| Drinking water works sludge(DWS) is an inevitable by-product produced from drinking water treatment plants. The most recognized feature of DWS lies in its local, easy and large availability. The management of DWS has become an increasingly important issue of concern to the public and it has been arranged into the country’s "12th five-year plan". Reuse of DWS should be an effective way to alleviate its environmental and social problems. In this work, experimental and theoretical investigation of the treatment of DWS with the assistance of ultrasound for improving its phosphorus and ammonium removal ability were conducted based on an overall understanding of the applications of DWS. The aim of this study is to explore the potential for reuse of DWS and to probe the feasibility of "controlling waste by waste". The main contents of this study are as follows:1. The acidification treatment of DWS with the ultrasound assistance and the discussion of intensification mechanism.This part mainly focuses on the optimization of operating conditions and the mechanisms of ultrasonic cavitation. The effects of acid concentration, ultrasonic treatment time, ultrasound power, agitation intensities, solid-liquid ratio and granularity on the recovery efficiencies of aluminum and ferric ions from DWS were examined and the optimum parameters were determined. Moreover, various methods including iodine releasing, metal foil etching and thermoelectric probe were used to detect the average intensity of cavitations in reactor. The results show that the acid concentration, ultrasound power, agitation intensities and the solid-liquid ratio are all important factors influencing wave attenuation. Less attenuation in reactor is obtained at the optimum conditions, leading to higher ultrasonic intensity and well wave propagation. It was justified by kinetics analysis that the surface chemical reaction, film diffusion and inertlayer diffusion of the recovering process were intensified by the effects of turbulence, spot energy and micro-disturbances generated from cavitation. Further analyses indicate that inter-layer diffusion is the controlling steps of recovering process without ultrasound and its rate constant increased greater than that of surface reaction or film diffusion under ultrasonic condition. It is concluded that, in coagulant recovering process, the use of ultrasonic can lead to inspiring results in the improvement of yield and kinetics.2. Acid coagulant-recovered DWS residual(DWSR) for phosphorus removal from wastewater.In view of insighting into the efficiencies and mechanisms of phosphorus removal, the properties of DWS, DWSR as well as the calcinated DWS(C-DWS) and DWSR(C-DWSR) were characterized by means of XRF, ICP, XRD, FT-IR, SEM, EDS and BET nitrogen adsorption methods. Results show that the overall contents of aluminum and iron in DWSR were significantly lower than that of DWS, but the proportion of oxalate-extractable aluminum and iron in DWSR is a little bit higher. Part of the amorphous aluminum and iron hydroxide in DWS converted into soluble aluminum and iron sulfate(DWSR). The contents of aluminum and iron were enriched slightly and some alumina/iron oxide formed in C-DWS and C-DWSR.To verify the efficiencies of DWS, C-DWS, DWSR and C-DWSR on the phosphorus removal in wastewater, a series of batch tests were carried out at different pH, initial concentration, sludge dose, adsorption time and temperature. The results show that the phosphorus removal capacity of DWSR and C-DWSR is more dependent on pH and much higher than that of DWS and C-DWS at an optimum pH. Futher semi-continuous tests indicate that competitive ions in wastewater have insignificant effects on phosphorus removal by DWSR and that a neutral pH level of effluent is achieved due to the buffering capacity of actual wastewater.Five kinds of adsorption isotherm models are adopted to fit the adsorption equilibrium data for DWS, C-DWS and C-DWSR. The correlation coefficient(R2) values are highest for the Langmuir plots. The Freundlich, D-R, and Temkin plots also give high R2 values. The values of constant BT of Temkin model reflect the different adsorption capacity among DWS, C-DWS and C-DWSR. The mean adsorption energy E of C-DWSR is about 70% of that of DWS and C-DWS, indicating a different adsorption mechanism. Different models of adsorption are used to describe kinetic data and calculate rate constants. Results indicate that pseudo second-order kinetic model describes phosphorus adsorption better for all tested sludge samples than pseudo first-order kinetic model and the rate constant k2 of DWSR is 20 times bigger than that of DWS. Further analyses also implies involvement