| Extracellular polymeric substances(EPS) are general property of microorganisms in natural environments and responsible for the structural and functional integrity of microbial aggregates. Some general functions have been attributed to EPS such as the formation of gel-like network keeping the biofilm bacteria together, the protection of bacteria against noxious influences, and even the reservation of carbon or energy. The components of EPS have the properties o f flocculation, adsorption, hydrophilic, hydrophobic, biodegradable, and play a crucial role in the field of environmental engineering. In this work, two studies have been carried out based on the properties of EPS. The first part studied the heavy metal biosorption behavior and mechanism of a bioflocculant MBFGA1 secreted by Paenibacillus polymyxa GA1. The second part studied a foaming method of dewatered sludge at a moisture content of 80-85 % and the drying behavior of the sludge wit h appropriate foaming pretreatment.In the flocculating process, composite flocculant is an effective way to improve the flocculating activity. The composite of MBFGA1 and poly aluminum chloride(PAC) makes a significant sense on enhancing flocculating rate, reducing MBFGA1 dosage, and especially decreasing the residual labile monomeric aluminum, which is meaningful for controlling of the biotoxicity of PAC. Two quadratic polynomial models with the response variables of flocculating rate and labile monomeric aluminum concentration were established by response surface methodology, respectively. The optimal flocculating conditions were MBFGA1 at 109.37 mg/L, PAC at 81.87 mg/L, initial p H at 8.5, times duration of stir 72.5 min and ambient temperature at 24.3 °C. Fourier transform-infrared spectra(FTIR) and environmental scanning electronmicroscope(ESEM) analysis indicated that MBFGA1 holded a large number of functional groups, which had strong capacity on binding aluminum and reducing its residual level during coagulation-flocculation.In view of the biosorption behavior of residual aluminum, MBFGA1 was further applied to the treatment of heavy metal wastewater including Cu, Zn, Pb and Cd. The precipitation of the heavy metal hydroxide build a suspension which could be separated by the process of flocculation and sedimentation. The heavy metal hydroxide increased with the dosage of Na OH, and the variation of Zeta potential of the suspension exhibited three phases, including increasing phase, plateau phase and decreasing phase. According to the comparation of the three phases, the plateau phase is a reasonable stage for adding the MBFGA1. On one hand, MBFGA1 can improve the separation of the precipitation of the heavy metal hydroxide by flocculation process. On the other hand, MBFGA1 can play a role of biosorption for the dissolved ions of heavy metal. The combination of flocculation and biosorption of MBFGA1 notably improved the heavy metal removal efficiency.Comparing with the Cu, Zn and Cd, MBFGA1 has the best removal rate on the sample of Pb wastewater. When the dosage of MBFGA1 at 5000 μL, Pb removal rate and corresponding biosorption capacity reach up 95.9 % and 566.7 mg/g, repectively. The optimal operating conditions were Na OH 3957 μL,MBFGA1 1518 μL,stirring time 95 min. The values of Pb removal rateand total organic carbon(TOC) in verification test operated under optimal condition were greater than 98 % and lower than 62.6 mg/L, respectively. FTIR and X-ray diffraction(XRD) indicated that the main functional groups present on MBFGA1 were carboxyl, amine and hydroxyl, which played vital roles in biosorption of Pb by means of surface complex reaction. Meanwhile, the phosphate group from the fermentation broth of MBFGA1 improved the removal of dissolved ions of Pb by precipitation and sedimentation.Foaming pretreatment has long been recognized to promote drying materials wit h sticky and viscous behaviors. A novel approach, Ca O addition followed by appropriate mechanical whipping, was employed for the foaming of dewatered sludge at a moisture content of 80-85 %. In the convective drying, the foamed sludge at 0.70 g/mL had the best drying performance at any given temperature, which saved 35-41 % drying time for reaching 20% moisture content compared with the non-foamed sludge. Considering the maximum foaming efficiency, the optimal Ca O addition was found at 2.0 wt%. For a better understanding of the foaming mechanisms, the foamability of sludge processed with other pretreatment methods, including Na OH addition(0-3.0 wt%) and heating application(60-120 °C), were investigated while continuously whipping. Their recovered supernatant phases were characterized by p H, surface tension, soluble chemical oxygen demand(s COD), protein concentration, polysaccharide concentration and spectra of excitationemission matrices(EEM). These comparative studies indicated that the sludge foaming was mainly derived fro m the decreased surface tension by the surfactants and the promoted foam persistence by the protein derived compounds. Further, a comprehensive analysis of the sludge drying characteristics was performed including the surface moisture evaporation, the effective moisture diffusivity and the micromorphology of dried sludge. The results indicated that the drying advantages of foamed sludge were mainly attributed to the larger evaporation surface in a limited drying area and the more active moisture capillary movement through the liquid films, which resulted in longer constant evaporation rate periods and better effective moisture diffusivity, respectively. |
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