The metabolism of polyphosphate accumulating organisms (PAOs) and glycogen accumulating organisms (GAOs) in enhanced biological phosphorus removal (EBPR) system under the tropical climate

• Main Author: Wang, Li
• Other Authors: Zhou Yan
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Environmental engineering
• Online Access: https://hdl.handle.net/10356/149119
• Institution: Nanyang Technological University

Enhanced biological phosphorus removal (EBPR) system has been widely applied in wastewater plants due to the economy and sustainability virtue. However, high temperature (>30 ℃) enhanced biological phosphorus removal (EBPR) is challenging because glycogen accumulating organisms (GAOs) can easily outcompete polyphosphate accumulating organisms (PAOs) under high temperature conditions. The nature of the carbon source is also known to impact the PAO-GAO competition, though previous studies have not assessed how carbon source impacts PAO metabolism at high temperature. This study investigated the effects of different carbon sources on two acetate/propionate enriched Accumulibacter PAO cultures at high temperature and compared the performance of carbon transformation with low temperature EBPR cases reported in literature, and revealed several key metabolic differences. Besides the common volatile fatty acids (VFAs) such as acetate and propionate, PAOs also utilized butyrate and iso-butyrate, but hardly used valerate and its isomer, iso-valerate. When acetate and propionate are limited, butyrate and iso-butyrate could be used as supplementary carbon source for EBPR. In addition, PAOs, under high temperature, seemed to prefer propionate compared with other VFAs. Nevertheless, high aerobic glycogen replenishment was realized with propionate as the sole carbon source anaerobically, a trait not previously observed at low temperatures, which may be one of the reasons for EBPR failure in the long-term operation with propionate as feed. A combined substrate of acetate, propionate and perhaps butyrate seems to be a better carbon source choice since the total VFA uptake rate was the highest when they were present together in the anaerobic phase, and this increased the aerobic P removal efficiency and reduced the glycogen recovery compared to propionate as sole substrate. GAOs have been previously observed to be more competitive for anaerobic volatile fatty acid (VFA) uptake as compared to PAOs at high temperatures. However, previous evidence regarding the impact of temperature on GAOs has exclusively focused on only one group of GAOs, namely Candidatus Competibacter phosphatis (or Competibacter). Defluviicoccus vanus GAO are another very important GAO group that have been found in EBPR plants, however, their competitiveness at high temperature for anaerobic VFA uptake was previously unknown. In this thesis, Defluviicoccus vanus GAO were enriched in two reactors at high temperature (30℃) and fed with acetate or propionate as sole carbon source to investigate their competitiveness for anaerobic VFA uptake at high temperature. The uptake rate of acetate or propionate in both GAO reactors was much lower than those of Candidatus Accumulibacter phosphatis PAOs operated under similar conditions, showing that PAOs displayed a competitive advantage at high temperature, unlike previous observations with Competibacter GAOs. The capacity of Defluviicoccus vanus GAOs to utilize other VFAs such as butyrate, iso-butyrate and valerate was also assessed. This study provides new insight into our understanding of GAOs in EBPR processes, where the impact of temperature on the PAO-GAO competition is highly dependent on the phylogenetic identity of the GAO present in the system. Butyrate, as the supplementary or the sole carbon source, could be a promising carbon source for EBPR under tropical climate. When the carbon source of PAO reactor was gradually changed from acetate to butyrate, the abundance of Candidatus Accumulibacter phosphatis first decreased from 37.4% to 8.4% and then recovered to 13.9% after acclimation. Besides, Rhodocyclaceae increased from 2.0% to 14.5% who also played an important role in P-removal. Thus, a relatively stable P removal performance was realized throughout the whole operation period. Nevertheless, butyrate had negative impact on GAOs. The biomass and microbial diversity continually decreased in GAO reactor, and Candidatus Competibacter phosphatis reduced from 27.3% to 6.2% and the dominant population was replaced by Zoogloea. With addition of butyrate as carbon source, the total amount of synthesized PHAs reduced in both PAO and GAO cultures and the composition of PHA was greatly changed. The presence of novel PHA (PHH) may disturb the microbial activity in aerobic phase, where GAO culture was more severely affected. Glycogen turnover seemed to be limited in both reactors. This could reduce the GAO metabolism in both cultures and avoid the metabolism switching from PAO’s to GAO’s. Furthermore, the biomass growth rate of PAO culture was higher than that of GAO when fed with butyrate, which also provides PAO competitive advantage. All above results indicate that butyrate could not be well metabolized by GAOs but could provide PAOs with competitive advantage. The anaerobic butyrate uptake models by PAO and GAO were established, and the differences of their metabolic pathways were also studied based on metagenomic genes. The results show anaerobic carbon transformation of PAO culture could be predicted by the random model where the acetyl-CoA and butyryl-CoA are randomly combined while the prediction of the selective model is more consistent with the experimental results of GAO culture. A higher gene expression of carbon transformation was found in PAO culture than that in GAO culture which suggests butyrate is more easily metabolized and converted to PHB and PHH by PAO. New strategies to promote EBPR performance at high temperatures are proposed based on the results of this work.