Anaerobic digestion of herbal waste: a waste to energy option

Herbal waste produced during the manufacturing of herbal products is a potential feedstock for anaerobic digestion due to high amount of organic matter that can be transformed into biogas as an energy resource. Therefore, the present study was undertaken to convert herbal waste produced during the manufacturing of common of Ayurveda products into biogas through anaerobic digestion process using batch test study under controlled mesophilic temperature conditions of 35 °C with food to inoculum ratio of 0.75. The maximum biomethane potential (BMP) of 0.90 (gCH₄COD/g COD_fed) and sludge activity of 0.70 (gCH₄-CD/gVSS) was exhibited by WS herbal waste owing to its high chemical oxygen demand (COD) of 4 g/g and better solubilization potential of the organic matter showing change in volatile suspended solids (ΔVSS) of 79%. On the other hand, the waste derived from the TA herb, exhibited the least biogas yield of 0.55 (gCH₄COD/g COD_fed) and sludge activity of 0.40 (gCH₄-CD/gVSS), albeit with higher organic matter present. This was due to the possible hindrance of waste solubilization by the presence of lignin. The waste derived from VVL and PE showed intermediate BMP and sludge activity. The methane generation rate constant (k), a key indicator of the biodegradation potential, was also evaluated. The k values showed similar trend as of BMP values ranging from 0.081 to 0.15 d^-1 thus indicating the influence of presence of lignin and the change in ΔVSS. The present study proves anaerobic digestion to be an alternative treatment method to be a milestone for management of herbal wastes and can be successfully implemented on real-scale systems.