Anaerobic degradation of digestate based hydrothermal carbonization products in a continuous hybrid fixed bed anaerobic filter

Highlights

• AD of digestate HTC liquor and slurry through a fixed bed anaerobic filter reactor.

• The reactor was operated for 300 days with increasing OLR up to 10 gCOD/L/day.

• Biogas production peaked at FOS/TAC ratio 0.25 while was the most stable at 0.15.

• No detrimental difference between the biogas production from HTC liquor and slurry.

• Digestate based hydrochar as supplement material for anaerobic digestion.

Abstract

This study investigates the suitability of continuous hybrid fixed bed anaerobic filter reactor for treating sewage and agro-industrial digestate hydrothermal carbonization (HTC) products; hydrochar and HTC liquor (HTCL). The reactor was operated for 300 days under mesophilic conditions at different organic loading rates (OLR); maximum OLRs of 7.4 and 10 gCOD/L/d were reached while treating HTC liquor and slurry, respectively. 15 g/L hydrochar were added to the reactor as a supplement while treating HTCL solely thus increasing the biogas production up to 153%. The reactor was fed with HTCL and hydrochar with an increasing mixing ratio, and the co-digestion impact was dependent on hydrochar concentrations. The results of the study indicate that the hybrid fixed bed anaerobic filter reactor is a promising anaerobic digestion configuration for treating HTCL and overcoming the HTC upscaling challenges, and the suitability of digestate hydrochar utilization as supplement material for anaerobic digestion.

Introduction

Wastewater treatment plants generate huge amounts of sludge that is commonly treated through anaerobic digestion (AD) for biogas production (Elagroudy et al., 2020). Digested sludge, named digestate, is commonly managed through agricultural application, incineration or landfilling. However, such management approaches are challenged by environmental and economic concerns, associated with digestate nitrogen and phosphorus concentrations and heavy metal content, in addition to transportation and storage costs (Marin-Batista et al., 2020, Vaneeckhaute et al., 2017). Thermochemical processes, specifically hydrothermal carbonization (HTC), are under investigation for managing digestate while achieving energy recoveries (Aragón-Briceño et al., 2017, Park et al., 2019), without the need for energy-intensive dewatering (Ahmed et al., 2021). HTC converts high-moisture organic feedstock into hydrochar, which is a carbon-rich solid product of the process, liquor and gas streams. The process is realized by applying temperatures ranging between 180 and 250 °C under subcritical conditions and self-generated pressure from 10 to 50 bar (Ischia and Fiori, 2020).

Digestate hydrochar is considered a poor-quality solid fuel due to its high ash content, and slagging and fouling behavior (Parmar and Ross, 2019). Marin-Batista et al. (2020) treated digestate hydrochar through acid leaching for phosphorus recovery and observed that the ash content was reduced up to 50%, yet still the hydrochar quality as a solid fuel was fairly interesting with higher heating values ranging between 21 and 23 MJ/kg. About the other main HTC product stream, the HTC Liquor (HTCL) contains a high load of organic compounds that solubilize during the HTC process (Merzari et al., 2019). HTCL treatment is one of the main challenges for HTC process upscaling; managing digestate HTCL through AD has proved to be a promising approach, as it improves the economics, energetics and the environmental performance of the HTC process (Ahmed et al., 2021). Most of the previous studies investigated the batch AD of HTCL with a limited study on the continuous AD. To the best of our knowledge, only Nyktari et al., 2017, Weide et al., 2019, Wirth et al., 2015, Wirth and Mumme, 2014 investigated the continuous operation of the AD process of HTCL. Wirth and Mumme (2014) compared a continuously stirred-tank reactor and an anaerobic filter for the AD of corn silage HTCL and favored the anaerobic filter for its more stable operation. Anaerobic filter reactors were also investigated for treating sewage sludge HTCL by Wirth et al. (2015) at mesophilic and thermophilic conditions at varying organic loading rates and hydraulic retention time and their results showed stability in the biogas production. Nyktari et al. (2017) investigated the AD continuous flow of HTCL through anaerobic fixed-film digesters, while Weide et al. (2019) investigated that through two anaerobic stages of a continuous stirred-tank reactor with sludge recirculation followed by an expanded granular sludge bed.

Several studies investigated AD enhancing by adding hydrochar as a supplement (Ferrentino et al., 2020, Lucian et al., 2020, Ren et al., 2020, Wang et al., 2020, Zhao et al., 2018). Pagés-Díaz and Huiliñir (2020) added municipal solid waste (MSW) hydrochar at different concentrations to HTCL obtained from sewage sludge and MSW co-HTC treatment. Their results showed that hydrochar concentration from 1 to 10 g/L shortened the lag phase, while methane yields were almost doubled at 15 g/L. It is reported that hydrochar enhances methane yields due to its large fraction of anaerobically degradable carbon and abundant surface functional compounds (Mumme et al., 2014, Xu et al., 2020). Xu et al. (2018) reported that hydrochar increases the conversion of volatile fatty acids (VFA) to methane, as 50 mg of VFA were removed by adding 1 g of hydrochar at 4 g/L hydrochar concentration. Hence, hydrochar addition accelerates the degradation of intermediate acids and methanation during AD, which is a rate-limiting step in the AD of HTCL (Wirth et al., 2015). Hydrochar can act as carrier for bacteria in AD and consequently increases the active biomass in the system (Xu et al., 2018). Furthermore, it enriches the microorganisms that degrade aromatic and phenolic organics (Usman et al., 2020); such organics are present in HTCL, they are hard-to-degrade via AD and can inhibit methanogenesis (Merzari et al., 2019).

While hydrochar addition can enhance the methane yields and consequently the energy recoveries from HTCL, processing HTC slurries through AD can be a promising solution for managing streams whose hydrochar is of poor quality as a solid fuel, like digestate. On these bases, this work aims to investigate the suitability of fixed bed anaerobic reactors for treating HTC products under continuous flow and mesophilic conditions at different organic loading rates (OLR). The reactor suitability was assessed through monitoring biogas production and degradation efficiencies, while being fed with digestate based HTC liquor and hydrochar obtained from sewage and agro-industrial wastewater. The novelty of this work is that it investigates the AD and HTC coupling through the operation of a hybrid continuous flow anaerobic reactor treating digestate HTCL and hydrochar; this to debottleneck HTC applications to sewage digestate and contribute to sludge reduction.