The use of fossil fuels, emission of greenhouse gases (GHG) into the atmosphere, and waste pose a problem to the environment and public health that urgently needs to be dealt with. Among numerous chemical activating agents that can be added to anaerobic digestion (AD) to enhance nutrient removal and increase the quality and quantity of biomethane, iron chloride (FeCl) is the one that has the lowest cost and is the most environmentally friendly. This state-of-the-art review aims to revise the influence of FeCl on the Brunauer-Emmett-Teller (BET) surface area of biochar and its ability to increase methane (CH) yield and remove contaminants from biogas and wastewater. The novelty of the study is that FeCl, an activating agent, can increase the BET surface area of biochar, and its efficacy increases when combined with zinc chloride or phosphoric acid. Regarding the removal of contaminants from wastewater and biogas, FeCl has proven to be an effective coagulant, reducing the chemical oxygen demand (COD) of wastewater and hydrogen sulfide in biogas. The performance of FeCl depends on the dosage, pH, and feedstock used. Therefore, FeCl can increase the BET surface area of biochar and CH yield and remove contaminants from wastewater and biogas. More research is needed to investigate the ability of FeCl to remove water vapor and carbon dioxide during biogas production while accounting for a set of other parameters, including FeCl size.