Performance of rice straw as mono- and co-feedstock of Ulva spp. for thalassic biogas production

The seasonal availability of Ulva spp. (U) poses a problem for the continuous operation of thalassic (TH) biogas digesters. Hence, rice straw (RS) was tested as an alternative substrate because of its abundance in Asian countries. The anaerobic monodigestion (AMD) of RS was performed under freshwater (FW) and TH conditions to investigate the TH biogas production performance using terrestrial biomass. Biological hydrolysis (BH-P) and 3% NaOH (NaOH-P) pretreatments were employed to minimize the limitation of biomass hydrolysis in the methane fermentation process. The BH-P [FW = 62.2 ± 30.9 mLCH(4) g(−1)VS (volatile solids); TH = 75.8 ± 5.7 mLCH(4) g(−1)VS] of RS led to higher actual methane yield (AMY) than NaOH-P (FW = 15.8 ± 22.8 mLCH(4) g(−1)VS; TH = 21.4 ± 4.2 mLCH(4) g(−1)VS) under both conditions (P = 0.008), while AMY of FW BH-P was comparable (P = 0.182) to TH BH-P. Thus, TH and BH-P was applied to the anaerobic co-digestion (ACD) of U and RS of varying mixture ratios. All ACD set-ups resulted in higher AMY (25U:75RS = 107.6 ± 7.9 mLCH(4) g(−1)VS, 50U:50RS = 130.3 ± 10.3 mLCH(4) g(−1)VS, 75U:25RS = 121.7 ± 2.7 mLCH(4) g(−1)VS) compared with 100% RS (75.8 ± 5.7 mLCH(4) g(−1)VS) or 100% U (94.8 ± 6.8 mLCH(4) g(−1)VS) alone. While the AMY of 50U:50RS was comparable to 75U:25RS (P = 0.181), it is significantly higher (P = 0.003) than its estimated methane yield (EMY; 85.3 mLCH(4) g(−1)VS), suggesting a synergistic effect on ACD of U and RS under 50:50 ratio. The results show that RS can be used as an alternative mono-feedstock for TH biogas production, and a high AMY can be obtained when RS is used as co-feedstock with U.