Pre-treatments to enhance biogas yield and quality from anaerobic digestion of whiskey distillery and brewery wastes: A review

In order to encourage industrial growth based on sustainability, the replacement of fossil fuels with renewable sources has gained global importance. Anaerobic digestion (AD) fulfils the requirements for a sustainable alternative fuel, and is also an environmentally friendly waste treatment method. It requires less energy than other methods such as gasification or pyrolysis due to its low operating temperature. Whiskey distillery and brewery waste streams are classed as high strength organic wastes due to their high BOD/COD content, thus rendering them a suitable feedstock for anaerobic digestion. Due to large global alcohol production, millions of tonnes of solid and liquid waste is discharged annually, so the potential for waste-to-energy conversion can make anaerobic digestion an attractive treatment option for the waste streams of distilleries and breweries rather than diversion to landfill or incineration. However, these waste streams are lignocellulosic, containing high fractions of lignin and crystalline cellulose, meaning pre-treatments prior to anaerobic digestion can significantly enhance the biogas yield and organic matter degradation. Acid pre-treatment and enzymatic pre-treatment are particularly promising, with improvement in quality up to 74% CH4 for AD of spent grain, with 16% increase in biogas yield, and up to 87% reduction in COD. However, industrial application of pre-treatments prior to anaerobic digestion remains limited. This review collates the literature to date on pre-treatments applied prior to anaerobic digestion of whiskey distillery/brewery wastes as well as current industrial practices and different reactor configurations. A particular focus is placed on the impact of pre-treatments on biogas yield in order to highlight potential enhancements in biogas yields for industrial implications.