b State Key Laboratory of Pollution Control and Resource Reuse,Tongji University, 1239 Siping Rd., 200092, Shanghai, China.
c College of Marine Ecology and Environment, Shanghai Ocean University,999 Hucheng Ring Rd., 201306, Shanghai, China.
dWater Research Institute, Shanghai Academy of Environmental Sciences, 508 Qinzhou Rd, 200233 Shanghai, China. )
* Corresponding author
E-mail: lhsaes@163.com
Tel.: 86-21-64085119
Abstract
The harvesting of microalgae is the main bottleneck of its large-scale biomass production, and seeking an efficient, green, and low-cost microalgae harvesting technology is one of the urgent problems to be solved. Microbubble air flotation has been proved to be an effective measure, but the generation of the right size of microbubbles and the mechanism of microbubbles-algal cells attachment are still unclear. In this study, microbubble air flotation was used as a harvesting method for Microcystis cultured in agricultural wastewater. The process mechanism of microbubble air flotation harvesting microalgae in wastewater was fully revealed from three aspects (the design of bubble formation, the adhesion law, and the recovery rate of microalgae under different working conditions). The results show that the length of the release pipe is the main factor affecting the proportion of microbubbles with particle size less than 50 μm. In the process of adhesion, when the particle size of microbubbles is 0.6~1.7 times the size of Microcystis , the adhesion efficiency of microbubbles to Microcystis is the highest. Under the conditions of pressure 0.45 MPa, gas-liquid ratio 5% and release pipe length 100 cm, the harvesting performance ofMicrocystis was the best. Microbubble air flotation has better harvesting performance of Microcystis with higher density. By understanding the mechanism of microbubble flotation, the technical parameters of microbubble flotation for harvesting energy microalgae are optimized to provide support for the development of efficient and low-cost devices and equipment for collecting microalgae.
Keywords:Microcystis, Microbubble flotation, Agricultural wastewater, Mechanisms
1.Introduction
With the gradual depletion of fossil energy and the environmental pollution caused by the exploitation and utilization of fossil energy, finding clean and renewable new energy is an urgent problem to be solved in today’s field (Yoro et al., 2021). Energy microalgae, as a renewable biomass energy source, has become a hot spot because of its short growth cycle, large biomass per unit area and strong environmental adaptability (Peter et al., 2022; Siddiki et al., 2022). In the process of cultivating energy microalgae, it is necessary to provide growth elements including water, inorganic nutrients and CO2, whose cultivation cost accounts for more than 70% of the total cost of microbial diesel production (Rawat et al., 2011), which restricts the development of microalgae biomass energy technology. However, China, as the largest livestock breeding and consumer in the world (Aravani et al., 2022), with a large amount of aquaculture wastewater with high nitrogen and phosphorus content, the microalgae photosynthetic bioreactor was introduced into the microalgae photosynthetic bioreactor as the medium for energy microalgae after preliminary treatment, which could synergistic achieve the goal of deep purification of aquaculture wastewater and reduction of the culture cost of microalgae biomass energy (Lopez-Sanchez et al., 2022). However, from the perspective of large-scale production of energy microalgae, separation from the aqueous growth medium is difficult and cost as microalgae are small (3~30 μm) in dilute suspension, and have a specific gravity similar to that of their medium (Ali et al., 2021). In the energy microalgae industry chain, the cost consumed in the harvesting process accounts for about 20%~30% of the whole cultivation cost of microalga (Rawat et al., 2011). Therefore, considering the impact of energy microalgae harvesting technology on oil extraction and subsequent finishing operation, the method must not contaminate the biomass or create atoxic medium for recycle (Uduman et al., 2010).
At present, the harvesting technologies of energy microalgae mainly include centrifugation, flocculation, filtration, traditional air flotation, etc. (Zhou et al., 2021). Most current harvesting methods have either economic or technical limitations, which include high energy costs, flocculant toxicity, or non-feasible scale-up (Fasaei et al., 2018; Fuad et al., 2018; Laamanen et al., 2021; Oh et al., 2001; Rawat et al., 2013). Flotation recently emerging as a promising alternative based on its good scale-up potential due to technical and economic parameters, such as lower energy and maintenance costs. Traditionally flotation is done either by air addition through a diffuser (dispersed air flotation) or through pressurization (dissolved air flotation) (Aulenbach et al., 2010). Dissolved air flotation separation is more efficient than dispersed air flotation, is more commonly used, and is also proven on a large scale (Christenson & Sims, 2011).The traditional air flotation method requires the addition of chemical flocculant (Laamanen et al., 2021), which has a good harvesting effect, but has a high cost. Moreover, the quality of energy microalgae will decrease due to the cytolysis caused by aluminum salt (Teixeira et al., 2010).
It has been shown that smaller bubbles result in higher capture efficiency (Hanotu et al., 2012). Molina et al. present possibly the closest technique to micro-flotation for algal harvesting (Molina et al., 2003). One of the efficient ways of facilitating bubble–particle interaction in the liquid rather than merely passing the bubbles through the liquid without it adhering and lifting the particles out of solution, is to generate the right size of microbubbles. The operational parameters such as pressure, gas/liquid ratio, and set-up of the release pipe can affect the size of microbubbles. So far, studies on microalgae microbubbles flotation have mainly focused on the optimization of conditions, including hydraulic loading rate, initial algal concentration, air to solids ratio, pH, salinity, and the type of flotation. However, the mechanism of micro flotation for harvesting energy microalgae has not been revealed. The harvesting performance of the flotation process depends on the attachment of microalgae cells onto microbubbles. Therefore, a solid understanding of the attachment mechanism is necessary to perform this harvesting operation.
This work is aimed at performing an experimental investigation of the interactions between microalgae cells and microbubbles to gain a deep insight into the mechanism governing microalgae cell to microbubbles attachment in the flotation process. The morphological characteristics and motion law of microbubbles under different operational parameters were analyzed, and the response relationship between the particle size of microbubbles and the energy microalgae was clarified. The technical parameters of harvesting energy microalgae by microbubble air flotation were optimized to provide a basis for the application of microbubble air flotation harvesting technology in microalgae industrialization.
2.Material and Methods