Proceedings

Calcium nitrate dosage results in improved biogas quality in anaerobic digesters

Stoeck, T.1, Filker, S.1, Breiner, H-W.1, Ettl, M.2 and Doppelbauer, G.2, 1 University of Kaiserslautern, Germany, 2 Yara Industrial, Germany

(free)

Abstract: 
The major goal in the optimization of anaerobic fermentation processes for biogas production is to
maximize CH4 yield while minimizing H2S formation. Based on empirical observations of industrial plant
operators the dosage of an ultra-pure calcium nitrate Ca(NO3)2 solution to anaerobic fermenters helped
to achieve this goal. To study effects of Ca(NO3)2 on the anaerobic digester microbiome, we sampled
a control fermenter and a Ca(NO3)2-treated fermenter of an operating plant. We used molecular
methods (DNA-metabarcoding and gene expression analyses) to study the structures and functions of
microbial communities. We found enhanced denitrification and methanogenesis and a decrease in
sulphate reduction. Surprisingly, also a notable increase of chlorophyll-degradation activity was
recorded during Ca(NO3)2 dosage. For microbial community structures, we found shifts with time in the
experimental fermenters but also in the control fermenter. However, we found no evidence that these
shifts can be attributed clearly to Ca(NO3)2 treatment. Other factors such as the varying quality of the
organic waste fed to the fermenter presumably have a much stronger effect on microbial community
structures. Based on our results, here we propose a conceptual model for the mode of action of
Ca(NO3)2 as a tool for the production of improved biogas quality in anaerobic fermentation processes.

Introduction:
The conversion of organic waste into biogas is an anaerobic process mediated by a complex microbial
community. The anaerobic fermentation process involves four main phases: hydrolysis, acidogenesis,
acetogenesis and methanogenesis. In the first phase particular organic polymers, such as
carbohydrates, lipids and proteins, are hydrolysed into sugars, fatty acids and amino acids. In the
acidogenesis and the acteogenesis, these less complex molecules are further degraded into
intermediates including volatile fatty acids (VFAs), acetate, alcohols, carbon dioxide (CO2) and
hydrogen (H2). In the final phase (methanogenesis), methane (CH4) is generated either from acetate (=
acetoclastic methanogenesis) or from hydrogen and carbon dioxide (= hydrogenotrophic
methanogenesis) (Klang et al. 2015). Besides methane, several other gases are produced during the
anaerobic fermentation process. These include among others carbon dioxide (CO2) nitrogen (N2),
nitrous gases (NOx) or laughing gas (N2O) as results of denitrification. But also, hydrogen sulphide
(H2S) is produced as a product of sulphate reduction through sulphate reducing bacteria (SRB).
Methane is the most valuable component of biogas produced through anaerobic fermentation because
of its energy content and its environmental compatibility after combustion. In contrast, hydrogen
sulphide is highly toxic and a threat to the health of fermenter workers in case of leakages (Truong et
al. 2006). Furthermore, hydrogen sulphide is highly corrosive causing notable economic damage
especially in combined heat and power plants (CHPs). In microbial anaerobic fermentation processes, hydrogen sulphide impairs the production of methane through substrate competition (Schieder et al. 2003). Furthermore, expensive measures are usually necessary to remove hydrogen sulphide from
biogas prior to combustion. Thus, one major aim in anaerobic digester operation is to maximize the
production of methane while minimizing the production of hydrogen sulphide. Empirical observations
and reports of industrial plant operators repeatedly confirmed an improvement of biogas quality after
dosage of a calcium-nitrate Ca(NO3)2 solution to the anaerobic fermentation process. These
improvements included (i) increased yield of methane, (ii) reduction in hydrogen sulphide production,
(iii) significant substrate savings, (iv) improved viscosity and (v) a more robust plant operation. In this
study, we aimed to reveal the biological effects of Ca(NO3)2 treatment on microbial community
structures and their metabolic activities to infer a mechanistic concept for the effects of Ca(NO3)2 on the
anaerobic fermentation process.

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