The Oxygenic Photogranule Process for Aeration-Free Wastewater Treatment.

This study presents the oxygenic photogranule (OPG) process, a light-driven process for wastewater treatment, developed based on photogranulation of filamentous cyanobacteria, nonphototrophic bacteria, and microalgae. Unlike other biogranular processes requiring airlift or upflow-based mixing, the OPG process was operated in stirred-tank reactors without aeration. Reactors were seeded with hydrostatically grown photogranules and operated in a sequencing-batch mode for five months to treat wastewater. The new reactor biomass propagated with progression of photogranulation under periodic light/dark cycles. Due to effective biomass separation from water, the system was operated with short settling time (10 min) with effective decoupling of hydraulic and solids retention times (0.75 d vs 21-42 d). During quasi-steady state, the diameter of the OPGs ranged between 0.1 and 4.5 mm. The reactors produced effluents with average total chemical oxygen demand less than 30 mg/L. Nitrogen removal (28-71%) was achieved by bioassimilation and nitrification/denitrification pathways. Oxygen needed for the oxidation of organic matter and nitrification was produced by OPGs at a rate of 12.6 ± 2.4 mg O2/g biomass-h. The OPG system presents a new biogranule process, which can potentially use simple mixing and natural light to treat wastewater.

The importance of filamentous cyanobacteria in the development of oxygenic photogranules.

Microorganisms often respond to their environment by growing as densely packed communities in biofilms, flocs or granules. One major advantage of life in these aggregates is the retention of its community in an ecosystem despite flowing water. We describe here a novel type of granule dominated by filamentous and motile cyanobacteria of the order Oscillatoriales. These bacteria form a mat-like photoactive outer layer around an otherwise unconsolidated core. The spatial organization of the phototrophic layer resembles microbial mats growing on sediments but is spherical. We describe the production of these oxygenic photogranules under static batch conditions, as well as in turbulently mixed bioreactors. Photogranulation defies typically postulated requirements for granulation in biotechnology, i.e., the need for hydrodynamic shear and selective washout. Photogranulation as described here is a robust phenomenon with respect to inoculum characteristics and environmental parameters like carbon sources. A bioprocess using oxygenic photogranules is an attractive candidate for energy-positive wastewater treatment as it biologically couples CO2 and O2 fluxes. As a result, the external supply of oxygen may become obsolete and otherwise released CO2 is fixed by photosynthesis for the production of an organic-rich biofeedstock as a renewable energy source.