How return sludge increase biogas production from cow manure? Ambar Pertiwiningrum, Ratih Kusuma Wardani, Joko Wintoko, Rachmawan Budiarto, Margaretha Arnita Wuri and Misri Gozan E3S Web Conf., 181 (2020) 01006 Published online: 24 July 2020 DOI: 10.1051/e3sconf/202018101006