Anaerobic model for high-loaded or high-temperature digestion – additional pathway of acetate oxidation

Abstract

Current digestion models cannot properly simulate processes that are operated under high loadings or high temperatures. A modification to existing models has been implemented by adding important missing degradation pathways, to accommodate these high loaded and high temperature anaerobic systems without artificially recalibrating the model parameters. This degradation pathway relies on the use of an alternate acetate oxidizing mechanism that is more tolerant to ammonia than the aceticlastic pathway. Inhibition values have been estimated and a logistic function has been used to apply ammonia inhibition. The model also relates metabolic activity to unionised species such as undissociated acetic acid as substrate (though not obligatory for all organisms) and unionized ammonia as inhibitor. The model also incorporates all important chemical species and activity coefficients in the equilibrium chemistry module (such as the phosphate buffer), resulting in more accurate pH predictions which is crucial for proper modeling of CO 2 and NH 3 stripping. This model can now be used to simulate processes that are operated under conditions where free ammonia inhibition can be an important factor for process efficiency and substrate conversions.

Maximizing Gas Production at Blue Plains AWTP for High-Loaded Digestion Process

“…The effect of THP can be modeled by converting portions of particulate biodegradable COD (X sp ) to soluble COD and VFA, and active biomass to X sp , while tracking the release of inert materials and nutrients (Wett et al, 2009). This model was further improved by incorporating ammonia inhibition of aceticlastic methanogens and alternative degradation pathways through acetic acid oxidation (ACOX) and hydrogenotrophic methanogenesis that are more tolerant of higher ammonia levels (Wett et al, 2012). Figure 1 shows the modified conceptual schematic used to model the anaerobic conversion of biomass to methane.…”

“…However, increased feed concentration and destruction of protein and other complex organic materials also result in higher pH and stoichiometric production of total ammonia-nitrogen (TAN) in the reactor, which combine to elevate the free ammonia (NH 3 ) levels above what is typically seen in conventional mesophilic digesters (Kepp et al, 2001). Free ammonia has inhibitory effects on the methanogens that are responsible for completing the final step in anaerobic digestion systems where acetic acid is converted to methane gas (Eldem, 2005Wilson, 2009Wett, 2012. Compared to hydrolysis-limited conventional digestion (Siegrist, 2002, Wilson, 2009Stallman, 2012, Blue Plains advanced digestion is expected to be methanogenesis-limited, and therefore, minimizing the unionized ammonia level becomes a major concern while developing a process control strategy.…”

Modeling Thermally Pretreated Mesophilic Anaerobic Digestion

“…The effect of THP can be modeled by converting portions of particulate biodegradable chemical oxidation demand (COD, X sp )) to readily biodegradable COD (rbCOD) and volatile fatty acids (VFA), and active biomass to X sp , while tracking the release of inert materials and nutrients (Wett et al, 2009). This model has been further improved by incorporating ammonia inhibition of aceticlastic methanogens (AM) and alternative degradation pathways through acetic acid oxidizers (ACOX) and hydrogenotrophic methanogens that are more tolerant of higher ammonia levels (Wett et al, 2012). The NH 3 inhibition was modeled using the logistic model and the parameters were calibrated using results from bench-scale digesters fed with thermally-hydrolyzed Blue Plains sludge (THD) running in parallel with a conventional mesophilic digester .…”

“…Free ammonia has inhibitory effects on the methanogens that are responsible for completing the final step in anaerobic digestion systems where methane gas is formed from intermediary substrates (Eldem, 2005, Wilson, 2009, Wett, 2012. Compared to hydrolysis-limited conventional digestion (Siegrist, 2002, Wilson, 2009, advanced digestion with THP pretreatment is expected to be methanogenesis-limited, and therefore, minimizing the unionized ammonia level becomes a major concern while developing a process control strategy.…”

The kinetics of process dependent ammonia inhibition of methanogenesis from acetic acid

“…However, the contrasting responses of acetate utilization kinetics and methane production rate to increased NH 3 levels suggests that the causeeeffect relationship between acetate degradation and methane production differs among THD and MAD reactors. In addition, methanogenic 16S rDNA clone library analyses performed on the THD and MAD reactor communities show a difference in the dominant methanogenic communities, favoring obligate hydrogenotrophic species in the THD biomass and favoring species capable of performing aceticlastic methanogenesis in the MAD reactor community (Wilson, 2010;Wett et al, 2012). Based on the differing sensitivities to ammonia exhibited by these biomass samples and the differences in microbial community, it is thus hypothesized that methanogenesis during THD is less dependent on ammonia-sensitive aceticlastic methanogenesis, and rather, non-aceticlastic methanogenesis from HAc is important for THD.…”