Co-digestion of Deproteinized Dairy Waste With Pig Slurry: Effect of Recipe and Initial pH on Biogas and Volatile Fatty Acid Production

Protein recovery from dairy waste leaves large amounts of deproteinized cheese whey, which could be further exploited for chemicals and energy carrier production in anaerobic digestion. The aim of this study was to evaluate the effect of deproteinized cheese whey (scotta or scotta permeate) in co-digestion with pig slurry at different initial pH values on biogas and volatile fatty acid production. Five levels of dairy waste (0, 25%, 50%, 75%, 100%, with amounts of pig slurry complementary to 100) were tested, in factorial combination with five values of initial pH (6.5, 7.0, 7.5, 8.0, 8.5), in laboratory mesophilic, in-batch, static conditions. The presence of dairy waste in the recipe induced anaerobic fermentation and a drastic drop in pH. The addition of pig slurry allowed the accumulation of large amounts of volatile fatty acids (up to 35–40 g l−1, at neutral pH, in the recipe with 25% dairy waste/75% pig slurry), especially propionic and valeric. Methanogenesis began when hydrogen production had stopped, and after pH adjustment at neutrality. The formulation 75% dairy waste/25% pig slurry had the highest methane (CH4) yield per volume unit of feedstock (10.3 ml CH4(stp) ml−1, on average). Increasing percentages of pig slurry reduced the CH4 yield per volume unit of feedstock, while increasing the specific CH4 yield. The addition of pig slurry to reactors digesting undiluted deproteinized dairy wastes, although inappropriate for optimizing CH4 yield, nevertheless allows to obtain high concentrations of volatile fatty acids.