Abstract:
Lactate, propionate, butyrate and acetate were classified as the intermediate organic acids occurring in an anaerobic fermentation before production of the desired final product, methane. To understand the overall behavior of the process, it needs to study the hnetics of substrate consumption and product formation. In this study, a model was considered as a tool to describe the behavior of the process. In order to obtain the complete model that can describe the interaction of intermediate organic acids in biogas production, lactate degradation model was needed to be developed and integrated with the existing models of butyrate, propionate and acetate degradations for methane production that were proposed by Chanchana (1 7) and Kittikorn (11). Therefore, in this study, the lactate degradation model was developed based on the experimental data of lactic acid digestion by using mixed cultures from pineapple peel fermentation that was proposed by Malinee [9]. In the model, modification of Monod's model by the effects of substrate and product inhibitions was performed to illustrate the specific growth rate of lactate bacteria, which consumed lactate as its substrate. After the results of the model and experimental data matched, the pathways of lactate degradation and kinetic constants were found. The best fitting model contained three groups of bacteria consuming lactate, which were LB 1 (lactate-utilizing bacteria for propionate and acetate formation), LB2 (lactate-utilizing bacteria for butyrate formation) and SRB 1 (sulfate-reducing bacteria for acetate formation). LB1 had the highest growth rate with maximum growth rate (pm,) equal to 2.15 dayq' and saturate coefficient (Ks) equal to 1.3 1 g-LactateIL. LB2 had a slow growth rate with value of p,,, at 0.51 day" and value of Ks at 2.9 g-LactateIL. SRB1, not only consumed lactate but also consumed propionate and butyrate. This bacteria had lower ability to consume lactate than LB1 but higher than LB2 (p,,, = 0.71 day-' and Ks = 0.07 g-LactateIL). In the case of inhibition effects, propionate and acetate were considered as product inhibitor for lactate bacteria. Because lower concentration of propionate caused inhibition effect (KLiP = 0.23 g-Propionatell and KLiA = 3.83 g-AcetateIL), it exhibited stronger inhibition effects on lactate degradation. Moreover, lactate was also accounted as the substrate with slight inhibition effect at KLi equal to 10.3 g-lactate/L.