Volatile fatty acids (VFAs) are valuable intermediate products in anaerobic digestion and hold significant potential for carbon recovery in wastewater treatment plants (WWTPs). This study investigates the effect of varying hydraulic retention times (HRTs) on VFA production, methane generation, and CO₂ emissions during semi-continuous fermentation of primary sewage sludge. Using six 2-liter reactors operated under mesophilic conditions, HRT was adjusted by altering daily feed volumes, simulating retention times ranging from HRT 1 to HRT 8.

Results revealed that VFA concentration increased with longer retention times, reaching a peak at HRT 6. Beyond this point, VFA levels declined, indicating increased conversion to methane. Methane production was negligible at lower HRTs but rose significantly at HRTs 5-8, with the highest average methane volume observed at HRT 8. Similarly, CO₂ emissions increased steadily with retention time. These findings suggest that HRT is a key parameter for optimizing carbon recovery from sludge. An HRT of 3 appears optimal for maximizing VFA yield before methanogenesis dominates. The results contribute to ongoing efforts to shift WWTPs toward more sustainable, resource-oriented operations through controlled fermentation and internal carbon reuse.