Novel Fibrous Fillers In Biofilm Process For Wastewater Treatment

The excessive emissions of nitrogen and phosphorous compounds to waterbodies can give rise to eutrophication, oxygen depletion and even water source toxicity for life. Accordingly, the total nitrogen (TN) and total phosphorous (TP) removal from contaminated waters has become an imperative demand according to public health and environmental protection. Actually, the more stringent regulations for nitrogen and phosphorous removal from wastewater have been implemented in China. Without a doubt, it is a markedly hard challenge to reduce wastewater discharge and restore water environment for the society as a whole. Particularly, it is imperative for the sewage treatment plants to expand scale and upgrade technical level. Thus, the wide investigation has been carried out with regard to the new wastewater treatment materials and process and old system optimizing. Especially, the biofilm technology has attracted great attention in the wastewater treatment. Here, polyacrylonitrile-based activated carbon fiber (PAN-ACF) and activated carbon fiber felt (PAN-ACFF) are investigated as biofilm carriers in wastewater treatment process considering the characteristics of high specific surface area, good biocompatibility, and chemical stability.1. PAN-ACF40g with the specific surface area1200m2·g-1as the biofilm carrier was used to wastewater treatment in two small-scale tests for artificial sewage and organic industrial wastewater.(1) The influence of hydraulic retention time (HRT) and reflux ratio (R) on chemical oxygen demand (COD), ammonia nitrogen (NH4+-N), and TN removal was studied in the aerobic biofilm reactor for artificial sewage treatment. The evaluation on various HRT showed that the highest removal efficiencies of COD, NH4+-N, and TN was obtained at8h in contrast to the HRT of16,12,4, and2h. The removal efficiencies according to various R displayed that the removal of COD, NH4+-N, and TN reached maximum with the R of1:1compared to R of1:2,2:1, and4:1; and the average values were94.8%,98.0%, and62.2%, respectively; wherein the filler’s contribution power per unit mass was1.99(gCOD·d-1)·g-1,0.20(gNH4+-N·d-1)·g-1, and0.12(gTN·d-1)·g-1. In addition, the COD removal efficiencies at four influent levels was measured at HRT=8h and R=1:1and the results showed that the highest (6.43(g·d-1)·g-1) filler’s contribution power per unit mass was obtained at the influent COD of1478.3-1652.4mg·L-1.(2) In small-scale experiments for organic industrial wastewater treatment with the influent BOD/COD of0.3. The COD removal efficiencies were tested at different dissolved oxygen (DO) of3.0,4.0,5.0, and6.0mg·L-1; and HRT of20,15,10, and5h. The results showed that the COD removal efficiencies were higher than75%with13<HRT<19h and 5.0<DO<6.0mg·L-1. when HRT and DO was5h and6.0mg·L-1respectively, the influent COD concentration varied in1851.3-1947.3mg-L"1with the highest average COD removal rate75.3%and the filler’s contribution power per unit mass was6.1(gCOD·d-1)·g-1.(3) To sum up, the small-scale tests indicated that the microbial populations in the PAN-ACF biofilm reactor were very rich. Besides, PAN-ACF filler greatly increased from60to463.6g after hanging biofilm. The dried fillers can be reused again as biofilm carriers.2. The pilot-scale experiments for sewage treatment in MUCT biofilm reactor.(1) A complex fiber filler (CFF) carrier was produced with the characteristics of specific surface area1450±100m2·g-1, dry weight70g·m-1, and sized Φ5×50cm. The CFF was composed of an outer-coat made by polypropylene net-cloth with loops (ultra-fine polypropylene and terylene fiber) on outer surface and an inner-core made by PAN-ACFF.(2) A step-feeding pilot-scale modified University of Cape Town process (MUCT) included pre-anoxic/anaerobic/anoxic-l/anoxic-2/oxic tank. The total volume of the MUCT biofilm reactor was0.54m3, whereas the volume ratio of Vpre-anoxic:Vanaerobic:Vanoxic:aerobic was1:1:2:5. Three reflux systems were included as follows:nitrification liquor recycle (R1), sludge return (R2) and nitrosation liquor recycle (R3). The mixed liquor suspend solid (MLSS) was2.5±0.5g·L-1in the anoxic and anaerobic tanks, and1.5±0.5g-L"1in the oxic tanks. Each tank was filled with the CFF fixed-bed with the total filler mass3.36kg and the dosing ratio of Tpreanoxic:Tanaerobic:Tanoxic:Taerobic was2:2:4:16. (3) The inlet and output concentrations of COD, NH4+-N, NO3-N, NO2-N, and TP were tested by three kinds of working conditions, which included: HRT:12,8, and6h; the step-feeding ratios between pre-anoxic tank and anaerobic tank:1:9,2:8, and3:7; R1:200%,300%, and400%; R2=R3:20%,40%, and60%. The sludge residence time (SRT) of anaerobic/anoxic/aerobic/secondary settling tank were6,10,10, and7d, respectively. The influent loads were1384.8±80.4gCOD·d-1,113.3±8.9gNH4+-N·d-1, and25.1±2.5gTP·d-1. During working condition3, the removal rates of COD, NH4+-N, and TP were93.2±0.8%,87.4±2.1%, and90.2±3.7%, respectively, which is the highest among three working conditions. During the treatment, the effluent COD concentration achieved Grade1level-B of "Cities Sewage Treatment Plant Pollutant Discharge Standard"(GB18918-2002); the effluent NH4+-N concentration was close to Grade1level-B; and the effluent TP concentration reached the Grade2. As for the filler’s contribution power per unit mass, it was384.2±23.5(gCOD·d-1)·kg-1,29.5±2.6(gNH4+-N·d-1)·kg-1,6.8±0.9(gTP·d-1)·kg-1.(4) The NO3--N and NO2--N concentrations in the MUCT biofilm reactor were measured in3working conditions. The results showed that the influent NO3--N and NO2-N concentrations were low and the effluent concentrations were high. The highest accumulative rate was achieved in working condition1and lowest accumulative rate was in working condition3. The net removal rates of NH4+-N were31.4%,52.7%, and78.0%, respectively among three working conditions.(5) To sum up, the pilot-scale tests showed that the MUCT biofilm reactor performed well on pollutants removal with CFF which showed the advantage of easy cleaning for reuse and need no periodic backwashing. However, sludge emission was carried out regularly for all the tanks in order to obtain high biological phosphorus removal.3. The engineering experiments with the CFF as biofilm carrier. A new ecological technique "CFF+aeration+EM bacterial agent" was developed from a contaminated artificial lake treatment, by which the water quality was markedly promoted from Inferior Grade5standard to Grade5standard according to "Environmental Quality Standards for Surface Water"(GB3838-2002). More importantly, a large number of fish appeared and the project successfully passed the government test after one month.In summary, PAN-ACF and CFF as biomembrane carrier were first developed and successfully used in wastewater treatment apparatus. Furthermore, the novel MUCT biofilm reactor with combined advantages of activated sludge and biofilm method has displayed some potential in pollutants removal. However, further studies are also needed on the mechanism discussion and parameters optimization.