As it has already been mentioned, reactor configuration is considered to be crucial for the overall performance of fermentative hydrogen production process. It is presumed that it influences the reactor’s microenvironment, microbial population, hydrodynamic behavior, substrate-consortia contact, etc. (Venkata, 2009). In general, reactors for fermentative hydrogen production can operate in either batch or continuous mode. Batch mode fermentative hydrogen production has been shown to be more suitable for research purposes (Chen et al., 2002; Lee et al., 2002), but any industrially feasible process would most likely have to be performed on a continuous or at least semi-continuous (fed or sequencing batch) basis.

CSTR, is the most commonly used continuous reactor system, offering simple construction, ease of operation and effective homogenous mixing as well as temperature and pH control. In a conventional CSTR, biomass is well suspended in the mixed liquor, which has the same composition as the effluent. However, in this type of reactor, biomass has also the same retention time (SRT) as the HRT, and thus, its concentration in the mixed liquor as well as the hydrogen production is limited, since high dilution rates might cause biomass washout. However, it was recently found that hydrogen-producing biomass in a CSTR could be self­granulated or flocculated under proper conditions (Fang et al., 2002b; Zhang et al, 2004). Another approach to increase the biomass concentration in a CSTR is to immobilize biomass in biofilms or artificial granules made of various support materials such as cuprammonium rayon (Kim, 2002), polyvinyl alcohol (Kim et al, 2003, 2005), polyacrylamide and anionic silica sol (Kim et al., 2003,

2005) .

Another category of continuous flow reactors are the systems characterized by physical retention of the microbial biomass, which offer several advantages compared to the conventional CSTR systems. In these systems, the SRT is independent of HRT due to physical retention of the microbial biomass inside the reactor, allowing high cell concentrations and thus high hydrogen volumetric production rates with relatively small reactor volumes. Physical retention of microbial biomass could be accomplished by several different means, including the use of naturally forming flocs or granules of self-immobilized microbes, microbial immobilization on inert materials, microbial-based biofilms or retentive membranes (Hallenbeck and Ghosh, 2009). However, a potential problem with these types of reactors is the loss of hydrogen through the formation of methane due to extended retention of the biomass inside the reactor, permitting the establishment of slow-growing methanogenic populations. Different types of reactor used for continuous hydrogen production, are presented in Table 13.6. Up to now, a comparative study of reactor performance in terms of hydrogen productivity could not be done, since the operational parameters along with reactor configuration, in all these studies, are different.

Type of reactor Microorganisms Feedstock H2 production rate Maximum H2 yield References Continuous stirred tank reactor (CSTR) Mixed culture Glucose 0.54 L/L/d 1.7 mol/mol glucose Lin and Chang, 1999 Upflow anaerobic sludge blanket reactor (UASB) Sludge from wastewater treatment plant Sucrose 6.67 L/L/d 1.5 mol/mol sucrose Chang and Lin, 2004 Packed bed reactor (PBR) Anaerobic sludge Sucrose 5.35 L/L/d 0.7 mol/mol sucrose Li ef a/., 2006 Anaerobic sequencing batch reactor (ASBR) Sludge from wastewater treatment plant Glucose 5.52 L/L/d 73.8 mL/gCOD Cheong ef a/., 2007 Fixed bed bioreactor with activated carbon (FBBAC) Sludge from wastewater treatment plan Sucrose 31.68 L/L/d Chang ef a/., 2002 Anaerobic fluidized bed reactor (AFBR) Activated sludge and digested sludge Glucose 29.04 L/L/d 1.8 mol/mol glucose Zhang ef a/., 2007b Polymethylmethacrylate (PMMA) immobilized cells Anaerobic sludge Sucrose 43.2 L/L/d 2.25 mol/mol sucrose Wu and Chang, 2007 Carrier-induced granular sludge bed (CIGSB) Sludge from wastewater treatment plant Sucrose 223.4 L/L/d 4.02 mol/mol Lee ef a/., 2006 Fluidized bed reactor (FBR) Sludge from wastewater treatment plant Sucrose 31.72 L/L/d Wu ef a/., 2007 Membrane bioreactor (MBR) Municipal sewage sludge Sucrose 40.08 L/L/d 1.51 mol/mol hexose Lee ef a/., 2007 Rhomboidal Enterobacter cloacae IIT-BT 08 Sucrose 1.855 mol/L/d Kumar and Das, 2001