| The experimental micro-polluted raw water was from Foshan Dongping river with the concentration of ammonia nitrogen(NH3-N) 1.5mg/L to 2.5mg/L. The effects of hydraulic retention time(HRT), the ratio of air to water, shocking load, other factors etc on NH3-N removal rate were discussed to determine the optimal parameters with suspended filler as biological carrier and iron oxide coated sand(IOCS) as filter. Using SEM and other analysis methods, the removal efficiency of biological pretreatment coupling modified sands to micropollutant NH3-N was investigated. The results are shown as below:At the bio film formation stage in the process of biological pretreatment:after 7d for biofilm formation, a yellow brown biofilm adhered to the surface of suspended fillers. The main organisms were zoogloea, and the biomass was 70 nmolP/g. Under the condition of continuous aeration, when the air-water ratios were 0.5:1 and 1:1, the removal rate to NH3-N were 75% and 66% separately. But intermittent aeration was more conducive to the removal of ammonia nitrogen. The optimal operation parameters were that:the ratio of rest time to aeration time was 3:1, the removal rate to NH3-N was 81.41%. The temperature of the raw water was 25～28 ℃, and NH3-N can be removed more then 80% stably. HRT was 1h, the removal rate was 65%～70%.At the stable operation stage in the process of biological pretreatment:the suspended filler attached microorganisms were of physical adsorption and bio-chemical actions, and the removal rate to NH3-N was 80%-90%. When the concentration of NH3-N in the raw water was more than 2mg/L and HRT was 1h, nitrite nitrogen was accumulated to 0.15～0.25mg/L which met the drinking water quality standard. Zeta potential peak value of raw water was-21.0mV, and biological pretreatment can reduce the absolute value of Zeta potential to 15.5mV effectively. Bio-floc which falled off at the pretreatment stage had obvious flocculation effect, moreover the number of particles in effluent decreased by 26.8% than influent’s.After biological pretreatment, biofilm on modified sands was in the stable phase on the fourth day. The removal rate of B-IOCS to NH3-N was 80%-88%. But biofilm formation on modified sand (IOCS) needed 7d without biological pretreatment, and the removal rate of ammonia nitrogen was 77%-84%. DO in the biological raw quartz sands (B-RQS) filter was reduced to 0.97mg/L, while DO in B-IOCS filter was reduced to 2.12mg/L. Therefore on the IOCS surface, the biological adsorption was firm, and the biochemical effects was strong. After biological pretreatment coupling modified sand, the removal rate of B-IOCS to ammonia nitrogen was 90%. The particle size in the effluent was 1-10nm, and turbidity was below 0.5NTU.When the filtration rate was 4m/h, biofilter had the highest removal rate to ammonia nitrogen. When the NH3-N of influent for 0.47-0.74mg/L, the NH3-N contained in B-RQS effluent can not meet the standard of drinking water in the first beginning 7 days, and the removal rate to NH3-N was 77.3%. The average removal rate of B-IOCS filter to NH3-N reached 84.67%. The removal effect of B-RQS to nitrite nitrogen is poor, and the average concentration in the effluent is about 0.18mg/L, but the concentration of nitrite nitrogen in the B-IOCS effluent was 0.006mg/L. The denitrification effect of B-IOCS was obvious, the average concentration of nitrate nitrogen in effluent was 4.17mg/L, while the B-RQS’s was 3.85mg/L. Biological pretreatment coupling B-IOCS to the wide range of ammonia nitrogen concentration had strong adaptability, and the removal rate to high NH3-N concentration was 65%.The surface of RQS was smooth. After biofilm formation, its surface structure was still of less porosity and roughness. Microorganisms were easy to be lost, and the removal rate to NH3-N was low. But the IOCS had a larger specific surface, and its adsorption performance was better. The biofilm on the surface of IOCS was firm and even comparatively, and biological action was obvious, so the removal rate to NH3-N was 90.83%. |
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