Study on the efficiency of anaerobic digestion of poultry manure and food waste in simultaneous digestion

Powering various industries with fossil fuels has resulted in the global generation of vast quantities of pollutants. Consequently, extensive research has been conducted to identify alternative energy sources; the significant strides made in recent decades toward discovering new, clean energy sources underscore the importance of these concepts and their associated fields of study. In many developing nations, including Iran, the demand for animal products has driven the rapid industrialization of poultry farming, simultaneously generating large volumes of waste. Globally, anaerobic digestion (AD) is employed to convert organic waste into biogas—a form of renewable energy. This technology holds significant commercial potential; as technological advancements progress, energy recovery rates are becoming increasingly competitive with those of fossil fuels.The carbon-to-nitrogen (C/N) ratio is a critical factor in the anaerobic digestion process, and a low ratio can negatively impact system performance. Poultry manure has a high protein content, which—depending on the organic loading rate—leads to the release of significant amounts of ammonium (approximately 3,000–8,000 mg/L) during hydrolysis, resulting in a low C/N ratio of 5–10. Therefore, to achieve an optimal C/N ratio, co-digestion with carbon-rich wastes—such as food waste or other agricultural residues—should be employed. Anaerobic digestion of poultry waste yields a more sustainable outcome: it eliminates offensive odors, preserves fertilizer value, and produces a valuable fuel—methane. Poultry waste differs from most other types of animal waste, as it consists of a heterogeneous mixture of manure, bedding materials, feed, debris, and feathers. Given this complexity, extracting data from the anaerobic digestion process presents a challenge. Anaerobic co-digestion is frequently employed to optimize biogas production and reduce digestion time. This study investigated the 90-day anaerobic digestion of poultry manure (320 mL) using anaerobic sludge from the Babol treatment plant (30 mL) as inoculum in five 500-mL glass reactors (working volume: 350 mL); varying amounts of food waste were added to reactors N2 through N5 to increase the organic loading rate. The reactors operated at a constant temperature of 33 °C with magnetic stirring at 100 rpm, and biogas production and pressure were monitored daily. In reactor N1 (poultry manure and sludge), gas production began after 60 hours, whereas the addition of food waste reduced the onset of gas production to 4 hours. Among the reactors tested, N4 demonstrated the best performance, achieving an organic loading rate of 4.17 kg VS/m³·d and a biogas yield of 850 mL/g VS. Results indicated that reactor N4 utilized 58% of the organic matter for methane production. In reactors N2 through N5, acetic acid constituted the largest fraction of volatile fatty acids (VFAs), while in reactor N5, ammonia accumulation reached the tolerance threshold (54 mg/L). However, the system stability index (VFA/ALK) did not exceed the critical threshold of 0.8 in any of the digesters, thereby preventing total system failure. The trend of changes indicated that pH levels dropped over time across all digesters, falling below 6 in the heavily loaded digesters N4 and N5—a condition that can inhibit methanogenic microorganisms. Notably, an interesting discrepancy was observed between these two digesters: despite maintaining a higher pH than N4, digester N5 experienced a significantly higher accumulation of VFAs, reaching 580 mg/L.