OICC Press International Journal of Recycling of Organic Waste in Agriculture 2251-7715 15 2 2026 06 30 Co-Composting MSW and Sewage Sludge: Comparing Vertical vs. Horizontal Methods with Different Aeration Rates in Lab-Scale 10.57647/ijrowa.2026.18322 EN Tahere Taghizade Firozjaee Faculty of Civil Engineering, Shahrood University of Technology, Shahrood, Iran Hasan Aminirad Faculty of Civil Engineering, Babol Noshirvani University of Technology, Babol, Iran Hossein Jalalianei Faculty of Civil Engineering, Shahrood University of Technology, Shahrood, Iran Farshad Golbabaei Kootenaei Faculty of Civil Engineering and Architecture, Shahid Chamran University of Ahvaz, Ahvaz, Iran Journal Article 2026 06 30 Purpose: This study investigated the co-composting of municipal solid waste (MSW) and sewage sludge (SS) in lab-scale vertical and horizontal reactors to compare their performance and identify optimal operational parameters for enhanced bio-safety. Method: The study compared the reactor configurations using two distinct aeration flow rates (22 and 66 L·min⁻¹) and two sizes of wood chip bulking agents (10 mm and 20 mm), resulting in eight experimental runs. Results: All eight treatments achieved the required thermophilic temperature (>55 ºC for at least 2 days). However, the Horizontal reactor method resulted in a shorter overall composting period. Notably, the treatment with the lowest aeration rate and largest bulking agent size (AVCCP2: Vertical, 22 L·min⁻¹, 20 mm) achieved the longest sustained thermophilic phase of 7 days. This optimal thermal condition confirmed the complete removal of pathogenic microorganisms (Salmonella spp., enteric viruses, and viable helminth eggs) and led to the fastest rate of C/N ratio reduction. Conclusion: The pilots AVCCP2 and AHCCP2 (Horizontal, 22 L·min⁻¹, 20 mm) demonstrated the maximum efficiency, primarily due to their long sustained thermophilic duration. AVCCP2 showed the best performance in terms of sanitization and pathogen control, achieving Class A compost standards and confirming the potential of vertical reactors for real-scale application with enhanced bio-safety. Highlights  ●        Vertical and horizontal in-vessel co-composting of municipal solid waste (MSW) and sewage sludge (SS) were comparatively investigated. ●        Optimization of aeration rate and bulking agent size was essential for efficient process progression. ●        Aerating at a lower rate significantly prolonged the thermophilic phase, ensuring effective pathogen removal. ●        A higher reduction rate of the C/N ratio was observed in treatments utilizing the smaller particle size of the bulking agent. ●        The increase in pH value was notably greater in the vertical reactors compared to the horizontal reactors.