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

Enhanced methane production with co-feeding spent coffee grounds using spare capacity of existing anaerobic food waste digesters

With increasing coffee consumption worldwide, the efficient and sustainable management of spent coffee grounds (SCG) has become increasingly challenging. This study investigated the anaerobic co-digestion of small amounts of SCG with food waste (FW) at increasing co-feeding ratios of 1:100-1:10 (volatile solids basis) to assess the possibility of SCG treatment using the spare capacity of existing anaerobic digesters. Co-feeding SCG increased methane production compared to FW mono-digestion in the tested range of co-feeding ratios without compromising process stability. Methane yield did not further increase when the SCG/FW ratio increased above 4%, and process failure occurred at a 1:10 co-feeding ratio without trace element supplementation. The enhanced methanogenic performance was attributed to increased protein removal efficiency, which was potentially related to the promotion of peptide hydrolysis. The overall results suggest that co-feeding appropriate small amounts of SCG to FW digesters can be a realistic sustainable option for SCG management.

Emerging potential of spent coffee ground valorization for fuel pellet production in a biorefinery

Abstract

The global market for fuel pellets (FPs) has been steadily growing because of a shift to coal substitutes. However, sustainability and the availability of biomass are the main issues. Various kinds of bio-wastes can be valorized through cutting-edge technologies. In the coffee industry, a valuable organic waste called spent coffee grounds (SCGs) is generated in bulk. SCG can be divided into two components, namely spent coffee ground oil and defatted spent coffee grounds (DSCG). SCG and DSCG can be used to produce FPs with excellent higher heating values. This review highlights that burning FPs composed of 100% SCG is not feasible due to the high emission of NOx. Moreover, the combustion is accompanied by a rapid temperature drop due to incomplete combustion which leads to lower boiler combustion efficiencies and increased carbon monoxide emissions. This was because of the low pellet strength and bulk density of the FP. Mixing SCG with other biomass offers improved boiler efficiency and emissions. Some of the reported optimized FPs include 75% SCG + 20% coffee silverskin, 30% SCG + 70% pine sawdust, 90% SCG + 10% crude glycerol, 32% SCG + 23% coal fines + 11% sawdust + 18% mielie husks + 10% waste paper + 6% paper pulp, and 50% SCG + 50% pine sawdust. This review noted the absence of combustion and emissions analyses of DSCG and the need for their future assessment. Valorization of DSCG offers a good pathway to improve the economics of an SCG-based biorefinery where the extracted SCGO can be valorized in other applications. The combustion and emissions of DSCG were not previously reported in detail. Therefore, future investigation of DSCG in boilers is essential to assess the potential of this industry and improve its economics.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s10668-022-02361-z.

Energetic Valorization of Cereal and Exhausted Coffee Wastes Through Anaerobic Co-digestion With Pig Slurry

In the past years, there has been steady growth in work relating to improve resource efficiency through waste minimization and bioenergy recovery to mitigate climate change. Agro-food industries produce large amounts of bio-waste, challenging innovative energetic valorization strategies in the framework of circular economy principles. Anaerobic digestion (AD) technology is an interesting route to stabilize organic matter and produce biogas as a renewable energy source. This study involves continuous co-digestion of pig slurry (PS), cereal and exhausted coffee wastes (CECW) performed in a continuously stirred tank reactor, with a hydraulic retention time (HRT) of 16 days under at mesophilic conditions (36.9 ± 0.3°C). The experimental trials, were designed to include different cereal and exhausted coffee liquor (CECL) shares in the feeding mixture, corresponding to different PS to CECL ratios (PS:CECL), respectively: 100:0 (T0), 90:10 (T1), 80:20 (T2), and 70:30 (T3), in terms of percentage of inlet feeding rate (v:v). The results obtained for the feeding rate (70:30) yield to the highest specific methane production (SMP = 341 ml.gVS−1) led to a 3.5-fold improvement in comparison with the reference scenario. The synergetic effect between the microbial consortia of PS and the high carbon to nitrogen ratio (C/N) of CECL explain the improvements achieved. The maximum soluble chemical oxygen demand (SCOD) reduction (84.0%) due to the high content and soluble chemical oxygen demand to total chemical oxygen demand ratio (SCOD/TCOD) corroborate the results achieved. The digester stability, evaluated by specific energetic loading rate, was below the limit (0.4 d−1). Results from ANOVA showed a significant effect of CECL on the resulting GPR and SMP values. Additionally, Tukey's “Honest Significant Difference” method, confirmed statistically significant differences between the trials T3-T0, T3-T1, T3-T2, and T2-T0. Thus, co-digestion of PS and of CECL seems to be a promising approach for bioenergy recovery and promoting biowastes circularity.

