Experimental Research On Partial-denitrification Processes For Nitrogen-rich Rare Earth Mine Wastewater

The strategic value of rare earths is enormous.The Weathered Crust Ion Adsorption Type Clay Ore of China is mainly located in the southern Jiangxi.In the process of ionic rare earth mining,ammonium sulfate is used for in-situ leaching.During the process of mining and extraction,a large amount of ammonia nitrogen tail water containing rare earths will be produced.Ammonia and nitrate coexist in the tail water of ammonia nitrogen.Nitrification-denitrification processes is the mainstream nitrogen removal method and it's costly.Therefore,it is necessary to develop a new type of biological nitrogen removal technology.Partial-denitrification/anammox processes is a hot spot of nitrogen removal,it has broad application prospects in the ammonia nitrogen tail water of rare earth mines.In order to promote the practical application of partial-denitrification in the ammonia nitrogen tail water of rare earth mines,the started-up and operational method of partial-denitrification has been studied and reported,and simulated actual wastewater,the short-term and long-term effects of heavy rare earth element Y(?)on the partial denitrification processes were studied and reported.Further,the species selection mechanism and nitrite accumulation mechanism of partial-denitrification bacteria,as well as the action mechanism and toxicity mechanism of rare earth ions on microorganisms,were explored.The main results are as follows.With the start-up experiment,it is found that the reactor can be started-up quickly and operate stably by controlling the effluent nitrate concentration in the above the critical value.After the reactor is started-up successfully,nitrate reduction ratio(NRR)is 90%,nitrite accumulation ratio(NAR)is 60%,Thauera abundance is 50%.The electron donor competition promotes the enrichment of Thauera.The higher ratio of nitrate reduction rate to nitrite reduction rate results in the accumulation of nitrite.Batch experiment showed that when Y(?)concentration ranging 1?50 mg/L was employed,the concentration of effluent NO₂^--N did not change significantly.It was also found that,bacterial activity was promoted when Y(?)concentration ranged 1?10 mg/L but inhibited when Y(?)concentration was elevated to a range of 20?100 mg/L.The study further revealed that extracellular adsorption of Y(?)had occurred within sludge biomass which led to the observed inhibition on bacterial activity.Addition of Y(?)made the bacteria produce more extracellular polymeric substance(EPS),and encapsulated the bacteria to resist the toxicity of Y(?),and EPS affected the mass transfer of substrate.Long-term experiments revealed that 5 mg/L Y(?)could gradually decrease the denitrification efficiency.The study further revealed that denitrification performance within the reactor could be restored when Y(?)addition is ceased,but NO₂^-accumulation was not restorable.Metagenomics analysis showed that the observable poor performance in denitrification process was due to Y(?)accumulation which had blocked relevant transporter channels on the cell membrane,and thereby affecting substances transmission which subsequently inhibited activities of denitrification-related enzymes and ultimately leading to microbial deaths.The denitrification function of the reactor could be restored in that,Y(?)which was accumulated in the periplasm could be excreted,and the periplasmic nitrate reductase(Nap)activity in the cytoplasm was also to be restored.The nitrite accumulation function of the reactor could not be restored in that,Y(?)which was accumulated in the nitrate transporter(Nar K)could not be excreted,and the membrane-bound nitrate reductase(Nar)activity in the cytoplasm was also not able to be restored,the phenomenon resulted in decreasing in the nitrate reduction rate,or the decrease in the abundance of progressive onset(PO)strains with nitrite accumulation function in Thauera and the increase in the abundance of rapid,complete onset(ROC)strains that can directly reduce nitrite.such that there was no accumulation of nitrite.Therefore,using partial-denitrification to treatment the ammonia nitrogen tail water of rare earth mines,when the water quality fluctuates,the impact of rare earth ion concentrations greater than 50 mg/L on partial-denitrification should be avoided.During the long-term operation,it should be ensured that the concentration of rare earth ions in the effluent is no more than 5 mg/L,the cumulative toxicity of the rare earth ion on microorganisms should be avoided.