Preparation Of Carbon Releasing Material And Study On Its Nitrogen Removal Efficiency As Filler Of Biological Aerated Filter

The excessive emissions of nitrogen brought increasingly serious environmental problems. National "12th Five-Year Plan" adds ammonia nitrogen as a new controlled indicator and the goal is emissions reduction of10%. With increasingly stringent emission standards, sewage treatment plants face enormous challenges of the nitrogen removal capacity, and is often exposed the problem of backward technology and lack of carbon source for denitrification. Because of the advantages of low energy consumption, low land use and high treatment efficiency, Biological Aerated Filter (BAF) attracted widespread attention. Extra carbon source need to be dosed as electron donors in biological denitrification process. Overdose of the traditional carbon source may impact effluent quality, so the process requires precise control dosing and costs high. Carbon-releasing process of cellulose substances cannot be effectively controlled, and it takes a long hydraulic retention time for denitrification. It’s a new focus that a new kind of carbon releasing material (CRM) is developed, not only as carbon source for denitrification, but also as a biological carrier.This thesis focuses on lack of carbon source for denitrification. Combined biological aerated filter process, a new kind of carbon releasing material is prepared, both as filler in BAF and providing carbon source for denitrification. First, starch was fixed to two kinds of carrier, polyvinyl alcohol (PVA) and sodium alginate (SA), by immobilization technique. Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used for analysis of their physical and chemical characteristics. Moreover, their COD releasing abilities and effects on denitrification were investigated. Then response surface methodology (RSM) was used to optimize the raw material recipe of better CRM. At last, nitrogen removal efficiency of BAF with the CRM prepared in accordance with the optimum mix and the influence of operation parameters on nitrogen removal efficiency was investigated. The main conclusions are as follows:(1) Preparation and evaluation of carbon releasing material with PVA and SATwo kinds of CRM containing starch were prepared with PVA and SA by immobilization technique. FTIR and SEM analysis indicated that starch was mixed better with PVA than with SA. Carbon-releasing process of PVA material entrapped starch follows second-order kinetic equation and correlation coefficient was0.9978. Mass transfer coefficient K was8.43mg/(h·g·L). The saturated COD released from per gram CRM with PVA was99.60mg/L. Under the conditions of temperature of18～22℃, initial NO3-N concentration of35mg/L and pH of7.2-8.0, the average denitrification dynamics rate was equal to18.5g/(m3·d) of CRM with PVA and15.5g/(m3·d) of CRM with SA. In summary, CRM with PVA performed better.(2) Formulation optimization of raw materials used to prepare CRM with PVAThe impact of different concentrations of raw materials on the performance of carbon release was investigated. Then the formulation of raw materials used to prepare CRM with PVA was optimized by RSM evaluation. According to ANOVA of the response surface quadratic model, the terms of X1, X3,X1X2, X2X3, X12, X22, X32had significant effects on mass transfer coefficient K of carbon-releasing process. X1was concentration of PVA; X2was concentration of starch; X3was concentration of SA. The minimum value of K was3.89mg/(h·g·L), calculated by Design-Expert software. At this point, concentration of PVA was4.31%, concentration of starch was10.16%, and concentration of SA was0.59%. (3)Study on nitrogen removal of the CRM with PVA as filler of BAFNitrogen removal efficiency of two-stage BAF that the first with ceramic and the second with the CRM with PVA was investigated. Besides, the influence of operation parameters such as HRT and gas-water ratio of the first stage, were studied. Under the conditions of temperature of19.6～24.6℃, influent pH of7.2～8.0,COD of174.67-214.67mg/L and NH4+-N concentration of28.84～39.32mg/L, the results showed that the system performed best, when HRT and gas-water ratio of the first stage was8h and7:1, HRT of the second stage was12h. At this point, the average effluent NH4+-N concentration was7.54mg/L; the average effluent TN concentration was19.61mg/L.