Continuous hydrogen production from cassava starch processing wastewater by two-stage thermophilic dark fermentation and microbial electrolysis
Introduction
Hydrogen is an attractive energy carrier due to its potential for higher efficiency of conversion to usable power, low generation of pollutants and high energy density [1]. It is possible to produce hydrogen from organic waste, wastewater through fermentation using microorganisms, which has proven to be a relatively simple and inexpensive process [2]. Dark hydrogen fermentation process has high hydrogen production rate. Moreover, various organic waste materials such as food waste, rice straw, cheese whey, corn stalk waste and cassava starch processing wastewater can utilize for hydrogen gas production by dark fermentation process [3]. The hydrogen yield from dark fermentation of organic waste such as vegetable kitchen waste, palm oil mill effluent, co-digestion of food waste and sewage sludge, cassava pulp hydrolysate and starch were 38 [4], 340 [5], 123 [6], 342 [7] and 125 [8] ml H2 gCOD−1, respectively. Thermophilic dark fermentation is energetically more favorable for biological H2 production and higher H2 yield than mesophilic dark fermentation [9]. The promising advantage of thermophilic dark fermentation is better hydrogen production rate and enhances the hydrolysis rate of the complex substrate. In addition, thermophilic can more effectively utilize complex carbohydrates such as cellulose and starch [10].
Successful hydrogen production from cassava starch processing wastewater by dark fermentation was achieved under thermophilic conditions. Xie et al. [11] obtained the continuous hydrogen production rate and yield were 3.45 L H2 l−1d−1 and 130 ml H2 gCOD−1 by mixed culture under thermophilic conditions (55 °C) at optimum values of 2 days HRT and OLR of 25.2 g COD l−1 d−1 in the anaerobic sequencing batch reactor (ASBR). In conventional hydrogen fermentation process could recover only 30–35% of the energy containing an organic waste converting to hydrogen. The rest of the energy remains in the liquid as VFA mainly butyric acid, acetic acid, lactic acid and propionic acid about 65–70% of the energy containing the organic waste. Microbial electrolysis process can convert acetic and butyric acid to hydrogen gas in a device called a microbial electrolysis cell (MEC). In an MEC, bacteria attached to an anode oxidize acetic and butyric acid, releasing electrons via a circuit to the cathode where hydrogen can be formed from protons in water [12]. Two-stage processes which integrated thermophilic dark fermentation with microbial electrolysis cell which eventually helps in improving gaseous energy recovery [13]. In addition, dark fermentation well complements MEC because fermentation efficiently breaks down complex forms of organic compounds into simple acids that anode-respiring bacteria can utilize in MEC [14]. Lu et al. [15] reported that an overall hydrogen recovery of 96% of the maximum theoretical yield of 0.125 g H2 g COD−1. Wang et al. [16] also reported that 41% increase in overall hydrogen yield from cellulose by integrated fermentation with MEC. However, integrated thermophilic dark fermentation with thermophilic microbial electrolysis cell is still a lack of information, especially for continuous operation.
In this study, the potential of hydrogen production from cassava starch processing wastewater was demonstrated using a two-stage thermophilic dark fermentation and thermophilic microbial electrolysis cell. The continuous operation of two-stage thermophilic dark fermentation and thermophilic microbial electrolysis from cassava starch wastewater processing using two phase up-flow anaerobic sludge blanket reactor was investigated.
Section snippets
Feedstocks and inoculums
The cassava starch processing wastewater (CSWW) used in this study collecting from National Starch and Chemical Co., Ltd., Kalasin, Thailand. CSWW was stored at the temperature of 4 °C for later use. The physiochemical characters of the cassava starch processing wastewater and hydrogen production effluent were shown in Table 1. Thermoanaerobacterium thermosaccharolyticum PSU-2 was used for hydrogen production by thermophilic dark fermentation [13]. The stock culture of T. thermosaccharolyticum
Effects of applied voltage on hydrogen production by MEC
Effect of applied voltages from 0.1 V to 0.8 V on MEC performance was investigated for improving net energy recovery. Maximum hydrogen yield of 245 ml H2 gCOD−1 and net energy recovery of 90% was obtained from applied voltage of 0.6 V H2 production rate, COD removal and hydrogen recovery of 0.6 V applied MEC was 61.48 ml H2 gCOD−1 d−1, 39% and 26.4% respectively. Applied voltage of 0.6 to MEC reactor was optimum in term of hydrogen production and net energy recovery. Hydrogen yields from
Conclusions
Continuous two-stage thermophilic dark fermentation and microbial electrolysis were efficient processes for hydrogen production from cassava starch processing wastewater. Applied voltage of 0.6 was optimum for microbial electrolysis cell (MEC). Hydrogen yield of continuous single stage MEC was 182 ml H2 gCOD−1. The hydrogen yield by two-stage dark fermentation and microbial electrolysis from cassava starch processing wastewater was 260 ml H2 gCOD−1 and 205 ml H2 gCOD−1, respectively. Hydrogen
Acknowledgement
The authors would like to thank Research and Development Institute Thaksin University (Grant No. 2559A10502044), Research Group for Development of Microbial Hydrogen Production Process, Khon Kaen University and Thailand Research Fund Senior Research Scholar (Grant No. RTA5980004), and Graduate Research Fund from the National Research Council of Thailand (Grant No. กบง./2559-ท 6.8) for the financial support.
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