## 4 Taking Advantage of Process Synergies
### Co-Digestion at Large Scale  
Another relevant advantage of co-digestion is that it attains a better use of equipment and cost-sharing because existing facilities can be adapted, or new ones can be built to process multiple waste streams in a single unit [162; 163]. The biogas production of existing plants can be increased by treating other materials from different industrial and agronomic sectors. However, implementing this approach for already-operating plants implies the introduction of several modifications in the facility to endow the system with the necessary flexibility for storing, pre-treating and feeding the co-substrate into the reactor. The plant must cope with the available co-substrates found in the surroundings, otherwise transportation costs would cancel out any benefit associated with the higher biogas production. The integration of co-digestion in WWTPs can be a solution for reducing the operating costs and also for increasing the electricity produced by the plant and the share dedicated for self-consumption. Digestion plants have a delicate balance between waste treatment, energy production and economic feasibility, with the latter being dependent on the plant scale and revenues obtained from the different products. In recent years, the number of large-scale digestion plants installed has increased significantly, providing economic and environmental benefits [153]. There exist many literature reports about the successful performance of co-digestion with sewage sludge and manures under a laboratory scale [22; 68; 75; 164]. Large-scale reports are less abundant, and therefore those found in the literature such as  
Figure 2: Co-substrates used in anaerobic digestion and expected problems in reactor dynamics. [PERSON] et al. [165], [PERSON] et al. [166] and [PERSON] et al. [167] represent a valuable source of knowledge. The benefits of co-digestion are associated with greater energy production thanks to the higher treatment capacity, organic loading increase and higher methane yields [5]. The increase obtained in biogas production may cover the whole energy demand of the WWTP [168; 169] and can be high enough, depending on the type and amount of co-substrate added, to become an efficient way of obtaining surplus energy and be considered as an eco-friendly and economically viable approach [170]. However, when attempting large-scale co-digestion strategies in already existing plants such as digesters in WWTP, operating problems become frequent as they are usually related to the high variability of co-substrate composition, changes in technical routines, maintenance and the installation of additional equipment to deal with these materials [171]. A co-digestion experience at the WWTP of Viareggio and Treviso (Italy) was described by [PERSON] et al. [165], where the source-sorted OFMSW was treated with sewage sludge, reporting a 50% increase in biogas production when increasing the organic loading of the reactor from 1.0 to 1.2 kg VS/m\({}^{3}\) d, and a five-fold increase in monthly biogas production in the second plant studied. However, not all reports present this significant of an improvement. At the Lansdowne WWTP in the municipality of Prince George, British Columbia, Canada, [PERSON] et al. [172] reported the results of a short co-digestion assay with source-sorted food wastes from supermarkets. The average daily biogas production was increased by just 8-10%, but several operational problems were highlighted and associated with this practice, such as the clogging of the hose connecting the chopper pump and the sludge recirculation line, needing manual maintenance for clearing up the line. Accumulation of fibrous scum was also reported near the digester floating roof and the visual presence of impurities in the biosolids was also indicated. In another large-scale study, a two-year experience was reported by [PERSON] et al. [173] using OFMSW separately collected for this aim. These authors performed their co-digestion study with sewage sludge at the Rovereto WWTP.