Andreas Lemmer

Repeatability of a laboratory batch method to determine the specific biogas and methane yields

To evaluate the potential of a substrate to form biogas, it is essential to have a fast and accurate method to determine the specific biogas yield (SBY) and specific methane yield (SMY). The objectives of this study were to assess the repeatability of the Hohenheim Biogas Yield Test (HBT) and to assess the influence of sample withdrawal and preparation on the determined SBY and SMY. Samples of dried maize whole plant material, maize cob, and power fodder, with up to six different treatments for preparation, were analyzed for their SBY and SMY using a discontinuous fermentation assay. Further, chemical composition of the whole plant sample was examined and different calculation models were used to predict SMY. The HBT showed a strong repeatability and a high precision for SMY. Two of the tested chemical composition models showed a satisfactory performance compared to the mean of the measured methane values. Significant effects caused by different sample treatments could be observed.

Mineral substances and macronutrients in the anaerobic conversion of biomass: an impact evaluation.

Optimal nutrient supply in the digester is essential for efficient biogas production. The aim of this study was to analyze the effects of various micro‐ and macronutrients on the process stability by a field test. The digestates of 25 biogas plants across the federal state of Baden‐Württemberg, Germany, were investigated. Collected data including trace elements, macronutrients, and volatile fatty acids (VFA) concentrations, as well as the organic loading rate and hydraulic retention time were subjected to statistical analysis. High variations in the concentrations within the different biogas plants were observed. Statistically significant effects of substrate constituents and process parameters on the stability of the anaerobic digestion process were found. Several micro‐ and macronutrients and the relationships between these elements, as well as the process parameters propionic acid, acetic acid, and acetic acid equivalent were tested. Ni, Mo, and S had a consistent statistically significant effect, while the organic loading rate and Se only showed an effect limited to the acetic acid concentration and the acetic acid equivalent. No statistically significant effect could be shown for Fe, Co, and Na. Most of the significant interactions between the tested elements contained Ni, Fe, and Co. This shows that a balanced relation between the concentrations of these elements is of greater importance than the presence of individual elements for a digester to be able to operate at high organic loading rates and maintain low VFA concentrations.

Energy balance of a two-phase anaerobic digestion process for energy crops.

This article deals with the digestion of energy crops in a two‐phase biogas process based on an anaerobic leach‐bed reactor combined with an anaerobic filter. The biogas process is a microbiological conversion of biomass into methane and carbon dioxide. This process is carried out by different microorganisms and can be divided into four steps which normally take place in only one digester. To be able to digest difficult energy crops by mono‐digestion and to meet the different needs of the several bacteria, which take part in the four‐step process of the methane production, the process was divided into two phases: (i) an anaerobic batch leach‐bed phase, where the leachate was produced and (ii) an anaerobic filter, where the organic fraction of the leachate was converted into biogas. Considering the results of the experiments, the two‐phase digestion of energy crops exhibited stable digestion behavior. No biological imbalance of the process, e.g. due to a sudden change of substrate, was detected either in the leach bed or in the anaerobic filter. Variation in suitability for two‐phase fermentation with an anaerobic batch leach‐bed reactor was observed for various substrates. The different substrates varied in their influence on acid formation and concentration as well as an influence on the course of the pH value.Therefore, an effect on the distribution of energy to the phases could be observed.

Is the continuous two-stage anaerobic digestion process well suited for all substrates?

Two-stage anaerobic digestion systems are often considered to be advantageous compared to one-stage processes. Although process conditions and fermenter setups are well examined, overall substrate degradation in these systems is controversially discussed. Therefore, the aim of this study was to investigate how substrates with different fibre and sugar contents (hay/straw, maize silage, sugar beet) influence the degradation rate and methane production. Intermediates and gas compositions, as well as methane yields and VS-degradation degrees were recorded. The sugar beet substrate lead to a higher pH-value drop 5.67 in the acidification reactor, which resulted in a six time higher hydrogen production in comparison to the hay/straw substrate (pH-value drop 5.34). As the achieved yields in the two-stage system showed a difference of 70.6% for the hay/straw substrate, and only 7.8% for the sugar beet substrate. Therefore two-stage systems seem to be only recommendable for digesting sugar rich substrates.

Effect of different pH-values on process parameters in two-phase anaerobic digestion of high-solid substrates

