## 1 Introduction  
Depleting oil reserves, increased energy dependency, and the environmental impact of fossil fuel use has led to research on sustainable energy resources with a cleaner footprint [1; 2; 3]. Biofuel, in particular biodiesel, is a feasible diesel fuel replacement because it can be directly employed without modification of the engine structure [4; 5; 6]. Biodiesel generally refers to the mixture of fatty acid methyl esters and are derived from lipid substances originated from triacylglycered-containing feedstocks [7; 8; 9]. A review of life cycle analysis on liquid biofuel systems has concluded that conventional biofuels (from grains and seeds) can provide moderate greenhouse gas (GHG) reduction benefits in measures,such as 'per GJ fossil fuel displaced', 'per ha land use', etc., owing to high land requirements [10]. The use of oleaginous microalgae biomass will help minimize this arable land requirement and increase GHG emission reductions per hectare for any potential bioenergy application. Microalgae has a high photosynthesis efficiency, oil content, biomass productivity, and growth rate when compared with other terrestrial oilseed crops [11; 12]. Despite these advantages, more substantial greenhouse gas emissions, increased nutritional needs, and increased use of water were recorded to algae associated with existing terrestrial bioenergy feedstocks from a life cycle analysis perspective [13; 14; 15]. However, those views have been contradicted by others because of limitations in the assumptions of those studies [16]. Microalgae biomass is distinct in many ways from typical feedstock, viz. the cell wall and plasma membrane chemistry, water presence, and tiny cell size [17]. As such, the process of lipid or oil extraction from microalgae needs special attention. Typically, lipid extraction methods use solvents such as toluene, hexane, butanol, ethanol, methanol, or ionic liquids, to extract lipids from intact, chemically treated, or mechanically ruptured cells [18; 19]. The mechanical methods include oil press, microwave-assisted extraction, and ultrasonic-assisted extraction [20; 21; 22]. The chemical methods include solvent extraction, Soxhlet extraction, supercritical fluid extraction, and accelerated solvent extraction [22; 23]. The used organic solvents help in extracting oil by rupturing cell walls and disrupting the interaction forces between the lipid and tissue matrix [24]. An appropriately designed solvent or solvent matrix usage will significantly increase the efficacy of the extraction process. This work focused on ultrasonic, solvent, and Soxhlet extraction processes. Ultrasonic irradiation has been used to accomplish enhanced mixing between reacting components. [PERSON] et al. [24] employed the ultrasonication to extract lipids from spent coffee ground with three different solvents (hexane, chloroform, or methanol). [PERSON] and [PERSON] [25] studied the effect of the operating conditions of ultrasonic flow systems on cell disruption of two algal strains, _Scenedesmus dimorphus_, and _N. oculata_. On the other hand, the modified Folch lipid extraction protocol [26] is generally used as a chemical extraction method. Hexane is primarily used in this process due to its less toxicity than chloroform, minimal affinity towards non-lipid contaminants, and apparent higher selectivity towards neutral lipid fractions [27]. However, a combination of several solvents is generally used for maximizing the yield. This study investigated two different microalgae species, _Chlorella_ sp. and _Spirulina_ sp., which are widely available in Malaysia and the southeast Asian region and possess low lipid contents. Usually, microalgae contain lipid in the range of 20-50% [28; 29]. _Chlorella_ is a freshwater green unicellular alga with fast growth and an easy cultivation process. The lipid content of _Chlorella_ is about 14-18.7% by the weight of the dry biomass in usual growth conditions [30; 31]. On the other hand, _Spirulina_ sp.