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English Issue Biogas Journal | Spring_2021 Diagram 3: Process monitoring at an agricultural large-scale biogas plant (2.5 MW el, install ) over a ten-month period VOA / TIC Nordm.; VOA Eq-GC/TIC + 0,3 VOA / TIC Nordm. VOA Eq-GC/TIC TIC TIC Test days Comparison between the VOA Nordmann/TIC Nordmann (red) and the new VOA Equ GC value/TIC (green) and also the TIC value (blue). Diagram 4: Process monitoring of the agricultural biogas plant in diagram 3 shown over a period of 10 months, the VOA/TIC ratio according to Nordmann (red) is shown in turn as a reference, and also the acetic acid reference values (VOA Equ GC) shown alone without the TIC value as a ratio (dotted, grey), moreover the new propionic acid / acetic acid value shown as a ratio (PA/AA, green) in the modified mode VOA / TIC Nordm. PA/AA modified VOA Eq-GC VOA / TIC; PS/ES mod VOA Eq-GC [mg/I] Test days Handling samples accurately As recently as 2014, Purser at al. modeled a modified mathematical correction factor for the VOA/TIC value according to Nordmann and Ripley, so that it then forced to the zero point. However, the previous titration points were still considered feasible for both methods and were not questioned. In the meantime, there have been publications by Hinken, Voß, Weichgrebe and Rosenwinkel about titration according to Nordmann (for example in Biogas Journal 6_2012) which emphasize accurate pre-treatment of samples by filtration or centrifugation. In addition, the measurement result is greatly affected by the age of the sample (CO 2 outgases and changes the carbonate buffer) and by representative sampling from the reactor. A comparably large variance of the measurement according to Nordmann that depends on the laboratory is also reflected in an interlaboratory comparison made by the Bavarian State Research Center for Agriculture (Köcker and Henkelsmann, Biogas Journal 1_2011). Thus, in an extreme case, a measurement of same sample led to VOA/TIC values of between 0.11 and 0.88, resulting in an averaging range of between 0.3 and 0.45. This disastrous divergence among the laboratories shows that a comparison between the different laboratories with VOA/TIC Nordmann can only be done with great uncertainty. Rieger and Weiland cautiously write “… Even if a limit value of ≤ 0.3 is considered as relatively certain for a stable process, this value is still mainly significant 30
Biogas Journal | Spring_2021 English Issue for the observation of its long-term development…” (2006). They also point out that a mere pH check in a well-buffered renewable raw materials biogas plant with or without liquid manure (TIC>7,000) is insufficient because the pH value hardly changes. There is no universal validated reference to estimate the VOA/ TIC ratio for a safe biogas process. Our own experience shows that the slurry-corn biogas plants (TIC value between 9,000 and 25,000) such as those of Rieger and Weiland with an acceptable range with a VOA/TIC value of
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