## 1 Introduction  
Renewable energy resources are efficient and sustainable, contributing to the decrease in global warming. In this category, bioenergy can be produced using organic wastes, helping them to enter the regional circular economy and thus increasing the efficiency of locally available resources. Anaerobic digestion (AD) is a process that provides a bioenergy carrier gas as it can efficiently deliver biogas/biomethane from organic wastes and has all the advantages of renewable energy while reducing greenhouse gas (GHG) emissions [1, 2]. Presently, in Europe, municipal waste management includes AD of organic waste streams (domestic and industrial) as a frequent practice, as it is now a mature technology for producing biogas [3]. Combining sustainable microalgae cultivation with an anaerobic digestion of remaining microalgae to obtain biodiesel and biogas is a smart strategy to make specific biofuels and manage wastes properly. The microalga _Chlorella protothecoides_ can grow heterotypically or autotrophically, with demonstrated improved productivity in biomass and oil, as well as high environmental sustainability [4]. The oil content accumulated in microalgae differs from one species to another and depends on the cultivation mode used. Heterotrophic cultivation of microalgae presents several advantages over the autotrophic mode, the most notable being a higher oil productivity. Heterotrophic microalgae growth can still be combined with autotrophic microalgae cultivation to absorb the produced carbon dioxide and to avoid its emission to the atmosphere [5]. The biodiesel from microalgae is a third-generation biofuel; therefore, it does not compete with food production, as is the case with edible vegetable oils. The biodiesel production process is quite old and is based on extracting the oil from microalgae biomass and then making it react with alcohol to allow the transesterification into fatty acid methyl esters (FAMEs). The cellular debris from microalgae, after oil extraction, is a substrate adequate to convert into biogas through the AD process [6]. However, microalgae cells can resist biodegradation and the organic matter inside them will not be available for the anaerobic microorganisms to convert it, thus making hydrolysis the limiting anaerobic digestion step. There is some research on different pretreatment methods to increase the methane yield of microalgal biomass which highlight mechanical methods, such as ultrasound and hydrothermal (T > 100 \({}^{\circ}\)C, P > 1 bar) methods, and biological, enzymatic methods [7]. The pretreatment choice depends on the microalgae genera as they have considerably different cell walls. In the case of _Chlorella_ sp., the cell wall is medium robust, as its structure consists of only two layers compared with other genera which have three or four layers, and its composition is made of carbohydrate polymers with hemicellulose, galactose, and rhamne [8]. Moreover, the cultivation medium, e.g., with or without glucose, and the environmental conditions, e.g., with light or in the absence of light, used to grow _Chlorella_ cells may impact the cell wall structure and composition [8]. Hence, testing simple pretreatments should be enough to release intracellular soluble organic matter and increase the efficiency of AD. Although the microalgae theoretical methane yield was estimated to be in the range of 0.48-0.80 L CH\({}_{4}\)/g VS [9], experimental results until now have only achieved the first figure. Much attention has been given to autotrophically grown _Chlorella_ sp. [7]; therefore, more experimental data will be needed on the application of pretreatments, particularly for the heterotrophically grown microalga. The AD process starts with breaking down organic matter through a set of biochemical reactions accomplished by bacteria and archaea, producing an energy-carrying gas consisting of carbon dioxide and methane [3].