Use of biochars in anaerobic digestion
Introduction
In recent years, considerable attention has been paid to the potential use of carbon-rich materials as soil amendments and for long term carbon storage (Lehmann, 2007). These materials, produced by thermochemical conversion of biomass and known as biochars, are expected to increase the storage capacity of soils for water and plant nutrients, to reduce greenhouse gas emissions, and to improve crop yields (Lehmann et al., 2009). Furthermore, an enormous carbon offset potential was calculated because of its high biological stability (Roberts et al., 2010). Today, several techniques for biochar production are investigated. The main attention is paid to pyrolysis (Lehmann, 2007) and hydrothermal carbonization (Libra et al., 2011) leading to biochars, which are termed pyrochar and hydrochar, respectively.
However, relatively high costs are a major drawback that impedes broader use of biochars (Roberts et al., 2010, Libra et al., 2011). One option to overcome this restraint is to achieve further economic benefits by expanding the biochar value chain. Such an additional application for biochars could be the use as additive in anaerobic digestions, which was subject of the present work. Interestingly, a recent study by Case et al. (2014) showed that biochar can reduce greenhouse gas (GHG) emissions in production of energy crops, which are widely used as feedstock for anaerobic digestion. Furthermore, digestate from anaerobic digestion was shown to be a suitable feedstock for hydrochar production (Mumme et al., 2011). Thus, system integration of biogas and biochar promises several synergies.
Although biogas production through anaerobic digestion has been established for some decades, there is still need for optimization of this process in terms of process stability, higher methane yields, and inhibition problems (Ward et al., 2008). A well-known problem concerning anaerobic digestion of N-rich materials is inhibition by excess ammonia (Rajagopal et al., 2013, Yenigün and Demirel, 2013). Nevertheless, N-rich organic materials, such as animal wastes (Rajagopal et al., 2013) and slaughterhouse by-products (Hejnfelt and Angelidaki, 2009), are very attractive feedstocks for biogas production. Several solutions to reduce ammonia concentrations in anaerobic digestion have been reported in the literature, for instance ammonia stripping (Yenigün and Demirel, 2013) and struvite precipitation (Rajagopal et al., 2013). Another approach is the addition of inorganic particles to the reactor, including zeolites. These minerals can remove ammonia (NH3) and ammonium ions (NH4+) through adsorption and ion-exchange on their reactive surfaces (Borja et al., 1993, Borja et al., 1996, Ho and Ho, 2012). Moreover, these particles are also suitable for attachment of microorganisms because of their well-defined microporous structure (Fernández et al., 2007). Both mechanisms are presumed to be mainly responsible for process enhancement, which leads to an increase in methane production (Montalvo et al., 2005). Similar to zeolites, addition of charcoal or activated carbon to anaerobic digestions of cattle dung or swine manure has been reported to cause an increase in biogas yield (Geeta et al., 1986, Kumar et al., 1987, Desai and Madamwar, 1994). The authors speculate that the higher yields were caused by biofilm formation on the particle surface. Based on the review of the literature, it appears likely that biochar can be an effective additive for enhancement of biogas production.
The overall aim of the present study was to investigate the behavior of biochars in anaerobic digestion. This includes a comparison of two different biochars derived from pyrolysis (pyrochar) and hydrothermal carbonization (hydrochar). Further objectives were to describe the impact of biochar on biogas production, to determine the anaerobic degradability of the biochars, to evaluate the biochars potential for mitigation of ammonia inhibition, to describe the impact on the microbial community, and to analyze the properties of the biochar-containing digestates.
Section snippets
Origin and properties of biochars, zeolite and inoculum
For comparison purposes, both a pyrochar and a hydrochar were selected for the experiment. The pyrochar was taken from a larger biochar sample used in an inter-laboratory ring trial on biochar chemical analysis, which was conducted in 2013 within the scope of the EU COST Action TD1107 “Biochar as option for sustainable resource management” (Schmidt, 2014). The pyrochar sample, labeled “BC 2”, was produced in early 2013 in a 500-III pyrolysis screw reactor (PYREG GmbH, Dörth, Germany) operated
Effects of biochars on uninhibited anaerobic digestion
During the 63 days of the experiment, all fermentations were observed to produce methane-containing biogas. The intensity and course of biogas and methane production, however, varied to a large extent. In fermentations without additional TAN (P0, H0, C0), the use of hydrochar (H0) increased the gas rates considerably (Fig. 1a and b). H0 showed a rapid start-up reaching peak production already on day 1. This can be explained by the relatively large fraction of readily available carbon such as
Conclusions
It can be concluded that biochars possess the ability to catalyze anaerobic digestion by mitigation of mild ammonia inhibition, support of archaeal growth and methanization of the biochars’ labile carbon. The suitability of a syringe-based BMP test to determine the behavior and degradability of biochars in anaerobic digestion was demonstrated. Compared to zeolite, the pyrochar and hydrochar used in this study showed less process stabilizing capacity. However, as the biochars were not
Acknowledgements
The research conducted by the authors was supported by grants commissioned from the German Federal Ministry of Research and Education to Project Management Jülich (PtJ). Additionally, the authors would like to thank Laureen Herklotz, Jaqueline Götze, and Christoph Prautsch for their support in chemical and microbial analyses. Furthermore, the authors like to express their gratitude to Gerald Dunst from Gerald Dunst Kulturerden GmbH (Austria) and Hans-Peter Schmidt from ithaka institute for
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