A. Ahmad, E. Evelyn, D. Andrio, D. Avriliani, M. Dalil, and A. Hamzah, “Sustainable Biohydrogen Production from Palm Oil Mill Effluent: Effect of Hydraulic Retention Time in a Hybrid Anaerobic System”, Leuser J. Environ. Stud., vol. 4, no. 1, pp. 69–78, Apr. 2026.