Reactor configuration and its operation mode are the main parameters that can affect the overall hydrogen production in dark fermentative systems. Reactor configuration is very important for microenvironment, microbial population, hydrodynamic behavior, etc. To date many of the laboratory scale dark fermen­tative experiments were in batch mode because it can be easily operated and flexible. But for industrial applications, it is important to operate the reactor continuously to keep continuous usage of the industrial waste and wastewater in accordance with continuous hydrogen production. Many of the studies were done by using continuous stirred tank reactor (CSTR). Suspended cell culture systems are advantageous because of the good mass transfer between microorganisms and substrates. But the disadvantage is the wash-out of cells at low hydraulic retention time conditions. Immobilization is an important technique to improve the system performance for continuous systems. Immobilization techniques can generally be divided into three main categories; adsorption (biofilm formation), encapsulation and entrapment. It is important to select an economical and durable method. Every immobilization technique and each material have their own advantages and disadvantages [89]. It is shown that immobilization can increase the system per­formance of dark fermentative hydrogen production [90]. For different immobi­lization techniques (attachment, granulation, flocculation and entrapment) ranging from 0.93 to 7.33 l/l/h at low HRT values between 0.5 h and 4 h. is achieved where suspended systems give hydrogen production rates of 0.15-0.58 l/l/h at 6 h HRT, demonstrating that immobilization of microorganisms can greatly increase reactor performance and biomass retention [91, 92].