Continuous fermentative hydrogen and methane production from Laminaria japonica using a two-stage fermentation system with recycling of methane fermented effluent

In this study, a two-stage fermentation system to produce H2 and CH4 from Laminaria japonica was developed. In the first stage (dark fermentative H2 production, DFHP), response surface methodology (RSM) with a Box-Behnken design (BBD) was applied for optimization of operational parameters, including cycle-frequency, HRT, and substrate concentration, using an intermittent-continuously stirred tank reactor (i-CSTR). Overall performance revealed that the degree of importance of the three variables in terms of H2 yield is as follows: cycle-frequency > substrate concentration > HRT. In the confirmation test, H2 yield of 113.1 mL H2/g dry cell weight (dcw) was recorded, corresponding with 96.3% of the predicted response value under desirable operational conditions (cycle-frequency of 17 hr, HRT of 2.7 days, and substrate concentration of 31.1 g COD/L). In the second stage, an anaerobic sequencing batch reactor (ASBR) and an up-flow anaerobic sludge blanket reactor (UASBr) were employed for CH4 production from H2 fermented solid state (HFSS) and H2 fermented liquid state (HFLS), respectively. The CH4 producing ASBR and UASBr showed a stable CH4 yield and COD removal until a HRT of 12 days and OLR of 3.5 g COD/L/d, respectively. Subsequently, for recycling of CH4 fermented effluent from the UASBr (MFEUASBr) as diluting water in DFHP, the tap water and MFEUASBr mixing ratio (T/M ratio) was optimized (a T/M ratio of 5:5) in a batch test using heat pretreated MFEUASBr at 90 °C for 20 min, resulting in the best performance. Although slight decreases of H2 yield (7.6%) and H2 production rate (3.5%) were recorded, 100% reduction of alkali addition was possible, indicating potential to maximize economic benefits. However, a drastic decrease of H2 productivity and a change of liquid-state metabolites were observed with the use of non-heat pretreated MFEUASBr. These results coincided with those of the microbial analysis, where non-H2 producing bacteria, such as Selenomonas sp., were detected. The results indicate that pretreatment of MFEUASBr may be required in order to recycle it in DFHP.

► Marine algae Laminaria japonica was used for hydrogen and methane production using continuous two-stage fermentation system.
► The solid state and liquid state from DFHP system were suitable for methane production.
► Achievement of successful H2 production without addition of alkaline solution by recycling of MFEUASBr as diluting water.
► The two-stage fermentation system achieved biogas conversion by 80.9% (H27.1%, CH4 ASBR 35.1%, CH4 UASBr 38.7%) and COD removal by 89.3%.