Abstract
Domestic wastewater treatment by purple phototrophic bacteria (PPB) is based on the assimilative uptake of organics and nutrients into the bacterial biomass. Thereby, it strongly depends on the carbon/nutrients ratio of the wastewater. The physiological COD/N/P ratio for PPB growth in domestic wastewater makes the addition of an external organic carbon source necessary in order to allow for an efficient process. However, PPB need a source of alkalinity (as CO2) to grow on reduced organics that serves as an electron acceptor since biohydrogen production (an alternative electron sink) is inhibited by ammonium. A preliminary experiment showed that high nutrients-loading wastewater was limited by CO2 imbalance, leading to poor removal efficiencies. Subsequently, the effect of the oxidation state of the organics added as external organic carbon sources to PPB reactors treating low nutrients-loading domestic wastewater has been analyzed. Three organics were used as additives to PPB development in four consecutive batches: acetate (more oxidized), ethanol and butyrate (more reduced). The PPB population was settled and the general performance under the three situations, in terms of organics, N and P assimilation, and growth kinetics was not significantly different irrespective of the external organic carbon source. The reactors were dominated by PPB, though reduced organics allowed for dominance of Rhodopseudomonas palustris, whereas oxidized organics caused co-dominance of R. palustris and Rhodobacter capsulatus. Thereby, alkalinity (as bicarbonate), and not the oxidation state of the organics, is the key parameter for the efficient treatment of domestic wastewater by PPB.
Highlights
Purple phototrophic bacteria (PPB) are a wide group of anoxygenic phototrophs that are highly metabolically versatile
Thereby, it can be inferred that the initial reduction of nutrients does not affect the biomass development and it is a suitable strategy for enabling efficient domestic wastewater (DWW) by PPB
It has been demonstrated that the general performance of the PPB mixed cultures was not affected by the oxidation state of the external organic carbon source
Summary
Purple phototrophic bacteria (PPB) are a wide group of anoxygenic phototrophs that are highly metabolically versatile This versatility led PPB to be proposed as key actors in a novel platform for anaerobic wastewater treatment with resource recovery [1]. Resources 2020, 9, 88 interesting metabolic pathway as it entails organics and nutrients assimilation into biomass with almost complete recycle of C and electrons, thereby not producing CO2 These bacteria use preferentially low energy infrared light, contrary to other photosynthetic microorganisms as algae and cyanobacteria. Incident photons (from light) generate an electron transport chain (ETC) and proton gradient and allow energy capture as adenosine tri-phosphate (ATP) [2] Both the Tricarboxylic Acid (TCA) and the Calvin-Bensom-Bassham (CBB) cycles are involved in the central metabolism.
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