Early warning indicators for mesophilic anaerobic digestion of corn stalk: a combined experimental and simulation approach

Abstract

Background: Monitoring and providing early warning are essential operations in the anaerobic digestion (AD) process. However, there are still several challenges for identifying the early warning indicators and their thresholds. One particular challenge is that proposed strategies are only valid under certain conditions. Another is the feasibility and universality of the detailed threshold values obtained from different AD systems. In this article, we report a novel strategy for identifying early warning indicators and defining threshold values via a combined experimental and simulation approach.

Results: The AD of corn stalk (CS) was conducted using mesophilic, completely stirred anaerobic reactors. Two overload modes (organic and hydraulic) and overload types (sudden and gradual) were applied in order to identify early warning indicators of the process and determine their threshold values. To verify the selection of experimental indicators, a combined experimental and simulation approach was adopted, using a modified anaerobic bioconversion mathematical model (BioModel). Results revealed that the model simulations agreed well with the experimental data. Furthermore, the ratio of intermediate alkalinity to bicarbonate alkalinity (IA/BA) and volatile fatty acids (VFAs) were selected as the most potent early warning indicators, with warning times of 7 days and 5-8 days, respectively. In addition, IA, BA, and VFA/BA were identified as potential auxiliary indicators for diagnosing imbalances in the AD system. The relative variations for indicators based on that of steady state were observed instead of the absolute threshold values, which make the early warning more feasible and universal.

Conclusion: The strategy of a combined approach presented that the model is promising tool for selecting and monitoring early warning indicators in various corn stalk AD scenarios. This study may offer insight into industrial application of early warning in AD system with mathematical model.

Keywords: Anaerobic digestion; BioModel; Corn stalk; Early warning.

Fig. 1

A comparison of experimental and simulated CH₄ yields (a1, a2) and CH₄/CO₂ ratios (b1, b2) related to the laboratory experiments. Data presented in subplots a1 and b1 represent R1, while those in subplots a2 and b2 represent R2

Fig. 2

A comparison of experimental and simulated acetic acid (a1, a2), propionic acid (b1, b2), butyric acid (c1, c2), and total VFA (d1, d2) concentrations related to the laboratory experiments. Data presented in subplots a1, b1, c1, and d1 represent R1, and those in subplots a2, b2, c2, and d2 represent R2

Fig. 3

A comparison of experimental and simulated total alkalinities (a1, a2), intermediate alkalinities (b1, b2), bicarbonate alkalinities (c1, c2), BA/TA (d1, d2), IA/BA (e1, e2), and VFA/BA (f1, f2) ratios related to the laboratory experiments. Data presented in subplots a1, b1, c1, d1, e1, and f1 represent R1, while those in subplots a2, b2, c2, d2, e2, and f2 represent R2

Fig. 4

A comparison of experimental and simulated early warning indicator values in R1 during gradual overload (a) and sudden overload (b), and in R2 during gradual overload (c). Negative values indicate a decrease, while positive values indicate an increase in the value of the respective indicators, at the points of abrupt changes and relative to steady-state values