Modelling and optimisation of the biogas yield after hybrid alkaline-ultrasonic pre-treatment in the early stages of anaerobic digestion of pot ale to shorten the processing time

Whiskey distillery wastewater (pot ale) is classified as a high organic content wastewater and its year-long large discharge volume makes it a suitable substrate for anaerobic digestion from environmental and economical perspectives. Prior to anaerobic digestion, a hybrid alkaline-ultrasonic pre-treatment was performed in order to alter the lignocellulosic structure of the pot ale. Effects of alkaline dose (0-3 M NaOH), amplitude ratio (40-100%) and exposure time (1-3h) of ultrasonic pre-treatment on CH4, CO2 and H2S generation within the first 2 days of anaerobic digestion were investigated at a lab scale batch reactor. Response surface methodology (RSM) was adopted as a process modelling and optimisation tool. Significant enhancements in the hydrolysis rate constant and methane yield were achieved in the early stages of digestion. The highest methane yield of the first 2 days digestion was 333 +/- 5 mL/g VS after implementation of ultrasonic pre-treatment at 70 % amplitude for 3 h in isolation leading to 48 +/- 4.4 and 56 +/- 3.5 % reduction in chemical and biological oxygen demand respectively. The optimum pre-treatment conditions according to combined numerical and graphical optimisation, to maximise CH4 yield while minimising the H2S generation was identified as 40 % amplitude ratio, 1-2.5 h exposure time and 0 - 0.6 M NaOH. (C) 2020 The Authors. Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.

Automated summary

The study demonstrates that after alkaline-ultrasonic pre-treatment, significant enhancements in hydrolysis rate constant and methane yield were achieved in the anaerobic digestion of whiskey distillery wastewater. Optimal conditions to maximize methane yield while minimizing hydrogen sulfide generation were identified as 40% amplitude ratio, 1-2.5 h exposure time, and 0-0.6 M NaOH.