| Theoretically, in the EBPR system, a portion of the supernatants at the anaerobic stage enriched with phosphate can be removed in sidestream. The phosphorus(P) in the supernatant was recovered by chemical-P precipitation. The advantages of this method are: achieving large amount of recycled P, decreasing phosphate load in the aerobic period, and improving the chemical oxygen demand(COD) to P ratio in the subsequent process. Nevertheless, the effect of such chemical-P recovery on the main stream needs further investigation.In this study, the chemical-P recovery from supernatants of anaerobic stage in sidestream was performed in the lab-scale anaerobic–aerobic sequencing batch reactor(SBR) and continuous flow reactor, which were fed with synthetic municipal wastewater. The influence of chemical-P recovery in sidestream on EBPR was discussed and the mechanism were analyzed. Additionally, the application conditions of chemical-P recovery in sidestream in EBPR system was achieved and the operation situation of the system under such conditions were also discussed. This study gives us insight into the mechanism of biological phosphorus removal, providing guidances to the development of the chemical-P recovery from supernatants of anaerobic stage in sidestream.The main results and conclusions are made as follows:1. The optimal deprivation frequency in SBR system was once three cycles. The optimal conditions of chemical phosphorus precipitation in sidestream was as follows: the molar ratio 1.40:1 of ferric ion to phosphate, stirring intensity for 275 r/min with 30 s in coagulation and 60 r/min with 18 min in flocculation, and precipitation time of 20 min. The efficiency of chemical phosphorus precipitation was more than 97%. Thus, the effects of iron ions and its compounds on biological P removal were highly eliminated.2. When chemical-P recovery from supernatants of anaerobic stage in sidestream was conducted in SBR system, more than 31% of P was recovered in the influent, and good COD removal efficiency(85.70 ± 3.15%) was achieved. Meanwhile, the sludge settleability was improved and P content of the sludge also decreased. However, the efficiency of phosphorus removal was timing-dependent, which was improved from 95.26% to 97.22% in the short-term but decreased in the long-term. When chemical-P recovery from supernatants of anaerobic stage in sidestream was applied to the continuous flow process, more than 34.26% of P was recovered in the influent, and good COD removal efficiency(83.06 ± 2.24%) was achieved. The efficiency of phosphorus removal was improved from 93.81% to 96.14%. While the sludge settleability and the activity of EBPR deteriorated when chemical-P recovery terminated. It’s necessary to set appropriate deprivation ratio in consideration of both chemical-P recovery and the stability of EBPR performance during the process of chemical-P recovery in sidestream of the biological treatment system.3. The main effect of chemical-P recovery from supernatants of anaerobic stage in sidestream on the biological phosphorus removal system was due to the decreasing of intracellular polyphosphate in sludge, which increased glycogen consumption and poly-β-hydroxyvalerate(PHV) synthesis. The stoichiometric ratios of Prelease/HAcuptake decreased from 0.39 mol-P mol-C-1 to 0.10 mol-P mol-C-1, Glycogendegraded/HAcuptake and PHV/Poly-β-hydroxyalkanoates increased from 0.39 mol-C mol-C-1 to 1.22 mol-C mol-C-1 and 6% to 22%, respectively. The metabolic shifted from polyphosphate-accumulating metabolism to glycogen-accumulating metabolism. Polyphosphate-accumulating organisms(PAOs) declined to less than 1% of the population, while glycogen-accumulating organisms proliferated to almost 20% to get more carbon source. The microbial community structure and diversity changed remarkably, the proportions of β-Proteobacteria and γ-Proteobacteria were increased first and then decreased, but α-Proteobacteria strictly increased.4. There are three prerequisites when persistently conducting chemical-P recovery in sidestream in the biological treatment system. Firstly, the performance of biological phosphorus removal should be stable. Secondly, phosphorus recovery should be performed with an appropriate deprivation ratio. Moreover, the phosphorus recovery should be in intermittent operation. The chemical-P recovery performed in the system with deprivation ratio of 40%, inducing the enhancement of EBPR. More than 96% of P-removal efficiency in mainstream and 46.7% of P-recovery in sidestream can be achieved. The activity of the sludge declined during the process of phosphorus recovery. The sludge settleability variated with the components of extracellular polymeric substances, and sludge volume index(SVI) values decreased first and then increased. The bacterial community structure changed significantly. The population of PAOs decreased but the activity was reinforced. In the recovery periods, PAOs had a competitive advantage and proliferated rapidly. |
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