| Along with the rapid development of the society and industry, Fe3+ and Mn2+ coud be present in an increasing number of water body. Recently, the possible effects of Fe and Mn on sludge characteristics have been considered important and investigated by many researchers, but their role in the biofilm system is not well understood. Biofilm processes are attractive in wastewater treatment with their complex microecosystem, high organic loading rates, and strong anti-shock loading capabilities. Extracellular polymeric substances (EPS), the representative components of microbial aggregates, are responsible for maintaining the integrity of the biofilm structure and functions. The existence of metal cations with various concentrations may stimulate the microorganisms to create a stress reaction rapidly changing the EPS characteristics and the biofilm micromorphology, which ultimately influences the stability of the biofilm structure as well as mass transfer in biofilm matrix. Therefore, the effects of Fe3+ and Mn2+ on the biofilm micromorphology and activity as well as the EPS composition and characteristics in biofilm were investigated in self-designed fixed bed biofilm reactor (FBBR). The mechanism of action of Fe3+ and Mn2+ with biofilm and EPS was explored by applying scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The following conclusions could be drawn from the results:(1) The biofilm accretion was conducted under the intermittent operation mode in the FBBR. The monitoring results of pH, DO, CODcr and NH3-N suggested that the FBBR has successfully started-up in about 10 days, and the reactor kept stable operation. The pH and DO reached 6.8-7.8 and 2-6 mg/L respectively, CODcr and NH3-N removal rates could be stabilized at more than 80% and 75% respectively. The microscopic examination showed that the biological species in biofilm system is rich, indicating the excellent operational performance of the FBBR.(2) Iron ions influence biofilm development and activity not only by directly impacting the microbial physiology but by indirectly affecting the EPS constituents in biofilm. The result indicated that 2 mg/L of Fe3+ promoted the biofilm mass, but 16 mg/L of Fe3+ affected the biofilm development and biofilm activity adversely. SEM observation indicated 16 mg/L of Fe3+ led to significant reduction of the filaments, great stimulation of the EPS secretion in biofilm. The result of the EPS composition suggested 2 mg/L of Fe3+ increased soluble EPS and loosely bound EPS which contributed to the microbial aggregation, while 16 mg/L of Fe3+ stimulated TB-EPS as a protective shield for microbes but imposes a steric hindrance in biofilm. FTIR and XPS analysis demonstrated that Fe3+ exerted a significant influence on the -CONH-groups of protein (PN), the C-O groups of polysaccharide (PS) and the phosphate groups of nucleic acids in EPS. Furthermore, Fe3+ may bridge with the carbon or oxygen atom in the functional groups of EPS in biofilm.(3) The Mn2+ augmentation contributes to the biofilm formation, and the compactness of the biofilm structure with the biofilm development, which could physico-chemically function in following ways:1) Mn is an essential nutrient for microorganisms; 2) Mn2+ combines with EPS to form cross-linkage; 3) and Mn compound acts as both core and carrier. Continuously dosing 10 mg/L of Mn2+ showed little effect on the removal of both CODcr and NH3-N which reached 85.4±1.5% and 76.3±1.9% respectively in an operating cycle in the biofilm maturation period. Additionally, with the different types of EPS contents, the production of TB-EPS can protect microorganisms against the adverse influences. Attention, Mn2+ not only promoted the biofilm maturation also enhanced the stabilization of the organic matter removal. Furthermore, Mn2+ may bridge with the oxygen atom of O=C and C-O in the functional groups of EPS in biofilm. |
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