Powdered activated carbon amendment in percolate tank enhances high-solids anaerobic digestion of organic fraction of municipal solid waste

Abstract

Despite many studies recently investigated the addition of conductive materials for enhancing biomethane recovery in anaerobic digestion, there have been limited efforts to adopt this strategy in high-solids anaerobic digestion (HSAD) with percolate recirculation. In this study, two identical lab-scale mesophilic (36 °C) HSAD systems were operated with the organic fraction of municipal solid waste (OFMSW) under similar operating conditions, including food to microorganism ratio and daily percolate recirculation rate. The percolate tank of the test HSAD system was amended with 15 g/L of powdered activated carbon (PAC). After 30 days of operation, the cumulative biomethane yield for the test reactor was 17 % higher than the control reactor (109 vs. 93 L CH₄/kg VS). Furthermore, PAC addition showed additional benefits by lowering free ammonia nitrogen (FAN) and volatile fatty acids (VFAs) concentrations that can inhibit anaerobic digestion. Nonetheless, the preliminary economic analysis suggested that the HSAD amended with PAC is practically challenging and will only be economically feasible if PAC can be recycled and reused for the long-term operation or several cycles.

Introduction

The ongoing increase of the world’s population growth and urbanization would cause a 70 % increment of annual solid waste generation from ∼2.0 billion tons in 2016 to 3.4 billion tons in 2050 (Hoornweg and Bhada-Tata, 2012; WorldBank, 2017, 2018). Moreover, the rising trend of the population would have led to the further global food demand and duplication or even triplication of current universal demand for biomass-based renewable energy by 2050 (Johansson et al., 2012). Moreover, growing population and urbanization would also instigate the landfilling of municipal solid waste (MSW) due to a lack of sustainable waste management practices and policies. Reportedly, the extensive open-air processing and storage of wastes in landfills cause emissions of greenhouse gases (GHGs), which poses serious risks to human well-being and global warming (WHO, 2018). So far, various efforts have been attempted to study and implement different practical and economic pathways to mitigate the menaces mentioned above. One of the most practical ways of diverting MSW from landfills is its valorization using thermochemical and biochemical technologies into various value-added products, such as compost, chemicals, and biofuels (Kumar and Samadder, 2020; Lim et al., 2016; Olatunji et al., 2019; Zhang et al., 2019).

Among various biochemical technologies, high-solids anaerobic digestion (HSAD) has been accepted as a practical and eco-friendly approach in addressing both environmental challenges and energy crisis through the waste diversion from landfills and the recovery of bioenergy (Dastyar et al., 2015; Guilford, 2009; Rezaee et al., 2020). Various advantages of HSAD include: (i) lower water consumption and energy demand, (ii) capability of handling higher organic loading rates (OLRs), and (iii) alleviating the requirement of dewatering of residuals (Fagbohungbe et al., 2015). However, compared to wet-type digesters (also known as low-solids anaerobic digestion, LSAD), heterogeneous and variable characteristics of the feedstock often lead to inferior process kinetics, lower energy recovery and solids removal from HSAD process (Fagbohungbe et al., 2015). The evolutionary timeline of the HSAD process reveals that, to date, numerous studies have aimed to investigate the significance of various operating parameters for HSAD systems, including temperature, food to microorganism (F/M) ratio, external and/or internal mixing, recirculation of percolate and biogas, etc. (Chen et al., 2018; Guilford et al., 2019; Guo et al., 2020; Wang et al., 2018; Westerholm et al., 2020). Notably, designs of several commercial HSAD systems adopted percolate/leachate recirculation to eliminate mass transfer limitations in the process via homogenization of organics and nutrients (Fagbohungbe et al., 2015).

The process of AD consists of a series of biochemical reactions: hydrolysis, acidification, acetogenesis, and methanogenesis (Ai et al., 2018; Elyasi et al., 2015). The initial hydrolysis and fermentation of organics materialize acetate and other electron carriers that can function as the precursors for methanogenesis (Stams and Plugge, 2009). The electron carriers like formate or hydrogen are transformed into biomethane via the interaction between bacteria and methanogens via indirect or mediated interspecies electron transfer (MIET), which is a very slow process indeed. Recently, the amendment of anaerobic digesters with conductive additives has shown significant potential to boost the biochemical process kinetics, enhance organics degradation, and improve biomethane recovery (Guo et al., 2018; Tsui et al., 2020). Conductive materials could promote direct interspecies electron transfer (DIET), a syntrophic partnership that streamlines the direct transfer of electrons from the bacterium to the methanogen (Baek et al., 2018; Barua and Dhar, 2017; Ren et al., 2020; Sharma et al., 2019). Various conductive additives, such as graphite, granular activated carbon (GAC), powder activated carbon (PAC), biochar, magnetite, carbon fibers, carbon cloth, etc., have been explored to enhance the AD process (Barua et al., 2019; Cheng et al., 2019; Indren et al., 2020; Lei et al., 2018). However, there have been limited studies on the application of conductive materials to improve the performance of HSAD processing the organic fraction of municipal solid waste (OFMSW). Moreover, to the best of the authors’ knowledge, no studies investigated the amendment of conductive additives in HSAD systems with percolate recirculation.

The present work investigates the effects of PAC on biomethane recovery from OFMSW in an HSAD system with percolate recirculation. The first objective of this research is to explore how the addition of PAC material influences HSAD process kinetics and degradation of OFMSW, compared to an unamended control reactor. The second objective is to examine the impact of PAC on the potentially inhibitory factors, including pH, accumulation of volatile fatty acids (VFAs), and free ammonia nitrogen (FAN) levels. In this study, PAC was added to the percolate tank of an HSAD system, while in previous studies, conductive additives were directly added into the digester tank along with feedstock and inoculum (Chowdhury et al., 2019a; Cuetos et al., 2017; Pan et al., 2020b; Ryue et al., 2019b; Xu et al., 2015b; Zhang et al., 2017c). Thus, this present study investigates whether recirculation of percolate amended with conductive materials can enhance the HSAD system.