# Biomethane Potential Test: Influence of Inoculum and the Digestion System
## 1 Introduction  
Biogas plants in Germany receive a bonus payment for power generation, which is guaranteed by the German Renewable Energy Source Act (EEG) for a period of 20 years after installation [1]. Many biogas plants will run out of this bonus payment during the next ten years. Thus, biogas plants have to work more efficiently for economic reasons. The biochemical methane potential (BMP) test is a common method to determine the maximum theoretical methane yield of a substrate. It has already been used for several years for dimensioning biogas plants [2]. High accuracy of the BMP test will be necessary to precisely predict economic viability in the future. Various protocols are available, all with the aim of achieving a high reproducibility of the BMP test. Besides the European standards ([PERSON] et al. [3] and [PERSON] et al. [4]), the VDI Standard 4630 [5] is the most commonly used protocol. These protocols show the same basic structure: (i) performing three technical repetitions, (ii) applying prescribed inoculum/substrate ratio (ISR), (iii) measuringmethane yield of pure inoculum as a blank, and (iv) measuring methane yield of standard substrates as positive control. In each of the protocols it is pointed out, how important the inoculum, the digestion system, and the working precision are to achieve a high reproducibility. Various inter-laboratory tests were conducted to check the accuracy of the BMP by using different digestion systems, inocula, and protocols. [PERSON] et al. [2] conducted an inter-laboratory test with 17 laboratories using starch, cellulose, and gelatin as substrates. The results showed a coefficient of variation (CV) of around 8%-11%. Similar results were reported by a German inter- laboratory test of KTBL/VDLUFA based on VDI Standard 4630. This test was done with different substrates and around 30 laboratories over several years. The CV excluding outliers was 8%-12% [6]. The impact of repeating BMP tests over several years was small, it only helped to reduce the number of outliers [7]. The Ecole Polytechnique Federale de Lausanne carried out inter- laboratory-tests with stricter specifications and a repetition after a few months [4]. The results without eliminating the outliners revealed a CV of 15%-17% [8]. [PERSON] et al. [9] investigated differences in the CV when using different BMP measuring protocols. Free choice of the used protocol was compared to the fix protocol for each laboratory, using a mineral substrate, NaHCO3 as pH buffer and an ISR \(>\) 2. The results showed a deviation of about 20% between the free and the fixed protocol. The measurement variability was not affected by different measuring protocols. [PERSON] et al. [10] reported similar differences by comparing protocols of VDI 4630 [5] and [PERSON] et al. [11]. Several studies were carried out to find reasons for deviations in measurement results (Table 1). Special focus was laid on the effect of the used inoculum. One of the main categorization criteria was the origin of the inoculum. [PERSON] et al. [12] and [PERSON] et al. [13] reported differing microbial community composition for different inocula from full scale biogas plants. Thus, inocula from different origins might affect the methane yield depending on their microbial community composition. [PERSON] et al. [14] tested sludge from chicken manure and municipal wastewater treatment plants as inocula using similar substrates. The inoculum from wastewater treatment led to a higher biodegradability of the inoculum/substrate mixture. [PERSON] et al. [15] revealed converse behavior, they detected a higher methane yield for inocula from a municipal solid waste landfill plant compared to inocula from wastewater treatment and other origins, when using animal by-products as substrate. [PERSON] et al. [16] investigated the methane yields of two different inocula. Higher methane yields were also achieved with inocula taken from anaerobic digesters. [PERSON] et al.