Simultaneous biohydrogen (H2) and bioplastic (poly-b-hydroxybutyrate-PHB) productions

Hydrogen is the highest energy density among the fuel types and energy sources. In essence, PHB is one of the known poly-hydroxy alkanoates (PHAs). PHB is an alternative to synthetic plastics as its physical properties are equivalent to those of polypropylene and also readily biodegradable (Arun et al., 2006). PHB from a single-cultured commercial-grade substrate is often more expensive due to its production cost and substrate. In contrast, PHB is easily synthesized from VFAs without requiring the β-oxidation, glycolysis pathways, with the support of fewer enzymes and metabolic reactions. The use of acid-rich wastewater as the substrate for PHB production can simultaneously reduce the production cost and the treatment of wastewater (Venkata Mohan et al., 2010). Agricultural wastes (i.e., rice husk (RH) and rice straw (RS)) are abundant renewable sources around the world and especially in Asia. Approximately 87% of the total global rice production is from Asian regions (Saini et al., 2015). Rice straw and husk are the by-products of rice cultivation for more than 50% of the total weight of the rice crop. They are usually disposed of traditionally in open fields in Asia (Sen et al., 2016). We evaluated the biohydrogen (H2), bioplastic (PHB) production utilizing various wastes under Dark Fermentative (DF), Photo Fermentative (PF) and subsequent DF-PF. Potential bio H2 and PHB producing microbes were enriched and isolated. The higher bio H2 and PHB producing substrate (RH and RS) under DF were used for subsequent DF-PF studies. The effects of substrate concentration (10-100%), pH (5.5-8.0) were examined in batch mode under DF and PF conditions. The maximum bio H2, PHB production, was found at 100% RS hydrolysate (H) (1.53±0.04 mol H2/mol glucose and 9.8±0.14 g/L PHB). In subsequent DF-PF, the highest amount of bio H2, PHB, was recorded at 100% RSH (1.82±0.01 mol H2/mol of glucose and 19.15±0.25 g/L PHB) at pH 7 by Bacillus cereus (KR809374) and Rhodopseudomonas rutila. Subsequent DF-PF biological H2, PHB production from renewable biomass such as RHH and RSH has considerable potential with respect to the sustainable management of global energy and environmental issues.