Anaerobic co-digestion of organic rich wastes and wastewater sludge has become an attractive economic possibility for water utilities as it enhances biogas production. The suitability of the organic rich waste depends on its biodegradability as well as on its synergetic effect on the anaerobic digestion process. The feasibility of sewage sludge (SS) treatment via co-digestion was studied in a semi-continuous mode at mesophilic conditions (36 ± 1 °C), with a hydraulic retention time of 17 days and an average organic loading rate of 0.94 ± 0.05 gVS Lreactor day−1, using the liquid fraction of pre-treated exhausted coffee biowaste (LECB) as a co-substrate. An anaerobic co-digestion trial (T1) was performed using as feeding mixture 80% SS and 20% LECB (v:v) and compared against a reference scenario of mono-digestion of SS (T0). The stability along assays was ensured by monitoring the digestate characteristics (pH, electrical conductivity, total alkalinity and ammonia content) and the specific energy-loading rate (SELR). Along the operation time of T1, methane yield and VS removal were significantly higher in comparison to mono-digestion of SS. Results showed that the addition of the co-substrate had a positive effect on specific methane production (3 times higher) and methane content (12% higher), indicating this is a feasible strategy towards self-sufficient wastewater treatment plants.