Synergistic Anaerobic Co-digestion of Poultry Manure and Typha domingensis for Biomethane Production: An Evidence-calibrated Comprehensive Review

Poultry manure presents substantial bioenergy potential but its nitrogen-rich composition, variable bedding content and propensity for ammonia-associated instability constrain anaerobic mono-digestion. Typha domingensis Pers., a wetland macrophyte that can form problematic dominant stands in particular ecosystems, provides an alternative lignocellulosic feedstock where removal is ecologically justified. Combining these residues could improve feedstock balance, redistribute inhibitory nitrogen and generate useful biomethane while linking wetland management to waste valorisation. Yet the term synergy requires stronger evidence than a rise in total gas production. This critical narrative review integrates species-relevant studies of aquatic-plant digestion, poultry-manure co-digestion, inhibition biology, reactor engineering, nutrient recovery and environmental assessment, with literature eligible through 29 August 2026. The evidence supports the digestibility of harvested cattail biomass and demonstrates that poultry manure can benefit from appropriately selected carbonaceous co-substrates. It does not establish a reproducible, quantitatively verified synergistic biomethane response for the specific poultry manure–T. domingensis pairing. The major uncertainties concern plant-part and harvest-season variability, effective rather than nominal carbon-to-nitrogen balance, free-ammonia exposure, lignocellulose accessibility, inoculum adaptation, trace contaminants, and the translation of batch biochemical methane potential into continuously loaded systems. A defensible synergy assessment must compare mixtures with both mono-digestion controls at equivalent volatile-solids loading, report methane rather than uncorrected biogas, account for experimental uncertainty and distinguish yield improvements from faster kinetics or higher volumetric throughput. The review develops a conditional engineering and ecological framework for feedstock characterisation, proportion screening, pretreatment selection, stability control and digestate management. It concludes that co-digestion is a plausible research proposition, not an already validated optimisation route, and identifies staged experiments needed to establish net methane, environmental and economic benefits.