The phototrophic metabolic behaviour of Candidatus accumulibacter.

The phototrophic capability of Candidatus Accumulibacter (Accumulibacter), a common polyphosphate accumulating organism (PAO) in enhanced biological phosphorus removal (EBPR) systems, was investigated in this study. Accumulibacter is phylogenetically related to the purple bacteria Rhodocyclus from the family Rhodocyclaceae, which belongs to the class Betaproteobacteria. Rhodocyclus typically exhibits both chemoheterotrophic and phototrophic growth, however, limited studies have evaluated the phototrophic potential of Accumulibacter. To address this gap, short and extended light cycle tests were conducted using a highly enriched Accumulibacter culture (95%) to evaluate its responses to illumination. Results showed that, after an initial period of adaptation to light conditions (approximately 4-5 h), Accumulibacter exhibited complete phosphorus (P) uptake by utilising polyhydroxyalkanoates (PHA), and additionally by consuming glycogen, which contrasted with its typical aerobic metabolism. Mass, energy, and redox balance analyses demonstrated that Accumulibacter needed to employ phototrophic metabolism to meet its energy requirements. Calculations revealed that the light reactions contributed to the generation of, at least more than 67% of the ATP necessary for P uptake and growth. Extended light tests, spanning 21 days with dark/light cycles, suggested that Accumulibacter generated ATP through light during initial operation, however, it likely reverted to conventional anaerobic/aerobic metabolism under dark/light conditions due to microalgal growth in the mixed culture, contributing to oxygen production. In contrast, extended light tests with an enriched Tetrasphaera culture, lacking phototrophic genes in its genome, clearly demonstrated that phototrophic P uptake did not occur. These findings highlight the adaptive metabolic capabilities of Accumulibacter, enabling it to utilise phototrophic pathways for energy generation during oxygen deprivation, which holds the potential to advance phototrophic-EBPR technology development.

The effect of seed sludge on the selection of a photo-EBPR system.

The phototrophic-enhanced biological phosphorus removal system (photo-EBPR) was recently proposed as an alternative photosynthetic process to conventional phosphorus removal. Previous work showed the possibility of obtaining a photo-EBPR system starting from a culture already enriched in polyphosphate accumulating organisms (PAOs). The present work evaluated whether the same could be achieved starting from conventional activated sludge. A sequencing batch reactor inoculated with sludge from a wastewater treatment plant (WWTP) was fed with a mixture of acetate and propionate (75%:25%) and subjected to dark/light cycles to select a photo-EBPR system containing PAOs and photosynthetic organisms, the oxygen providers for the system. The results showed that it is possible to obtain a photo-EBPR system starting from a WWTP sludge, although the process is slower than when started with a sludge already enriched in PAOs. After 15 days of operation, the system could remove 60 ± 2 mg-P/L of phosphorus (approximately 67% of the concentration at the end of dark period) in the light period, from which 13 ± 1 mg-P/L was removed during the phase without external air supply. These results indicate that a photo-EBPR system can be obtained independently of the seed sludge initially used, provided that a suitable operating strategy is implemented, i.e. by imposing conditions that favour the growth and coexistence of PAOs and photosynthetic microorganisms.

The impact of operational strategies on the performance of a photo-EBPR system.

A novel Phototrophic - Enhanced Biological Phosphorus Removal (Photo-EBPR) system, consisting of a consortium of photosynthetic organisms and polyphosphate accumulating organisms (PAOs), was studied in this work. A sequencing batch reactor was fed with a mixture of acetate and propionate (75%-25%) and subjected to dark/light cycles in order to select a photo-EBPR system containing PAOs and photosynthetic organisms, the latter likely providers of oxygen to the system. The results from the selection period (stage 1) showed that the photo-EBPR culture was capable of performing P release in the dark and P uptake in the presence of light, under limited oxygen concentrations. During the optimization period, the aeration period, which was initially provided at the end of the light phase, was gradually reduced until a non-aerated system was achieved, while the light intensity was increased. After optimization of the operational conditions, the selected consortium of photosynthetic organisms/PAOs showed high capacity of P removal in the light phase in the absence of air or other electron acceptor. A net P removal of 34 ± 3 mg-P/L was achieved, with a volumetric P removal rate of 15 ± 2 mg-P/L.h, and 79 ± 8% of P removal from the system. Also, in limiting oxygen conditions, the P uptake rate was independent of the PHA consumption, which demonstrates that the organisms obtained energy for P removal from light. These results indicated that a photo-EBPR system can be a potential solution for P removal with low COD/P ratios and in the absence of air, prospecting the use of natural sunlight as illumination, which would reduce the costs of EBPR operation regarding aeration.