Microbial Ecophysiology of Whey Biomethanation: Comparison of Carbon Transformation Parameters, Species Composition, and Starter Culture Performance in Continuous Culture

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Abstract
Changes in lactose concentration and feed rate altered bacterial growth and population levels in a whey-processing chemostat. The bacterial population and methane production levels increased in relation to increased lactose concentrations comparable to those in raw whey (6%) and converted over 96% of the substrate to methane, carbon dioxide, and cells. Sequential increases in the chemostat dilution rate demonstrated excellent biomethanation performance at retention times as low as 25 h. Retention times shorter than 25 h caused prevalent bacterial populations and methane production to decrease, and intermediary carbon metabolites accumulated in the following order: acetate, butyrate, propionate, lactate, ethanol, and lactose. Bacterial species dominated in the chemostat as a function of their enhanced substrate uptake and growth kinetic properties. The substrate uptake kinetic properties displayed by the mixed chemostat population were equivalent to those of individual species measured in pure culture, whereas the growth kinetic properties of species in mixed culture were better than those measured in pure culture. A designed starter culture consisting of Leuconostoc mesenteroides, Desulfovibrio vulgaris, Methanosarcina barkeri, and Methanobacterium formicicum displayed biomethanation performance, which was similar to that of a diverse adapted mixed-culture inoculum, in a continuous contact digestor system to which 10 g of dry whey per liter was added. Preserved starter cultures were developed and used as inocula for the start-up of a continuous anaerobic digestion process that was effective for biomethanation of raw whey at a retention time of 100 h.