In many publications, primary fermentation is described as a limiting step in the anaerobic digestion of fibre-rich biomass [Eastman JA, Ferguson JF. Solubilization of particulacte carbon during the anaerobic digeston. J WPCF. 1981;53:352–366; Noike T, Endo G, Chang J, Yaguchi J, Matsumoto J. Characteristics of carbohydrate degradation and the rate-limiting step in anaerobic digestion. Biotechnol Bioeng. 1985;27:1482–1489; Arntz HJ, Stoppok E, Buchholz K. Anaerobic hydroysis of beet pulp-discontiniuous experiments. Biotechnol Lett. 1985;7:113–118]. The microorganisms of the primary fermentation process differ widely from the methanogenic microorganisms [Pohland FG, Ghosh S. Developments in anaerobic stabilization of organic wastes-the two-phase concept. Environ Lett. 1971;1:255–266]. To optimize the biogas process, a separation in two phases is suggested by many authors [Fox P, Pohland GK. Anaerobic treatment applications and fundamentals: substrate specificity during phase separation. Water Environ Res. 1994;66:716–724; Cohen A, Zoetemeyer RJ, van Deursen A, van Andel JG. Anaerobic digestion of glucose with separated acid production and methane formation. Water Res. 1979;13:571–580]. To carry out the examination, a two-phase laboratory-scale biogas plant was established, with a physical phase separation. In previous studies, the regulation of the pH-value during the acid formation was usually carried out by the addition of sodium hydroxide [Cohen A, Zoetemeyer RJ, van Deursen A, van Andel JG. Anaerobic digestion of glucose with separated acid production and methane formation. Water Res. 1979;13:571–580; Ueno Y, Tatara M, Fukui H, Makiuchi T, Goto M, Sode K. Production of hydrogen and methane from organic solid wastes by phase separation of anaerobic process. Bioresour Technol. 2007;98:1861–1865; Zoetemeyer RJ, van den Heuvel JC, Cohen A. pH influence on acidogenic dissimilation of glucose in an anaerobic digestor. Water Res. 1982;16:303–311]. A new technology without the use of additives was developed in which the pH-regulation is executed by the pH-dependent recycling of effluent from the anaerobic filter into the acidification reactor. During this investigation, the influence of the different target pH-values (5.5, 6.0, 7.0 and 7.5) on the degradation rate, the gas composition and the methane yield of the substrate maize silage was determined. With an increase in the target pH-value from 5.5 to 7.5, the acetic acid equivalent decreased by 88.1% and the chemical oxygen demand-concentration by 18.3% in the hydrolysate. In response, there was a 58% increase in the specific methane yield of the overall system. Contrary to earlier studies, a marked increase in biogas production and in substrate degradation was determined with increasing pH-values. However, these led to a successive approximation of a single-phase process. Based on these results, pH-values above 7.0 seem to be favourable for the digestion of fibre-rich substrates.

Do two-phase biogas plants separate anaerobic digestion phases? – A mathematical model for the distribution of anaerobic digestion phases among reactor stages

In this article a mathematical model is introduced, which estimates the distribution of the four anaerobic digestion phases (hydrolysis, acidogenesis, acetogenesis and methanogenesis) that occur among the leach bed reactor and the anaerobic filter of a biogas plant. It is shown that only the hydrolysis takes place in the first stage (leach bed reactor), while all other anaerobic digestion phases take place in both reactor stages. It turns out that, besides the usually measured raw materials of the acetogenesis and the methanogenesis phases (organic acids), it is also necessary to analyze the process liquid for raw materials of the acidogenesis phase, i.e., sugars, fatty acids, amino acids, etc. The introduced model can be used to monitor the inhibition of the anaerobic digestion phases in reactor stages and can, thus, help to improve the control system of biogas plants.

Effects of mechanical treatment of digestate after anaerobic digestion on the degree of degradation

The aim of this study was to increase the biogas production from different substrates by applying a mechanical treatment only to the non-degraded digestate after the fermentation process in order to feed it back into the process. To evaluate this approach, digestates were grounded with a ball mill for four different treatment time periods (0, 2, 5, 10 min) and then the effects on the particle size, volatile organic substances, methane yield and degradation kinetic were measured. A decrease of volatile fatty acids based on this treatment was not detected. The mechanical treatment caused in maximum to a triplication of the methane yield and to a quadruplicating of the daily methane production.

Biological hydrogen methanation – A Review

Surplus energy out of fluctuating energy sources like wind and solar energy is strongly increasing. Biological hydrogen (H2) methanation (BHM) is a highly promising approach to move the type of energy from electricity to natural gas via electrolysis and the subsequent step of the Sabatier-reaction. This review provides an overview of the numerous studies concerning the topic of BHM. The technical and biological parameters regarding the research results of these studies are compared and analyzed hereafter. A holistic view on how to overcome physical limitations of the fermentation process, such as gas-liquid mass transfer or a rise of the pH value, and on the enhancement of environmental circumstances for the bacterial biomass are delivered within. With regards to ex-situ methanation, the evaluated studies show a distinct connection between methane production and the methane percentage in the off-gas.

Influence of different substrates on the performance of a two-stage high pressure anaerobic digestion system

The two-stage autogenerative high-pressure digestion technique is a novel and promising approach for the production of gaseous fuels or upgraded biogas. This new technique is described in the patent DE 10 2011 015415 A1 and integrates biogas production, its upgrading and pressure boosting in one process. Anaerobic digestion under elevated pressure conditions leads to decreasing pH-values in the digestate due to the augmented formation of carboxylic acid. Model calculations carried out to evaluate the two-stage design showed that the pH-value in the pressurized anaerobic filter has a major influence on the methane content of the biogas produced. Within this study, the influence of the nitrogen content as one of the most important buffering substances on the performance of the system has been tested. The results show that higher NH4 contents lead to higher pH-values in the digester and as a consequence to higher methane contents.

High-pressure anaerobic digestion up to 100 bar: influence of initial pressure on production kinetics and specific methane yields.

ABSTRACT To ensure an efficient use of biogas produced by anaerobic digestion, in some cases it would be advisable to upgrade the biogenic gases and inject them into the transnational gas grids. To investigate biogas production under high-pressure conditions up to 100 bar, new pressure batch methane reactors were developed for preliminary lab-scale experiments with a mixture of grass and maize silage hydrolysate. During this investigation, the effects of different initial pressures (1, 50 and 100 bar) on pressure increase, gas production and the specific methane yield using nitrogen as inert gas were determined. Based on the experimental findings increasing initial pressures alter neither significantly, further pressure increases nor pressure increase rates. All supplied organic acids were degraded and no measurable inhibition of the microorganisms was observed. The results show that methane reactors can be operated at operating pressures up to 100 bar without any negative effects on methane production.