A critical review of the VFA/TA (FOS/TAC) method

Transcription

1 A critical review of the VFA/TA (FOS/TAC) method By Mario A. Rosato Excerpt and adaptation from the book of the same author Managing Biogas Plants, CRC Press, and from the article Redimensioning the importance of the FOS/TAC test, published by Agronotizie and Bioprocess Control Academy. Among all the many techniques employed for field monitoring of biogas plants, none has created more "rural legends" than the so-called VFA/TA test (a.k.a. FOS/TAC in the German technical literature). Some owners of biogas plants and scholars trust this simple analysis with almost religious faith, while in the practical operation of biogas plants very seldom the canonical VFA/TA quotient corresponds to the optimum plant production. In this article, we will analyze the logical flaws of the VFA/TA method and a case study of a biogas plant where it proved to be not only useless, but also detrimental. What the VFA/TA is The abbreviation VFA stands for volatile fatty acids, which are usually expressed in equivalent milligrams of acetic acid per liter. The abbreviation TA stands for Total Alkalinity, expressed in mg equivalent of calcium carbonate per liter. According to the proponents of this method, if the VFA/TA ratio falls in the range of values between 0.23 and 0.3, the anaerobic digestion process is stable. With a ratio of less than 0.23, the plant begins to "feel hungry" and requires the increase of the ration of feedstock to digest, while a value greater than 0.3 would indicate the beginning of "indigestion". The advantage of the VFA/TA method is that it is easy to measure both the VFA and the TA with a technique called titration. The same requires a few minutes, and does not require expert personnel when using an instrument called automatic titrator. Probably this ease of operation has popularized the VFA/TA method, but at the same time opened the door to the risk of biological collapse of the anaerobic system, because it is limited to the monitoring of a single parameter, and it bases on a logical fallacy, as will be explained later on. The logical flaws in the VFA/TA method It is a well-established principle of the scientific method that a conclusion can be true if it is based on two propositions which are both true. Nevertheless, the conclusion can be false even if both propositions are true, in other terms: it is a necessary but insufficient condition. On the other hand, a conclusion driven from false propositions sometimes can be true, but just for pure coincidence. The VFA/TA method bases on the suppositions that a given titration point of the anaerobic sludge represents the concentration of VFA as if it was acetic acid, and another titration point represents the total alkalinity as calcium carbonate equivalent. If the alkalinity is such that it compensates the concentration of acids, preventing the ph from dropping, i.e. if the ratio VFA/TA is in a given range, then the process is stable. Such reasoning is too simplistic. To understand it better, consider the following analogy with the clinical practice: imagine a doctor Mario A Rosato 23 rd August 2017 Page 1 of 5

2 trying to diagnose the state of health of a person using only the relationship between cholesterol and glycosides. We know very well that monitoring both cholesterol and glycosides is a routine analysis, but they are just indicators of state of health of a person. For the physician to diagnose a patient, a long list of indicator parameters are required, to which macroscopic aspects such as heartbeat, blood pressure, temperature, infections in place, etc. must be added. Trying to define the state and stability of an anaerobic plant on the basis on the sole VFA/TA ratio has the same significance of trying to measure a person s health based on the sole cholesterol / glycoside ratio. In the extreme case, a subject with both parameters high could seem healthy, although he/she would be at risk of heart attack and diabetes. The logical flaws of this method, and of the consequent plant management style, are the following: a) The VFA are not composed of just acetic acid. A given value of VFA measured as acetic acid equivalent can lead to disastrous consequences if the real composition of the VFA contains too much propionic acid, a strong bacterial inhibitor present in some organic waste. Therefore, the first proposition of the method is generally true, but when it is not, the plant owner will lose profits, in spite of measuring good VFA/TA values. This will be explained with a real case study in the next section. b) The acetic acid is precursor of just 70% of the overall methane produced by the anaerobic digestion. Trying to optimize the plant towards the path acetic fermentation plus acetochlastic methanogenesis will upset the equilibrium of the microbial ecosystem. Result: the total biogas production may improve, but its methane composition and the overall efficiency of conversion will drop. The degradation of acetic acid produces 50% methane and 50% carbon dioxide: stoichiometry is inescapable! c) Carbonates and bicarbonates are not the only source of alkalinity in anaerobic sludge. For instance, a high concentration of ammonium will increase the alkalinity, lowering the VFA/TA ratio and giving the false result of a hungry plant. Nevertheless, under certain conditions ammonium can be an inhibitor of the anaerobic digestion, so overfeeding the plant may lead to the production of more ammonium and to upsetting the process. The titration method allows any way to obtain some information: an order of magnitude idea of the total VFA concentration and of the total alkalinity, which are variables that need to be analyzed separately, within a context. It is useful to remember that a common practice in process engineering is to define the state of a system, where "state" means a set of variables, which characterize the system s behavior at a given time. In the case of single-stage anaerobic digestion (i.e., in a single digester) it is necessary to monitor at least twenty parameters to characterize a state and, therefore, to know the margin of stability and efficiency of the process. In systems with several stages (two or more tanks), each reactor will have its own optimum state and hence, the combinations of relevant variables to define and control the whole system will increase proportionally. Mario A Rosato 23 rd August 2017 Page 2 of 5

