Shifting the anaerobic reactor from methanogenesis (producing CH4) to acidoge — nesis (to produce H2) is important to make the process more feasible with wider application potential. Bioaugmentation is generally applied to improve the start up of a reactor, to enhance performance efficiency, to protect the existing micro­bial community against adverse effects or to compensate for organic or hydraulic overloading [99, 125]. In this way, the bioaugmentation strategy was applied to an operating anaerobic bioreactor (producing CH4) to shift to acidogenic pro­cess so as to produce H2 [25]. For this purpose selectively enriched H2-producing mixed consortia (in immobilized form) was used as augmenting inoculum. After augmenting, the H2 production rate almost doubled (Fig. 6). Bioaugmentation with co-cultures Clostridium acetobutylicum X9 and Ethanoigenens harbinense B49 showed to improve cellulose hydrolysis and subsequent H2 production rates from carboxymethyl cellulose [67].