Chemical and ecotoxicological evaluation of biochar produced from residues of biogas production
Introduction
The most frequently used raw materials for biogas production are mainly agricultural and food waste, animal feces and sewage sludge [1]. In the process of fermentation, those materials are used in various proportions and at various temperatures (mesophilic, thermophilic) [2]. Apart from biogas, another product of this anaerobic fermentation is the post-fermentation digestate (RBP), which is often separated into liquid and solid phases [1]. RBP can be composted, or landfilled, but most often it is used as a fertilizer in agriculture. However, research shows that the direct application of RBP to soils may be limited not only because it may contain heavy metals, polycyclic aromatic hydrocarbons (PAH) [3], seeds of weeds or infrequent parasites [2] but also because of the high mobility of its nutrients [4], which causes leaching of the nutrients from the soils and increases the eutrophication of water. Given the drawbacks of deriving fertilizer from RBP, the conversion of RBP into biochar, a popular alternative in recent years, may be an interesting solution. An effect of RBP conversion into biochar is the immobilization of organic carbon and nutrients in soil [5], [6]. Additional advantages of the use of biochar is an increase in crop yields, the limitation of nutrient leaching [5] and the reduction of bioavailability of heavy metals and PAHs, which are present in feedstock and soil [7].
An increase in the production of biochars from waste materials requires an in-depth analysis of their properties in the context not only of the potential benefits but also of the potential threats to the environment. Apart from the estimation of the contaminant content, such as heavy metals or PAHs, in biochars, it is also important to determine how biochar added to soil will affect living soil organisms. Recent studies show [8], [9], [10] that in spite of their acceptable content of contaminants, biochars may have an adverse effect on living organisms. In light of the information given above, a strict control of biochars appears to be necessary. Biochars should not be applied to soils without prior characterization comprising the physicochemical analyses, content of contaminants and effects on organisms. Permissible content of different pollutants such as heavy metals or PAHs in sewage sludge or composted organic waste (which may be used as a fertilizer) are strictly defined and respected in most countries. The situation is different in the case of RBP, for which there is lack of legal regulations. The exception is Switzerland where the amount of PAHs in RBP is clearly specified. The same is the case with biochar. Only in Switzerland, the European Biochar Certificate (voluntary industry standard in Europe) is obligatory for the use of biochar in agriculture. This regulation concerns the properties and permissible content of different contaminants in biochar [11].
RBP is a specific material, and due to its increasing production may be interesting to use for the biochar production. Previous studies have shown [12] that biochar produced from RBP can have better properties than the original RBP, but there is no information about the environmental risks associated with its use.
The objective of this study was to determine the content of PAHs, heavy metals, and of the level of toxicity of biochars produced from RBP at various temperatures towards various groups of organisms. The study was conducted with the use of RBP acquired from three different biogas plants operating under various production regimes. This study may prove that biochar produced from RBP may have better properties for agricultural use and be less toxic than the RBP itself. This is particularly important considering insufficient studies that have been reported for this type of material.
Section snippets
Biochar preparation
Three different residues from biogas production were used in the present study. The biogas plants mainly differed in the type of the process temperature (mesophilic: 32–37 °C, thermophilic: 52–55 °C) and the separation at the solid and liquid fractions (Table 1). RBP was collected during spring 2014 after the fermentation process of the pipeline drain in the case of liquid fraction and separator; this is the solid fraction. The samples were mixed, dried at 30–35 °C for approximately 10 days (solid
Trace metal content
The content of heavy metals was distinctly related to specific types of biochar (Table 2). The highest content of Cr (from 28.2 to 38.9 mg/kg), Zn (from 40 to 301 mg/kg), Cu (58.0–77.7 mg/kg) and Ni (from 10.9 to 33.6 mg/kg), irrespective of the pyrolysis temperature, was noted for biochar from the non-separated digestate PI. The content of Zn and Cu was 2-fold higher compared to the other biochars. For Cr and Ni, the difference in their levels among the particular biochars depended on the
Conclusion
Biochars produced from RBP were characterized by low levels of heavy metals (excluding Ni and Cd) and PAHs, conforming to the permissible norms of Germany's Federal Soil Protection Act and Switzerland's Chemical Risk Reduction Act. The contents of PAHs and metals depend on the type of feedstock used for pyrolysis, which is probably the main factor determining the content of these contaminants in the biochars. The most toxic biochars were obtained from non-separated RBP produced under mesophilic
Acknowledgements
The project was funded by the National Science Centre granted on the basis of the decision number DEC-2012/07/E/ST10/00572.
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