Characterization Of Extracellular Polyphosphate In The EBPR System

Extracellular polymeric substances (EPS) matrix of microorganisms have been receiving increasing interests in the field of wastewater treatments. Increasing evidence points to the important roles of extracellular polymeric substances (EPS) in the phosphorus removal in EBPR process, a significant level of phosphorus was found in the EPS at the end of the aerobic phase, but lack of in-depth understanding. Therefore, in this dissertation, polyphosphate, the major phosphorus form in EPS, was regarded as the key research object to explore the characteristic of phosphorus in EPS in-depth. The main contents and results are as follows:1. The characteristics and mechanisms of the interactions between humic acids (HA)(one of the main components of EPS) and two model polyP compounds (sodium tripolyphosphate, STPP; and sodium hexametaphosphate, SHMP) with various chain lengths have been investigated. First, the thermodynamic parameters of the binding between HA withpolyP could be calculated by isothermal titration calorimetry at different pHs. Then,31P nuclear magnetic resonance technique was employed here to prove the formation of complex structure between HA and polyP. To further characterize the interaction, spatial configuration changes of HA after binding with polyP were evaluated using laser lighting scattering analysis. Results show that a stable polyphosphate-HA complex would be formed through the noncovalent interactions, and hydrogen bond might be the main driving force for the binding process, which might be formed between the proton-accepting groups of polyP (P=O and P-O") and the oxygen containing functional groups in HA. Association between long-chain poly-P (SHMP) and HA might be more likely to occur than the association between short-chain polyP (STPP) and HA. This work proved the interaction between polyP and HA, which was one of the main components of EPS, suggesting potential interaction may exist between polyP and EPS. The polyP-accumulating ability of EPS was more than physical interception, the binding interaction between polyP and EPS may be the main driving force.2. High resolution imaging and multi-elemental analytical methods, in combination with chemical compositional analyses were applied to elucidate the characteristics of phosphorus substances in EBPR sludge. The elemental distribution and composition of both intracellular and extracellular phosphorus were first investigated using scanning transmission electron microscope coupled with energy dispersive X-ray spectrometer. Results showed that three different types of polyP were found, in which polyP granules were largely within the cytoplasm, dispersive polyP associated with cell membranes and extracellular polyP predominantly in EPS. A positive relationship of the distribution and density were observed between EPS and phosphorus. The extracellular phosphorus found in the EPS region was composed of short-chain polyP and calcium phosphate precipitates, in which polyP occupied the predominant proportion. This work applied an in situ visualization method to explore the complicated EBPR system, offering a new insight into the morphology and composition properties of EPS.3. A full-factorial central composite design used in response surface methodology (RSM) was chosen to evaluate the effects of substrate concentration, temperature, pH and aeration rate on the EPS contribution to phosphorus removal (the percentage of total phosphorus (TP) in EPS per TP in sludge, TPEPs/TPsiudge). Based on the RSM results, the interacting effects of these factors were elucidated, an empirical model representing the relationship between the percentage of TPEPs/TPsiudgc and independent variables was obtained based on the statistical tests, and this model was suitable for predicting the percentage of TPEPs/TPsludge, the results of the optimum actual values were as follows:substrate concentration 545.7 mg COD/L, temperature 23.6?, pH 7.58, aeration rate 0.079 m3/h while the response predicted was 6.89%.The influential priorities of the four factors were obtained by grey relational analysis method, the influence priority of the four factors were:aeration rate> temperature> substrate concentration> pH. This work provided useful information of the contribution of EPS to the phosphorus removal in the EBPR process.