Five bacterial strains (Chryseomonas luteola, Fusobacterium mortiferum, Enterobacter agglomerans,Klebsiella oxytoca and an unidentified Gram-negative rod) were grown on raw baker's yeast effluent to assess the influence of environmental factors on biodegradation processes. A 3×4×3 factorial design was used to determine the effects of time, pH and COD concentration, at four different temperatures. Total volatile fatty acid production was chosen as the most representative indicator of biodegradation. Results showed that the strains differed greatly in their ability to produce anaerobic digester intermediary metabolites, under defined environmental conditions. The study showed that the degradation of the complex compounds in baker's yeast effluent could be enhanced by changing environmental factors. The most positive responses were obtained at the higher COD concentrations (30 g l−1), the higher pH values (6.0), after 24 to 48h incubation time and at the higher temperatures (35°C). The most positive effect (+355.00) was found forChryseomonas luteola at a 48h incubation time, COD concentration of 30 g l−1, pH of 6.0 and temperature of 35°C. The volatile fatty acid yields obtained by the strains differ from the statistical indications, but provide a valuable reference of the actual concentrations obtained during the experimental study. The factorial design represented the effects of environmental changes, while the quantitative TVFA data set gave experimental data. This study showed that the manipulation of various environmental factors in biologically controlled systems, such as anaerobic digesters, could further enhance the biodegradation efficiency of the microbial population in the raw effluent.

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