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Albeit the biodiesel production from suspended microalgal system has gained immense interests in recent years, the domineering limitation of being economically infeasible has hindered this technology from partaking into a large-scale operation. To curtail this issue, attached growth system had been introduced by various studies; however, those were still unable to alleviate the socio-economic challenges faced in commercializing the microalgal biomass production. Thus, this study had developed a novel approach in cultivating-cum-harvesting attached Chlorella vulgaris sp. microalgae, whilst using solid organic waste of palm kernel expeller (PKE) as the supporting and alimentation material for microalgal biofilm formation. The effects of three variables, namely, PKE dosage, light intensity, and photoperiod, were initially modelled and later optimized using Response Surface Methodology tool. The derived statistical models could predict the growth performances of attached microalgal biomass and lipid productivity. The optimum growing condition was attained at PKE dosage of 5.67 g/L, light intensity of 197 μmol/m2 s and photoperiod of 8 light and 16 dark hours/cycle, achieving the microalgal density and lipid content of 9.87 ± 0.05 g/g and 3.39 ± 0.28 g/g, respectively, with lipid productivity of 29.6 mg/L day. This optimum condition had led to the intensification of biodiesel quality with a high percentage of monounsaturated fatty acid, i.e., oleic acid (C18:1), encompassing 81.86% of total fatty acid methyl ester components. Given that the positive acquisition of PKE as an excellent supporting material in enhancing the microalgal density and lipid productivity that had resulted in the commercially viable biodiesel quality, this study served as a novel revolution in augmenting the microalgae and solid waste utilities for sustainable energy generation. [Display omitted] • Palm kernel expeller as the nutrients and platform for attached microalgal growth. • Optimum cultivation condition to enhance microalgal density and lipid productivity. • FAME profile unveiled biodiesel quality with 81% of monounsaturated fatty acid. [ABSTRACT FROM AUTHOR]

Microalgae have been recognized as a potential feedstock for biodiesel production due to their fast growth rate and ability to accumulate lipid within their cells. However, high lipid content and biomass productivity are usually difficult to achieve simultaneously. Instead of stressing the microalgae during cultivation stage, the stress environment such as nutrient starvation, salinity and light effect were introduced to Chlorella vulgaris after harvesting to study the effect on their lipid content. From the results attained, one day of nutrient starvation with 6.0 g/L of salinity stress under dark room condition had shown the highest lipid content of 38.8% (dry weight basis). The lipid content was recorded at 40.28% (dry weight basis) when the working volume was scaled up. In addition, the fatty acids identified in the extracted microalgae lipid were mainly consisted of linoleic, linolenic and palmitic acid, which were commonly found in corn oil, soybean oil and sunflower oil. • High biomass productivity and lipid are difficult to achieve simultaneously. • The stress towards the lipid content of microalgae after harvesting was studied. • Triple stress condition resulted the highest lipid content of microalgae. • Fatty acids of microalgae has similar profile as corn, soybean and sunflower oil. [ABSTRACT FROM AUTHOR]

Changing the growth environment for microalgae can overall lead to the fundamental alteration in cellular biochemicals whilst attaching onto palm kernel expeller (PKE) waste to form adhesion complex in easing harvesting at stationary growth phase. This study had initially optimized the PKE dosage, light intensity and photoperiod in maximizing the attached microalgal productivity being attained at 0.72 g/g day. Lipid content increased progressively from pH 3 to pH 11, with the highest value observed at pH 11. Meanwhile, in terms of protein and carbohydrate contents, the highest values were obtained by cultivation medium of pH 5 with 9.92 g and 17.72 g, respectively followed by pH 7 with 9.16 g and 16.36 g, respectively. Moreover, the findings also suggested that the low pH mediums utilized polar interactions in the formation of complexes between PKE and microalgae, whereas at higher pH levels, the non-polar interactions became more significant. The work of attachment was thermodynamically favourable towards the attachment formation with values greater than zero which was also aligned with the microscopic surface topography, i.e., revealing a clustering pattern of microalgae colonizing the PKE surface. These findings contribute to comprehensive understanding of optimizing growth condition and harvesting strategy of attached microalgae in attaining the cellular biochemical components, facilitating the development of efficient and sustainable bioresource utilization. [Display omitted] • Attached microalgae onto palm kernel expeller in producing cellular biochemicals. • PH factor positively militated biochemical components of attached microalgae. • Low dispersive and polar energy surface yielded optimal microalgal productivity. • Microalgae biolayer formed cluster upon reaching stationary growth phase. [ABSTRACT FROM AUTHOR]