Mechanical Pre-treatments Evaluation of Cattle Manure Before Anaerobic Digestion

Abstract

Pre-treatments are often applied in anaerobic digestion in order to improve the performance of the process: increased methane yield and rate. Physical pretreatments are the most commonly used for anaerobic digestion of agricultural waste. The main purpose of this work was to evaluate the effects of different lab-scale mechanical pretreatments on cattle manure. Three mechanical pre-treatments were sequentially applied to cattle manure: shredding, mixing and blending. An original set of analytical procedures was performed before and after mechanical pretreatments, in order to assess physical, chemical and biological properties of the feedstock. Thus, shredding and mixing pre-treatments were not significantly efficient in terms of methane production yield and rate. In contrast, methane production rate increased with blending treatment, and methane yield was slightly improved as well. An increase of soluble chemical oxygen demand (COD) was also reported with blending pretreatment. The reduction of particle size was mainly between 31.5 and 4 mm for all pretreatments and it led to an increase of water retention capacity of the cattle manure. Therefore, mechanical pre-treatments do not only affect the biochemical characteristics of agricultural waste. It also influences its structure and reactivity. In this context, blending pretreatment showed as the most performant pretreatment among the evaluated pretreatments with an increase of 18% on methane rate and a higher COD solubilisation. Graphic Abstract Mechanical pre-treatments are present in almost all biogas plants treating organic waste. However they have been far less investigated than thermal chemical or biological pre-treatments. The objective of this work was to characterize different mechanical effects (shredding mixing and blending) on cow manure. An original characterization procedure has been used including biochemical and physical parameters. Our results show that the methane yield and methane production rate was improved by the pre-treatments the most important effect being blending. Shredding mostly reduced the size of the large particles but its effect on the biochemical characteristics was limited.