of film and inter-particle diffusion mechanisms as rate controlling steps at lower and higher concentrations, respectively, and film diffusion having a much higher impact on DWSR than on DWS. Thermodynamic analysis results reveal that the phosphorus removal by DWS or C-DWS is a spontaneous and endothermic process with increased entropy and that it is a spontaneous and exothermic process with increased entropy for DWSR or C-DWSR.To further examine the possible mechanisms of phosphorus removal by sludge samples, the metal leaching test and the measurement of phosphorus content in sludge particles before and after adsorption and in flocks formed in water and phosphorus aqueous were performed, as well as the FT-IR spectra of the sludge samples before and after adsorption for the purpose to explore the identity of the phosphate impregnated sludge. The decrease of phosphate concentration in solution accompanied with an increase in pH indicates that ligand exchange between hydroxyl on the surface of sludge and phosphate in solution is the dominating mechanism for DWS and C-DWS. The simultaneous decreases in phosphate and aluminum concentration in solution imply that precipitation rather than adsorption play a crucial role in phosphorus removal by DWSR and C-DWSR. The results of this study show that DWSR is effective for phosphorus removal from wastewater based on its aluminum and ferric ions.3. Preparation and characterization of ammonium adsorbent derived from DWS.The work included the preparation methods of ammonium adsorbents and the characterization of their surface properties. A simple and efficient approach learned from a conventional hydrothermal method was proposed. It was mainly composed of extraction and synthesis process with the assistance of ultrasonic. According to different processes, we prepared sample M-DWS1# and M-DWS2#, both of which have much higher ammonium removal capacity than that of DWS. The process of M-DWS2# lies in its higher yield and less waste discharge compared with that of M-DWS1#. The properties of DWS and M-DWS were characterized by means of XRF, ICP, XRD, FT-IR, SEM and EDS. The BET specific surface area(SSA), zeta potential, iso-electric point(IEP) and cation exchange capacity(CEC) of sludge samples were also determined. It was found that most of the lamellar structures of DWS were converted into the small particles of M-DWS2# and into the spherical units with 30 nm diameter in M-DWS2#, respectively. The CEC and SSA of M-DWS were many times higher than that of DWS, and the IEP of M-DWS decreased compared with that of DWS. More siloxane surface and micro-pore structure formed in M-DWS should be contribution to the increase in SSA. The decrease in IEP can be ascribed to the large amounts of broken bonds(Si-O-) generated at the external surface of oxides and to the isomorphic substitutions of Al3+ for Si4+ in the silicon(aluminum)-oxygen tetrahedral sheet.4. The performance of ammonium adsorption of M-DWS1# and M-DWS2# and the discussion of their adsorption mechanisms.Batch tests were conducted to examine the effects of initial pH, initial concentration, sludge dose, adsorption time and temperature on ammonium removal efficiencies of M-DWS1# and M-DWS2#. It was found that the initial pH affected ammonium adsorption on M-DWS1# and M-DWS2# greatly. Under an optimum pH of 7-8, the highest ammonium removal rate of 90% for M-DWS1# and 80% for M-DWS2# were achieved at an initial concentration of 50 mg/L. Data obtained from batch studies were also applied to commonly used adsorption isotherm models. Langmuir isotherm model is found to be better fitted with M-DWS1#, while, Frendlich isotherm fits well to M-DWS2#. Further analysis indicates that the maximum adsorption capacity of M-DWS1# and M-DWS2# evaluated from the Langmuir isotherm is 6.11 mg/g and 5.10 mg/g, respectively, and that their mean adsorption energy E values are in the range of 8-16kJ/mol, inferring ion exchange adsorption for ammonium removal. Adsorption kinetic data of M-DWS1# and M-DWS2# were best described by pseudo second-order kinetic model and inter-particle diffusion mechanisms is the rate controlling step. It is speculated that ammonium removal by M-DWS1# and M-DWS2# is spontaneous, exothermic physical adsorption of increased entropy based on their thermodynamics parameters. It is point out according to the result of zeta potential, FT-IR and EDS that both ion exchange and molecule adsorption are the adsorption mechanisms of ammonium onto M-DWS1# and M-DWS2# under different pH conditions. Interestingly, M-DWS2# can remove both ammonium and phosphate from wastewater efficiently. |
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