Identification of Critical Problems in Biochemical Methane Potential (BMP) Tests From Methane Production Curves

Abstract

Biochemical methane potential (BMP) tests are routinely conducted in academia and industry to determine the methane potential of a given substrate. Although many guidelines have been proposed to standardize BMP tests, results published in the peer-reviewed literature show that critical flaws in experimental design or execution are still common. Therefore, a powerful but simple method for evaluating the quality of BMP measurements is proposed in this work: the specific methane production (SMP) curve from any BMP trial should have a similar shape for most substrates, and significant deviation from this typical response is usually associated with a lack of reliability in resulting BMP estimates. In this study, some common experimental mistakes were reproduced to demonstrate this concept and establish relationships between flaws in experiments, SMP curves, and BMP values. The studied flaws were inoculum storage (2 weeks and at different temperatures), inoculum dilution with water (no dilution to 1:2 dilution), and inoculum-to-substrate ratio (from 2.00 to 0.05). Common kinetic models were used to better assess SMP curves. All flaws exhibited an impact on the SMP curves, but excessive dilution and extremely low inoculum-to-substrate ratio had the largest impacts on both SMP curves and BMP. In the most extreme case (ISR of 0.05), there was a clear lag phase of more than 10 days, and the resulting BMP was 15% lower than for the reference case. Together these results demonstrate the utility of the proposed method. The ultimate goal of this research is to help less experienced researchers identify and address problems in their experimental procedure, ultimately contributing to more accurate BMP measurements. The general approach is both simple and useful, and should always be part of a quality check for BMP tests.

Development and Validation of a Low-Cost Gas Density Method for Measuring Biochemical Methane Potential (BMP)

“…As in the other experiments, the contribution of this error in inoculum-only bottles to BMP was small. Bottles showed expected patterns in SMP curves [22], and all methods resulted in similar curves (GDt and gravimetric results are shown for two substrates in Figure 5). Both the GDt and gravimetric results clearly show a lag phase in the cellulose curve, but none for FI3.…”

“…This RSD value was higher than the maximum of 5% recommended by [10] for homogenous substrates. All SMP curves followed the typical pattern over time [22], and GD t and gravimetric curves were similar (not shown). The GD t approach provides a single estimate of biogas composition for each bottle, which is then applied over the duration of a BMP trial (see Section 2).…”

“…For inoculum-only bottles, where total biogas volume was only slightly more than headspace volume, the bottle headspace must have contained a significant amount of N 2 at the end of the trial, and the correction cannot be applied as described in Equations (6) and (7). All SMP curves followed the typical pattern over time [22], and GDt and gravimetric curves were similar (not shown). The GDt approach provides a single estimate of biogas composition for each bottle, which is then applied over the duration of a BMP trial (see Section 2).…”

Power and Limitations of Biochemical Methane Potential (BMP) Tests

“…Therefore, the conditions applied in a BMP test aim at the best possible environment for microbial growth. This includes using inoculum from a well-functioning digester (Raposo et al, 2011), including a positive control (such as microcrystalline cellulose) to check the performance of the inoculum (VDLUFA, 2011;Holliger et al, 2016), choosing a suitable inoculum-to-substrate ratio (ISR) to avoid both over-or under-loading of the process (Koch et al, 2019), providing a suitable and constant temperature combined with a gentle mixing, and removing the oxygen from the headspace by flushing with an inert gas prior to incubation (Koch et al, 2015a). Additionally, tests continue until methane production rate is very low to avoid an underestimation of the BMP due to only partial degradation of slowly degradable substrates (Holliger et al, 2016).…”

Biomethanation Potential (BMP) Study of Mesophilic Anaerobic Co-Digestion of Abundant Bio-Wastes in Southern Regions of Tunisia

“…Shapes of the plotted curves in Figures 3 and 5 have similar behaviors. Koch et al have proposed a simple method for evaluating the quality of BMP measurements, but it is powerful, which is the bio-methane production curves from the BMP test should have similar shapes for most assays [64]. Furthermore, Figures 3 and 5 show an initial phase characterized by a low production rate, followed by a high production rate phase until a steady state is reached and is maintained in the final decaying phase.…”