3 A study case of a plant where the VFA/TA was ideal, but the process was collapsed It can happen (more often than actually perceived, especially in plants fed only on silage) that maintaining the VFA/TA relationship in the range of acceptable values does not allow, however, a stable operation of the plant. The graphics in Figure 1, taken from a real case of a plant, whose identity will be kept anonymous in respect of the owner s privacy, are quite eloquent. Figure 1. Although the organic loading was controlled in order to maintain the VFA/TA ratio within the range of acceptable values (by performing the test twice a day!), the production of the plant was not only small, compared to its nominal capacity, but also unstable. Figure 1 represents a situation, fortunately not widespread but not even rare, which requires some further reasoning on another of the limitations of the VFA/TA method: its absolute lack of selectivity. Please note that, by definition, the parameter VFA is expressed in milligrams of equivalent acetic acid per liter. In a "healthy" anaerobic digestion process, most of the volatile fatty acids are constituted by acetic acid with small proportions of butyric acid and propionic acid, and then the single VFA parameter measured by titration is a fair indicator of the state of the biological process. Acetic acid is the immediate precursor of methane and carbon dioxide, two gases that represent the last stage of degradation of organic matter, and then a certain concentration of acetic acid is required for the process to proceed in full efficiency. Once the anaerobic bacteria consume all the acetic acid present, they cannot produce any more biogas and if, by contrast, the concentration of acetic acid is too high, the bacteria "suffer of indigestion" and stop producing methane. Propionic and butyric acids are always present in small amounts and are also converted into acetic acid by some groups of bacteria, but if these latter are missing or are Mario A Rosato 23 rd August 2017 Page 3 of 5

4 inhibited for some reason, the accumulation of acids other than acetic, beyond a certain threshold, is a potent inhibitor of the methanogenesis process. It is important to remark that propionic acid is used as a preservative by the baking and other packaged foods industry, indicated on the package label as "preservative E280", demonstrating the antibiotic power of said substance. In summary, the VFA/TA test is able to detect the total volatile fatty acids "as if" they were acetic acid, but does not distinguish between the different volatile fatty acid kinds. Therefore, it is a good indicator of the health of the bacterial system... but only if it is already healthy! In figure 1 it is possible to observe that, between day 18 and day 23, the VFA/TA ratio was maintained equal to 0.25 (i.e. very close to the ideal value according to the scientific literature). Nevertheless, the production of energy is absolutely unstable, shifting from the maximum peak to the minimum peak of the entire period under consideration. The explanation of such anomalous behavior is that at the time, total volatile fatty acids were mainly constituted by butyric acid and propionic acid, and the methanogenic phase of the digestion process was compromised because of a series of reasons too long to discuss here, correctly diagnosed after performing some simple biological tests. According to the criterion of the VFA/TA ratio, when measuring a value equal to 0.25, the plant would have to receive its nominal daily ration (about 50 ton / day of silage). In the case under study, feeding the plant with a reduced ration (about 12 ton / day) was already enough to increase the VFA/TA ratio during the following days, while the production of energy continued to fluctuate without reaching a stable operation. In this case, biological activity of the digester was inhibited and the plant was running at reduced capacity, in spite of the VFA/TA ratio was in the range defined as ideal in the literature. Conclusions Managing a biogas plant means becoming a "bacteria farmer". In the same way that livestock requires regular veterinary inspection and monitoring of the quality of the fodder also an anaerobic digester requires a "healthy and balanced" diet. The practice of monosubstrate digestion usually involves the frequent collapse of biological processes. It is only with the installation of his own laboratory and training of his staff, that the manager of the biogas plant will ensure the full efficiency of the anaerobic digestion process. Monitoring several biological constants will help to prevent biological collapses in advance and therefore will ensure the profitability of the investment. The field laboratory, however, cannot be a single instrument. When already available, the automatic titrator should be employed together with other instruments, in order to get as much information as possible on the state of the digestion process. If the plant does not have a laboratory, the minimum field laboratory should include a moisture analyzer, a muffle oven and a kit consisting of at least three laboratory batch reactors, or alternatively three continuous reactors, with an appropriate temperature control system and flow measurement of the biogas produced. The titrator is an optional. Carrying out routine biological tests is at the reach of anybody and provides much more information about the substrate and the microbial ecosystem, than any other physicochemical test. Mario A Rosato 23 rd August 2017 Page 4 of 5

5 Figure 2. An example of minimalist anaerobic digestion laboratory, composed of: moisture analyzer, muffle oven, spectrophotomer and AMPTS (Automatic Methane Potential Test System with 15 batch reactors and net methane volume counter) Mario A Rosato 23 rd August 2017 Page 5 of 5