Biogas recovery from two-phase anaerobic digestion of food waste and paper waste: Optimization of paper waste addition

Highlights

• Biogas was recovered from the co-digestion of FW and PW using TPAD.

• The effect of PW% on the long-term performance of TPAD was investigated.

• The decreasing trends of ammonia and alkalinity were well simulated.

• Higher C/N raised the microbial yields, which aggravated the nitrogen deficiency.

• PW:FW = 40:60 (TS%) is recommended for bioenergy augmentation via TPAD.

Abstract

In order to optimize the biogas recovery from the co-digestion of food waste (FW) and paper waste (PW), the effect of PW content on two-phase anaerobic digestion (TPAD) was investigated. The mixtures of FW and PW, with the ratios of 10:0, 8:2, 6:4 and 5:5 (total solids), were fed into TPAD to recover biomethane. After the long-term expriment, it is elucidated that the methanogenesis in TPAD was stable for PW ≤ 40%. When PW = 50%, NH₄HCO₃ was added to the methanogenic phase to provide nitrogen.

As the indicators of the stability of the anaerobic process, the ammonia and alkalinity in the methanogenic phase were simulated for their decreasing trend. The simulation results quantified the nitrogen deficiency in the methanogenic phase for PW = 50%. Also, the comparison of alkalinity and ammonia revealed that ammonia was the major contributor to the alkalinity. Furthermore, via stoichiometric calculations, high C/N ratios were found to increase the microbial yield and exacerbated the nitrogen deficiency. In the energy estimation, adding PW showed significant increase only when PW ≥ 40%. It was concluded that 40% was the optimal PW content for bioenergy augmentation from co-digestion of FW and PW using TPAD.

Introduction

The amount of municipal solid waste produced increases with the process of urbanization. In the OFMSW, FW and PW are the two major categories that can be separated from the source of waste or by the mechanical sorting system (Hoornweg and Bhada-Tata, 2012; López et al., 2010; Romero-Güiza et al., 2014). As the fossil fuel supply becomes depleted, the chemical energy recovered from such waste biomass has potential to be stored in biofuels to fill the gap between the energy supply and the energy requirements (Gaurav et al., 2017). Biomethane is a gaseous biofuel which can be collected from the biogas emitted from the anaerobic digestion process and purified by removing other gaseous components. High-solid wet anaerobic digestion is considered one of the most effective and economical biomass conversion technologies (Aboudi et al., 2017; Dolan et al., 2011; Qiang et al., 2012). As an anaerobic digestion process, TPAD achieves the separation of the acidogenic phase and the methanogenic phase, which makes it possible to collect the intermediate fermentation products, such as hydrogen and organic acids, in the acidogenic phase (Luo et al., 2011; Shen et al., 2013).

There have been numerous reports on the anaerobic treatment of FW, with many focused on the co-digestion with nitrogen-rich biomass (Mata-Alvarez et al., 2014; Tandukar and Pavlostathis, 2015; Uçkun Kiran et al., 2014). The co-substrates in most of those studies were sewage sludge or manure from poultry, swine, cattle or human beings (Cavinato et al., 2010; Kim et al., 2004; Owamah et al., 2014; Wang et al., 2014). The basic aim of co-digestion is to adjust and optimize the carbon to nitrogen (C/N) ratio (Cook et al., 2017; Zhen et al., 2015; Zhou et al., 2013). From this point of view, due to its rich abundance in carbohydrates such as lignocellulose, FW has been generally regarded as the carbon-rich material to be combined by nitrogen-rich (protein-rich) materials. However, despite the acidic pH, its element composition suggests that FW could be classified as a nitrogen-rich biomass. FW contained a sufficient amount of proteins and the anaerobically digested effluent contained excess ammonia, with an approximately equivalent amount of alkalinity (Kobayashi et al., 2012a; Qiang et al., 2012). It has also been concluded that there is no indication of methanogenic failure in the co-digestion of FW and waste activated sludge at any ratio (Heo et al., 2004). On the contrary, the researches on the co-digestion of FW with other carbon-rich (lignocellulosic) materials were infrequently reported. Some cases of co-digesting FW and lignocellulosic biomass included a third nitrogen-rich waste, such as the ternary combinations of FW/card package/cattle slurry or FW/corn stover/chicken (Li et al., 2013; Zhang et al., 2012). But the feasibility of co-digesting FW only with carbon-rich material was less investigated.

As another principal category in the OFMSW, PW includes toilet/tissue paper, office printing paper, newsprint paper, paper bag, cardboard, etc. (Gonzalez-Estrella et al., 2017b). By applying the mechanical sorting systems with specific optical sensors, separating different kinds of paper products has become feasible in practice (Rahman et al., 2014). In fact, PW can be used as the conjugate material of FW because it can be generated, collected and sorted at the same site as FW. Conventionally, the sorted PW is reused to produce sanitary paper. Meanwhile, the effect of PW content on anaerobic digestion was less focused on. The studies on the digesting performance varied by PW contents is required because an uncertain fraction of PW is commingled in the collected OFMSW. Generally, the PW and FW are commingled so tight that separating the PW is costly, unnecessary and technically impossible. Moreover, the anaerobic digestion of OFMSW could be seen as the terminal in the life-cycle of paper products. With reference to these facts, investigatig the co-digestion of FW and PW helps to unveal the feasibility of digesting unseparated OFMSW and the potential of biomethane augmentation.

In a batch experiment on waste paper, the liquid-state anaerobic digestion harvested a higher methane yield, at 312.4 L/kg-VS_fed, than that of the solid-state anaerobic digestion of 15 L/kg-VS_fed (Brown et al., 2012). Other results from batch experiments found that methane can be recovered from 80% of cardboard, 80% of office paper and 70% of the co-digestion of cardboard and FW (Asato et al., 2016; Gonzalez-Estrella et al., 2017a). It has been reported that the toilet paper was fully degraded on the reject side of the membrane bioreactor (Chen et al., 2017). Mixing 15% and 30% waste paper (wet basis) to biowaste showed little change in the thermophilic VS removals (Fonoll et al., 2016). Still, few studies have been reported on the effect of mix ratios on the performance of TPAD during the co-digestion of FW and PW.

It is believed that the performance of the anaerobic digestion of OFMSW could be optimized by properly adjusting the PW content in the anaerobically treated OFMSW. In order to optimize the biomethane recovery from the co-digestion of PW with FW, the effect of mixing different contents of PW was investigated on TPAD. The OFMSW was made up of 0%, 20%, 40% and 50% (TS) of PW and the rest was FW. The effect of PW contents in OFMSW is focused on biomethane recovery, organic removal and long-term stability. Bivariate linear regression is used to simulate the ultimate ammonia and alkalinity concentrations after external NH₄HCO₃ was added. The effect of substrate C/N ratios on the microbial reproducing yields and energy estimation are also discussed.