Publication: Development of granular microalgae through granulation process for efficient separation.
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Date
2025-08
Authors
Kabir Ahmad, Syahirah Faraheen
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Abstract
Microalgae is a type of phytoplankton which possesses a high level of photosynthetic efficiency and growth rate. Chlorella sp. are frequently used microalgae species in industries as well as for research purposes as it has exceptional temperature tolerance and ability to thrive in extreme climates. It is currently being recognised as a potential source of feedstock for bioenergy products. However, the separation of microalgae from its growth medium is the major issue that restricts the harvesting processes due to microalgae’s lower density. This research is carried out to develop denser and robust microalgae granules that can be harvested by self flocculation. In order to address the knowledge gap, a preliminary investigation of microalgae cultivation was conducted to determine the optimal conditions for achieving higher microalgae growth rates and productivity. Response Surface Methodology (RSM) method was employed to investigate the effect of inoculum concentration of microalgae cells, photoperiod, aeration rates, and pH towards microalgae growth and biomass productivity. The optimum growth rate and biomass productivity were obtained at 0.3215 d-1 and 0.2591 mg/L/d respectively, with the following cultivation conditions; inoculum concentration percentage of 11.1%, exposed under continuous (24 hr) illumination, aeration rates of 2.4 L/min and pH of 6. Subsequently, the optimized conditions were applied to cultivate microalgae in an 8 L sequencing batch reactor (SBR). Cultivation under optimal environmental conditions resulted in the increase of microalgae concentration of initially 50 mg/L to 2978.19 mg/L, corresponding to microalgae productivity of 44.4 mg/L/d with SVI30 and settling velocity of 58.51 mL/g and 19.2 cm/h respectively. The amount of EPS increased promptly from 10.62 μg/mL to 63.12 μg/mL within 42 days which led to development of microalgae granules which can be harvested by self-flocculation within 15 min. This research also investigates the effect of carbon availability on microalgae growth in a SBR. Three 2 L sequential batch reactors with different carbon types namely reactor A-mixotrophic cultivation mode (C₆H₁₂O₆+ NaHCO₃+ Bold basal nutrient), reactor B-autotrophic cultivation mode (C₆H₁₂O₆+ Bold basal nutrient), reactor C-heterotrophic cultivation mode (NaHCO₃+ Bold basal nutrient) were operated. After 40 days of operation, the findings showed that microalgae developed under mixotrophic conditions had better overall results. Subsequently, the optimized growth and mixotrophic conditions were applied to cultivate microalgae in an 8 L SBR under sterilized conditions. The cultivation of microalgae in SBR under these conditions resulted in the successful formation of denser and resilient microalgae granules with SVI30 of 13.7 mL/g and settling velocity of 27.9 cm/h, harvested within 5 min. The microalgae concentration has increased from an initial value of 500 mg/L to 4511 mg/L, resulting in a microalgae productivity of 100.2 mg/L/d. The harvested microalgae granules were analysed and evaluated for their potential to be used as bio pigments, bio-polymers, and bio-adhesives materials. The microalgae granules demonstrated potential for resource recovery, with enhanced chlorophyll retention and a maximum PHA yield of 0.7221 g PHA/g biomass. The granules produced bio-oil with 21.4 wt.% yield, predominantly composed of phenolic compounds. Overall, this study concluded that, microalgae able to self-flocculate and form dense granule without addition of bacteria or chemical flocculant, which can easily be harvested under optimum conditions and the biomass harvested can be used in wide range of applications.