| Coal is one of the most important energy in China which has a significant influence on our social and economic development. However, coal mining and utilization process caused a serious harmness to local water. Based on pollution investigation and monitoring of4water samples in Shaanbei Shenmu coal mine, the water quality in Shaanbei coal mine were evaluted by methods of single factor water quality identification index and comprehensive water quality identification index. The results showed:coal powder pollution was much serious and color suspended solid (SS), COD, NH4+-N,NO3-,SO42-,Cr6+,Cu2+, Fe3+, Mn2+, Pb. Cd elements in waters were much more than the reference sample, especially heavy metals Cr. Cd. COD and TP were high enough to pollute envrionmental seriously. The water quality near mud caused by coal gangue in Ludaogou was the worst. Its pollutants exceeding the standard were SS. PO43-, COD, Cr, Cd and its comprehensive water quality identification index was3.561.According to the characteristics of polluted water in coal mining area and the advantage of Shaanbei with sufficient sunlight, symbiotic algae-bacteria biofilm system was employed to remediate this polluted water. The carriers for attachment of microorganisms and algae included spherical packing and elastic three-dimensional packing. Cr (Ⅵ) was hired as a representative of diverse heavy metals. To simulate natural light, three fluorescent tubes were added on the oxidation ponds with a light:dark than14:10. The removal efficiency and removing mechanism of Cr(Ⅵ), COD, NH4+-N. TP, SO4-2-by symbiotic algae-bacteria biofilm were examined. The results was presented as following:(1) Six Cr-tolerant strains were isolated from the active sludge in a oxidation ditch to test their cellular growth mass in different Cr(Ⅵ) concentration liquid medium after24h,48h and72h growth. A high Cr-tolerant strain was choosed from them to removal Cr(Ⅵ) in wastewater. The cellular mass of six strain were decreased with increasing initial Cr(Ⅵ) concentration in most cases. The high Cr-tolerant strain had a maximum Cr(Ⅵ) removal efficiency(76.4%) in a best concentration range (about654mg/L) and the highest Cr(Ⅵ) removal rate was18.9mg/(gSS·h) after24h growth. A higher ratio of biomass to amount of Cr(Ⅵ) was benefit to improve the removal of Cr(Ⅵ) when the Cr(Ⅵ) concentration was lower. However, when the Cr(Ⅵ) concentration was higher, it did not help much for improve the removal of Cr(Ⅵ). All of them presented excellent bioaccumulation abilities, however, it seemed that the resistance and the removal potential of heavy metal had no direct connection.(2) When HRT was14.3d and carrier was spheric packing, the algae-bacteria biofilm had a better removal efficiency of98.3%and was tolerant to21.2mg/L Cr(Ⅵ) with a removal potential of88.2%. Cr(Ⅵ) content reacheed286μg/g (dry weight mud) in the sediment of influent side. Cr(Ⅵ) was reduced to Cr(Ⅲ) by microbes. Cr(Ⅲ) turned into Cr(OH)3and then precipitated into sediment. The bioaccumulation potential of Cr(Ⅵ) by Spirogyra (12.08μg/g dry weight) is far less than Eichhornia crassipes's (79.41μg/g dry weight).(3) When the concentration of COD in influent was100mg/L. the COD removal efficiency was beyond80%. In oxidation pond the heterotrophic bacteria and algae cooperated with each other to reduce heavy metal and COD. The best removal efficiency of NH4+-N was65.3%with a6mg/L influent concentration. Nitrogen was removed by assimilation anabolism and nitrification. The influent TP concentration was1.7mg/L of which over60%was reduced by assimilation and precipitation.(4) When the inflow COD was50mg/L, the dissolved oxygen(DO) was3.5mg/L in reactor for photosynthetic oxygenation by algae which was unsuitable for the reduction of SO42-. After increasing the COD concentration to100mg/L, the diurnal average DO was1.8mg/L and minor night DO was0.37mg/L in which sulfate reduction could take place. The max removal was65.5%with a70.7mg/Linfluent sulfate.(5) Algae-bacteria biofilm system went through three periods from start-up to stable operation:hydrophyte stage, Eichhornia crassipe stage and Spirogyra biofilm stage. Eichhomia crassipe could inhibit the formation of Spirogyra biofilm. Green algae grew better in Eichhornia crassipe stage than Spirogyra biofilm.When Eichhornia crassipe was taken out from the reactor Spirogyra began to attach to carriers. Spirogyra prefer to immobilize on extracellular polymeric substances (ESP) excretived by bacteria. The length of Spirogyra filament had a good linear relationship with time(d):y=48.77x.(6) Illumination and depth of water were important to algae amount, whereas concentration of COD and Cr(Ⅵ) were key factors for bacterial growth. Addition of the tridimensional elastic carrier could increase the algae amount of upper reactor and the bacteria concentration both upper and lower reactor which was helpful to strengthen anoxic and anaerobic environment for better denitrification and sulfate reduction.(7) A shorter hydraulic retention time (HRT) had little effect on Cr(Ⅵ) removal. For a7.2h HRT, Cr (Ⅵ) removal rate could run up to2.4mg/(L·d). The system had reached the maximum COD removal capability of12.5mg/(L·d). Less HRT could impact nitrogen removal potential and was destructive to TP removal unless with a longer HRT than14.3d, however, sulfate-reducing bacteria(SRB) could still survive under a Cr(Ⅵ) loading rate of3.1mg/(L·d) and remove7.5mg sulfate per litre within a day.(8)The dosage of Cr-resistance strains had a impermanency improvement for increasing Cr(Ⅵ) removal efficiency. It could be used in a hazardous waste pollution event rather than in a continuous water treatment svstem.
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