Impact of headspace pressure on the Biochemical methane potential (BMP) experiments

• BMP tests were conducted using manometric and automatic volumetric test systems. • All the conditions produced more than 352 nmL/g of VS of biomethane. • At highest headspace pressure a 43.21% reduction in biomethane yield was observed. • Smaller headspace resulted in a higher initial concentration of biomethane. Biochemical Methane Potential (BMP) tests are fundamental for assessing the methane production capacity of organic substrates used for anaerobic digestion (AD). However, reproducibility of such tests still represents a significant challenge due to variations in experimental setups, the measurement techniques for biogas volume, as well as inoculum characteristics. To address these issues, standard protocols are used to ensure the reproducibility of results, but such protocols still have some gaps that can impact the test. This research aims to evaluate the impact of headspace pressure in pressurized and non-pressurized bottles, using manometric and automatic volumetric BMP systems to generate those different conditions, varying the filling volume of the bottles (from 30% to 70%) and the inoculum-to-substrate ratio (ISR) (from 2 to 6), to highlight its influence on methane yield. The results indicate that high headspace pressure, caused by a high filling volume and a low ISR, correlates with reduced methane production. The headspace composition leads to the assumption that CO 2 dissolves into the liquid phase at high-pressure, leading to pH alterations that could inhibit the process. Our findings demonstrate that headspace pressure must be carefully controlled in BMP tests, especially when using manometric systems, to avoid underestimation of methane potential.