The anaerobic digestion process stands out as an alternative for clean energy production and as a pathway for mitigation of greenhouse gas emissions from agro-industrial industries. For the anaerobic digestion process to proceed efficiently, the chemical constitution of the feedstock and their biodegradability must be carefully selected and if necessary adjusted using appropriate pre-treatments. Tests were carried out to determine the Biochemical Methane Potential (BMP) of the following feedstocks: (a) primary effluent from a wastewater treatment plant (WW), (b) brewer’s spent grains from a craft brewery (BSG) and horse manure (HM), in order to evaluate their possible use in co-digestion systems and investigate the influence of each product on the final production of biomethane. The BMP tests included mono-digestion of wastewater treatment plant effluent (WW) as a reference feedstock and co-digestion of: WW with horse manure (90% WW + 10% HM), WW with horse manure brewer’s spent grains (85% WWTP + 10% HM + 5% BSG). The three alternative feedstocks were evaluated in duplicate for a period of 47 days. The digestion of WW yielded a cumulative biogas production of approximately 1200 mL of biogas containing 60.5 mL of biomethane (5% v/v). In contrast, the co-digestion of WW-HM over the same period, produced 2880 mL of biogas with 840 mL of biomethane (29% v/v). The co-digestion of WW-HM-BSG resulted in accumulated biogas production of around 2660 mL of biogas containing 600 mL of biomethane (22.6%). The tests showed that digestion of WW had a lower biogas yield and a low biomethane content and both parameters could be improved by co-digestion with HM and BSG. From both feedstocks tested in co-digestion, HM yielded the higher improvement in biogas and biomethane yields while supplementation with BSG did not improve the results obtained with WW + HM. This behavior may be associated with the extensive degradation of starch and protein components during beer fermentation while horse manure still contains a higher amount of nutrients available for the digestion process. Therefore, co-digestion of WW with 10% HM was established as best feedstock composition to maximize both biogas and biomethane yields. These results represent a promising alternative for reusing wastewater treatment plant effluent combined with and additional carbon source to produce sustainable energy.