Fermentative hydrogen production from cassava stillage by mixed anaerobic microflora: Effects of temperature and pH

Abstract

Fermentative hydrogen production from cassava stillage was conducted to investigate the influences of temperature (37 °C, 60 °C, 70 °C) and initial pH (4–10) in batch experiments. Although the seed sludge was mesophilic anaerobic sludge, maximum hydrogen yield (53.8 ml H₂/gVS) was obtained under thermophilic condition (60 °C), 53.5% and 198% higher than the values under mesophilic (37 °C) and extreme-thermophilic (70 °C) conditions respectively. The difference was mainly due to the different VFA and ethanol distributions. Higher hydrogen production corresponded with higher ratios of butyrate/acetate and butyrate/propionate. Similar hydrogen yields of 66.3 and 67.8 ml H₂/gVS were obtained at initial pH 5 and 6 respectively under thermophilic condition. The total amount of VFA and ethanol increased from 3536 to 7899 mg/l with the increase of initial pH from 4 to 10. Initial pH 6 was considered as the optimal pH due to its 19% higher total VFA and ethanol concentration than that of pH 5. Homoacetogenesis and methonogenesis were very dependent on the initial pH and temperature even when the inoculum was heat-pretreated. Moreover, a difference between measured and theoretical hydrogen was observed in this study, which could be attributed to homoacetogenesis, methanogenesis and the degradation of protein.

Introduction

Ethanol is an important renewable fuel that can contribute to the reduction of negative environmental impacts by fossil fuels [1]. China is the third largest ethanol producer in the world [2], and cassava-derived ethanol production has been increasing due to its economic benefits compared to other ethanol-producing crops in the country [3]. However, large amounts of wastewater (cassava stillage), containing high concentrations of organics and suspended solids are produced during the ethanol production process. In addition, the waste temperature could be as high as 90 °C after distillation separation. Our previous study showed that cassava stillage is suitable for thermophilic anaerobic treatment and that the contained organics could be converted to fatty acids, hydrogen, carbon dioxide and consequently to methane [4]. Compared with methane, the intermediate product, hydrogen is a more important industrial commodity which is widely used for syntheses of ammonia, alcohols and aldehydes [5]. Hydrogen could be produced by fermentation from organic wastes such as household solid waste [6], sewage sludge [7] and winery wastewater [8]. It was reported that the carbohydrate contained in the wastes was effective and reliable for fermentative hydrogen production [9], [10]. Thus cassava stillage should be of interest as a potential substrate for hydrogen production.

Considering the high temperature of cassava stillage, thermophilic and extreme-thermophilic conditions may be more suitable without additional costly pre-cooling facilities. Fermentative temperature can affect hydrogen production by influencing the microorganism composition and activity of enzymes such as hydrogenases [11]. It was reported that the hydrogen yield obtained under thermophilic condition could be as high as 2.4 mol H₂/mol hexose, much higher than the value of less than 2.0 mol H₂/mol hexose from mesophilic condition [12], [13], [14], [15]. The maximum hydrogen yields of 2.68 and 2.47 mol H₂/mol hexose were obtained at 75 °C [16] and 70 °C [17], respectively. However, when using cassava starch as substrate, Lee et al. [18] found that the hydrogen production obtained at 37 °C was six times higher than that of 55 °C. The suitable temperature range for hydrogen production from cassava stillage is still unknown. During the fermentative hydrogen production, high concentrations of volatile fatty acids such as butyric and acetic acids can be accumulated in the system and result in the drop of pH if not properly controlled. pH variation may alter the quantity of hydrogen production, aqueous product, substrate utilization efficiency and microbial community. Many studies were conducted on the effect of initial pH under mesophilic condition [19], [20], [21], but the obtained results were inconsistent and the optimal pH differed from one study to another. For example, Khanal et al. [19] reported that acidic initial pH was favorable for hydrogen production, while Lee et al. [20] found that alkaline initial pH was suitable for hydrogen production through batch tests. The disagreements in the optimal temperature and pH in different studies may be resulted from the differences in substrates, seed sludge and operation conditions. In addition, there are few studies focusing on the effect of pH on hydrogen production under thermophilic condition. It is necessary to investigate the effects of temperature and pH on hydrogen production from the same substrate by certain seed sludge.

Currently, few reports are available on the systematical investigation of hydrogen producing characteristics of mesophilic anaerobic sludge from real wastes under different temperature and pH ranges. Mesophilic anaerobic sludge as seed sludge for hydrogen production can easily be obtained since most of the anaerobic reactors were operated under mesophilic conditions [22], [23], [24]. By pretreatments (e.g. heat, acid and base), the methanogens in the mesophilic anaerobic sludge can be inhibited while hydrogen producing bacteria can be enriched [25]. Homoacetogens and methanogens are two hydrogen consuming microorganisms and will lead to the lower hydrogen yields if they are not inhibited [7]. However, the inhibition effectiveness of pretreatment on their activities is still questionable; as hydrogen consumption was still observed even when the inoculum was pretreated [26]. Further study is necessary to elucidate the activities of homoacetogens and methanogens under different fermentation conditions.

This study was conducted to explore the feasibility of hydrogen production from cassava stillage by mesophilic anaerobic sludge. The effects of temperature (mesophilic, thermophilic and extreme-thermophilic) and initial pH (4–10) on the hydrogen production were investigated and the involved mechanisms were discussed. In addition, the activities of homoacetogens and methanogens in the hydrogen production process were also discussed.