Batch mesophilic anaerobic co-digestion of spent goat batch mesophilic anaerobic co-digestion of spent goat straw bedding and goat cheese whey: Comparison with the mono-digestion of the two sole substrates

Spent livestock bedding is a valuable resource for the production of green energy (methane) in rural areas. Comparison and evaluation of batch anaerobic digestion and co-digestion of different mixtures of goat straw bedding (SGSB) and goat cheese whey were carried out. Biochemical methane potential (BMP) tests of the 100% SGSB, 95% SGSB-5% whey, 90% SGSB-10% whey, 85% SGSB-15% whey and 100% whey were found to be 423 +/- 7, 354 +/- 9, 371 +/- 2, 293 +/- 1, 274 +/- 2 mL CH4 g(-1) VS. Two different kinetic models were evaluated. The logistic model revealed a decrease in the maximum methane production rate (R-m) from 34.7 +/- 1.5 to 14.1 +/- 0.9 mL CH4 g(-1) VS.d(-1) when the percentage of whey in the mixture increased from 0 to 15% as a consequence of the increased ammonia released during the co-digestion of increased concentrations of whey. The lowest value for the maximum methane production predicted by the model (P) was found for 100% whey (274 +/- 10 mL CH4 g(-1) VS). A two-substrate model was applied to describe the evident existence of rapid and slowly degradable material. Regarding the hydrolysis kinetic constants predicted by this model, considerable increases in the rapid biodegradation stage (k(rapid)) were observed when comparing to the values found for the slow (k(slow)) biodegradation stage in all the cases tested. The increases between both constants rose from 5 to 42% when the percentage of whey increased.