Monitoring and Analysing Biological Processes in Digesters

Large parts of the Hohenheim University, which is one of the oldest universities in Stuttgart, are located in the Hohenheim Castle. The main focus of the university is on economics, natural sciences, and agricultural sciences. It has an exceptional reputation, not only in Germany but throughout the world, especially in the agricultural sciences.

A number of state institutes affiliated to this university as well as many other specialized research stations have played key roles in practice-oriented research for about 130 years. One among them is the State Institute of Agricultural Engineering and Bioenergy (Figure 1) founded in 1883 in the name of Royal Württemberg Institute of Machine Inspection, which has been handling biogas research projects for nearly 30 years.

Figure 1. View of the State Institute of Agricultural Engineering and Bioenergy, which is affiliated with the Institute of Agricultural Engineering.

Bioenergy Is Booming

As early as the 1930s and 1940s, the energy reserved in biomass was used to fuel vehicles through the process of “wood gasification.” However, wood gasification was nearly stopped completely due to the availability of petroleum to produce fuel. Nevertheless, the limited availability of fossil fuels and the budding support for abandoning nuclear power have now increased the significance of renewable energy sources.

Biogas and its Generation

Bioenergy, as well as the energy source biogas, is continuously available, renewable, and can be stored. Biogas involves an exhilarating type of power generation, specifically for agriculture. Currently in Germany, close to 5000 biogas plants supply a total of 1716 MW of power to the power grid, which is nearly equal to the power produced by 1.5 medium-sized nuclear power plants.

The base materials for producing biogas are solid and liquid manure from livestock and renewable raw materials (Figure 2). Biogas is produced during the decomposition of organic substances through a four-step anaerobic digestion process, which involves hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Hydrolysis and acidogenesis are the primary fermentation, and acetogenesis is the secondary fermentation. Prior to refining, flammable biogas is in the form of a water-saturated gas mixture largely comprising carbon dioxide and methane. The oxidizable methane component plays a vital role in the use of biogas as it releases energy after being burned.

Figure 2. Liquid and solid manure from approximately 300 livestock units and renewable raw materials are the base materials for the biogas production.

Alcohols, hydroxycarboxylic acids such as lactic acid, and short-chain fatty acids (volatile fatty acids (VFAs) such as butyric acid, and propionic acid) are largely produced during the hydrolysis and acidogenesis steps. During the acetogenesis and methanogenesis steps, these acids are first converted into acetic acid and then into biogas. The fermentation process can be monitored by observing the acetic acid to propionic acid ratio as well as the absolute concentration of these volatile organic acids (VOAs). The total concentration of these organic acids in the fermentation substrate can also be indicated in terms of equivalent amount of acetic acid. For this purpose, the different molecular weights have to be taken into account [1].

If the acetic acid equivalent is less than 1000 mg/L, then the fermentation process is intact because at these values, the production of acid during the primary fermentation happens at the same rate as the degradation of acid by methanogenesis, thus attaining equilibrium. However, if this process is disturbed by disinfectants, then the methanogenic microorganisms are normally inhibited first.

Consequently, there is a significant increase in the acid concentration in the fermentation substrate either due to this inhibition or due to “overfeeding.” The pH of the substrate drops when the buffer capacity of the digester exceeds due to the acid increase. If the pH is less than 6.8, the metabolism of the methanogenic microorganisms is stopped entirely, affecting the biogas production process [2].

Apart from acid concentration, the buffer capacity (total inorganic carbon and TIC value) of the fermentation substrate also plays an important part in the fermentation process. The TIC value quantifies the pH-stabilizing carbonate/hydrogen carbonate as well as ammonium/ammonia buffer, and is a measure of the optimum point to which hyperacidity in the biogas plant can be buffered before the pH actually starts to decrease. Operators work hard to achieve the goal of operating the biogas plants at optimal conditions by employing modern process instrumentation and the accompanying analysis equipment.

Research of Digester Biology at the Hohenheim University

As a cornerstone in the field of bioenergy research, Hohenheim University has the most advanced biogas laboratory in Europe. The research biogas plant known as “Unterer Lindenhof” (Figure 3) was constructed in 2007 in the Swabian Mountains as a supplement to the laboratories that already existed at the Institute of Agricultural Engineering in Hohenheim.

