Anaerobic digestion of crude glycerin (CG) along with animal waste has been an excellent option for increasing the production of biogas and methane to achieve efficiency in the treatment of both residues. This study aimed to evaluate improvements in specific productions of biogas and methane, reductions in solid and fibrous components in substrates prepared with dairy cattle manure and CG (containing 14 % glycerol). With these residues, experimental substrates were prepared and placed in 25 batch digesters. Initial content of the TS in the influent was 4 % and CG was added in increasing doses (0, 5, 10, 15 and 20 % relative to total solids (TS) of the influent). Results were submitted to ANOVA and orthogonal contrasts to assess the effects of linear and quadratic order and thereby estimate the optimal CG doses through the adjusted models. The highest values for specific production of methane (0.19 and 0.26 L g −1 of TS and volatile solids (VS) added, respectively) were reached with the CG inclusions of 6 and 8 %, respectively. Total production of biogas with the inclusion of 6 % CG was 11 % higher when compared to the control treatment. The largest reduction in VS (48 %) was achieved with the addition of 4 % CG. Addition of CG at levels between 3 and 8 % improved the efficiency of the process of anaerobic digestion with dairy cattle manure.

The objective of this study was to determine the optimal inclusion doses of crude glycerin in anaerobic co‐digestion with dairy cattle manure processed at 10, 17, and 24 days of retention in order to maximize reductions in solid and fibrous constituents and increase the productions of biogas and methane. Semi‐continuous digesters were fed with daily loads of substrates containing doses of 0, 5 and 10% (based on total solids, TS) of crude glycerin and dairy cattle manure. The highest specific productions of biogas and methane with the 10‐day HRT were also achieved in substrates with 0% glycerin, reaching values of 0.91 and 1.27 L/g of TS and solids (VS) added, respectively. In substrates retained for 17 and 24 days the values were maximal with addition of glycerin doses between 2.7 and 3.6% resulting in yields of 1.72 and 2.42 L of biogas per gram of TS added and 2.37 and 3.31 L of biogas per gram of VS added, respectively. For methane production, the optimal inclusion doses of crude glycerin with HRTs of 10, 17, and 24 days were 0.0, 3.7, and 3.3%, generating 0.70, 1.43, and 1.86 L of methane per gram of VS added. The inclusion of crude glycerin in co‐digestion with dairy cattle manure is not recommended for digesters operated with retention time of 10 days. However, when substrates remain for 17 or 24 days this inclusion promotes reductions of solids constituents while maximizing the specific productions of biogas and methane. © 2017 American Institute of Chemical Engineers Environ Prog, 37: 1305–1311, 2018