Enriching and aggregating purple non-sulfur bacteria in an anaerobic sequencing-batch photobioreactor for nutrient capture from wastewater

Purple non-sulfur bacteria (PNSB), a guild of anoxygenic photomixotrophic organisms, rise interest to capture nutrients from wastewater in mixed-culture bioprocesses. One challenge targets the aggregation of PNSB biomass through gravitational separation from the treated water to facilitate its retention and accumulation, while avoiding the need for membranes. We aimed to produce an enriched, concentrated, well-settling, nutrient-removing PNSB biomass using sequencing batch regimes (SBR) in an anaerobic photobioreactor. The stirred tank was fed with a synthetic influent mimicking loaded municipal wastewater (430-860 mg COD Ac L Inf -1 , COD:N:P ratio of 100:36:4-100:11:2 m/m/m), operated at 30°C and pH 7, and continuously irradiated with infrared (IR) light (>700 nm) at 375 W m -2 . After inoculation with activated sludge at 0.1 g VSS L -1 , PNSB were rapidly enriched in a first batch of 24 h: the genus Rhodobacter reached 54% of amplicon sequencing read counts. SBR operations at volume exchange ratio of 50% with decreasing hydraulic retention times (48 to 16 h; 1 to 3 cycles d -1 ) and increasing volumetric organic loading rates (0.2 to 1.3 kg COD m -3 d -1 ) stimulated the aggregation (compact granules of 50-150 μm), settling (sedimentation G-flux of 4.7 kg h -1 m -2 ), and accumulation (as high as 3.8 g VSS L -1 ) of biomass. The sludge retention time (SRT) increased freely from 2.5 to 11 d without controlled sludge wasting. Acetate, ammonium, and orthophosphate were removed simultaneously (up to 96% at a rate of 1.1 kg COD m -3 d -1 , 77% at 113 g N m -3 d -1 , and 73% at 15 g P m -3 d -1 ) with a COD:N:P assimilation ratio of 100:6.7:0.9 (m/m/m). Competition for substrate and photons occurred in the PNSB guild. SBR regime shifts sequentially selected for Rhodobacter (90%) under shorter SRT and non-limiting acetate concentrations during reaction phases, Rhodopseudomonas (70%) under longer SRT and acetate limitation, and Blastochloris (10%) under higher biomass concentrations. We highlighted the benefits of a PNSB-based SBR process for biomass accumulation and simultaneous nutrient capture at substantial rates, and its underlying microbial ecology. Graphical abstract Highlights PNSB were highly enriched (90%) in an anaerobic stirred-tank photobioreactor. The mixed-culture SBR process fostered PNSB biomass aggregation and accumulation. PNSB sludge reached 3.8 g VSS L -1 and a sedimentation G-flux of 4.7 kg h -1 m -2 . PNSB enabled a high simultaneous removal of COD (96%), N (77%), and P (73%). Rhodobacter , Rhodopseudomonas , and Blastochloris competed for acetate and photons.