Effect Of C/N Ratio And Ca²⁺ On Biofilm And Component Of Extracellular Polymeric Substances

Due to the special rock types and calcium-rich geochemical background of karst regions, the karst water contains a lot of Ca2+. In most parts of northern and southern limestone areas of China, the hardness of drinking water generally exceeds the sanitary standard for drinking water. In industry, it is usually adding Ca(OH)2to adjust the pH as a result that the wastewater contains large amounts of calcium. However, since Ca2+is not toxic, its impact on biological treatment of water is often ignored as a conventional metal ion, and therefore it has not been adequately studied. Biofilm process is a method which use organized groups of microorganisms attached on the surface of support (biofilm) to treat wastewater. As a major component of biofilm, extracellular polymeric substances (EPS) can not only enhance the ability of microbial cells to resist the external environment, but also a key factor affecting the stability of the biofilm. Therefore, it is necessary to study the effects of Ca2+on biofilm and its EPS. Carbon and nitrogen are the main constituent elements of carbohydrates and proteins in EPS, respectively. The ratio of carbon and nitrogen (C/N) would affect the component of EPS, thus it would affect the interaction between EPS and Ca2+or the interaction between biofilm and Ca2+. Therefore, it is necessary to study the effects of C/N ratio on the component of EPS. However, the effects of C/N ratio on the component of EPS and the effect of Ca2+on biofilm and its EPS in wastewater treatment are rarely reported.In this paper, the effect of C/N ratio on the degradation effect of biofilm and the component of EPS was studied by building a sequencing batch biofilm reactor (SBBR) to culture biofilm. And under a more suitable C/N ratio condition, the effects of Ca2+on the morphological structure, the degradation effect of biofilm and the component of EPS were investigated. The results showed that:(1) For the effects of C/N ratio on the degradation effect of biofilm and the component of EPS, the biofilm was cultured under different C/N ratio condition through adding the carbon sources (sucrose) and nitrogen sources (ammonia). The removal efficiency of COD and ammonia nitrogen was discussed. While the regularity of C/N on component of EPS was studied using three carriers (PC board, PP ring and aldehyde PVA). Results suggested that:When the C/N (mass ratio) reduced from8to2.67by adding ammonia, the removal efficiency of COD was maintained at around90%, the degradation efficiency of ammonia declined; When the C/N (mass ratio) increased from8to24by adding sucrose, the removal efficiency of COD declined, the degradation efficiency of ammonia changed unstably. For both cases, the amount of EPS and protein in biofilm reduced gradually with increasing C/N ratio; Adding ammonia could increase the amount of protein in EPS; Adding sucrose could increase the amount of polysaccharide and also reduce the ratio of protein and polysaccharide (PN/PS) in EPS.(2) For the effects of Ca2+on the morphological structure and the degradation effect of biofilm, biofilm was cultured through adding Ca2+to the reactor under the condition of the C/N (mass ratio) of8. The morphological structure of biofilm was observed using optical microscope and scanning electron microscope. The removal efficiency of COD, ammonia nitrogen, total nitrogen and total phosphorus were discussed in different concentration of Ca2+. Results suggested that:A moderate increase of Ca2+concentration can promote microbial attachment and biofilm formation. It also can improve the structural integrity of biofilm and increase in internal holes and channels of biofilm. Continuously increasing a small portion of Ca2+(1to2mg/L) could facilitate the removal of organic compound during the initial period of biofilm formation (about1Od). However, the removal efficiency of ammonia nitrogen would be weakened as the concentration of Ca2+exceeds120mg/L; the removal of organic compound and total nitrogen would be reduced significantly when the concentration of Ca2+exceeds280mg/L. This suggests that a higher concentration of Ca2+would inhibit the activity and degradation of biofilm. The removal efficiency of total phosphorus would be the lowest when the concentration of Ca2+is120mg/L and the removal efficiency would be increase with the concentration of Ca2+continue to increase due to the precipitation generated with Ca2+and phosphate.(3) For the effects of Ca2+on the component of biofilm and its EPS, EPS was extracted from biofilm under different concentration of Ca2+, and protein and polysaccharide in EPS were determined. The content of Ca2+in biofilm was also analysis to explore the role of Ca2+on biofilm and its EPS. Results suggested that: Ca2+could not promote the secretion of EPS while it could increase the dry weight of the biofilm. When the concentration of Ca2+is too high, the secretion of EPS in biofilm would be reduced. And the enrichment of Ca2+in biofilm and its EPS reduced. Thus the quality of biofilm had also declined. The analysis of Fourier transform infrared spectroscopy (FTIR) showed that the effect of Ca2+on functional groups in biofilm is not significant. Furthermore, moderate concentration of Ca2+would also be increase the mechanical strength of the biofilm.