Improving energy balance of wastewater treatment plants using pre-treated coffee waste as a co-substrate in anaerobic digestion process

In this study, the strategy for improving energy balance of wastewater treatment plants using pre-treated coffee waste represented by coffee spent grains (SCG) as a co-substrate to sewage sludge (SS) was proposed. Hydrodynamic cavitation at a pressure of 5 bar with a duration of 20 and 30 min was chosen as a pre-treatment method. The anaerobic digestion was conducted in a batch system under mesophilic conditions; the substrate-to-inoculum ratio was 0.69-0.71. Two co-digestion series with cavitated SCG were provided, differing in terms of caffeine content. Moreover, the co-digestion series supplied with raw SCG and mono-digestion of SS were conducted as reference experiments. The obtained results indicated that, compared to SS mono-digestion, a significant yielding growth by 12% was found in the series supplied by a mixture of SS and SCG with the highest content of caffeine cavitated for 30 min. Therein, the methane yield was 462.3 ± 21.3 mLCH₄/gVS accompanied by improved methane production rate by 9% and shortening of the lag phase. The evaluation of energy balance with the significant energy profits of 39% was found. Moreover, the results of the economic analysis indicate that only in this case the cost-effectiveness of the technology was achieved.