1.1 Biocatalyst

Biocatalyst (inoculum) selection and its pre-treatment plays a vital role in select­ing requisite microflora for efficient H2 production [4, 7, 15, 30, 71, 72, 73]. Inoculum preparation affects both start up and the overall efficiency of H2 pro­duction. Typical anaerobic mixed cultures cannot produce H2 as it is rapidly consumed by H2-consuming or CH4-producing bacteria (MB) [74]. The most effec­tive way to enhance H2 production from anaerobic culture is to restrict or terminate methanogenesis by allowing H2 to become a metabolic end product. Physiological differences between H2-producing bacteria (AB) and H2-consuming bacteria (MB) forms the main basis for the preparation of the inoculum to start up the acido — genic H2-producing process [72]. Spore-forming H2-producing bacteria can form spores which protect them when they are in an adverse environment (high tempera­ture, extreme acidity and alkalinity), but methanogens have no such capability [72]. Some of the pretreatment methods normally used for selective enrichment of an H2-producing inoculum are listed in Table 3. Methanogenesis could also be elim­inated by maintaining short retention times (2-10 h) during reactor operation [75, 76] as H2-producing bacteria grow faster than the methanogens [72]. Combining different pre-treatment methods also showed a positive effect on the H2 production process [4, 21, 30, 38, 71]. In spite of good enhancement in H2 production, marked reduction in substrate degradation efficiency was observed after applying pretreat­ment methods [4, 30, 71], which can be attributed to the inhibition of MB. The methanogenesis function is required to metabolize intermediates generated from the acidogenic process. Untreated anaerobic inocula showed low H2 yield in spite of effective substrate removal leading to CH4 formation due to the presence of MB.

The influence of various pre-treatment methods applied individually and in combi­nation on H2 production and substrate degradation patterns from the treatment of diary based wastewater under acidic conditions is illustrated in Fig. 2