Designing a biochar-based pretreatment method for distillery effluents entering constructed wetlands

Abstract

Spent lees is an important distillation waste by-product created during whisky production, characterised by low pH and high dissolved copper (dissCu) content. Constructed wetland systems (CWs) are used in some Scotch whisky distilleries to treat spent lees prior to surface water discharge to prevent substantial environmental risks of spent lees in aquatic ecosystems, by buffering the acidic nature and decreasing the dissCu and organics content. However, as these wetland systems are biological communities (containing bacteria, fungi, plants, etc.), direct disposal of spent lees into CW ecosystems presents a challenge given its nature. This study investigates the efficacy of modified biochar (waste) sourced from whisky cask cooperage operations, as an effluent pretreatment method. The aim is to transform this raw biochar into NaOH-modified biochar, which will be capable of buffering the effluent pH and removing dissCu from spent lees before it enters CWs. To enhance raw biochar, NaOH modification and additional heat treatment were utilised which increased the alkalinity, heavy metal removal capacity and pH buffering potential. Experiments with 0.2% and 2% NaOH treated biochar demonstrated its treatment performance using a diverse range of spent lees samples with varying characteristics, including pH (4.1–4.4), dissCu concentrations (33.8–59.5 mg/L), and total organic carbon (685–754 mg/L). Column studies further supported the efficacy of the modified biochars, showing significant pH elevation and efficient dissCu removal (of 5.72 mg of Cu per g of biochar) when treating spent lees. This dissCu removal capacity compares favorably with other wood-derived biochars studied in the literature, which have reported between 2.75 and 7.44 mg/g. Surface characterisation techniques (EDX, XPS, FTIR, XRD) showed how the Cu ions microprecipitated on the modified biochar surface, validating its remediation potential. This study highlights the potential of biochar as a sustainable pretreatment solution for distillery effluents, paving the way for enhanced and more circular waste management practices within the Scotch whisky industry.