Citation: Krishnamoorthy, A.; Rodriguez, C.; Durrant, A. Sustainable Approaches to Microalgal Pre-Treatment Techniques for Biodiesel Production: A Review. Sustainability 2022, 14, 9953. https:// doi.org/10.3390/su14169953 Academic Editor: Raf Dewil Received: 11 July 2022 Accepted: 5 August 2022 Published: 11 August 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). sustainability Review Sustainable Approaches to Microalgal Pre-Treatment Techniques for Biodiesel Production: A Review Amarnath Krishnamoorthy, Cristina Rodriguez * and Andy Durrant School of Engineering and Computing, University of the West of Scotland, Paisley PA1 2BE, UK * Correspondence: cristina.rodriguez@uws.ac.uk Abstract: Microalgae are a potential source of numerous nutritional products and biofuels. Their applications range from the food industry to the medical and fuel sectors and beyond. Recently, the conversion of biomass into biodiesel and other biofuels has received a lot of positive attention within the fossil fuel arena. The objective of biorefineries is to focus on utilising biomass efficiently to produce quality biofuel products by minimising the input as well as to reduce the use of chemical or thermal pre-treatments. Pre-treatment processes in biorefineries involve cell disruption to obtain lipids. Cell disruption is a crucial part of bioconversion, as the structure and nature of microalgae cell walls are complex. In recent years, many research papers have shown various pre-treatment methods and their advantages. The objective of this paper was to provide a comprehensive in-depth review of various recent pre-treatment techniques that have been used for microalgal biodiesel production and to discuss their advantages, disadvantages, and how they are applied in algal biorefineries. Keywords: biofuel; microalgae; pre-treatment; biomass; cell disruption; lipids; biodiesel; renewable energy 1. Introduction Natural resources are a significant part of the economic structures that meet the requirements of humanity. With the increasing human population, economic production is also constantly growing, which paves the way for research into the creation of new products and the innovation of current materials in an attempt to overcome the energy crisis. The energy crisis is one of the greatest current concerns for the world’s stability and peace. Countries with developing economies that have limited natural resources need to secure fuel supplies. Fossil fuels, such as coal, petrol, natural gas, etc., have been viewed as fundamental energy sources [1], and they are used in very large amounts around the world. However, our long-term dependence on fossil fuels has challenged the lowering of greenhouse gases and has paved the way for global warming. The increase in the earth’s overall temperature due to various human activities and natural causes has also contributed to the phenomenon of global warming. Some data have shown that the increase in the global temperature may result in increased health risks in future generations [2]. In order to retain clean ecosystems and maintain stability, renewable and eco-friendly biofuels are needed to replace fossil fuels [3]. These replacements are derived from natural resources, such as microalgae [4]. Algae comprise macroscopic and microscopic organisms, with some macroscopic organisms growing to a length of 10 m, and some microscopic organisms growing to a few micrometres in size. Microalgae are considered to be a fascinating resource for industries, as they are helpful for producing multitudinous products because of their high growth rate, photosyn- thesis efficiency, and process optimisation. They have already been used in commercial industries, such as in animal feed, food, therapeutics, cosmetics, and biofuel [5–9]. The main advantages of culturing microalgae are that they can be cultured with minimal space, fewer nutrients, and minimal water (saline or brackish water) [10,11]. Microalgal cells are Sustainability 2022, 14, 9953. https://doi.org/10.3390/su14169953 https://www.mdpi.com/journal/sustainability