###### Abstract
In addition, secondary sludge or waste-activated sludge may need the application of pre-treatments for facilitating microbial degradation under anaerobic conditions, but this increases the energy demand. On the other hand, when dealing with livestock wastes, it is usually accepted that the methane yield of these organic materials is not high enough to make digestion attractive. Nitrogen-containing compounds may cause inhibition leading to poor performance. Therefore, adding a co-substrate capable of balancing the C/N ratio and trace element content will significantly increase methane production [6; 7] and energy valorization. Improving the efficiency of anaerobic digestion is of great relevance when considering this process to be a suitable alternative for energy production. This is a key aspect if this technology is to play a relevant role in decarbonizing the economy. The valorization of organics into energy and valuable end-products allows the reduction of the carbon footprint of different human activities. Low-quality resources are in this way re-integrated into the global economy as energy, nutrient cycling or valuable organics. However, not all attempts to increase the efficiency of anaerobic digestion are to be considered adequate. A careful evaluation of the energy demand of the whole process should be carried out. The transformation of organics into biogas may require additional equipment based on co-substrate characteristics and introducing pre-treatment units, which would translate into further energy demands [8], probably making the whole treatment chain unfeasible. There are several pre-treatment options for improving the degradation of organics and enhancing the hydrolysis stage. Still, not all of these alternatives find commercial applications due to their excessive energy demand, the limited capacity for recovering energy and the detrimental effect of some chemical compounds generated during the pre-treatment. Major achievements recently attained in anaerobic digestion deal with new technologies capable of accelerating the hydrolysis stages (thermal, mechanical pre-treatments, ultrasound application, additions of chemicals). Novel techniques have been developed such as the application of pulsed electrical fields, high-voltage pulsed discharges and electrooxidation [9; 10; 11]. The success of the industrial implementation of these technologies keeps a close relationship between biogas production improvement and the energy demanded during pre-treatment. Other alternatives for improving anaerobic digestion are the supplementation of carbon conductive materials, adsorbents, nanomaterials and trace elements to enhance organic degradation [12; 13; 14; 15; 16]. However, any type of material added to the system may be subsequently released into the environment, creating interactions with biota which may result in adverse effects due to the presence of co-contaminants [17]. Although anaerobic digestion is a widely extended technology, several factors prevent the number of installed units from growing worldwide at a higher pace. These are related to high installation costs and operational complexities. The economy of scale favors large industrial plants, but this option is not always possible due to social opposition and constraints due to substrate transport. Recently, several reports have been published in the literature regarding the costs associated with these treatment plants, the efficiency of the process and the enhancement of biogas production in an attempt to increase economic feasibility [18; 19; 20]. There is vast experience at a large scale in co-digestion of sewage sludge and livestock farm wastes and extensive literature regarding research work also dealing with this subject [21; 22; 23]. However, better performance and faster conversion rates are still needed to improve plant financial balance and search for configurations that allow the finding of a mid-point between process conversion efficiency and plant operating costs.