Nitrogen Removal In Oligotrophic Reservoir Water By Mixed Aerobic Denitrifying Consortiums And Immobilized Denitrification

Nitrogen has been identified as a major pollutant of concern in drinking source water.The excess nitrogen into reservoir waters not only causes eutrophication,but also poses a great threat to water security.The confirmation of aerobic denitrification in oligotrophic reservoir suggests that it should be possible to use aerobic denitrifying strains to remove nitrogen and improve water quality.Regarding the nitrogen removel of oligotrophic source water as the breakthrough point,the aerobic denitrification technology and in-situ water monitoring experiment have been used to study:（1）The sources and seasonal variations of nitrogen pollution in Lijiahe Reservoir,and the actual impact on the reservoir water quality;（2）The denitrification and decarbonization characteristics of the two kinds of mixed aerobic denitrifying consortiums in oligotrophic source water as well as the influencing factors;（3）The optimal immobilized time of the mixed consortiums by polyurethane foam as biomass carrier and its denitrification performance and reusability in source water.The main results and conclusions were showed in the following:（1）During the one-year monitoring period,inflow runoff was the main source of nitrogen and phosphorus pollutants in Lijiahe Reservoir.The contribution rates of total nitrogen（TN）,total phosphorus（TP）and permanganate index in the upstream water to the total pollution load were respectively up to 99.41%,99.51%,99.55%.The average concentration of TN in each water layer of the main reservoir area was all above 2.0mg/L,which exceeded surface water Grade III water standard（1.0mg/L）,and the primary nitrogen was nitrate（NO₃^--N）;the average concentration of TP was 0.01⁰.18mg/L,over-standard in autumn and winter.Nitrogen excess,which was a major cause of eutrophication,has induced the massive cyanobacterial blooms with maximum biomass concentration of 2.95×10⁸ cells/L in summer,seriously damaging the ecological balance of the reservoir.It is of great significance to seek effective bioremediation to remove nitrogen and improve reservoir water quality.（2）The growth and denitrification characteristics of natural consortium A and artificial consortium B which were respectively isolated from the source water and sediment of Zhoucun reservoir were studied in Lijiahe Reservoir source water.The results showed that the consortium A removed 40.43%of nitrate,30%of TN and 81.13%of the total organic carbon（TOC）in oligotrophic source water with extra 15 mg/L TOC during 24³6 h aerobic cultivation.The consortium B removed 41.59%of NO₃^--N,39.84%of TN and 76.37%of TOC in oligotrophic source water with extra 10 mg/L TOC after 16 h cultivation.The ammonium（NH₄⁺-N）and nitrite（NO₂^--N）were not accumulated.Within a certain range,adding carbon source was helpful for simultaneous removal of nitrogen and carbon in source water.Adding a small amount of phosphorus promoted bacterial growth.Under a lower temperature（10°C）,although the rate of growth and denitrification were slowed down,there was a slightly negative influence on final nitrogen removal efficiency.According to the sequence analysis of 16S rDNA,the dominant strains of natural consortium A were Acinetobacter（28.26%）,Pseudomona（9.90%）,Comamonas（4.38%）,Brevundimonas（2.71%）.The oligotrophic consortium A and B removed nitrogen and carbon mainly through self-assimilation and aerobic denitrification..（3）The nitrogen removal performance of the immobilized mixed-consortiums B was also explored in micro-polluted source water.The bio-enhanced denitrification of source water was achieved through immobilizing by polyurethane foam.The results revealed that after around 6 days of immobilization,approximately 30%³5%of NO₃^--N and 25%³0%of TN in the system could be removed by the carriers,with 9cycles of reuse.Mixed consortiums could adhere well to the surface of the carrier and internal pores.Depending on aerobic denitrification and carrier adsorption,nitrogen was reduced in oligotrophic water.Polyurethane carrier immobilization consortium B can persist long-term stability in the system to become dominant species,which could ease the problem of free state agents easily drained during the strengthening process.