Abstract
Domestic and industrial wastewaters contain organic substrates and nutrients that can be recovered instead of being dissipated by emerging efficient technologies. The aim of this study was to promote bio-hydrogen production and carbon fixation using a mixed culture of purple phototrophic bacteria (PPB) that use infrared radiation in presence or absence of an electrode as electron donor. In order to evaluate the hydrogen production under electrode-free conditions, batch experiments were conducted using different nitrogen (NH4Cl, Na-glutamate, N2 gas) and carbon sources (malic-, butyric-, acetic- acids) under various COD:N ratios. Results suggested that the efficiency of PPB to produce biogenic H2 was highly dependent on the substrates used. The maximum hydrogen production (H2_max, 423 mLH2/L) and production rate (H2_rate, 2.71 mLH2/Lh) were achieved using malic acid and Na-glutamate at a COD:N ratio of 100:15. Under these optimum conditions, a significant fixation of nitrogen in form of single-cell proteins (874.4 mg/L) was also detected. Under bio-electrochemical conditions using a H-cell bio-electrochemical device, the PPB were grown planktonic in the bio-cathode chamber with the optimum substrate ratio of malic acid and Na-glutamate. A redox potential of -0.5 V (vs Ag/AgCl) under bio-electrochemical conditions produced comparable amounts of bio-hydrogen but significantly negligible traces of CO2 as compared to the biological system (11.8 mLCO2/L). This suggests that PPB can interact with the cathode to extract electrons for further CO2 re-fixation (coming from the TCA cycle) into the Calvin cycle, thereby improving the C usage. It has also been observed during cyclic voltammograms that a redox potential of -0.8 V favours considerably the electrons consumption by the PPB culture, suggesting that the PPB can use these electrons to increase the biohydrogen production. These results are expected to prove the feasibility of stimulating PPB through bio-electrochemical processes in the production of H2 from wastewater resources, which is a field of special novelty and still unexplored.
Highlights
IntroductionThe high content of organics and nutrients in industrial and domestic wastewaters is a valuable resource for energy and products recovery (Puyol et al, 2017a)
Typical wastewater systems entail the dissipation of the contamination
The following section include all results generated after exploring the physiology of phototrophic bacteria (PPB) for selecting those culture conditions, including nitrogen and carbon sources, for achieving an optimal conversion of an extracellular source of electrons into hydrogen production and CO2 fixation
Summary
The high content of organics and nutrients in industrial and domestic wastewaters is a valuable resource for energy and products recovery (Puyol et al, 2017a). Upgrading of existing WWTP as resource recovery systems by implementing novel technologies, are mandatory steps considering economic and environmental benefits and recent policies within the circular economy. The biological accumulation of nutrients and their subsequent recovery, has received great attention as an environmental friendly and certainly cost-effective process (Batstone et al, 2015). Purple phototrophic bacteria (PPB) have shown significant accumulation of organics and nutrients from wastewater through assimilative processes (Batstone et al, 2015). The use of PPB in the Partition-Release-Recovery concept proved to be far superior to other phototrophic organisms (as algae or cyanobacteria), since they achieve high growth rates and are not inhibited by O2 (Muñoz and Guieysse, 2006)
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