Methanogenic pathway and microbial succession during start-up and stabilization of thermophilic food waste anaerobic digestion with biochar

Highlights

• Methane production was significantly enhanced in biochar reactors by up to 18%

• Biochar addition resulted in low VFA levels and enhanced process stability.

• Biochar addition increased overall archaea to bacteria ratio.

• Biochar addition promoted the growth of electroactive Clostridia.

• DIET pathway and acetoclastic methanogenesis was enhanced in biochar reactors.

Abstract

One of the major obstacles for thermophilic anaerobic digestion is the process instability during start-up. This study proposed the use of a cost-effective additive, biochar, to accelerate and stabilize the start-up of thermophilic semi-continuous food waste anaerobic digestion. The results showed that the reactors with biochar addition resulted in up to 18% higher methane yield as compared to the control reactors (without biochar). The key microbial networks were elucidated through thermochemical and microbial analysis. Particularly, the addition of biochar promoted the growth of electroactive Clostridia and other electroactive bacteria, while the absence of biochar promoted the growth of homoacetogenic Clostridia and syntrophic acetate oxidizing bacteria. It was revealed that biochar promoted direct interspecies electron transfer between the microbes and was responsible for the faster degradation of volatile fatty acids. Furthermore, reactors with biochar also enhanced the thermodynamically favourable acetoclastic methanogenic pathway due to the higher abundance of Methanosarcina.

Introduction

The generation of food waste (FW) is about 1.3 to 1.6 billion tons annually, with it showing an upward trend due to continued global population growth and economic activities. (Ma and Liu, 2019). The traditional waste disposal methods, i.e. incineration and landfill, are increasingly being criticized as unsustainable technologies while the energy and nutrients recovery from FW using anaerobic digestion (AD) is becoming an attractive practice (Ma and Liu, 2019). AD is a biological waste treatment technology that decomposes organic matters into biogas (renewable energy) in an oxygen-free environment and produces biofertilizer simultaneously. FW is considered as an ideal substrate for AD because it mainly consists of carbohydrates, proteins and lipids (Liu et al., 2017). AD is typically operated at either mesophilic (35–40 °C) or thermophilic (50–55 °C) conditions (Braguglia et al., 2018). Mesophilic anaerobic digestion (MAD) is generally adopted in full-scale plants because of stable biogas yields and less energy requirement compared to thermophilic anaerobic digestion (TAD) (Braguglia et al., 2018). Regardless, TAD is gaining more attention in recent years because of its higher organic matter degradation rate and higher methane yield compared to MAD (Tian et al., 2019). For example, the organic loading rate (OLR) was found to be optimal at 1.5 gVS L⁻¹ day⁻¹ with a methane yield of 37 L mL gVS⁻¹ for MAD and 2.5 gVS L⁻¹ day⁻¹ with a methane yield of 541 mL gVS⁻¹ for TAD in the study of investigating the effect of OLR on AD of FW (Liu et al., 2017). Besides, TAD also reduces pathogen load and odour emission which makes the application of the digestate as fertilizers more favourable than MAD (Moset et al., 2015).

However, one major limitation of the full-scale TAD commercialization is the non-availability of seed sludge and the unstable start-up phase that sometimes leads to process failure (Shin et al., 2019). Therefore, the start-up of TAD is normally carried out with mesophilic inoculum through different strategies such as a one-step increase of temperature and step-wise increase of temperature (Shin et al., 2019). However, irreversible accumulation of volatile fatty acids (VFA), pH drop and even system failure are often reported because, on the one hand, the increase in temperature can improve hydrolysis and aggravate the accumulation of intermediates (De La Rubia et al., 2013); and on the other hand, methanogens are often severely impacted by the temperature shock thereafter leads to kinetic uncoupling within the biological steps (Yan et al., 2017). Furthermore, the degradation of VFA, such as propionate and butyrate, is also slow due to its positive Gibbs energy (Wang et al., 2018b). To overcome this, syntrophy between organisms where electrons are shuttled from one organism to another is therefore crucial for methanation of such complex substrate (Yan et al., 2017). One of the predominant electron transfer mechanism is interspecies hydrogen transfer (IHT), where H₂ serves as electron carrier between the substrate-oxidizing microorganism and H₂-utilizing methanogens (Park et al., 2018). For the syntrophic partners to grow, it requires the rapid consumption of H₂ as the accumulation (>10 Pa) can restrict the syntrophic system thermodynamically (Park et al., 2018). There is growing evidence showing that the syntrophic communities do not rely exclusively on IHT and that electron-donating and electron-accepting microbes could form electric current and transfer electrons via the direct interspecies electron transfer (DIET) (Park et al., 2018). Furthermore, the electron transfer speed is said to be 10⁶ times faster in DIET than IHT (Cruz Viggi et al., 2014). Therefore, establishing DIET can be an important strategy to avoid the irreversible accumulation of VFA.

Studies have explored the addition of conductive materials (CM) in AD to facilitate DIET and enhance methane production by compensating pili and other cell component involved in the exogenous electron transfer (Martins et al., 2018). In other works, batch study using glucose as substrate was successfully conducted to start TAD by adding granulated activated carbon (GAC) and carbon nano-tube (CNT) (Yan et al., 2017). To the author's knowledge, no study has been conducted using a complex substrate, such as food waste, to start TAD with mesophilic seed sludge by adding CM and the operation stability on semi-continuous TAD after start-up has yet to be verified. Besides, the price of most CM (e.g. US$2000/kg for CNT, US$0.31/kg for GAC, and US$0.08–0.12/kg for biochar) can be prohibitive, and the continuous addition may not be economically feasible in actual AD operation (Martins et al., 2018, Zhang et al., 2018a, Zhang et al., 2020). As an alternative, biochar, produced by thermochemical conversion of biomass waste, has been investigated as an additive in AD to enhance process stability and methane production, as well as to improve digestate quality for soil application and nutrient recycling (Usmani et al., 2016). In microbial studies, the mechanism of DIET enhanced by CM has primarily been studied in co-cultures where Geobacter species are one of the DIET partners (Chen et al., 2014, Wang et al., 2018a). However, it was highlighted in a review paper (Martins et al., 2018) that the majority of the studies involving CM did not detect or detected Geobacter in low percentage, yet it was still claimed that CM facilitated DIET. Therefore, it is also important to elucidate other potential candidates that could support DIET in CM.

Overall, the main objectives of this study were to (1) evaluate the performance of the start-up of food waste TAD with the addition of biochar, (2) find the optimal amount of biochar for start-up, and (3) elucidate both the methanogenic pathways and potential microbial species that could contribute to DIET. To achieve the objectives, semi-continuous AD with and without biochar addition was performed to compare the effect of biochar on the methane yield, VFA variation, and microbial community.