g·L-1. Both the enriched microalgae species are large filamentous algae species with excellent settleability; during steady state of the operation, their settleability could reach to as high as 100%.With different VER values, three systems were established during the extended experiments:(1) system one(VER=0.2) with Oscillatoria as the dominant species;(2) system two(VER=0.5) with both Oscillatoria and Melosira as the dominant species; and(3) system three(VER=0.7) with Melosira as the dominant species. By comparing the results with those from the exploratory experiments, it can be concluded that specific species’ enrichment could be achieved by manipulating the VER. Regarding the settleability, the three systems achieved a settleability of above 97% on day 32(VER=0.2), day 15(VER=0.5) and day 11(VER=0.7) respectively after the start up. Within the system, different settling time had no significant effect on the improvement of the settleability. These results suggested that high VER was the key factor to enrich fast settleable species form mixed culture.In wastewater purification, the dominate species showed different impacts on nitrogen and phosphorus removal. The Oscillatoria system(VER=0.2) achieved the best nitrogen reduction performance. The highest NO3--N removal efficiency was 98.7%, with an effluent NO3--N concentration as low as 0.04 mg·L-1. The Oscillatoria-Melosira system(VER=0.5) and the Melosira system achieved average NO3--N removal efficiencies of 83.7% and 64.1%, with effluent NO3--N concentration of 1.7 mg·L-1 and 3.7 mg·L-1 respectively. In contrast, all the systems achieved an extraordinary phosphorus removal performance, with phosphorus removal efficiency of above 99% and effluent PO43--P concentration consistently below 0.05 mg/L.The harvested biomass could have certain utilization potentials. Therefore, in the stable operation period during the extended experiments, the harvested microalgae biomass was determined for oil, protein and polysaccharide contents. The results showed both the Oscillatoria and Melosira had a low oil content(6.5% and 10% respectively), suggesting that the microalgae biomass did not have a good potential for biodiesel production. However, they have good potential to produce protein due to the high protein content of 47% and 41%.In conclusion, this study verified that fast settleable microalgae species could be enriched from mixed culture by applying certain selective pressures. High VER was the key selective pressure. In addition, VER has determinable effect on dominance of the selected species. The excellent settleability brings good potential to achieve high density microalgae cultivation and high-rate advanced wastewater purification. |
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