The New Mechanism Of Phosphorus Removal In The EBPR Systems

Enhanced biological phosphorus removal (EBPR) technology has been widely used for municipal wastewater treatment, and phosphorus-accumulating organisms (PAOs) were considered to play the key roles in this process. The biosorption of phosphorus by microbial extracellular polymeric substances (EPS) and biologically induced phosphate precipitation process are considered to influence significantly the phosphorus removal in EBPR process, while the mechanisms are still unclear. Therefore, in this dissertation, the crucial roles of the microbial EPS and biologically induced phosphate precipitation in phosphorus removal in EBPR process were explored in-depth. The metabolic shift of PAOs for treatment of wastewater with high calcium content was also investigated. The main contents and results are as follows:1. The species of extracellular phosphorus in EBPR sludge were charactersitics using 31P nuclear magnetic resonance (NMR) spectroscopy. Six extraction methods (e.g, Cation exchange resin, sonication, EDTA, heating, formaldehyde/NaOH and centrifugation) used for EPS extraction from EBPR sludge were compared according to the comprehensive analysis of the content and species of phosphorus in EPS and transformation of phosphorus species in extraction process. Results show that the extraction methods had a substantial influence on the species of phosphorus and their contents in the extracted EPS. Three main species of phosphorus including orthophosphate, pyrophosphate and polyphosphate identified by 31P NMR spectroscopy were found to be present in the EPS, which together accounted for about 7-10% of the total phosphorus in the activated sludge. Cation exchange resin method was found to be more appropriate for EPS extraction and the subsequent characterization of phosphorus in the EPS of EBPR sludge.2. The dynamic changes of the contents and species of intracelullar and extracellular phosphorus in the anaerobic-aerobic process were explored by 31P NMR spectroscopy, in order to elucidate the possible roles of EPS in phosphorus removal in the EBPR process. Results show that the intracellular polyphosphate (polyP) extracted by cold perchloric acid was confirmed to be involved in the cyclic degradation and synthesis in the EBPR process, and their average chain lengths were reduced to about 30 in the anaerobic phase and recovered to about 44 under aerobic conditions. However, the polyP in EPS had a much shorter chain length (about 7-10). High phosphorus content in EPS at the end of aerobic phase was found to contribute about 5-9% to phosphorus removal in the EBPR process. Based on these results, a modified EBPR metabolic model with the recognition of EPS contribution to phosphorus removal was proposed.3. The dynamic process and mechanism of biologically induced phosphate precipitation in the EBPR granular sludge systems were explored. Large amounts of phosphate precipitates were found to accumulate in the granular sludge, evidenced by scanning electron microscopy coupled with energy dispersive X-ray (SEM-EDX). Combineded with the analyses of X-ray diffraction, X-ray photoelectron spectroscopy and phosphorus L2,3-edge X-ray absorption near-edge structure spectroscopy, hydroxyapatite, amorphous calcium phosphate, brushite and struvite were found to be present in the precipitates with various fractions. Results indicate that phosphate precipitation occurred more easily within the granules, and the threshold of precipitation in the granules was considered to be about supersaturation index greater than-0.4 in the bulk. Magnesium was proven to strongly affect the evolution of amorphous calcium phosphate in EBPR reactor, and accordingly influence the phosphate precipitation. The backscattered SEM-EDX analysis shows that EPS also can influence the phosphate precipitation process in the EBPR granular sludge. The contribution of the biologically induced precipitation process to phosphorus removal was estimated to be above 15% at an influent calcium of 28 mg/L. The phosphate precipitates in the granules accounted for about 9.6% of total phosphorus in the granlues.4. The effects of calcium on the metabolic pathway of PAOs were explored. When the influent calcium concentration increased, the tendency and extent of extracellular Ca-phosphate precipitates were elevated, and large amount of intracellular inert Ca-polyP were synthesized. During this process, the microbial population remained almost stable.,The ratio of phosphorus released and acetate uptaken, the glycogen degraded per acetate uptaken and the PHA synthesized per acetate uptaken in the anaerobic period confirmed that the metabolic pathway of PAOs was shifted from the PAO metabolism to the GAO metabolism as the influent calcium concentration increased. The mechanism of the change of PAO metabolic pathway at a high calcium concentration was proposed.