Purple phototrophic bacteria granules under high and low upflow velocities

The application of granular biomass has enabled energy efficient, high-rate wastewater treatment systems. While initially designed for high-strength wastewater treatment, granular systems can also play a major role in resource recovery. This study focused on the formation of purple phototrophic bacteria (PPB) granular biomass during synthetic wastewater treatment. Liquid upflow velocity was applied as the driving force for granulation. Separate reactors were operated at either low (2-5m h^-1) or high (6-9m h^-1) upflow velocities, with sludge retention times (SRTs) ranging from 5-15d. Reactors produced anaerobic, photo-granules within ~50d. The sludge volume index (SVI₃₀) of the granules was 10mL g^-1 and average settling rates were greater than 30m h^-1, both metrics being similar to existing granular technologies. Granule sizes of 2-3mm were recorded, however the particle size distribution was bimodal with a large floc fraction (70-80% volume fraction). The extracellular polymeric substance (EPS) and alginate-like extract (ALE) contents were similar to those in aerobic granular biomass. Fluorescence in-situ hybridisation (FISH) imaging identified PPB bacteria dispersed throughout the granules with very few methanogens and an active core. Outer layer morphology was substantially different in the two reactors. The high-upflow reactor had an outer layer of Chromatiales and an inner layer of Rhodobacteriales, while the low-upflow reactor had lower abundances of both, and limited layering. According to 16s gene sequencing, PPB were a similar fraction of the microbial community in both reactors (40-70%), but the high upflow granules were dominated by Chromatiales (supporting FISH results), while the low upflow velocity reactor had a more diverse PPB community. Methanogens were seen only in the low upflow granules and only in small amounts (≤8%). Granule crude protein content was ~0.60gCP gVS^-1 (~0.45gCP gTS^-1), similar to that from other PPB production technologies. The growth of a rapid settling and discrete PPB granular biomass on synthetic wastewater suggests methods for resource recovery using PPB can be diversified to also include granular biomass.