Acetogenesis and methanogenesis liquid digestates for pretreatment of rice straw: A holistic approach for efficient biomethane production and nutrient recycling

The present work investigated the potential of acidogenesis digestate rich in volatile fatty acids (Bio-D-Acid) and methanogenesis digestate rich in ammonia (Bio-D-Meth) as a green method for the pretreatment of rice straw and biomethane production. After pretreatment with Bio-D-Meth for 48 h, a maximum hemicellulose reduction of 22.4% was recorded, while 48 h of Bio-D-Acid showed the lowest reduction of 15.7%. In addition, pretreatment for 48 h with Bio-D-Meth and Bio-D-Acid showed significant removal of lignin by 16.6 and 11.0%, respectively. SEM and FTIR confirmed major increase in straw porosity and degradation after all applied pretreatments. The highest significant biomethane content of 71.2% was recorded in 24 h Bio-D-Acid-pretreated straw. However, the highest significant yield of 249.1 L kg(-1) VS was recorded from the 48 h Bio-D-Meth-pretreated straw, which was 9.8, 19.6, and 27.0% higher than 24 h Bio-D-Meth-, 24 h Bio-D-Acid and 48 h Bio-D-Meth-treated straw, respectively. Importantly, pretreatment with Bio-D-Meth for 24 h showed the highest estimated net bioenergy output of 7.37 GJ ton(-1) dry straw, which represented 28.6 and 0.13% increase over that of the control and 48 h Bio-D-Meth-pretreated straw, respectively. The present study suggested application of Bio-D-Meth for 24 h as an efficient eco-friendly pretreatment method for efficient AD of rice straw.