The Coupled Effect of Light and Iron on the Photogranulation Phenomenon.

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.

Photogranulation in a Hydrostatic Environment Occurs with Limitation of Iron.

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.

Characterization of particles from ferrate preoxidation.

Studies were conducted evaluating the nature of particles that result from ferrate reduction in a laboratory water matrix and in a natural surface water with a moderate amount of dissolved organic carbon. Particle characterization included size, surface charge, morphology, X-ray photoelectron spectroscopy, and transmission Fourier transform infrared spectroscopy. Characteristics of ferrate resultant particles were compared to particles formed from dosing ferric chloride, a common water treatment coagulant. In natural water, ferrate addition produced significantly more nanoparticles than ferric addition. These particles had a negative surface charge, resulting in a stable colloidal suspension. In natural and laboratory matrix waters, the ferrate resultant particles had a similar charge versus pH relationship as particles resulting from ferric addition. Particles resulting from ferrate had morphology that differed from particles resulting from ferric iron, with ferrate resultant particles appearing smoother and more granular. X-ray photoelectron spectroscopy results show ferrate resultant particles contained Fe2O3, while ferric resultant particles did not. Results also indicate potential differences in the mechanisms leading to particle formation between ferrate reduction and ferric hydrolysis.