Ulva biomass as a co-substrate for stable anaerobic digestion of spent coffee grounds in continuous mode

Highlights

• Ulva was examined as co-substrate for anaerobic digestion of spent coffee grounds.

• Co-digestion with Ulva proved beneficial for continuous SCG biomethanation.

• Beneficial effect is more pronounced at higher organic and hydraulic loads.

• Reactor microbial community structure changed dynamically during the experiment.

• Network analysis provides a comprehensive view of microbial interactions.

Abstract

Ulva biomass was evaluated as a co-substrate for anaerobic digestion of spent coffee grounds at varying organic loads (0.7–1.6 g chemical oxygen demand (COD)/L d) and substrate compositions. Co-digestion with Ulva (25%, COD basis) proved beneficial for SCG biomethanation in both terms of process performance and stability. The beneficial effect is much more pronounced at higher organic and hydraulic loads, with the highest COD removal and methane yield being 51.8% and 0.19 L/g COD fed, respectively. The reactor microbial community structure changed dynamically during the experiment, and a dominance shift from hydrogenotrophic to aceticlastic methanogens occurred with increase in organic loading rate. Network analysis provides a comprehensive view of the microbial interactions involved in the system and confirms a direct positive correlation between Ulva input and methane productivity. A group of populations, including Methanobacterium- and Methanoculleus-related methanogens, was identified as a possible indicator for monitoring the biomethanation performance.

Introduction

Coffee is one of the most consumed crops. Annual coffee consumption is approximately 9 million tons worldwide (http://www.ico.org), and more than 8 million tons of spent coffee grounds (SCG) are generated yearly (Vardon et al., 2013). With continuous increase in global coffee consumption, handling SCG has become an increasingly challenging issue. Currently, most SCG is discarded as waste due to a lack of practical methods to efficiently handle the huge volume. If improperly treated, however, SCG residues can cause pollution problems because of the decomposition of readily biodegradable organics and the potential release of caffeine, tannin, polyphenols, etc. (Qiao et al., 2013, Vardon et al., 2013). The high organic content of SCG suggests its potential as an energy feedstock, and its conversion into biofuels has been investigated in several studies. Biogas production by anaerobic digestion (AD) is considered a practical approach for energy recovery from SCG. However, previous studies have shown that mono-digestion of SCG often fails to achieve long-term stability primarily due to a lack of nutrients and trace elements in the substrate (Kim et al., 2016). A viable option to mitigate this problem is adding a co-substrate that can improve biodegradability by amending the characteristics of the substrate mixture (Fonoll et al., 2015).

Ulva, commonly known as sea lettuce, is a green seaweed that causes significant macroalgal blooms worldwide. The worst Ulva bloom on record was observed in the Yellow Sea off the Chinese coast in summer of 2008. A massive green tide covered approximately 3800 km² (ca. 20 million wet tons of seaweed biomass) and deposited over 1.5 million wet tons of seaweed biomass on the beaches of the region (Gao et al., 2010). Because Ulva is readily biodegradable and rich in nitrogen and sulfur, its decomposition can lead to serious hygiene and environmental problems. Given the nutrient-rich nature of Ulva, AD represents an attractive option to treat harmful seaweed waste. However, low C/N and C/S ratios of Ulva can limit microbial activity and cause significant deterioration of methanogenic performance. Co-digestion with a co-substrate with high C/N and C/S ratios, such as SCG, may provide a practical solution to this limitation. Not like SCG, Ulva biomass does have spatial (between inland coastal areas) and seasonal (between summer and winter) variations in its availability. This limitation should be carefully considered in such co-digestion approaches for stable operation.

A recent study by the authors’ group evaluated the feasibility of co-digestion of SCG with different co-substrates in biochemical methane potential (BMP) tests (Kim et al., 2016). The results demonstrated that, although no synergistic effect was observed, co-digestion with Ulva significantly enhanced the reaction rate without loss in methane productivity. However, this co-digestion has not been examined for continuous process performance and stability, which is important for practical applications of the co-digestion strategy. Therefore, this study aims to investigate the potential of Ulva as a co-substrate to support stable AD of SCG in continuous mode, where fresh material is continuously fed to the digester and digestate is continuously removed. A reactor experiment was performed without any supplements for a more economical and convenient approach. The primary focus of this study is the response of the process to variations in operating conditions such as organic loading rate (OLR) and substrate composition. For deeper insights into the underlying microbial ecology, shifts in microbial community structure, associated with changes in process performance and operating conditions, were analyzed by next-generation sequencing.