6.4 Substrate composition

Biogas yield and the compositions of biogas are greatly influenced by the composition of feedstocks. AD of carbohydrates, fats, and protein yield 886, 1535, and 587 L biogas/kg-VS with methane content of approximately 50, 70, and 84%, respectively, [184]. Substrate to inoculum ratio (S/I), as well as biodegradability of the substrate, is another important factor affecting batch AD processes, especially at high solid content [192]. Too high S/I ratio may be toxic, while too low S/I ratio may prevent induction of the enzyme necessary for biodegradation [193]. Too high concentration of feedstock can cause inhibition or failure of AD [194] due to substrate inhibition. High S/I ratio can lead to overloads due to VFAs accumulation [192, 195] and long lag phase. Thus, a low S/I ratio is preferred in order to attain shorter lag phase [192, 196]. Owen et al. [197] proposed a standard S/I ratio to be approximately 1 g-VSsubstrate/g-VSinoculum.

ammonium ion (NH4

glucose (Eq. (11)).

acetate as the substrate [206].

121

+

DOI: http://dx.doi.org/10.5772/intechopen.85138

lignocellulosic materials

that is toxic to methanogens (pH 8.5) [186, 203].

Bio-hydrogen and Methane Production from Lignocellulosic Materials

7.1 Processes for fermentative hydrogen production

7. Processes for bio-hydrogen and methane production from

The abundance of lignocellulosic biomass makes it a viable feedstock for hydrogen (H2) and methane (CH4) production. Cellulose in lignocellulosic biomass can be saccharified to glucose then fermented to hydrogen and methane. In this section, summarized details on fermentative conversion process for hydrogen, i.e., dark fermentation and photo-fermentation, methane production, and AD are given.

The methods that are investigated widely for fermentative hydrogen production are dark fermentation, photo-fermentation, and a coupling system comprising dark fermentation and photo-fermentation [204]. Dark fermentation is an acidogenic fermentation process conducted under anaerobic conditions in the absence of light. Dark fermentation, as compared to photo-fermentation, is regarded as a more promising method [42], owing to its ability to utilize a wide range of biomass, its high hydrogen production rate, and its independence of lighting conditions [109]. Microorganisms used in dark fermentation are strictly anaerobic bacteria, particularly those in the genus Clostridium, and facultative anaerobic bacteria, e.g., Enterobacter spp. [205]. Mixed cultures, for example, sludge compost and sewage sludge, are also used [204]. In theory, the maximum HY obtained under dark fermentation is 4 mol-H2/mol-glucose when acetic acid is produced as a co-product (Eq. (9)). This is roughly equivalent to one third of energy recovery from the biomass [204]. The HY of 2 mol-H2/mol-glucose can also be obtained when butyric acid is produced as the co-product (Eq. (10)). However, when mixed culture is used, mixed acids are often produced, leading to a lower HY of 2.5 mol-H2/mol-

> C6H12O6 þ 6H2O ! 2CO2 þ 2CH3COOH þ 4H2 (9) C6H12O6 þ 6H2O ! 2CO2 þ CH3CH2CH2COOH þ 2H2 (10)

4C6H12O6 þ 6H2O ! 3CH3CH2CH2COOH þ 2CH3COOH þ 8CO2 þ 10 H2 (11)

Photo-fermentation is another process being investigated widely for hydrogen production from biomass. Unlike dark fermentation, photo-fermentation is a process that requires light to drive the conversion of organic substrates into hydrogen. Purple non-sulfur bacteria are a group of microorganisms responsible for hydrogen production under photo-fermentation. Examples of PNSB include Rhodobacter spp., Rhodopseudomonas spp., and Rhodospirillum sp. Photo-fermentation is a process known for its high substrate conversion efficiencies [206]. In theory, photofermentation can completely convert organic compound into hydrogen, i.e., 12 moles of hydrogen can be obtained from a mole of glucose (Eq. (12)), which is much higher than that obtained through dark fermentation (4 mol-H2/molglucose). However, when VFAs are used as the substrate, lower HYs in a range 1–10 mol-H2/mol-VFA are obtained (Eqs. (13)–(17)). In photo-fermentation, it was reported that PNSB showed an affinity toward VFAs, with malate and lactate being the most preferable substrate. Nevertheless, a good yield is also reported using

). This can possibly increase the pH of the digestate to a level

#### 6.5 Organic loading rate

Organic loading rate (OLR) is defined as the amount of VS or COD components fed per day per unit digester volume. Higher OLR can reduce the digester's size and the capital cost as a consequence. However, enough time (HRT) should be provided to the microorganisms for breaking down the organic material and converting it into gas [198]. An increase in OLR can result in higher hydrogen production efficiency [199]. However, a further increase in OLR beyond a certain level will result in substrate inhibition, leading to a lower MY [200]. Too high OLR can shift the metabolic to solventogenic phase [201]. Hobson and Bousfield [201] and Chandra et al. [185] reported that a total solid content of 8.0–10.0% is desirable for optimum MY.