Figure 3. The research biogas plant “Unterer Lindenhof” is located on the Swabian Alb (German state of Baden-Württemberg) and is part of the Research Station for Livestock Farming and Animal Breeding.

Research at this institute primarily focuses on process optimization of biogas plants, with a particular interest in the assessing the conditions of the digester. For this purpose, a number of parameters, such as the total carbon, nitrogen and sulfur content (Ctot, Ntot and Stot), organic dry matter (ODM) content, heavy metal content (Co, Cu, Ni, Se, Zn and Cd), total solid (TS) content and mineral content (Na, Mg, K, P and Ca), are appropriate. Samples from the digester (Figure 4) can be used to determine these parameters.

Figure 4. Various samples from the digester are ready for analysis

Determining the VOA/TIC Value by Titration

The VOA/TIC value is an important tool for assessing the fermentation process, and it can be determined cost-effectively using a simple titration method and the correct laboratory equipment. The samples are initially shaken and then easily centrifuged, or at least roughly filtered using a pleated filter. About 10 mL of the sample from the digester is aliquoted and transferred to a titration vessel or sample changer beaker. To this sample, 30 mL of distilled water is added. Dynamic equivalence-point titration is performed using 50 mmol/L of H₂SO₄ as titrant, and a combined pH glass electrode that is initially calibrated by using standard buffers at pH 4.00 and pH 7.00.

As per tradition, analysis of the titration is carried out at the fixed endpoints of pH 5.00 and pH 4.40. The equivalence point is attained between pH 3.50 and pH 4.00 and approximately correlates to the sum of all acids, i.e. the total acids including the VOAs. The VOA/TIC value is calculated by using an empirically determined formula, and the same holds good for the TIC and VOA values.

It can be presumed that the consumption of sulphuric acid up to the fixed endpoint 1 (pH 5.00) is due to the hydrogen carbonate and carbonate present in the sample. Therefore, the TIC value is calculated by using the amount of sulphuric acid consumed between the original pH and pH 5.00.

The consumption of sulphuric acid between pH 5.00 and pH 4.40 is credited to the VOAs. So the value of VOA is related to the difference between the fixed endpoint 2 (pH 4.40) and the fixed endpoint 1 (pH 5.00). The ratio of VOA to TIC gives the VOA/TIC value, where VOA/TIC values >0.3 (shown in Figure 5, example 1, value 0.727, blue curve) indicate that there is something wrong with the fermentation process.

Figure 5. Titration curves of three digester samples obtained with the tiamo titration and automation software.

The curve shape in examples 2 and 3 (red and green lines in Figure 5) is noticeably different from example 1 due to the low VOA value in these examples. The VOA/TIC values of 0.246 and 0.241 associated with these examples indicate that the process is biologically intact. Optimal VOA/TIC values may vary from one case to another based on the plant and the substrate used. Therefore, the optimal operating conditions and the related VOA/TIC value should be necessarily established.

During regular operations, the VOA/TIC value is established by means of endpoint titration without documenting the titration curves, which is completely adequate. However, for the purpose of research and development, interesting additional information can now be extracted from more than 2000 titration curves at the Hohenheim University (Figure 6).

Figure 6. The automated Metrohm VOA/TIC system that is installed at Hohenheim University consists of a 785 DMP Titrino with Exchange Unit and an 814 USB Sample Processor. Ms. Thomalla (in the picture) and Ms. Buschmann are responsible for the titrimetric analysis of the samples from the digester.

References

[1] Lemmer, A. Preissler, D. Zielonka, S. Oechsner, H.: Fermentation – alles ganz einfach?; Fachzeitschrift GWF – Gas/Erdgas, 719–722, 148 (2007) Munich.

[2] aid infodienst Verbraucherschutz, Ernährung, Landwirtschaft e.V.: Biogasanlagen in der Landwirtschaft; aid-Heft 1453, 48 pages, (2003) Bonn.

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.