Hydrogen is seen by many as the fuel of the future because it has a very high energy density, three times that of petrol or diesel, and because its use produces only water instead of greenhouse gases and other exhaust pollutants. Furthermore, using petrol and diesel in combustion engines waste at least two thirds of the energy in the fuel, whereas hydrogen can be used in fuel cells, which are about twice as efficient, so much more of the fuel’s energy is put to good use and less fuel is needed.
The major biological processes for bio-hydrogen production are bio-photolysis of water by algae, dark fermentation, photo-fermentation of organic materials and the sequential dark and photo-fermentation processes.
Waste water as rich potential source of energy
Typical wastewater systems entail the dissipation of the contamination. However, the high content of organics and nutrients in industrial and domestic wastewaters is a valuable resource for energy and products recovery. Hence, upgrading of existing WWTP (Waste Water Treatment Plant) as resource recovery systems by implementing novel technologies, are mandatory steps considering economic and environmental benefits and recent policies within the circular economy.
Among the competing technologies, the biological accumulation of nutrients and their subsequent recovery, has received great attention as an environmental friendly and certainly cost-effective process.
Purple Bacteria in Sewage
Purple phototrophic bacteria (PPB) have shown significant accumulation of organics and nutrients from wastewater through assimilative processes. PPB is a group of anaerobic facultative microorganisms, which can utilize infrared light (IR) as the main energy source. 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.
PPB are extremely versatile organisms due to their complex metabolic system, involving major C, N, S, P, and Fe pathways, which absorbs the IR energy through their photosystem, composed by carotenoids and bacteriochlorophyls. Anoxygenic photosynthesis generates practically all the energy required for growth via the so-called cyclic electron flow. In domestic wastewater treatment, the main metabolism follows photoheterotrophic growth on volatile fatty acids and sugars, although chemoheterotrophy (e.g., fermentation and anaerobic oxidation) can provide the necessary electrons for photoautotrophic growth (via hydrogen).
PPB can be used for the extraction of high value-added products from waste sources, such as biofuels like bio-hydrogen, bioplastics as PHA and single-cell proteins. The metabolic pathways to obtain the valuable bioproducts are catalyzed by variant enzymes. Monitoring the functionality of the involved bacteria and following-up their activity, can be of added value toward maximizing the bioproducts’ formation.
Purple bacteria ‘batteries’ turn sewage into clean energy
In a recent study, the optimum culturing conditions for maximizing the hydrogen production by a mixed culture of purple phototrophic bacteria in sewage has been analyzed.
This is the first study to demonstrate that PPB-which can store energy from light-when supplied with an electric current can recover near to 100 percent of carbon from any type of organic waste, while generating hydrogen gas for use as fuel.
Purple phototrophic bacteria make an ideal tool for resource recovery from organic waste, thanks to their highly diverse metabolism.
The bacteria can use organic molecules and nitrogen gas-instead of CO2 and H2O-to provide carbon, electrons and nitrogen for photosynthesis. This means that they grow faster than alternative phototrophic bacteria and algae, and can generate hydrogen gas, proteins or a type of biodegradable polyester as byproducts of metabolism.
Which metabolic product predominates depends on the bacteria’s environmental conditions-like light intensity, temperature, and the types of organics and nutrients available. In the study, the team manipulates these conditions to tune the metabolism of purple bacteria to different applications, depending on the organic waste source and market requirements. The uniqueness of the study includes the use of an external electric current to optimize the productive output of purple bacteria.
According to the authors, this was the first reported use of mixed cultures of purple bacteria in a bioelectrochemical system — and the first demonstration of any phototroph shifting metabolism due to interaction with a cathode. Capturing excess CO2 produced by purple bacteria could be useful not only for reducing carbon emissions, but also for refining biogas from organic waste for use as fuel.
In Conclusion: Though the aim of the study was to increase biohydrogen production by donating electrons from the cathode to purple bacteria metabolism, it seems that the PPB bacteria prefer to use these electrons for fixing CO2 instead of creating H2.