Optimization of substrate composition for biohydrogen production from buffalo slurry co-fermented with cheese whey and crude glycerol, using microbial mixed culture

Biohydrogen production from buffalo slurry (BS) co-fermented with cheese whey (CW) and crude glycerol (CG) was investigated using a suitable microbial community (F210) as inoculum. Mixture Design was used to find the optimal composition (%) of the three substrate components and to investigate the effect of the mixing ratio on Bio-H2 yields. Maximum H2 yield estimated through the model was around 117 mL H2/g VSadded, while the maximum experimentally detected was 111.6 ± 21.8 mL H2/g VSadded, obtained for a mixing ratio of substrate composition of 66% BS and 33% CW (R2 = 0.962; p-value = 0.0001). CW was clearly the most suitable substrate (with a relative contribution higher than 46%), but led to a rapid drop in pH from 6.5 to 4, while BS showed high buffering capacity by maintaining the pH above 6. Interestingly, the co-digestion of the different substrates decreased the H2 production lag phase λ; in particular the presence of BS shortened the lag period (λ < 3 h) and increased the degradation efficiency of CG. The results demonstrate the usefulness of the mixture design for finding the optimal substrate composition, using BS as co-fermentation substrate to obtain high H2 production yields. Moreover the response surface shows the possibility of mixing the substrates in different ways, while maintaining H2 production within an optimum range: 105-117 mL H2/g VSadded. This might offer a considerable advantage in the effective management of systems or processes, in which the substrates availability may change over the time.