1.2 Process Integration Approach

Utilization of remaining carbon present in wastewater from acidogenic H2 produc­tion (an organic acid rich effluent) for additional biogas (H2 or CH4) generation is one way to sustain the process. Integration of an acidogenic process with a terminal photo-fermentative process (for additional H2 production) [6, 7, 110] or acidogenic process (for additional H2 production) [86] or methanogenic process (for methane production) [23] were reported along with enhanced substrate degra­dation (Fig. 5). Soluble metabolites formed during methanogenic or from acidogenic processes could be utilized by photosynthetic bacteria [6, 7] or acidogenic cultures

[23] to produce additional H2. Photosynthetic bacteria can produce H2 by consum­ing organic acids which are abundant in the effluents generated from acidogenic H2 fermentation processes [4, 6, 110]. Theoretically, the maximum H2 yield may be obtained when glucose is converted to acetate as the terminal product through dark fermentation, then subsequently converted into H2 through photo-fermentation [113]. Integrated systems showed higher H2 yields compared to single-step fermen­tation [6, 13, 23, 73]. A two-stage process has been envisioned to obtain yields closer to the theoretical stoichiometric yield of 12 mol H2/mole glucose [86, 113]. However, the efficiency of both H2 production and substrate degradation were found to depend on the process used in the first stage along with the composition of the substrate [23]. The effluent from the first stage of operation generally contains ammonia, which inhibits the second stage process. This can be restricted by dilu­tion and neutralization (to adjust the pH to 7) prior to feeding [10]. Integration of an acidogenic H2 production process followed by a methanogenic anaerobic digestion for CH4 production facilitated an enhanced energy yield along with higher substrate removal efficiency [23, 75, 114, 115]. Integration of the acidogenic process with a photo-fermentation process showed a more positive influence over the correspond­ing methanogenic process integration (Table 4). This might be due to the presence of a relatively higher concentration of VFA bound residual carbon corresponding to the methanogenic process. Multi-stage process was often used to maximize H2 production. Initially, the process consisted of two stages, dark fermentation followed by photo fermentation [10] but three or even four stages have since been proposed in different configurations [109]. The acid-rich organic effluent generated from the initial process of dark fermentation was sent to photo-fermentative process followed by direct photolysis finally using microbial electrolysis cells to produce H2 at fourth stage.