Research On High-efficiency Biological Denitrification Technology Of Nitric Acid Wastewater

Nitric acid wastewater is widely used in metal processing,fluorine chemical industries,etc.It has the characteristics of low pH,high nitrate nitrogen concentration,and low carbon/nitrogen ratio（COD/TN,C/N）.It is difficult to deal with denitrification by traditional biological methods,and the increasingly stringent emission standards cannot be met.The continuous feed water batch reactor has been developed on the basis of the traditional sequential batch reactor,and has obvious advantages in load capacity.This study compared the operation effect of the continuous feed water batch reactor under different working conditions,optimized the reactor treatment matrix,improved the process technology,and solved the difficulties in engineering application,laying the foundation for the engineering application of the process.The research results obtained are as follows:1.Under conditions of sufficient carbon source,the reactor can be used for pH=1.3.And the nitric acid wastewater effectively removes 99%of TN.2.The optimum C/N ratio of sodium acetate,sodium acetate+methanol(COD _{sodium acetate}/COD _methanol=5/1)is between 3.5/1 and 3/1.The C/N of methanol as carbon source should not be less than 3.2/1,and need to add alkaline reagents.The dosage of the alkaline reagent was controlled at 0.79 g/L（NaOH/waste water）to stabilize the pH of the reactor at 7.4±0.1.At the same time,comparing the concentration of Total Dissolved Solids（TDS）and operating cost,it is concluded that methanol and potassium dihydrogen phosphate are better than sodium acetate and dipotassium hydrogen phosphate as carbon and phosphorus sources,respectively.The effluent TDS content is very low,only 1481 mg/L,the cost of tons of water treatment is only13.8 yuan.3.The aeration process more effectively blows off the microbubbles in the activated sludge than the agitation process,thereby better improving the sludge settling performance.At the same time,a part of the remaining COD in the tail water can be removed,and the degradation rate（COD/MLSS）is about 18.8 mg/（g·h）.And the dissolved oxygen generated by aeration can be reduced from oxygen saturation to anoxic state after standing for about 50 minutes,without affecting the denitrification in the next cycle of the reactor.4.The volumetric load and sludge load（NO₃^--N/MLSS）of the reactor can still remove more than 99%of the TN in the nitric acid wastewater after they are increased to 0.68 kg/（m³·d）and 0.61 kg/（kg·d）.At this time,the treatment rate of NO₃^--N can reach 28 mg/（L·h）.And through theoretical analysis,it is considered feasible to use the membrane effluent to improve the existing SBR process to the AO-MBR process.5.The denitrifying sludge can maintain high denitrification activity after stopping for 20 days,so the denitrification performance can be quickly recovered within 1-2 hours after the reactor is restarted.In contrast,pH-shock activated sludge has a slightly longer self-healing process.6.ORP can be used to effectively indicate the denitrification process,but the pH is not acceptable.When ORP<-250 mV,the denitrification effect is good;when-250mV<ORP<0 mV,the effluent NO₃^--N needs to be further tested to accurately judge the reactor treatment effect;if ORP>0 mV,it can be concluded that the denitrification effect of the reactor is not good.