Biofilm Formation And Nitrogen Removal Performances Of Modified Polyethylene Suspended Carriers

The hydrophilicity and biocompatibility of the polyethylene (PE) carrier are poor. It has drawbacks in some aspects, such as slow speed of biofilm growth, low amount of biofilm and low activity of biofilm, which influences its wastewater treatment performance. To solve this problem, hydrophilic and nutrient-release modified PE carriers were prepared through mixture modification respectively. Then, modified carriers were used to accelerate the start-up of the moving bed biofilm reactor (MBBR) process, and also used to enhance the nitrogen removal in the sequencing batch moving bed biofilm reactor (SBMBBR).The introduction of hydrophilic groups could improve the hydrophilicity of the PE carrier. The contact angel of the PE carrier decreased from 94.3°to 79°,77.8°,76.6° by doping starch, doping diatomite, doping starch and diatomite, respectively. Meanwhile, slow dissolution of starch gave modified carriers the ability of nutrient-release. Results of the start-up experiment showed that, the average growth rates of polysaccharide content and protein content on the composite carrier were 1.8 times and 1.9 times as quick as those of the PE carrier respectively on the first 36 days under the same conditions. The contents of polysaccharide and protein for the composite carrier were 68.4 mg/L and 69.5 mg/L,67.8% and 47.9% higher than those for the PE carrier on day 60. The higher contents of polysaccharide and protein indicated better biological and celluar activity of biofilm. After biofilm grew mature, the amount of attached biofilm on composite carriers was 2.3 ± 0.3 g/L,89.3% higher than that on PE carriers, indicating that the composite carrier enriched microorganisms. SEM and FISH results showed that different types of microbial communities formed alternate or symbiotic system and that the number of nitrifying bacteria was higher on the composite carrier. All these advantages of the composite carrier in biofilm formation made its effluent COD and NH4+-N reach the national standard on day 20 first,12 days earlier than that of the PE carrier. Obviously, the use of the composite carrier shortened the start-up period of the MBBR process.The density of the modified carrier increased to 0.972 g/cm-3 and its contact angle decreased to 72.4°with the addition of multi-modifiers. NH4+ -N removal of the two SBMBBRs process were better than that of the SBR process. Effluent NH4+ -N of the modified carrier first reached the national standard on day 18 in SBMBBR2,2 days earlier than that of the PE carrier in SBMBBR1 and 8 days earlier than that in SBR respectively. SBMBBR2 showed the highest TN removal, SBMBBR1 showed moderate performance, and SBR showed poor performace in nitrogen removal in the steady phase. Average TN removal efficiency of SBMBBR2 was nearly 20% higher than that of SBR in the steady phase. NH4+-N removal rates of the two SBMBBRs were faster in one cycle phase. The spatial heterogeneity of biofilm created favourable environment for simultaneous nitrification and denitrification (SND), therefore SBMBBRs exhibited better TN removal. Moreover, the advantages of the modified carrier in biofilm formation made TN removal efficiency of SBMBBR2 20.2% higher than that of SBR at the end of the aerobic phase and 18.4% higher at the end of the second anoxic phase in one cycle phase, respectively.