3.5. Energy balance analysis
The IE of supply chains was analyzed for eight scenarios of rice straw corresponding to farmers’ fields (FRM) and from a long term experiment at IRRI fields (LTI) during the wet and dry seasons, with manual (Man) and mechanized (Mec) supply chains. The IE of the supply chain was accounted for by the rice straw from rice cultivation, harvesting, collection, transportation, and storage. The IE allocation of the rice straw from rice cultivation came from the energy from rice seed, fertilizer, pesticide, fuel consumption, and labor while the IE of collection, transportation, and storage came from fuel consumption, labor, and HDPE for making canvas for storage. Adding to supply chain, the IE of AD was accounted for by the HDPE used for making containers for ensilaging, digesters, and reservoirs of biogas, as well as laborers. The OE was accounted for by the biogas and digestate base on the AD experimented results as above. The components of the IE and OE of the eight scenarios are shown in Table 5, with the IE indicated with a plus (+) sign and the OE with a minus (−) sign. For one ton of rice straw collected and used for AD, the result of this study demonstrates that a total IE required ranged from 4367 to 4756 MJ, while the total OE of biogas and digestate generated from AD was approximately 8134 MJ. As the net energy gain from utilizing rice straw for AD bioenergy production is between 3378 and 3767 MJ t−1 and a positive net energy balance between 71% and 86%. Fig. 3 show the energy balance network built in SIMAPRO for rice straw that was collected for AD from farmers’ fields in the Philippines via mechanized supply chain. One ton of rice straw required a total IE (in + value) of 4714 MJ. The IE for AD included 3475 MJ for HDPE and 379 MJ for labor. The OE obtained is 8134 MJ comprised of 7880 MJ from biogas and 254 MJ recovered from digestate. The OE and IE resulted in a net energy gain of 3420 MJ for each ton of rice straw. Energy balance network of the manual scenario was calculated in the same manner as for the mechanized supply chain; the total input energy of the manual scenario for one ton of rice straw was 4406 MJ. Of this total IE in the manual scenario, rice cultivation accounted for 440 MJ, in a total supply chain IE of 552 MJ. The IE of AD is the same for both scenarios. The net energy that resulted from the manual scenario is 3728 MJ per ton of rice straw.