Bioenergy generation from spent tea and potato wastes through anaerobic codigestion within bioeconomy approach

Abstract

INTRODUCTION Anaerobic digestion is a well-established process in which renewable energy carrier-biogas is simultaneously generated through upcycling of organic waste streams. Biogas is a green energy vector having a crucial role in the future sustainable bioeconomy development. Flexibility of these systems such as digestion of multiple feedstock enables not only energy production but also waste management and has an economic potential for the locals. The combination of the local wastes has a great concern within this perspective. Potato production is one of the major agricultural activities in worldwide including Turkey and nearly 100 ktons/year of peels are produced from potato industry globally. Turkey is one of the important tea producers worldwide, the production is over 225 thousand tons/year. Since tea is a popular beverage, spent tea waste amount is considerable potential in the country. The aim of this study was to optimize the inoculum amount in the anaerobic digesters treating spent tea and potato waste.MATERIALS&METHODS Within this scope, biomethane potential tests were set up for five different inoculum/substrate ratios (I/S: AD1:1.0, AD2:1.5, AD3:2.0, AD4:2.5, AD5:3.0) on the basis of volatile solids (VS). Anaerobic batch tests were conducted by Automatic Methane Potential Test System (AMPTS) II (Bioprocess Control, Sweden), active reactor volume was 400 mL. The reactors were operated in triplicates under mesophilic conditions for 20 days. The blank reactors including only standard inoculum without feedstock were also conducted and the background biomethane production was subtracted from the experimental setups. The results were reported based on the VS content of the feedstock in the reactors.RESULTS The results showed a clear effect of I/S ratio in biomethane production (Figure 1). Whereas increasing I/S ratio resulted faster biogas production in the system, the highest methane yield was recorded as 244 mLN CH4 gVS-1 in AD4 operated with I/S of 2.5. The methane yields were gradually increased with increasing I/S ratio and ranged between 225-244 mLN CH4 gVS-1 except AD5 in which the methane yield was 233 mLN CH4 gVS-1. There was not a significant difference on VS values between the reactors s at the end of the operation period (%1.9 ± 0.01). Soluble COD values were in the range of 2180 mg/L - 410mg/L. Alkalinity was measured as 4250 ± 45 mg/L showing there was not a limitation in the reactors regarding buffering acidity. I/S ratio is a key parameter affecting VFA production and methane generation rates. Since increasing amount of inoculum enables higher biomethane yield, while shortening the incubation period, it should be noted that, the optimum value for I/S ratio is highly dependent on substrate characterization and inoculum source.CONCLUSION In the sustainable circular economy approach, bioenergy has a major role in green energy production and optimization of optimization of the operating conditions for local feedstock has a crucial role. Results revealed that, the spent tea waste and potato waste are potential feedstock in anaerobic digesters whereasthe optimum I/S ratio is 2.5. These findings offer practical information to future applications of biomethane production with spent tea and potato waste as feedstock.Keywords: anaerobic co-digestion, bioeconomy, biomethane potential, tea waste, potato waste