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