Biorefinery of spent coffee grounds waste: Viable pathway towards circular bioeconomy

The circular bioeconomy plan is an innovative research based scheme intended for augmenting the complete utilization and management of bio-based resources in a sustainable biorefinery route. Spent coffee grounds based biorefinery is the emerging aspect promoting circular bioeconomy. The sustainable circular bioeconomy by utilizing SCG is achieved by cascade approaches and the inclusion of many biorefinery approaches to obtain many bio-products. The maximum energy recovery can be obtained by process integration. The economic analysis of the biofuel production from SCG is dependent on the cost of raw material, transportation, the need of labor and energy, oil extraction operations and biofuel production. The inclusion of new products from already established product can minimize the investment cost when related to the production cost. A positive net present value can be achieved via SCG biorefinery which indicates the profitability of the process.

Modulation of gut microbiota by spent coffee grounds attenuates diet-induced metabolic syndrome in rats

Coffee brewing produces spent coffee grounds as waste; few studies have investigated the health benefits of these grounds. This study investigated responses to spent coffee grounds in a diet-induced rat model of metabolic syndrome. Male Wistar rats aged 8-9 weeks were fed either corn starch-rich diet or high-carbohydrate, high-fat diet for 16 weeks, which were supplemented with 5% spent coffee grounds during the last 8 weeks. Rats fed non-supplemented diets were used as controls. High-carbohydrate, high-fat diet-fed rats developed metabolic syndrome including abdominal obesity, impaired glucose tolerance, dyslipidemia, and cardiovascular and liver damage. Body weight, abdominal fat, total body fat mass, systolic blood pressure, and concentrations of plasma triglycerides and non-esterified fatty acids were reduced by spent coffee grounds along with improved glucose tolerance and structure and function of heart and liver. Spent coffee grounds increased the diversity of the gut microbiota and decreased the ratio of Firmicutes to Bacteroidetes. Changes in gut microbiota correlated with the reduction in obesity and improvement in glucose tolerance and systolic blood pressure. These findings indicate that intervention with spent coffee grounds may be useful for managing obesity and metabolic syndrome by altering the gut microbiota, thus increasing the value of this food waste.

Co-feeding spent coffee grounds in anaerobic food waste digesters: Effects of co-substrate and stabilization strategy

Anaerobic digestion of spent coffee grounds (SCG) is considered disadvantageous, particularly under mono-digestion conditions, owing to slow degradation and nutrient imbalance. This study investigated the effect of co-feeding of SCG at a low ratio into food waste (FW) digesters, with the aim to determine whether SCG can be effectively treated and valorized using the spare capacity of existing digesters. Duplicate reactors showed stable performance under FW mono-digestion conditions but manifested severe deterioration in three volume turnovers after co-feeding of SCG (FW:SCG at 10:1 on a volatile solids basis). The reactors failed to recover despite repeated interrupted feeding and stabilization, and Ulva was added (FW:SCG:Ulva at 20:2:1) for nutrient supplementation. The two reactors subjected to different stabilization strategies (i.e., timing and intervals of interrupted feeding) responded differently to Ulva co-feeding: one recovered and maintained stable albeit suboptimal performance, whereas the other failed. Furthermore, the microbial communities developed differently in the reactors.

Effects of changes in temperature on treatment performance and energy recovery at mainstream anaerobic ceramic membrane bioreactor for food waste recycling wastewater treatment

An anaerobic ceramic membrane bioreactor (AnCMBR) has been attracted as an alternative technology to co-manage various organic substrates. This AnCMBR study investigated process performance and microbial community structure at decreasing temperatures to evaluate the potential of AnCMBR treatment for co-managing domestic wastewater (DWW) and food waste-recycling wastewater (FRW). As a result, the water flux (≥6.9 LMH) and organic removal efficiency (≥98.0%) were maintained above 25 °C. The trend of methane production in the AnCMBR was similar except for at 15 °C. At 15 °C, the archaeal community structure did not shifted, whereas the bacterial community structure was changed. Various major archaeal species were identified as the mesophilic methanogens which unable to grow at 15 °C. Our results suggest that the AnCMBR can be applied to co-manage DWW and FRW above 20 °C. Future improvements including psychrophilic methanogen inoculation and process optimization would make co-manage DWW and FRW at lower temperature climates.

Effect of Mild-Temperature Thermo-Alkaline Pretreatment on the Solubilization and Anaerobic Digestion of Spent Coffee Grounds

Mild-temperature thermo-alkaline pretreatment of spent coffee grounds (SCG) was studied to improve its solubilization and methanation. The simultaneous effects of NaOH concentration (0–0.2 M) and temperature (60–90 °C) were investigated. Significant solubilization of SCG was achieved by the pretreatment, particularly under high-NaOH-concentration and high-temperature conditions. However, adding NaOH above a certain concentration adversely affected the methane production. Therefore, the degree of solubilization (SD) correlated poorly with methane yield (Ym). Response surface models of SD and Ym were successfully generated. The maximum response of SD (36.4%) was obtained at 0.18 M NaOH and 90.0 °C, while that of Ym (263.31 mL CH4/g COD added) was obtained at 0.13 M NaOH and 70.5 °C. Hydrogenotrophic Methanospirillum species were the dominant methanogens in all the SCG digestion tests. It is likely that NaOH concentration had a more significant influence on the development of microbial community structure, particularly of methanogens than temperature.