Production of biogas from different organic materials is a most interesting source of renewable energy.The biomethane potential (BMP) of these materials has to be determined to get insight in design parameters for anaerobic digesters. Although several norms and guidelines for BMP tests exist, inter-laboratory tests regularly show high variability of BMPs for the same substrate. A workshop was held in June 2015, in Leysin, Switzerland, with over 40 attendees from 30 laboratories around the world, to agree on common solutions to the conundrum of inconsistent BMP test results. This paper presents the consensus of the intense roundtable discussions and cross-comparison of methodologies used in respective laboratories. Compulsory elements for the validation of BMP results were defined. They include the minimal number of replicates, the request to carry out blank and positive control assays, a criterion for the test duration, details on BMP calculation, and last but not least criteria for rejection of the BMP tests. Finally, recommendations on items that strongly influence the outcome of BMP tests such as inoculum characteristics, substrate preparation, test setup, and data analysis are presented to increase the probability of obtaining validated and reproducible results.

In order to enhance the efficiency of anaerobic digestion, the effects of ultrasounds, ozonation and thermal pre-treatment have been studied on waste activated sludge. The feature of this study was to carry out the comparison of the three pre-treatments in the same conditions and on the same sludge sample. Each treatment was tested in two conditions close to optimum conditions to maximise batch anaerobic sludge biodegradability. All treatments led to chemical oxygen demand and matter solubilisation and had little influence on mineral matter. In terms of solubilisation thermal pre-treatment was better than sonication or ozonation. But, in terms of batch anaerobic biodegradability, best results were obtained with ultrasounds with an energy of 6250 or 9350 kJ/kg TS and a thermal treatment at 170 or 190 • C. Moreover, treatments had effects on physicochemical characteristics of sludge samples: apparent viscosity decreased after all treatments but the reduction was more important with thermal treatment. Median diameter of sludge flocs were reduced after sonication, increased after thermal treatment and did not change after ozonation. Finally, capillary suction time (CST) increased after ozonation, increased highly after sonication and was reduced after thermal treatment.

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