Methods for the Evaluation of Industrial Mechanical Pretreatments before Anaerobic Digesters.

Different methods were tested to evaluate the performance of a pretreatment before anaerobic digestion. Besides conventional biochemical parameters, such as the biochemical methane potential (BMP), the methane production rate, or the extent of solubilization of organic compounds, methods for physical characterization were also developed in the present work. Criteria, such as the particle size distribution, the water retention capacity, and the rheological properties, were thus measured. These methods were tested on samples taken in two full-scale digesters operating with cattle manure as a substrate and using hammer mills. The comparison of samples taken before and after the pretreatment unit showed no significant improvement in the methane potential. However, the methane production rate increased by 15% and 26% for the two hammer mills, respectively. A relevant improvement of the rheological properties was also observed. This feature is likely correlated with the average reduction in particle size during the pretreatment operation, but these results needs confirmation in a wider range of systems.

An improved procedure to assess the organic biodegradability and the biomethane potential of organic wastes for anaerobic digestion.

In this study a fractionation procedure was developed and applied to evaluate the potential of some organic wastes (two cattle manures and two catch crops, fresh and after ensiling) for anaerobic digestion. This procedure was based on water extraction of the raw sample, which enabled the evaluation of the contributions of water-soluble and particulate phases to the investigated properties. Biomethane potential (BMP) and chemical oxygen demand (COD) were determined and used to assess the anaerobic biodegradability of raw materials. Analysis of structural carbohydrates, total Kjeldahl nitrogen, water-soluble carbohydrates, volatile fatty acids and pH were also included to explain the main phenomena involved in methane production from the tested biomass. Results show that the origin and the preparation mode had a significant impact on BMP distribution. Based on a COD balance, the biodegradability of the various feedstocks ranged from 45% to 75%. Biodegradability of fresh materials was negatively correlated with the sum of structural carbohydrates and lignin content. Among the feedstock used, the water-soluble phase represented 8-69% of the total COD and 7-46% to the total BMP. Solubilization of organic matter during ensiling was due to the production and accumulation of organic acids from particulate carbohydrates and organic nitrogen. This procedure detects kinetic and biodegradability differences among biomass and thus it can be useful for the design of anaerobic digestion plants. Furthermore, it can be applied to evaluate the efficiency of biomass pretreatments.