Elsevier

Bioresource Technology

Volume 239, September 2017, Pages 437-446
Bioresource Technology

Nutrient and heavy metal accumulation in municipal organic waste from separate collection during anaerobic digestion in a two-stage laboratory biogas plant

https://doi.org/10.1016/j.biortech.2017.05.026Get rights and content

Highlights

  • Solid digestate is exposed to element accumulation, except for N, P and Mg.

  • The loss of initial N, P and Mg load accounts up to 45%.

  • K load of feedstock was completely rediscovered in the solid digestate.

  • Element retention in the biogas plant has a decisive impact on nutrient content of digestate.

  • Heavy metal concentration in the solid digestate increases by factor 1.6.

Abstract

Municipal organic waste (MOW) is a promising feedstock for biogas plants and separate collection will increase available quantities. To close nutrient circles digestates shall be redistributed to arable land. However, less is known about digestate properties and how they are influenced during digestion. Therefore, changes in nutrient and heavy metal concentration in the solid digestate were investigated during anaerobic treatment of MOW in a two-stage laboratory biogas plant. Results show that the solid digestate is exposed to element accumulation, except for N, P and Mg. The loss of initial N, P and Mg load accounts up to 45%, which must be redistributed elsewhere in the digester system. K load of feedstock was completely rediscovered in the solid digestate. Heavy metal concentration in the digestate increases by factor 1.6 at average. The results emphasize that element retention in the digester system has a decisive impact on nutrient content of digestates.

Introduction

In the last decade biogas has become an important energy provider. In 2016, 17% of electricity generation from renewable energies was delivered by the biogas sector in Germany (AGEE-Stat, 2017). Until today, more than 8000 biogas plants have been constructed in Germany (FNR, 2017) promoted by the refunding policy of the Renewable Energies Law (Erneuerbare-Energien-Gesetz, EEG). With the EEG amendments from 2012 to 2016 the German government clearly focussed on organic waste recovery. Because of this new subsidise policy, refunds for biogas from organic waste are higher than for biogas from energy crops. Another motivation for waste recovery is the introduction of a nationwide separate collection of municipal organic waste (MOW) in Germany in 2015 under the Closed Substance Cycle and Waste Management Act (Kreislaufwirtschaftsgesetz, KrWG). Different studies show that about 9 million Mg of MOW, which can also be referred to as biowaste, could be provided by separate collection every year (Kern et al., 2012, Schüch et al., 2016). More than 100 biogas plants in Germany have specialized on the treatment of MOW (Kern et al., 2012). An increase of MOW utilisation as feedstock is expected (Schüch et al., 2016). The exploitation of the energetic potential of MOW through anaerobic digestion will result in a certain amount of digestion residues that are available for further usage. The application of digestates as soil amendment can be an interesting alternative to traditional fertilizers as they are suitable to replace nutrients and organic matter (Insam et al., 2015, Nkoa, 2014, Odlare et al., 2015, Odlare et al., 2011).

The quality of digestates is mainly influenced by the characteristics of feedstock (Al Seadi et al., 2013, Alburquerque et al., 2012, Zirkler et al., 2014). Following, the element composition of the ingestate can be an indicator for the potential nutrient recovery (Campuzano and González-Martínez, 2016). Residues from organic waste digestion are known to contain more micro nutrients than residues from energy crops digestion (Schattauer et al., 2011). However, Teglia et al. (2011) do not recommend direct use of digestates and application on land as soil amendment due to impurity contents. To obtain a high-quality product post-treatment of digestates is necessary (Al Seadi et al., 2013, Drosg et al., 2015). In Germany, final product requirements are determined by the German Biowaste Ordinance (Bioabfallverordnung, BioAbfV). The anaerobic digestion of organic waste can be realized by various technologies. Today, a strong preference of dry fermentation systems can be observed for biowaste in Germany because of technological improvements achieved during the last years of biogas boom and a high impurity tolerance. Therefore, a two-stage laboratory biogas plant with dry fermentation of feedstocks is investigated in this study. After digestion, residues are commonly separated into a liquid and a solid fraction (Fuchs and Drosg, 2013). Investigations on energy crops digestates show that the liquid fraction is rich in plant-available nitrogen and potassium (Drosg et al., 2015). It can be considered as a liquid fertilizer and be either spread directly or be further conditioned by e.g. ultra-filtration and reverse osmosis to receive higher nutrient concentrations (Fuchs and Drosg, 2013). The solid fraction is carbon-rich and contains phosphorus and other micro nutrients (Drosg et al., 2015, Insam et al., 2015). A few publications have dealt with digestate characterisation (Al Seadi et al., 2013, Teglia et al., 2011, Zirkler et al., 2014). Others have studied the fate of nutrients during anaerobic digestion (Massé et al., 2007, Möller et al., 2010, Möller and Müller, 2012, Schattauer et al., 2011). However, little information is available about solid digestate characteristics from anaerobic treatment of MOW as these residues are usually composted and further processed before application to land. Insam et al. (2015) suggest that instead of only generating biogas more emphasis shall be put on operating biogas processes with the objective of producing a highly functional soil amendment. To manage final amendment characteristics, it first needs to be clear how the biogas process affects digestate properties. With this study, knowledge on the fate of nutrients and heavy metals in the solid digestate during anaerobic treatment of MOW shall be improved. This is realized by continuous sampling of substrate during the digestion process and following chemical analysis of digestates. Knowledge gained shall be used to consider post-treatment of digestate with regard to its usage as organic soil amendment. This article therefore aims at showing properties of feedstocks and digestates from a two-stage laboratory scale biogas plant with dry fermentation of MOW, determining the development of nutrient and heavy metal content in the feedstock during the anaerobic digestion process and evaluating the impact of retention time and degradation rate on the digestate quality.

Section snippets

Feedstock

Three different feedstocks of source-separated municipal organic waste (MOW 1–3) have been treated by anaerobic digestion in the laboratory two-stage biogas plant under strict process monitoring and regular digestate sampling. Feedstocks were obtained from a local composting plant near Dresden (Germany). Test series took place in February 2015 (MOW 1), October 2015 (MOW 2) and April 2016 (MOW 3). Prior to digestion, feedstock was mixed with organic bulk material. Large plastic bags and

Characteristics of MOW feedstocks

Prior to the experiment initial characteristics of MOW from separate collection have been determined. Results are presented in Table 1. Characteristics differ between the three feedstocks. Total nitrogen (N) content ranges from 17 g kg−1 DM (MOW 1) to 21 g kg−1 DM (MOW 2). Phosphorus (P) content is highest in MOW 3 (3 g kg−1 DM). Potassium (K) concentration ranges from 9 g kg−1 DM to 12 g kg−1 DM in the feedstock samples. Calcium (Ca) content is highest for MOW 1 and 3 (20 g kg−1 DM) and lowest for MOW 2 (19 g kg−1

Conclusion

The results of this study emphasize that element retention in the digester system has a decisive impact on final element content in the digestate, especially for N, P and Mg. However, the comparison to other studies shows that the extent of element retention is depending on nutrient and heavy metal content of feedstock and process conditions during anaerobic treatment. Finally, results of this study underline the influence of feedstock quality on final digestate characteristics. Therefore,

Acknowledgements

We are grateful to the German Ministry of Research and Education (Bundesministerium für Forschung und Entwicklung, BMBF) for funding this research which is part of the project “Investigations on the utilisation of digestates from anaerobic treatment of municipal organic waste with regard to nutrient recycling and the impact of digestate application on plant and soil development” (VeNGA, FZK 03EK3527A). We further thank Martina Heinrich and Bastian Pötzsch for their support during practical work

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