RESUMEN
Cyanobacteria occasionally self-immobilize and form spherical aggregates. This photogranulation phenomenon is central for oxygenic photogranules, which present potential for aeration-free and net-autotrophic wastewater treatment. Light and iron are tightly coupled via photochemical cycling of Fe, suggesting that phototrophic systems continually respond to their combined effects. Thus far, photogranulation has not been investigated from this important aspect. Here, we studied the effects of light intensity on the fate of Fe and their combined effects on the photogranulation process. Photogranules were batch-cultivated with the activated sludge inoculum under three photosynthetic photon flux densities: 27, 180, and 450 µmol/m2·s. Photogranules were formed within a week under 450 µmol/m2·s compared to 2-3 and 4-5 weeks under 180 and 27 µmol/m2·s, respectively. Batches under 450 µmol/m2·s showed faster but lower quantity of Fe(II) release into bulk liquids compared to the other two sets. However, when ferrozine was added, this set showed substantially more Fe(II), indicating that Fe(II) released by photoreduction undergoes fast turnover. Fe linked with extracellular polymeric substances (EPS), FeEPS, diminished significantly faster under 450 µmol/m2·s, while the granular shape in all three batches appeared along with the depletion of this FeEPS pool. We conclude that light intensity has a major influence on the availability of Fe, and light and Fe together impact the speed and characteristics of photogranulation.
Asunto(s)
Cianobacterias , Hierro , Aguas del Alcantarillado , Oxígeno , Compuestos Ferrosos , Oxidación-ReducciónRESUMEN
Filamentous cyanobacteria are an essential element of oxygenic photogranules for granule-based wastewater treatment with photosynthetic aeration. Currently, mechanisms for the selection of this microbial group and their development in the granular structure are not well understood. Here, we studied the characteristics and fate of iron in photogranulation that proceeds in a hydrostatic environment with an activated sludge (AS) inoculum. We found that the level of Fe in bulk liquids (FeBL) sharply increased due to the decay of the inoculum but quickly diminished along with the bloom of microalgae and the advent of the oxic environment. Iron linked with extracellular polymeric substances (FeEPS) continued to decline but reached steady low values, which occurred along with the appearance of granular structure. Strong negative correlations were found between FeEPS and the pigments specific for cyanobacteria. Spectroscopies revealed the presence of amorphous ferric oxides in pellet biomass, which seemed to remain unaltered during the photogranulation process. These results suggest that the availability of FeEPS in AS inoculums-after algal bloom-selects cyanobacteria, and the limitation of this Fe pool becomes an important driver for cyanobacteria to granulate in a hydrostatic environment. We therefore propose that the availability of iron has a strong influence on the photogranulation process.
Asunto(s)
Microalgas , Aguas Residuales , Biomasa , Hierro , Aguas del AlcantarilladoRESUMEN
Oxygenic photogranules (OPGs), spherical aggregates comprised of phototrophic and nonphototrophic microorganisms, treat wastewater without aeration, which currently incurs the highest energy demand in wastewater treatment. In wastewater-treatment reactors, photogranules grow in number as well as in size. Currently, it is unknown how the photogranules grow in size and how the growth impacts their properties and performance in wastewater treatment. Here, we present that the photogranules' growth occurs with changes in phototrophic community and granular morphology. We observed that as the photogranules grow larger, filamentous cyanobacteria become enriched while other phototrophic microbes diminish significantly. The photogranules greater than 3 mm in diameter showed the development of a layered structure in which a concentric filamentous cyanobacterial layer encloses noncyanobacterial aggregates. We observed that the growth of photogranules significantly impacts their capability of producing oxygen, the key element in OPG wastewater treatment. Among seven size classes investigated in this study, photogranules in the 0.5-1 mm size group showed the highest specific oxygen production rate (SOPR), 21.9 ± 1.3 mg O2/g VSS-h, approximately 75% greater than the SOPR of mixed photogranular biomass. We discuss engineering the OPG process based on photogranules' size, promoting the stability of the granular process and enhancing efficiency for self-aerating wastewater treatment.