Anaerobic digestion affecting nitrous oxide and methane emissions from the composting process
Graphical abstract
Introduction
To achieve the well accepted goal of a circular economy, a proper management of organic waste is crucial. Recycling technologies need to be found where greenhouse gas (GHG) emissions to the environment are minimized and nutrients in the organic material returned to the soil. Organic residues contain nutrients, and the decomposition of organic waste also releases energy. The most common ways to treat organic waste are composting and anaerobic digestion (AD). Composting is aerobic degradation, and the energy produced as a microbial by-product during the degradation of organic matter is released as heat. Heat production is caused by microbial activity and therefore depends on moisture, aeration, and C/N ratio. Temperature in the compost is also dependent on ambient temperature and the size and shape of the composting system. Proper aeration is needed for a good composting process, but this also means that ammonia and other gases can be lost during the process. AD produces biogas that contains methane (CH4) and can be used for energy generation. There is almost no loss of nutrients or gas emission during the process as it happens in a closed container, but there is a risk of losses during later handling of the organic rest, digestate, e.g., via NH3, N2O and CH4 emissions. Digestate, is a good fertilizer with ample plant available nitrogen (Foereid et al., 2021; Odlare et al., 2014; Sogn et al., 2018). Because the water content in digestate is high, above 90%, it is often separated into a solid fraction and liquid fraction to ease storage and transport. In some cases, the solid digestate (DS) is also composted before application as a soil amendment.
Global warming is a world-wide concern. Most organic waste management releases some GHGs (Andersen et al., 2010; Swati and Hait, 2018), so it is important to minimize this as much as possible. Minimizing GHG emissions from digestate treatment is important to promote sustainable cities and communities (SDG 11) and make their consumption and production sustainable (SDG 12). It is also a climate action (SDG 13) both because it reduces GHG and because it promotes the use of waste for biogas.
Three GHG's are emitted from decomposition processes: carbon dioxide (CO2), CH4 and nitrous oxide (N2O). CO2 returns recently fixed carbon to the atmosphere and is therefore not a net addition. It is produced when microorganisms break down organic matter and therefore CO2 emissions often serves as an indication of the degradation. Strictly anaerobic methanogenic archaea use carbon (e.g., in form of CO2 or acetic acid) instead of oxygen as a terminal electron acceptor and produce CH4. Therefore, CH4 is produced in anaerobic processes, but also during composting, as anaerobic zones will usually occur. Some of the produced CH4 may then be oxidized e.g., by methanotrophic bacteria, before it is emitted to the atmosphere. N2O is produced during microbial transformations of ammonium (NH4+) and nitrate (NO3−): NH4+ is transformed into nitrite and then NO3− (nitrification) with some loss as N2O, and NO3− is then transformed into nitrogen gas (N2) via denitrification, also with some loss as N2O. How large the losses as N2O are in each step, depends on the conditions, but because there are two processes involved requiring different conditions, N2O emissions can be difficult to predict. Nitrification and denitrification can be performed by various microorganisms such as archaea, bacteria or fungi. Ammonia-oxidizing bacteria and archaea are known to perform the first step of NH4+ oxidation while nitrite oxidizing bacteria perform the further oxidation to NO3−. Denitrification is mainly known to be facilitated by heterotrophic bacteria but there are also autotrophic denitrifiers have been identified.
GHG emissions during composting of organic waste have been assessed. Substantial emissions of both CH4 and N2O have been found (Ermolaev et al., 2015; Zhu-Barker et al., 2017). There are only a few studies of GHG emissions during digestate composting (Li et al., 2018; Zeng et al., 2016). Dietrich et al. (2020) found that GHG emissions after applications of digestate to soil can be substantial, but applications of composted DS do not induce emissions. That raises the question of how large emissions from composting DS may be. Emissions of N2O may well be higher from DS because of an increased NH4+ to total nitrogen (N) ratio after AD. Emissions of CH4 are usually assumed to be lower after AD because most of the CH4 potential has been used up during the AD process (Brémond et al., 2021; Vergote et al., 2020). However, there are also some arguments why CH4 emissions may be higher from digestate composting than from composting the organic waste directly. Digestate contains a microbial community that is adapted for high CH4 production (He et al., 2000; Sundberg et al., 2013a) and the pH in digestate is elevated and more suitable than in the original residues (Kheiredine et al., 2014). Sometimes there is also some CH4 potential left in the digestate because the digestion process is not run to completion (Li et al., 2020). In addition, fresh organic material, usually added as bulking agents to provide structure (Ahn et al., 2011; Beck-Friis et al., 2000; Bustamante et al., 2013), may provide additional available carbon as CH4 potential.
The objective of this paper was to compare raw food waste composting to digested food waste composting. The composting process and emissions of the GHGs N2O and CH4 were compared.
Section snippets
Composting feedstocks
Romerike biogas plant (RBA) (60.18728, 11.39981) is a biogas plant treating food waste mainly collected from households in Oslo. The organic waste was grinded, sieved, screw pressed and heated up to 80°–100 °C before it was sampled as food waste (FW) in the biogas plant. The biogas plant in Romerike uses a thermal hydrolysis process as pre-treatment and a mesophilic process (38 °C) for the AD. DS was sampled after adding a polymer for flocculation and de-watering by a decanter centrifuge.
Cumulative CH4 and N2O emissions
Anaerobic pre-treatment of the feedstock for composting enhanced both cumulative CH4-C and N2O-N emissions, compared to composting raw food waste: Cumulative CH4-C emissions after 3 weeks were 12 times higher from the DSmix than from the FWmix (p < 0.001) (Fig. 1). Cumulative N2O-N emissions were almost 7 times higher for the DSmix than for the FWmix (p = 0.004).
Composting process - temperature, CO2, pH, C/N, EC
Temperature is an important indicator for the composting process as it is closely linked to microbial activity and the decomposition
Conclusion
The digestate composting process was faster and reached higher temperatures in the beginning than food waste composting where the elevated temperature lasted longer. Both CH4 and N2O emissions during composting were significantly higher from digested food waste than from raw food waste. CH4 was found to be the major contributor to GWP while N2O contributed much less to the GWP. The “import” of different microorganisms through feedstock, esp. methanogens in digestate, can enhance CH4 production
CRediT authorship contribution statement
Maria Dietrich: Methodology, Investigation, Formal analysis, Writing – original draft. Monica Fongen: Investigation, Resources. Bente Foereid: Funding acquisition, Project administration, Conceptualization, Investigation, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was funded by the Research Council of Norway through the RICEDIG project [grant number 283570]. The authors thank Romerike Biogas Plant for providing composting feedstocks and Tormod Briseid for useful comments on the manuscript.
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