Biogas Potential of Tuna-Processing Byproducts and Wastewater Sludges: Batch and Semi-Continuous Studies

Tuna-processing facilities produce substantial amounts of concentrated organic residues and sludges containing high levels of proteins, lipids, and nitrogen, which are not easily handled by conventional waste treatment methods. In this work, the anaerobic digestion (AD) performance of tuna-processing by-products (TPB1–2) and associated wastewater sludges (TWS1–3) was investigated using a combination of biochemical methane potential (BMP) tests, theoretical methane yield calculations based on the Buswell–Boyle equation, semi-continuous mono-digestion experiments, and 16S rRNA gene-based microbial analyses. Among the evaluated materials, TWS2 produced the highest methane yield (554.6 N mL CH4/g VS) and, when its annual production volume was taken into account, showed the greatest estimated energy recovery (approximately 1.88 × 106 kWh per year). By contrast, TWS3 exhibited the lowest methane yield (239.8 N mL CH4/g VS), which was attributed to the presence of lignocellulosic sawdust and its limited biodegradability. TWS1 showed a moderate level of performance, with an estimated biodegradability of 62.3%, which may have been influenced by the addition of ferric salts and polymeric coagulants during sludge conditioning. In the semi-continuous digestion experiments, reactors that were initiated under relatively high total ammonia nitrogen (TAN) concentrations achieved stable operation within a shorter period, with the acclimation phase reduced by approximately one hydraulic retention time. These trends were supported by the microbial community data, where an increase in Bacillota-associated families, such as Tissierellaceae and Streptococcaceae, was detected along with a clear shift in dominant methanogens from Methanothrix to the more ammonia-tolerant Methanosarcina. Taken together, it is suggested that, when ammonia levels are appropriately managed, mono-digestion of tuna-processing sludges can be operated at a moderate organic loading rate. The process stabilization and energy recovery in nitrogen-rich industrial wastes are closely linked to gradual microbial adaptation rather than immediate improvements in methane yield.