Introduction
In Castilla-La Mancha, the meat processing industry generated approximately half of the 104,431 tonnes of agro-industrial waste in 2019 (Castilla-La Mancha, 2024). These are highly biodegradable, making them ideal for methane production through anaerobic digestion (Rodríguez-Abalde et al., 2019). However, the hydrolysis of these lipids and proteins can inhibit this process (Rodríguez-Méndez et al., 2017) due to the release of ammonia and long-chain fatty acids (Rodríguez-Abalde et al., 2017). To avoid this, various strategies can be employed, such as co-digestion or a two-stage anaerobic digestion process. In the latter case, the hydrolysis and acidogenesis stages would occur first, obtaining H₂, volatile fatty acids (VFAs), and ammonium (NH₄⁺). NH₄⁺ is produced through ammonification under anaerobic conditions where protein hydrolysis occurs (Deng et al., 2024). Therefore, developing strategies to intensify the anaerobic digestion process, such as thermal pretreatment at different temperatures, will promote the formation of renewable gases (H₂ and CH₄), as well as NH₃, which is considered an energy vector and a carrier of H₂.
In this study, batch tests were performed to investigate the potential of producing renewable gases (H₂, NH₃, and CH₄) from meat processing waste. This will support the development of future strategies to intensify the anaerobic digestion process.
Material and Methods
Three different concentrations (12.5%, 25% and 50%) of the selected waste (manure, blood, wastewater, and sludge from the company wastewater treatment plant (WWTP)) were used to assess the potential of producing renewable gases. Tests were conducted according to Hollinger et al. (2016) and Carrillo-Reyes et al. (2020) for evaluating CH4 and H2 generation, respectively.
Results and Discussion
As shown in the analysis, blood and manure have a high COD. Blood also has a high NH₄⁺ content. As the concentration of the wastewater and sludge increases, CH4 production increases. In the case of wastewater, CH₄ production is too low due to the presence of recalcitrant and/or toxic compounds that are detrimental to methanogenic archaea. However, increasing the organic load of the sludge multiplies CH₄ generation by almost fivefold, from 50 to 217 mL CH₄. However, when blood is added, CH₄ production decreases as blood concentration increases, due to the accumulation of volatile fatty acids (VFAs), as evidenced by the detection of H2. Furthermore, the high N-NH₄⁺ content in blood may also contribute to the inhibition of archaea. In the case of manure, the maximum amount of CH₄ was produced when the waste concentration was 25%, resulting in an improvement in CH₄ production from 95 to 146 mL CH₄. Higher concentrations would lead to inhibitory processes, possibly related to an overload of organic matter. NH₄⁺ concentration was analysed throughout the trials, increasing along the different tests, highlighting the transformation of organic nitrogen into ammoniacal nitrogen. Nevertheless, an inhibitory concentration of NH₄⁺ (>1.5 g/L; Deng et al., 2024) was only achieved when blood was used. On the other hand, no H₂ production was observed during the biochemical H₂ test due to the unacclimatised inoculum used and the need for an initial waste pre-treatment. Future work includes waste pretreatment, co-digestion, co-fermentation, and using acclimated sludge to obtain H2.
Conclusions
Anaerobic digestion is a suitable method of treating waste to produce renewable gases (CH₄, H₂, and NH₃). The results obtained showed that WWTP sludge and manure were the most suitable materials for CH4 production, while blood had high potential for NH3 generation. However, intensification strategies such as co-digestion and two-stage anaerobic digestion are needed to optimise the production of these gases.
References
Carrillo-Reyes, J., Buitrón, G., Moreno-Andrade, I., Tapia-Rodríguez, A. C., Palomo-Briones, R., Razo-Flores, E., ... & Zaiat, M. (2020). Standardized protocol for determination of biohydrogen potential. MethodsX, 7, 100754.
Castilla-La Mancha. (2024). Plan de prevención y gestión de residuos de Castilla-La Mancha 2030.
Deng, Z., Sierra, J. M., Ferreira, A. L. M., Cerqueda-Garcia, D., Spanjers, H., & van Lier, J. B. (2024). Effect of operational parameters on the performance of an anaerobic sequencing batch reactor (AnSBR) treating protein-rich wastewater. Environmental Science and Ecotechnology, 17, 100296.
Holliger, C., Alves, M., Andrade, D., Angelidaki, I., Astals, S., Baier, U., Bougrier, C., Buffière, P., Carballa, M., & De Wilde, V. (2016). Towards a standardization of biomethane potential tests. Water Science and Technology, 74(11), 2515–2522.
Rodríguez-Abalde, Á., Flotats, X., & Fernández, B. (2017). Optimization of the anaerobic co-digestion of pasteurized slaughterhouse waste, pig slurry and glycerine. Waste Management, 61, 521–528.
Rodríguez-Abalde, Á., Guivernau, M., Prenafeta-Boldú, F. X., Flotats, X., & Fernández, B. (2019). Characterization of microbial community dynamics during the anaerobic co-digestion of thermally pre-treated slaughterhouse wastes with glycerin addition. Bioprocess and Biosystems Engineering, 42, 1175–1184.
Rodríguez-Méndez, R., Le Bihan, Y., Béline, F., & Lessard, P. (2017). Long chain fatty acids (LCFA) evolution for inhibition forecasting during anaerobic treatment of lipid-rich wastes: Case of milk-fed veal slaughterhouse waste. Waste Management, 67, 51–58.
