In today’s industry, about 80% to 90% of citric acid (C6H8O7) production is carried out using submerged fermentation method and Aspergillus niger as citric acid producing microorganism. The use of fermentation method provides advantages, including the utilization of agricultural waste as substrate. Rice husks can be used as substrate in citric acid fermentation because they contain high cellulose and hemicellulose. However, agricultural waste also contains lignin which can interfere with the hydrolysis of cellulose and hemicellulose into reducing sugars, therefore optimisation of pre-treatment process is needed to separate lignin in rice husk. In addition to the optimisation of pre-treatment process, optimisation of fermentation parameter variations to obtain maximum citric acid yield is necessary. Therefore, the aim of this narrative review is to desribe various pre-treatment methods that can be done to remove lignin from rice husks, and to provide information on the benefits of using the Central Composite Design method in the optimisation of various fermentation parameters.