Temperature effects on synergy in anaerobic co-digestion: cheese whey as study case. J. Jaimes-Estévez*, L. Castro*, H. Escalante* * Grupo de Investigación en Tecnologías de Valorización de Residuos y Fuentes Agrícolas e Industriales para la Sustentabilidad Energética (INTERFASE), Escuela de Ingeniería Química, Universidad Industrial de Santander, Carrera 27, Calle 9 Ciudad Universitaria, Bucaramanga, Colombia.
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Abstract Cheese whey (CW) is the main waste of cheese making process. Anaerobic codigestion (ACD) is an attractive alternative to solve this problematic, as it can stabilise organic matter in CW and produce biomethane that can be used to energy production. The aim of this research was to evaluate the ACD process of cheese whey/dairy manure (DM) mixture in order to determinate the synergistic effect under different temperature (T) ranges. Firstly, biomethane potential (BMP) and the volatile fatty acids (VFA) content were evaluated at different temperatures, inoculum/substrate ratio (ISR) and CWfraction, using a surface response methodology. The highest biomethane yields in psychrophilic, mesophilic and termophilic ranges were 0.45, 0.6 y 0.51 m3CH4/kg VSadded, respectively. The second stage was to determine the effect of temperature conditions under the synergy. In that case, the synergistic effects were higher at psychrophilic conditions. The results allowed to conclude that the synergy of the CW/DM mixture increase biomethane ACD yields, respect to the CW and DM mono-digestion, even at unfavourable environmental conditions. Keywords Anaerobic co-digestion, cheese whey, synergistic effects, temperature INTRODUCTION Dairy chain generates a residual liquid fraction denominated cheese whey (CW), which represents approximately 90% of the milk employed. According to the high organic load of CW, its disposal represents a challenge for small producers in developing countries that do not count with any kind of treatment plant. Anaerobic co-digestion (ACD) of CW with dairy manure (DM) is a strategy to reduce the acidification problems and to improve stability in the process. Low cost tubular digesters (absence of active mixing devices and/or active heating systems) are an attractive alternative to implement in small and medium dairy enterprises (SME). Benefits of tubular digesters are associated to energy obtainment and nutrients recovery, under local temperature. The performance and specific methanogenic activity (SMA) of ACD are highly influenced by temperature. In this regard, Latin America presents a diversity of geographical and metheorological conditions that leads to a wide temperature variety from psychrophilic (40 ºC) range. Studies about co-digestion focus their research mainly in the substrate/co-substrate ratio effects, C/N ratio and macromolecules content. These researches converge in the positive or negative interactions among substrates for biomethane production [1]. However, the majority of those studies do not evaluate
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the relationship between synergistic effect and temperature. Based on this background, the aim of this research was to evaluate the ACD process of CW/DM mixture in order to determinate the synergistic effect under psychrophilic, mesophilic and thermophilic temperatures. MATERIALS & METHODS In this study, fresh DM and digested dairy manure was used as co-substrate and inoculum, respectively. CW was obtained from a SME. BMP and VFA content were evaluated at different T (8, 15, 25, 35 y 42°C), ISR on VS basis (0.8, 1.0, 1.5, 2.0 y 2.2) and CWfraction (0.1, 0.3, 0.6, 0.9 y 1.0), using a factorial design (22 + central and axial points). Synergy effects was determine according to Labatut et al., [2]. Specific methanogenic activity (SMA) was measured for each studied temperature. The BMP, VFA and SMA were determined according to the protocol suggested by Refs [3], and [4], respectively. RESULTS AND DISCUSSION The adjustment of the factorial design explained the BMP and VFA behaviour in 92.19% and 95.64% respectively (Figure 1). IRS between 1.5 to 2.2, CWfraction around 0.6 to 0.9 (v/v) and T from 20 to 35 °C, favours biomethane production. Optimizing the BMP and minimizing the VFA content simultaneously, it was possible to obtain the most favourable conditions that were 37°C, 0.7 and 2.2, for T, CWfraction and ISR respectively.
Figure 1. Isothermal response surfaces (15. 25 and 35 °C) for BMP and VFA as function of ISR and CWfraction
Under those conditions it is possible to obtain a BMP of 0.6 m3 CH4/kg VSadded with a VFA content around 382.60 mg/L. BMP and synergistic effects as function f temperature are shown in figure 2a. It can be seen an opposite behaviour: i) BMP increases directly with T except in termophilics temperatures. ii) Synergy decreases with T increases. This behaviour shows that with a higher temperature it is necessary a lower synergy to obtain better biogas production. Results show that the mixture CW:DM improve the synergy, reaching better yields compared with mono-digestion potentials, even at lower temperatures conditions.
Figure 2. Behaviour of BMP, synergy (a) and SMA (b) at different temperatures
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SMA tendency is similar to BMP behaviour (Fig 2b). Results show that SMA is useful under psychrophilic temperatures. Based on above, synergy contributes AD to minimize limiting operational conditions. It means that ACD process is suitable even at zones under psychrophilic temperatures. CONCLUSIONS The relationship between synergistic effect and temperature shows that at lower temperatures, exist a higher synergy. Accordingly, success of ACD under psychrophilic temperatures depends on the interaction between blends. REFERENCES [1]. H. Escalante, L. Castro, V. Besson and J. Jaimes, “Feasibility of the anaerobic process of Cheese Whey in a plug flow reactor (PFR),” Ingeniería, Investigación y Tecnología, vol. VWIII, no. 3. In press, 2017. [2]. R. Labatut, L. Angenent and N. Scott, “Biochemical methane potential and biodegradability of complex organic substrates,” Bioresource technology, 102(3), pp. 2255-2264, 2011. [3]. Irini Angelidaki, M. Alves, D. Bolzonella, L. Borzacconi, J. L. Campos, A. J. Guwy, S. Kalyuzhnyi, P. Jenicek and J. B. Van Lier, “Defining the biomethane potential (BMP) of solid organic wastes and energy crops: a proposed protocol for batch assays,” Water Science & Technology -WST, pp. 927-934, 2009. [4]. S. Astals, D. Batstone, J. Mata-Alvarez and P. Jensen, “Identification of synergistic impacts during anaerobic co-digestion of organic wastes,” Bioresource Technology, vol. 169, p. 421–427, 2014.
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