Investigating various types of pre-treatments of whiskey distillery and brewery wastes for anaerobic digestion

Waste to energy conversion potential of whiskey distilleries and breweries is attracting more and more interest due to the high energy demands of manufacturing processes as well as environmental concerns regarding typical waste disposal methods. Whiskey distillery/brewery waste streams, commonly known as pot ale and spent grain, are classified as lignocellulosic material. A pre-treatment step prior to anaerobic digestion is therefore necessary to obtain sufficient lignin degradation and to achieve an increased hydrolysis rate. In this research project, a combination of alkaline pre-treatment with beating, microwave and ultrasonic pre-treatments were introduced to the literature as novel pre-treatment methods for whiskey distillery/brewery waste streams. Alkaline-beating hybrid pre-treatment was applied and optimised for both pot ale alone and a pot ale–spent grain mixture. The biogas generation of non-treated pot ale and pot ale-spent grain with a 50% inoculum on a wet basis was found to be 205 ± 21.4 ml biogas/g VS with 19.3% CH4 and 239 ± 3 ml biogas/g VS with 49.1± 2% CH4 respectively. A significant enhancement was seen after the implementation of a 1M NaOH and 7.5 min beating pre-treatment, and the biogas yields increased to 550±6 ml/g VS with a CH4 content of 54.3% and to 360±10 ml/g VS with 49.1% CH4 respectively. Response Surface Modelling was employed for modelling, statistical analysis and optimisation of the process. The most powerful design factor was identified as inoculum amount. Therefore, seeding ratio was scanned from 50 – 95% on wet basis with application of thermochemical pre-treatment. A 3-fold increase was achieved in biogas yield by reaching 2768±234 ml/g VS with combined 1 M NaOH alkaline and 240 W microwave pre-treatment prior to anaerobic digestion of pot ale with a 95% inoculum. On the other hand, no significant difference was seen in biogas yield after implementation of alkaline-ultrasonic pre-treatment. However, combined 3 M NaOH alkaline and 1 amplitude ultrasonic pre-treatment for 2 hours on pot ale offered a reduction in COD of 57±6.1% and a reduction of 60±3.3% in BOD. The mineral quality of the digestate (P, K, Ca, Mg, Fe, Mn, Zn and Cu) was analysed to assess it’s potential for agricultural use as a fertilizer; this compared favourably with an industrial digestate. A full-scale AD plant for industrial application was modelled based on the experimental results; CAPEX/OPEX estimations were obtained from published resources with data from 7 AD plant. It was determined that CSTR with an inoculum ratio of 65 – 95% in combination with CHP