Optimization of biomethanation focusing on high ammonia loaded processes

Han Wang

Research output: Book/ReportPh.D. thesisResearch

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Abstract

The toxicity effect of high ammonia is one of the most common problems, which cause imbalance and low biogas production rate in biogas plants. When protein-rich substrates (e.g. pig manure and mink manure, food waste, etc.) are used in biogas plants, lead to suboptimal utilization of the biogas potential and unstable biogas process. However, up to now, the solutions for alleviating ammonia toxicity effect have been proven either too expensive or time consuming for the full-scale biogas plants. Thus, sustainable and practical solutions to overcome the problem of ammonia inhibition efficiently are urgently required. In order to alleviate the toxicity effect of high ammonia levels, some new ideas-hypotheses were presented and evaluated in this thesis.
Firstly, preliminary modelling results from a previous study, have demonstrated that the increase of lipids’ concentration in ammonia-rich substrates, could theoretically mitigate the ammonia inhibition problem (Angelidaki et al., 1999). Therefore, the effect of co-digestion of cattle manure with lipids (i.e. glycerol trioleate (GTO)) under high ammonia levels (5 g NH4+-N·L-1) in anaerobic continuous stirred tank (CSTR) reactors (RGTO) was assessed. Additionally, for comparison purposes, a soluble carbohydrate (i.e. glucose) was also used as a co-substrate in an identical CSTR reactor (RGLU). At 5 g NH4+-N·L-1, relative methane production of RGTO and RGLU, was 10.5% and 41% compared to the expected uninhibited production, respectively. At the same time control reactor (RCTL), only fed with manure, reached 32.7% compared to the uninhibited basis production. Therefore, the hypothesis that the co-digestion of manure with lipids could alleviate the ammonia inhibition was not supported by the results. However, an “ammonia-LCFA synergetic inhibitory effect” was observed, which caused a deterioration of the inhibition effect in anaerobic digestion process. On contrary, the reactor where glucose was co-digested demonstrated higher tolerance to ammonia toxicity compared with the reactor where GTO was used.
Secondly, the problem of ammonia inhibition during biomethanation process could also be solved by microbiological methods. It is possible to promote the syntrophic acetate oxidation pathway during biomethanation process for counteracting ammonia inhibition. Therefore, the effects of different ammonia levels on pure strains of syntrophic acetate oxidation bacteria (SAOB) and hydrogenotrophic methanogens were evaluated. Furthermore, the effect of different ammonia levels on the syntrophic cultivation of SAOB and hydrogenotrophic methanogens was also assessed. The results showed that some hydrogenotrophic methanogens (79.1% of the theoretical methane production) were equally, or more resistant to ammonia toxicity compared to SAOB (11.1% of the theoretical methane production). In addition, the thermophilic hydrogenotrophic methanogens tested in the current study were more robust to high ammonia concentrations compared to the mesophilic hydrogenotrophic methanogens, which was contradictory to the results of some previous studies. Moreover, for SAOB, the resistance to ammonia toxicity could be improved by syntrophic cultivation with hydrogenotrophic methanogens, which indicated that at high ammonia levels, hydrogenotrophic methanogens seem to be the key players in the SAO pathway.
Thirdly, based on the same idea (promoting the syntrophic acetate oxidation pathway to alleviate ammonia inhibition), the hypothesis of bioaugmentation with high ammonia tolerant methanogenic archaea could be a new practical solution for fast recovery from ammonia inhibition. The results derived from this study clearly demonstrated a 31.3% increase in methane production yield in the CSTR reactor, at steady-state, after bioaugmentation. It indicated that this new solution to counteract ammonia inhibition was more practical and effective compared with other methods applied today in continuous reactors. Furthermore, bioaugmentation with an ammonia tolerant methanogen to alleviate ammonia toxicity could be applied for improving the efficiency of biomethanation process in full-scale continuous reactors.
Finally, an innovative method, where hydrogen is injected in the anaerobic reactor and subsequently been converted together with carbon dioxide to methane by hydrogenotrophic methanogens, could potentially be more tolerant to ammonia toxicity. Therefore, the effect of different ammonia levels on this hydrogen assisted biogas upgrading process under different hydrogen partial pressure (0, 0.25, 0.5 and 1 atm) in anaerobic reactors at both mesophilic and thermophilic temperature was evaluated. When the initial hydrogen partial pressure was 0.5 atm, the methane yield at high ammonia load (7 g NH4+-N L-1) was 41.0% and 22.3% lower than at low ammonia load (1 g NH4+-N L-1) in mesophilic and thermophilic condition, respectively. For the reactors without adding hydrogen, the methane yield decreased 65.0% (mesophilic) and 44.2% (thermophilic) when ammonia level increased to 7 g NH4+-N L-1. The results demonstrated that the hydrogen assisted biogas production and upgrading processes were inhibited by high ammonia levels. Nevertheless, the hydrogen assisted biogas upgrading process was still more robust to the increasing ammonia concentrations compared to the conventional anaerobic digestion processes. Under all the different ammonia concentrations tested in the current study, the optimal hydrogen partial pressure in batch reactors was 0.5 atm. Furthermore, at 0.5 atm of hydrogen partial pressure, the thermophilic methanogens seemed to be more robust to high ammonia concentrations (5 and 7 g NH4+-N L-1) compared with mesophilic methanogens.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark, DTU Environment
Number of pages53
Publication statusPublished - 2016

Projects

Innovative biogas process for ammonia-rich wastes

Wang, H., Angelidaki, I., Fotidis, I., Dechesne, A., Norddahl, B. & Schnürer, A. L.

Stipendie fra udlandet

01/11/201230/06/2016

Project: PhD

Cite this

Wang, H. (2016). Optimization of biomethanation focusing on high ammonia loaded processes. Technical University of Denmark, DTU Environment.